TROUBLE SHOOTING ELECTRICS
We all spend time looking after the mechanical and body work
of our cars but rarely touch the electrical systems.
Just with anything else, a little preventive maintenance will help prevent
The most common failures are due to poor
Take five minutes once a month to check for over heated and
loose terminals and wires.
Pay particular attention to;
The lighting and ignition switches, fuse box, relays, starter motor and
Any evidence of over heating and or loose joints should be dealt with
Multi block connectors should be checked and cleaned with WD40 and then packed
with petroleum jelly or silicon grease before plugging them back together.
Any suspect terminals should be replaced, preferably by crimping and soldering
the terminal to the cable.
(With simple test equipment!)
It wonít turn over! Where have we heard this one
As you try and start the engine look at the dash lights. If they go out or very
dim either the battery is flat or you have a poor connection on, it, the chassis
earth, main engine earth or starter motor.
The starter may or may not go click or rat tat tat.
If the dash lights remain at full brightness and you hear no click from the
starter motor then there is a failure between the ignition switch and the
starter solenoid. Check the inhibitor switch, ignition switch and any relays
that are in that circuit. Sometimes the inhibitor switch on auto boxes can go
out of adjustment or the wires fall off.
If the starter solenoid goes click and the lights remain bright then either the
motor itself is at fault or the contacts or the wiring on the solenoid are
To give you an idea if the charging circuit is working. Run the engine and turn
on all the lights, they should brighten up a little as you rev the engine from
idle. If they do not brighten up, either the battery is very well charged or the
alternator has stopped charging.
This only a very very crude form of observation and wonít work if your engine
idles above 800RPM but might give you hint if the battery is constantly going
If you have a situation where the battery is going flat there are going to be
1 The battery is at the end of itís life. An indication of this is if it
suddenly loses itís charge especially in cold weather.
2 The alternator has stopped charging.
3 If you are constantly starting the engine and not allowing the battery to
4 There is a drain or short in the electrical system.
Suspect batteries can be checked by doing a ďdrop testĒ. You apply a heavy
load to it by means of a proper tester and measure the voltage at the same time.
A good fully charged battery should maintain 11 volts for over 10 seconds with
very little gassing in any of the cells. A duff battery will immediately start
to gas and the voltage will start dropping.
The charging circuit can be checked by measuring the voltage across the battery
while the engine is running at about 2000RPM. The voltage should be about 13.5
Volts and should alter only a little when you put all the lights on.
Make sure the battery is kept fully charged. A 24 hour low rate trickle charge
is the kindest way to do this and the area must be well ventilated.
A drain or short in the system is included in the next part. Donít go away
A bit about test equipment first.
Probably the most useful bit of kit you can have is a ďtest lampĒ, merely a
simple bulb with some wires attached. A 5Watt and a 21Watt bulb with a couple of
wires with clips on the ends is really all you need. (In addition an old 50Watt
car bulb can be placed in series with the battery to trace stubborn short
circuits). At the most a Voltmeter to check the charging system and voltage
drops is useful. You can get a reasonable meter for about £15 Maximum. Stay
clear of the digital ones unless you are prepared to pay for an American
ďFlukeĒ these are the best all round digital meter as far as Iím
LEAKAGES OR DRAINS
If you find the battery is constantly going flat, and it and the charging system
is OK, then you have a leakage or drain. You can gauge how serious the leak or
drain is by how quickly the battery is discharged. If it goes down over a week
or two then the problem could be something small like a boot light staying on.
If it goes down overnight you have a serious drain and should attend to it at
The following method is the safest and most efficient way of determining
leakages or drains. You cannot damage any of the electrics whilst the lamp is in
place and even accidentally shorting live wires to earth will do no harm.
THIS IS NOT THE CASE WHEN THE BATTERY IS FULLY CONNECTED.
The first thing to do is disconnect one terminal of the battery and place your
5Watt test lamp in series with it.
It lights up ? You have a drain!
Make sure everything is turned off and that the interior light bulbs have been
It goes out ? you left something on or the interior lights are staying on.
Itís still alight ? Now go around disconnecting things. Start with the easy
things like the alternator, then remove the fuses one by one, the minute the
light goes out will be the last thing you disconnected. If it was the alternator
then one of the diodes has probably gone leaky.....not uncommon. If removing one
of the fuses puts the lamp out, put that fuse back and start, one by one,
disconnecting and reconnecting any wires connected to it until the lamp goes out
again, that wire will lead to your problem.
If none of these puts the lamp out move onto the ignition switch, then the
lighting switch and so on. Remember youír only concerned with those electrical
items that are fed from the battery, not the ignition.
Do each item in turn donít try and rip everything apart because otherwise
youíll confuse yourself.....be methodical do it step by step and remember to
reconnect any lead that doesn't help find the fault!
Youíve found the wire but donít know what it does!
Remove the test lamp and reconnect the battery, leave the wire you suspect
disconnected. Now go and find what part of the electrics doesnít do anything.
Horns, cigarette lighters, boot lights, hazard lights. These things are usually
run off a battery feed so are permanently powered up. Remember, something can
still appear to work OK and still be faulty.
For this I recommend you use a 21Watt bulb for the test lamp
and an old 50Watt head lamp in series with the battery. You will find that most
of the electrical items will at least try and work depending on the power they
draw and in any event only have one circuit on at a time. The head lamp in
series with the battery will begin to glow brighter as you switch more things
on. Please do not attempt to turn the engine over.
IF YOU CHOSE TO FULLY CONNECT THE BATTERY FOR THESE TESTS
YOU RUN THE RISK OF DAMAGING THE ELECTRICAL SYSTEM IF YOU
SHORT A LIVE WIRE TO EARTH.
First of all disconnect the ignition coil, and electric pump if you have one.
Remove the fuse that blows and put the 21Watt test lamp across the terminals. If
you know what item is blowing the fuse switch it on, if you donít know, go
around switching things on until the lamp lights.
Now disconnect anything you know of that is controlled by that switch. If the
lamp goes out then the item youíve just unplugged is the culprit otherwise,
disconnect any wires from that fuse, note! the wires must be those being fed by
the fuse, not powering it, until the lamp goes OUT, that wire should lead you to
the source of the short, perhaps a chaffed or cut cable that is touching the
Weird earth faults
Put the lights on and the radio starts up.......yep Iíve had that, but the
best one was on a car where the engine died when you turned left. It turned out
to be a poor earth under the dash board.
Here are some examples of poor earthing.
Dash lights glow or flash when you brake or indicate.
Brake lights glow when you indicate.
Side lights come on when you brake or indicate.
Headlights at half brightness and both main and dip filaments are glowing.
The gauges play up when you brake or indicate.
Indicators go dim when braking.
All the lights come on when you try and start the engine.
All the lights go off when you try and start the engine.
Poor earthing can lead to the most difficult of faults to trace. The best thing
to do is go over every single earthing point on the car. Unbolt the terminals
clean them and the connection point, any paint and rust, must be
removed. The metal must be bright and shiny. Use a new star or grip washer
between the terminal and the metalwork.
Use two engine earth lines, one near the starter or an engine mount and one from
the rear of the gear box. Use the proper braided earth cable, not battery wire.
These earths should be taken to the chassis, not a body panel. The same applies
to the battery earth cable.
GRP body work.
All items mounted on GRP must have a decent earth cable going to the nearest
proper earth point.
Heavy current items such as radiator fans and head lights should have their own
separate earth wires going directly to an earth point.
Well that is a brief walk through of fault finding, the most important thing is
to observe and think about what you see most electrical faults are caused by
silly little things.