Bertram Hill
  • Home
  • Contact
  • Motorsport
    • VSCC Rallies
    • VSCC Trials and Tours
    • Motor Sport Circuits
    • Clubs/Hill Climbs
  • Technical
    • Vintage and PVT Information
    • Classic Car Security
    • TVR Griffith
    • Making Exhaust Gaskets
    • TVR S3
    • Multimeter
    • Volvo V40 >
      • Volvo V40 Snapped Bonnet Cable
      • Volvo V40 Changing the Cam Belt
      • Volvo V40 Changing the Alarm Siren
      • Volvo V40 Locking Wheel Nuts Seized
    • Interesting Technical Sites
  • Links
    • Classic/Vintage Car Sales
    • Classic/Vintage Suppliers
    • Workshop Information Sites
    • Driving Links
    • Workshop Suppliers
    • Useful Sites
  • Purchase Prints

Using a  Multimeter to fault find on Classic and older cars

Picture
This information was originally published as an  article in TRaction the club magazine of the Triumph  TR Register and although biassed towards the sidescreen Triumph TR's hopefully  it will be useful to owners of other marques.
 
This information has  been gathered from personal experience and is believed to be accurate but there  is no guarantee that this is the case. Anyone using this information does so on  the understanding that its use is completely at their own risk and that no legal liability of any kind will be accepted by the author for errors or ommisions or consequential damage to persons or possessions. Prospective users should make their own considered judgement or seek specialist  advice as to the accuracy or otherwise of any statements made before using this  information in any way.

A little knowledge is a dangerous thing ! This is especially true with electricity, as without a great  deal of care it can cause untold damage to both person and possessions.  Automobile electrics are however rather more forgiving and while without care  you may damage some car components you are unlikely to kill yourself . 

Electricity is unseen so we  need instruments to indicate its presence the "Multimeter "is one of  these.  Wheras multimeters were once expensive and the domain of  electronics/electrical  specialists who understood the capabilities and limitations of these devices  they are now inexpensive and commonplace but unfortunately not all users  understand them. Hopefully the following will go some way to improve matters so  that enthusiasts will be able to use a multimeter more effectively.  In my experience many people are confused between Current measured in Amps and Voltage (or Potential Difference) measured in Volts. To make any sense of electrical measurement it is  vital that these two are understood. 

Before an electric current  can flow, we need something for it to flow through e.g. a piece of  wire, termed  a conductor. We also need something to drive the current through the conductor  that is a voltage across its ends.  What does this mean in practice. Well as electricity is invisible we can use a water analogy to   explain:  In Figure 1 the tank of water feeds a vertical tube with taps at the levels h , 2h and 3h. Each tap is  the same size. If tap A is opened the water spurts out to distance d from the  tube, if tap B is opened water now reaches distance 2d and likewise for tap C the water reaches to 3d.
Picture
Picture
                                    Figure  1                                                                                                                        Figure2                                 
                                                                              
As can be seen the higher  the head of water the greater distance the water spurts out. It is exactly the  same with electricity. If the head of water is now replaced with batteries as in  Figure 2 a single battery cause current I amps to flow, doubling the voltage by  having two batteries in series causes 2 x I amps to flow and likewise 3  batteries in series causes a current of 3 x I Amps. Returning to  the water analogy if we now imagine a domestic central heating system. If a  ground floor radiator develops a pin hole leak the water will spray out in a narrow jet as the pressure of water forces it out. If however in trying to stem  the flow we inadvertently pull a feed pipe out of the radiator (in addition to the loud blast in your ear from your loved one) the water gushes out in abundance as there is now a much bigger hole and thus much less resistance to water flow. In exactly the same way any conductor will have a certain resistance  to current flow, this could be a copper wire or perhaps a plastic clothes  line.  If the terminals of a car battery are shorted out with some copper cable there will probably be a loud crack and the cable will most likely burn out and you may  damage your battery -Dont try it.  If the experiment is now repeated with a plastic line  there will not even be a spark. This is because the plastic clothes line has a very high  resistance to current flow wheras the copper cable has a very low resistance. A long time ago Mr Ohm  discovered that there was a very simple relationship between current flow,  voltage and this  resistance:

I  (current in Amps) = V (potential difference in Volts) Divided by R (Resistance in Ohms).   
                                                         i.e.
I =V/R
If we substitute typical  values for a 12 volt car battery, a yard of thinish cable could have a  resistance of 1/10 of an ohm and the plastic line about 10 Million ohms .  If we return to Ohms equation of I = V/R   So for the cable I = 12/0.1  i.e 120 Amps ( a large current) but  For the clothes line I  =  12/10,000000 i.e. 1.2 Micro amps (1.2/1,000000) a barely measurable amount.   This is the basis of  all  measurements with a multimeter, understand this and you are half way there. 
 
For an electric current ( I) to flow through any conductor of resistance (R) there must be a driving force  or potential difference (V) across it. The current flowing, I =V/R.  Simples!

Multimeters can  measure voltage and current , voltage and current may be Direct as from a  car battery or Alternating as from   the mains supply. They also measure resistance. For Classic cars such as the TR  the vast majority of measurements will be either of DC Voltage or Resistance. 
 
An initial warning, with all multi-meter measurements when measuring Resistance ensure that all Voltage supplies are  switched off , if  not the measurements  will be affected and you could destroy the meter.  Always disconnect the vehicle battery before making resistance measurements. 

Before Looking at some typical electrical faults familiarize yourself with your meter. Try measuring  the dc voltage across your car battery, and try various smaller torch type  batteries so as to get used to measuring. You should also study your meter instructions to understand its capabilities:

Select the Resistance (Ohms) range on your meter and short circuit the two measuring probes together,  the meter should read zero 0 ohms. Try and measure your own resistance.   Holding the meter probes in each hand a fairly high reading of around 50,000  ohms or 50 K-ohms is typical, the value depends on how thick skinned you are!  This is one good reason when measuring resistance for keeping hands away from the metal probe ends to avoid modifying the readings. Now wet your hands and repeat the measurement, you will find it much lower as water although a  relatively poor conductor is better than skin. This is why electric shocks from the 240 volt ac house mains are much more serious in a bathroom or kitchen as  the water lowers your resistance and causes much greater current to flow, frequently a lethal amount. 

The 12 volt car battery by  itself cannot give you an electric shock as its voltage is too low but its capacity to burn out cables etc. and harm you through burning if still a  possibility as anyone who has experienced a vehicle electrical fire will testify. This is the reason that fuses are used for the various circuits and  electrical devices. In the event of a fault condition (low resistance) that could cause a cable to burn, the fuse fails instead and protects the cable. 
A reminder of Ohms law:                               I=V/R                 as R  is reduced  so I increases 
  
Next try measuring the  resistance of some car bulbs. Three typical 12 Volt automobile bulbs as shown here:
Picture
The 12 Volt 21/5 Watt rear  side/brake lamp bulb to the left, a typical Quartz Halogen Headlamp/Dipped beam  bulb to the right rated at 12 volt 60/55 Watt and a 12 volt 2.2 Watt dashboard  lamp in the centre. We will deal with "Watts" in more detail later for now just  think of it as the Power of the bulb or its light output,
large  Watts = bright  light and vice versa. 

The filaments of the headlamp/dipped beam  bulb  should have resistances of around 0.5 and 1.0 ohm, in other  words  a low value. If it measures a high value then the bulb has blown.  Similarly the  21/5 Watt bulb filaments should measure around 1.0 and 2.5 ohms. The  much lower power  dashboard lamp should measure around 7 ohms. If the resistance measures very much  more than this then the filaments are probably open circuit. Some meters are  rather inaccurate on the lower ohms ranges and may indicate  zero ohms for all of  the above. That is OK, only a high ohms reading indicates  a failed filament. This is a quick and easy method of checking if car bulbs are serviceable. 

Typical car electrical faults will now be looked at. 
Caution:   Modern  vehicles use  complex electronics to control a whole raft of electrical devices  including such obscure things as monitoring whether the lights are functioning  correctly. Do not attempt to follow the advice given below in tracing faults on  these  vehicles as the chance of causing very expensive damage is a distinct  possibility. Fortunately original TR's are quite simple in comparison and you  are most unlikely to cause any damage. 
In the first article it  was  shown that for an electric current " I " in Amps to flow through a conductor of  resistance " R" measured in Ohms we require a voltage or potential difference  across the ends of a conductor " V " volts. And that according to Ohms law:  I = V/R 
 
So that if we are able to  measure two of these quantities then the third can easily be calculated. A multimeter allows this to be acheived. In tracing electrical faults on classic  cars the two quantities most often measured are DC Voltage and Resistance. 
 
Moving on to a typical  fault likely to be found on TRs.
Fault  1: Dip headlamp on the nearside not working but main beam ok and off-side lights all ok.  Initially we need to look  at the circuit diagram, using the TR2 diagram below as an  example:

Picture
Triumph TR2 Circuit  Diagram Scanned from the TR2 Service Manual
Before measuring anything,  with a bit of logical investigation it is possible to narrow the fault down to a  few parts of the circuit. The relevant wiring to the nearside dip lamp is  highlighted in green:

  1.The wiring to both dip headlamps are  taken from a common terminal on the dip switch (dipper switch) so the dip switch  is obviously working otherwise the off side dip would also have failed. 

  2. Likewise the -12  volt  feed to the dipswitch must be present as without it neither dip nor main beam  bulbs would be illuminating. 

  3. So the fault must  lie  between "A" and "B" i.e. somewhere between the dipswitch terminal and the nearside dip lamp bulb. 

We can now narrow the fault  even further by measuring the resistance of the dip/main lamp bulb,  remove it  from its socket, clean its terminals and check for a low resistance on both  filaments. A high (greater than a few ohms) measurement would indicate  a blown  dip filament.  Assuming the dip filament  checks ok we can move back to the car and make a DC voltage measurement. Note,  that unlike moderns original sidescreen cars were wired with a Positive earth.   With the meter on an  appropriate DC volts range, switch on the dip beams and carefully measure the  voltage at the nearside dip/main beam bulb socket. With the positive meter lead  (Red) to the earth side of the socket measure the dip and main beam terminals in  turn (with the Black probe), be careful not to short circuit the meter probes  otherwise you may see some sparks and melt your probes. The main beam terminal  should read -12 volts (as it was working ok) the dip probably zero or less than  -12 volts. Check for corrosion, switch off the lamps and clean the socket and  lamp bulb as  necessary. If the dip beam socket terminal now measures -12 volts,  the fault  has been  located. 
  Corrosion of lamp sockets  is very common. ( if your car has been converted to Negative earth,  reverse the  polarities above). 

If the fault persists, now  identify the nearside dip beam connection on the dip switch and with dip beams  switched on, measure the voltage with the positive meter lead earthed  (battery  +ve is fine), it should measure -12 volts, if not the teminal will most likely  be corroded, clean it up and remeasure. 
With -12 volts on the dip  switch terminal but zero or low volts still on the dip lamp socket the fault  must lie in the wiring between these points i.e. between A and B or more  likely  within a connector in the circuit. 

Bullet Connectors 
Picture

Bullet connectors are used extensively in the side screen cars and are 
notoriously unreliable and liable to corrosion.  With the  battery disconnected and
the meter on a resistance range measure the  resistance from the dip switch
terminal "A" to the dip beam lamp socket terminal  "B", it should measure a
fraction of an ohm, anything higher indicates a faulty  connection. Trace the
cable route and check and clean any connectors in the  circuit and look
particularly for corroded bullet type connectors. Bearing in  mind the age of
the cars the connectors may have been changed to a more modern  plug/socket but
could even be just bare wires twisted together and insulated.  Wires and or
connectors may need resoldering. 


Copper wire that has been  in service for a while will most likely be oxidised to give a black appearance,  even under its insulation. The wire must be cleaned back to a  bright copper  colour with fine emery cloth otherwise it will not solder  effectively.   When the resistance  from A  to B is a fraction of an ohm, and with -12 volts at the dip beam terminal and a  good main/dip lamp bulb plugged in there is now no reason why the lamp should  not operate. It is possible for the wiring itself to fail within the insulation  but in my experience unless there has been some abrasion  or overloading etc.  This is most unlikely. 

So by a logical  sequence of  tracing the circuit, measuring voltage and resistance the fault has been  identified. This same sequence may be used to fault find almost any electrical  fault on a TR, summarising
  • From the circuit identify  the relevant wiring /components and pin point the likely culprits 
  •  Using the meter measuring  resistance and or dc voltage to drill down even further
  • Clean or replace corroded  connectors and terminals resoldering if necessary
  • Identify the fault 

What is a Watt?  Earlier, three common car bulbs  were shown with various power ratings ranging from the dashboard bulb at  2.2 Watt to the head lamp main beam filament at 55 Watt. So what is a watt? A  Watt defines the rate that work is expended or put another way the rate of   energy conversion. For a car bulb it measures the amount of energy used by the bulb, not the light output. The light output will depend on how efficient the 
bulb is at converting electrical energy into light. 1 Watt is defined as the power expended when 1  volt causes a current of 1 Amp to flow through a circuit.   This brings us to a  useful  formula: 

                                                                   
W=I xV  or Watts = Amps x Volts.

 So knowing the power of  say  a bulb e.g. 55  Watts and if used  on   a normal 12 volt circuit we can easily
  calculate the current flowing:

                                                                  W = I x V therefore I =  W/V  and in our example I = 55/12 , i.e. 4.58 Amps. 
So why is this useful?  Well  in the above example knowing the power of a bulb we can easily calculate the  current drawn and thus the thickness of cable required. It is usual to allow  some margin of safety, so for the headlamp bulb, cable rated at 10 amps   maximimum, should be adequate. 

Why do car batteries or  indeed any batteries eventually fail? Well, above, when a simplified circuit of  a battery driving current around a loop was shown some elements were initially  omitted, this diagram is more complete:
Picture
 Any circuit even a  loop of wire will have some resistance ("r2 and r3" left) this needs to be added. All  batteries also have some resistance even if this is very low. This is  represented by "r1" in the diagram.  When a car  battery is new and fully charged its internal resistance
r1 will be very low, a fraction of an ohm. 
To start a 2 litre  engine  on a cold day when the oil is thick can take a very large current. If we  assume  that the starter motor ( R) has a resistance of 0.1 ohm*  and the batteries resistance r1 is 0.01 ohm  then from Mr  Ohms  law:

I = V/Rtotal , i.e. I = V/R+r1           i.e  I = 12/ 0.11 = 109 amps 

If the battery  becomes discharged r1 could rise say to  0.2 ohm, recalculating:
                          
  I =12/0.3  i.e. 40 amps ,  a large  current but maybe not enough to turn  the cold engine. 

Additionally ,  in the cicuit we omitted the resistance of the  leads from the battery to the  starter motor and the main battery earth lead  represented by "r2 and  r3".  So a truer value  of current flowing
I is: V / (R +r1 +r2  +r3) 
So if either of these leads has a  corroded contact it could easily represent a resistance of 0.2 ohm,  or more, enough to  cause the car to fail to start. 

While on the subject of  circuit resistance, why is it that the 240 volt ac domestic supply is very  capable of killing you wheras the much higher 20,000 volts of the ignition HT  feeding the spark plugs seldom causes much beyond a shock. Well again all down to the internal resistances* "r" of the supplies.  For  the ac mains "r" is very low and not a limiting factor wheras for the ignition  HT circuit* "r" is very high so cannot  sustain a high  enough current to cause harm. 

A typical ignition low  tension (LT) circuit will be passing about 1 Amp through the contact breaker and  the ignition coil primary. 

So the power expended  by  the LT side is
12 x 1 = 12 Watts 

Now the high tension  (HT)  side of the ignition coil generates about 20,000 volts.   As the HT power out has to  be less than the LT power in (due to heat losses in the coil) if we assume 2  Watts is lost as heat we are left with 10 watts to fire the spark plugs. 
                         
    Now from above I = W/V, so the current flowing is 10/20,000 = 0.5 millli amps 
 
   *And the effective resistance of the coil secondary R =  V/I, i.e. 20,000/0.5 = 40,000 Ohms 

This is the reason that the  inclusion of a radio suppressor or a high resistance suppressing ignition  lead  each with a resistance around 5,000 ohms , makes little difference to the   quality of the spark. 

*With the ac mains supply, motors and in fact ignition  coils although internal resistances have been mentioned, internal impedance  would be the more correct term. With an alternating supply, coils and  transformers are involved and other factors termed inductance and capacitance  have to be considered along with resistance that gives rise to Impedance, but  this is beyond the scope of this article. 


Warning: Some electronic ignition  circuits are capable of delivering a much greater shock that could harm someone  with a weak heart etc. 

A bit off  the  subject, but a good way to test ordinary torch batteries NOT CAR BATTERIES.
 A normal AA battery has a  voltage of about 1.5 volts, 1.3 volts if a rechargeable cell. So with the meter  on dc volts first measure the voltage. If this is low then the cell is definitely discharged. If it measures OK it may still be unservicable because a  meter on its voltage range takes a negligable amount of current and does not   test the battery under normal working conditions. The batteries internal  resistance may have risen to prevent it delivering sufficient current to be  useful in service
.

Picture
AA, AAA Rechargeable cells  and a PP9 Battery 

Select the high DCamps  range on your meter and momentarily short circuit the torch battery,  I mean  extremely quickly to avoid discharging the cell.  A fully charged: AA  cell  should give between 5 -10 amps .
An AAA cell between 3-8   amps 
And a 9 volt PP9 cell  about  3 amps 

This is an extreme test  and  maybe frowned upon by some but I have used it successfully for over 30 years to  decide whether a torch cell is servicable. It is also about the only  time I use  the meter to measure DC current directly. 


  
Earlier the basic relationshipbetween  Voltage(V), Current(I) and Resistance(R) was shown to be: 
 
                        I = V / R,                             i.e. Amps =  Volts / Ohms 

And that Power W =V x I,                         i.e. Watts = Volts x Amps 
 
Tracing electrical faults can be rather daunting  to many but by  using a logical approach there is no reason why most electrical  faults on a TR  cannot be identified or at least narrowed down. Although  specific examples will  be explained it is the method of diagnosis that is  important not the specific  faults described.

A  fuse Blowing 
A fairly common fault in older cars is a fuse blowing, sometimes  the fuse itself can be faulty so changing it cures the fault  but much more  frequently it means that  something is  overloading the circuit and changing the fuse merely causes it to  blow again.  The Circuit Diagram below  has been kindly supplied by Autowire  www.advanceautowire.com
Picture
This shows the layout and components very much clearer than the  original factory diagram. The basic TR3A only has two fuses as shown, one  between A1and A2 the other from A3 to A4. Unlike modern cars some circuits are  not fused at all. The 3As circuits are powered thus: 
  •    The main battery Positive is earthed to the chassis and  engine, the battery negative (-VE) is initially wired to the starter motor. From   there it   connects to fuse A1 and the Ammeter.
  •      From the Ammeter it feeds terminal “A” on the control  box.
  •      Inside the control box after passing through a few turns of a low resistance sensing coil it emerges at terminal “A1”, for practical purposes  in   this example “A” on the control box may  be viewed as directly connected to “A1”.
  • From Control Box “A1” it then feeds both the ignition and lighting switches.
So all of the supplies for a TR3A pass through the ammeter with  the exception of the horns as they are fed directly from A2 via the fuse to A1. 
 The circuits connected to fuse A1-A2 are permanently live, those  connected  to fuse A3-A4 are via the ignition switch so are only live with the ignition on. If the wiring is traced from the lighting switch it can be seen  that apart for USA market cars there is no fuse, so a short circuit on  any light  could cause the loom to burn
out. Likewise tracing the ignition switch wiring,  this connects to fuse terminal “A3” and then directly to the ignition circuit,  so a short circuit in the coil  could also possibly causethe loom to fail. Fortunately ignition coils rarely fail that way but it does show the wisdom of fusing circuits to avoid a  major problem. Fitting an auxiliary fuse in the lighting circuit as for USA cars  is a wise move and could save an expensive burn up.   Even so both dip and main  beam headlamps are still not fused at all.  Contrast the TR with a modern car  where nearly every electrical  component has a separate fuse. 
 
From Ohms law a 5 Watt rear side light bulb takes 5/12 =0.42 Amps   when operating. The diagram above indicates that three lights  are connected in  parallel  so to give some degree of  safety and mechanical strength a cable rated at about 8 Amps capacity would be  used. Now if the bulb socket develops a short circuit to  earth  say of 0.1 Ohm,  the current now shoots up to 12/0.1= 120 amps.  The cable  and your loom would  then burn happily. If a fuse say of 30 Amps capacity had  been used (as for USA  cars) then this would rapidlyblow and protect the loom. 

Typical  Fault: Fuse between A3 and A4 blowing 
Looking again at the circuit the switched live  –VE from the  ignition switch feeds fuse terminal “A3” and the ignition circuit. As the  ignition  circuit is wired before the fuse it cannot affect it. So attention is drawn to the circuits after the fuse i.e. those  connected to “A4”, namely: 
  • The Wiper motor
  • The Fuel guage and sender
  • The brake lights
  • The direction indicators
     
    Remove each wire  from the A4 terminal and insert a new fuse. You can check that the  fuse is OK  using the Ohms range on your multimeter, it should  measure zero or a very low  value. Now replace each wire one at a time  until the fuse again blows. Although this wastes another fuse the fault has  now been narrowed down to  a single circuit.  Mark the offending wire and  reconnect the other good  circuits  to “A4”. 
    If we assume that the brake light circuit was causing the fault this  circuit can now be investigated further.  Looking agan at the wiring diagram above the  brake light circuit  comprises the brake switch, the brake light bulbs and the associated wiring. So  one of these components or the wiring is shorting to earth to cause the fuse to  blow. As with all resistance measurements first disconnect the battery and  earth one meter probe.  Select the Ohms  range on the meter and measure the resistance of the disconnected brake wire  light wire to earth, it will be a low reading. Then disconnect the other end of the wire i.e. its connection to the brake switch and the short circuit should  clear. If not the wire is shorting to earth somewhere. Replace the brake switch connection and now disconnect the output wire from the brake switch feeding the brake lights. If the short remains then the switch must be faulty. If the short circuit clears the fault  must now  lie beyond the switch. Reconnect the switch output wire and in turn disconnect  each brake light until the fault is isolated.
So summarising
If a fuse is  blowing first identify the offending circuit by disconnecting all output wires from the fuse and reconnecting in turn. Then systematically check  the resistance  of  the faulty circuit until the offending component/connector  or loom cable is found. 

A lamp or Component not functioning 
The most common fault is a light or other electrical item not  working. One procedure to trace this type of fault i.e. a failed  dip headlight has already been explained.  Another approach is to bypass the wiring loom and run parallel live and earth  feds direct to the component. Suppose the windscreen washer motor  stops working.  
  • Identify which wire to the  washer motor is earthed and which is live. 
  • Disconnect the live wire 
  • From  the battery –VE connect a flying lead direct to the motor live and see if it  runs. If it does there is a  fault in the live feed to the motor  so check the fuse, the, switch and  the  wiring
  •  If  it still fails to run reconnect the live and  then disconnect the earth lead from the motor 
  • From  the battery +VE (earth) connect a flying lead to the washer motor earth connection. If  it now runs the earth lead is faulty. Clean the connections and check the  wiring. 
  •  If it  still does not run the motor itself is faulty, so as a double check use your  meter on its DC volts rangeto see if 12  volts appears across  its terminals.
All electrical fault finding should follow a logical sequence to  trace faults: 

    1. Use  the wiring diagram to narrow down the likely components and/or wiring

    2. Use  the meter on its DC volts range to check that supplies are reaching the  component

    3. With the car battery disconnected use the meter on its resistance (Ohms  range) to  check wiring for continuity of the connecting cables  i.e   zero or very  low Ohms 
 
    4. Check for short circuits to  earth, again zero or low Ohms
 
     5. Check  for poor connections and high resistance connectors, terminals and wires (high  Ohms) 
 
Testing individual components 

Lamp bulbs  Ensure  the contacts are not corroded and with the meter on Ohms range check for a very  low reading. A high reading indicates  a blown bulb

Horns Check for corroded contacts. Disconnect existing cables and run cables direct from the battery to test

Switches   For example: lighting, dip beam, windscreen wiper motor, washer motor, horn etc.  Disconnect existing cables and measuring across the contacts with the switch ON the meter should indicate 0 or very  low Ohms. With the switch OFF the  meter should read an open circuit or a very high reading (keep fingers away from  the probes to avoid lowering the reading)

Ignition capacitor  Not easy to measure without specialized equipment, substitution with a known  good example is the easiest check. Do not assume that a new capacitor or indeed any other after market component is necessarily OK there are a lot of poor ones on the market

Control Box Unless you feel confident following the alignment in the Service Manual best left to a  specialized service agent or replace with  a known good   unit

Starter Motor If this is suspected, remove from the car and test on the bench. When connected to a good 12 Volt battery it should run at a high speed.   This is  not a definitive check as the motor will still need to be checked under  load ( i.e. on the car) but if it will not run on the bench  it is definitely  faulty.
 
Dynamo The  Ignition light should extinguish above idling speed. The ammeter gives a good  indication if the dynamo is charging as just after starting a strong positive  charge should be observed. If your car has a voltmeter  then the volts should be at least 13 volts above idling speed. Another check is to remove the Control box plastic  lid and observe  the “Cut Out” contact as the engine is revved above idling. The  contacts should  be seen to close and then open again as the revs drop back to  idle - this  corresponds to the ignition light being Off then  On.
 
Ignition Coil  Difficult to check reliably as they can fail with heat (as the engine warms up) and/or as the revs increase. Substitution with a  known good unit the best approach, again be aware of poor after market products.

Fuel guage sender Do  not attempt measurements with it in the tank due to the extreme volatility of  fuel. On the bench, with the meter on Ohms range  measure across the terminals as  the float arm is gradually moved from minimum to maximum. The reading should  gradually increase from  0 Ohms to around 70-90 Ohms. Any sudden very high  reading (or open circuit) indicates a damaged resistance track or the moving  contact losing contact  with the track. It is worth trying to clean the track  gently or the contact may be bent slightly to increase contact pressure before  buying a replacement.
 
Identifying electrical faults on a TRis not difficult provided you adopt a logical  approach and are patient.  
 
Best of luck!