Dummies Guide: what do the ignition light and voltmeter do?

[Tim Proctor]

I know it seems like a silly question, but:

Our voltmeter varies between 15v and 11v, not seeming to depend on speed or electrical load (lights, battery charging) and when it goes low, we seem to get fuel pump problems: car dies, we find air in the fuel pipe which needs purging. So, where should the volts be in normal cruising and what does the voltage tell us?

Similarly, the ignition light has been flashing from time to time, and we don't know what it means. It doesn't light up when we switch the ignition on, and we're sure that before last week's new alternator fitting, it did. Looking at the wiring diagram, we can see that it sits between parts of the circuit that would nominally be at 12v, so clearly, it lights when there is voltage difference sitting across parts of the '12v' circuit. Beyond that, we're a bit stuck.

Louise

[Goldstar]

Sounds like your alternator is cookedr Tim/Louise. The ign light gets a feed from the alternator diode Pack, I think it’s called the excited wire (assuming 18acr alternator) which it sounds like yours isn’t providing. Voltmeter is the other clue, it should be higher than 12.8 v when the alternator is operatimy correctly - have you got a spare?

[mallardstag]

Agree with Goldstar that you have a problem with your alternator (or the control unit if you have a MK1 with external control unit). With the engine running you should have a steady voltmeter reading of about 14.5V (maybe dropping to around 13V with lots of electrical load), without the alternator working the voltmeter will read battery voltage (11V in your case indicating a discharged/faulty battery and this will cause your fuel pump to stop working before the engine starts).

The ignition light sits between the alternator output and the ignition circuit - so that with the ignition on (12V output) and the alternator stopped/faulty (0V output) the light will glow. With the engine running and the alternator producing 14.5V then you will have the same (or very nearly the same) voltage on either side of the lamp and it won't glow.

I hope this clarifies it a bit for you.

Simon

[Kithmo]

Check the wiring first, it could just be the ign connection (the small one) to the alternator. The fact that the alternator has been replaced means the connection has been disturbed. If the connection is broken the alternator won't get "excited" and won't start charging.

[Tim Proctor]

The old alternator died last week too far from home to get back. We were close to a classic car specialist garage and so they fitted the new one, so we're a bit uncertain exactly how it's been wired up. Ruuning, the voltage will be high for ten or fifteen minutes, then will drop for a while, with or without a loss of fuel pump. The ignition light does not come on with the ignition switched on and motor stopped, but it does briefly light occasionally while running. Does this make it sound likely that the wiring is wrong, or perhaps we have a loose connection?

Louise

[Goldstar]

Yes but not sure which. Surely they should have been able to unplug one alternator and plug the plug into the back of the next. Can you post a picture of the plug that goes I got eh back of the alternator?

[Tim Proctor]

Yes but not sure which. Surely they should have been able to unplug one alternator and plug the plug into the back of the next. Can you post a picture of the plug that goes I got eh back of the alternator?

Will do, next time I'm down at the barn where it lives.

[Phil S]

Is the alternator on a mk1 with an external regulator or a later one 17acr type with an internal regulator.
if a mk1 then it's a regulator not alternator problem
if a 17acr type then it sounds like the one fitted is a dud.
wiring can't be incorrect as even if no plug the terminals are different sizes.
Phil

[sujitroy]

I have a MK1 Stag with the original Mk1 setup. (External regulator) While idling, my volt meter frequently drops below 12V. Only when I rev up the engine beyond sat 1500 rpm does the voltage rise to to about 13.5 Beyond starting the car, my ignition light does not come on. I though this behavior was normal for the voltage reading to fluctuate like this on a Mk1 stag. Am I to be corrected?

Sujit

[SteveD]

I have a MK1 Stag with the original Mk1 setup. (External regulator) While idling, my volt meter frequently drops below 12V. Only when I rev up the engine beyond sat 1500 rpm does the voltage rise to to about 13.5 Beyond starting the car, my ignition light does not come on. I though this behavior was normal for the voltage reading to fluctuate like this on a Mk1 stag. Am I to be corrected?

Mines the same.

[Tim Proctor]

Ours is Mk1/2, what else would it be? The new alternator is a 45 amp 16/17 ACR, which seems to be the correct choice. The low volts are not following low engine revs. Photos of connectors on https://www.ratsport.com/PBSCProduct.asp?ItmID=9667761

[Tim Proctor]

We have more experience of the problem. The volts dropping and the fuel pump stopping always happens soon after being stopped for a while. Never after a cold start in the morning but almost invariably after stopping for lunch or coffee. A few minutes into the run, the volts fall to below 11.5, the pump stops, the car dies, the fuel line is air-locked. After several minutes, the volts rise, and above 12, the pump restarts. We purge the air, and off we go.

Maybe the fuel pump is over-fussy, is it normal to stop at 11.5v?

Is the electric fan possibly doing something to pull volts down?

Still no ignition light when ignition on, engine off, but it does light when the volts are below 11 and the engine is running.

Battery is charging, otherwise it would long be flat.

[jakesmig]

Sounds like you have a definite issue with the Alternator. The ignition light is used to 'excite' the alternator and switches on the coil in the unit. If you take the light wire off the alternator and connect it to earth the light should illuminate when the ignition is switched on. When the engine starts the electronics in the alternator are activated by the voltage from the ignition light and the alternator starts to work, as long as the alternator is turning over and working correctly the ignition light will never, ever light or flicker above idle unless there is a short to earth in the ignition light cable..

A charged battery will power a car without input from an alternator for some hours once the engine is running, if you start the car on a full charge it will run virtually all day as long as you don't use wipers, lights etc too much. The air locking in the fuel line is a weird one, where does the air come from, unless it enters the system from the tank or leaking joints it is either evaporation or the pump is reversing and drawing air back in. Good luck with that one.

Start with earthing the ign light cable, if it lights you have an alternator issue, go from there. Fuel air lock, check for leaks that would let air in, or hot spots where evaporation would occur. If it didn't evaporate before and cause air locks then that option is unlikely now.

John

[wilf]

The change in behaviour after "cooling down" after a period of heat soak suggests some time spent double checking earths and all the other connections to that alternator in the engine may could be worthwhile. Many electrical problems are actually "mechanical".

[davidf]

Also check the engine earth strap - if that is not making a good connection, all sorts of strange effects are seen. Mind you, any problem there would normally affect starting, as a big current flows through it then.