Ignition light on dashboard stays on too long

[Robert Morrison]

Hi all. Had my fairly standard TR7 DHC some 30 years now, been a member of the register that time too, yet this is my first query posted. 

When turning the ignition key one stop all the lights in the cluster come on as they should. However, when turning the engine on, all the lights go off except the ignition light. 

If I rev the engine to 2,000 + rpm the light goes off and stays off until I stop and then restart the engine.

I found an old article on this forum from 2007 (https://www.tr-register.co.uk/forums/index.php?/topic/11189-glowing-ignition-light/&tab=comments#comment-72644 ) and this would suggest an issue with the alternator. I did buy a new alternator from Robsport 4 years ago - and the car won't have done 1,000 miles in that time. So, I hope it's not needing a new one again.

The only other thing I would add is that this issue only occurred after a local garage cut out the rusty floor pan on the passenger side and welded in the replacement panel that I bought from Rimmers. The garage is adamant that they only disconnected the battery to do the welding and didn't touch the alternator. 

Any thoughts?

[jerrytr5]

Certainly sounds like alternator but might be worth replacing the ignition light bulb, could be the contacts are not as good as they should be.

Really a question for RobH always good for electrical issues. Drop him a PM (personal message), he may not look at the 7 forum that often, or repost in the General Tech forum.

Jerry

[RobH]

It certainly sounds like the alternator Robert.  

The ignition light is connected between the battery supply and the alternator regulator supply and if it remains lit it means the alternator voltage at the regulator is lower than that of the battery.  It is odd though that the 'fault' goes away once you rev the engine and does not come back. That may point to a poor connection, possibly at the slip-ring, which clears with revs.

It is worth doing a simple check.  ( I am assuming you have the standard Lucas ACR  alternator )

If you have a test-meter (the dash gauge isn't accurate enough) ,  when you measure across the battery with the engine running at 3200 rpm there should be at least 13.6v rising to 14.4v as the revs are increased further.  If it is much lower or much higher there is a problem with the alternator. 

That does not necessarily mean you need to replace the whole thing.  It is fairly simple to replace the diode stack, regulator and slip-ring.   If you don't want to do that yourself, a local vehicle electrician should be able to do a bench check to confirm the fault and change whatever is necessary at lower cost than a new unit. 

( It is possible but unlikely that welding near to the unit without disconnecting it could have done something. Under the wrong circumstances, high voltage spikes could be induced into the car wiring by the welding arc and that could damage the rectifier stack or the regulator. Actually, disconnecting the battery without also earthing the connecting lead makes that more likely since there is then no longer a low-impedance connection to earth.  )

[John Morrison]

Welcome Bob

really good to hear you are still TR’ing

John

[microg]

I have a Reliant Scimitar as well as my tr and the scimitar does exactly the same as yours. You rev it and light goes out. Has new alternator as yours but fitted led bulbs in dash and was told these dont initially have enough resistance to get the alternator working but the extra load when revving it fixes it. Either its been like this for over a year with no probs  

[RobH]

44 minutes ago, microg said:

....led bulbs in dash and was told these don't initially have enough resistance to get the alternator working but the extra load when revving it fixes it.

That is a possibility (though it's actually too much resistance, not too little) .  In the case of your Scimitar you could fix that either by replacing the ignition lamp with an ordinary one or by bridging it with a low value resistor.  (180 Ohm  1 Watt should do it).

An alternator requires a small remnant of magnetic field in order to start generating.  Some may already have enough residual from the last time it was run but others require the extra current coming via the ignition lamp to provide enough. If the magnetism is very low then as you say, it can take some revs before there is enough voltage generated to 'bootstrap' the machine into life, since output from a winding is proportional to the velocity with which the lines of flux are cut. 

[Robert Morrison]

Hi guys.

First of all, a big thank you to everyone who has offered an explanation/solution. I'm afraid I don't possess the skills or have the equipment myself to do these tests, but I will take the information provided back to my garage and get them to look at the issue.

Secondly, apologies for the delay in replying. I've been away this last week and just getting up to date with things.

My car is garaged in a lockup about 7 miles away from where I live so I don't see it every day. But today was a nice day so I took the opportunity of taking it out for a run. On arrival, it started on the second attempt (so no flat battery) but although I ran it in neutral in the garage the light wouldn't go off at all. To me, that was probably now pointing to a faulty alternator. However, as soon as I pulled away from the lock up for the run the light went off. The car ran really nice. After stopping and restarting the engine the light went off after a few seconds at revs higher than 2,000 RPM.

Once I get it checked I will report back.

C'est la vie.

[Robert Morrison]

On 10/28/2022 at 9:38 PM, John Morrison said:

Welcome Bob

really good to hear you are still TR’ing

John

Hi John,

Long time no speak. Yes, I've had the car 30 years now. It went to Newcastle with me after Carlisle in 2003 and we have been in Edinburgh 10 years now. Don't get to use the car as much as I would like to. It seems that all my interests compete for the same weather!

 

Hope you & Doreen are fine.

Bob

[Robert Morrison]

On 10/28/2022 at 8:16 PM, RobH said:

It certainly sounds like the alternator Robert.  

The ignition light is connected between the battery supply and the alternator regulator supply and if it remains lit it means the alternator voltage at the regulator is lower than that of the battery.  It is odd though that the 'fault' goes away once you rev the engine and does not come back. That may point to a poor connection, possibly at the slip-ring, which clears with revs.

It is worth doing a simple check.  ( I am assuming you have the standard Lucas ACR  alternator )

If you have a test-meter (the dash gauge isn't accurate enough) ,  when you measure across the battery with the engine running at 3200 rpm there should be at least 13.6v rising to 14.4v as the revs are increased further.  If it is much lower or much higher there is a problem with the alternator. 

That does not necessarily mean you need to replace the whole thing.  It is fairly simple to replace the diode stack, regulator and slip-ring.   If you don't want to do that yourself, a local vehicle electrician should be able to do a bench check to confirm the fault and change whatever is necessary at lower cost than a new unit. 

( It is possible but unlikely that welding near to the unit without disconnecting it could have done something. Under the wrong circumstances, high voltage spikes could be induced into the car wiring by the welding arc and that could damage the rectifier stack or the regulator. Actually, disconnecting the battery without also earthing the connecting lead makes that more likely since there is then no longer a low-impedance connection to earth.  )

Hi Rob,

I managed to find a friendly neighbour who not only had a multi-meter but knew how to use it.

We started with measuring the battery voltage without the engine running. It was 12.9volts

After starting the engine, the volt reading didn't change at all until the rpm of the engine exceeded 3,000. At 3,200 rpm, the reading was 15.0volts.

Once it 'got going' the volt readings varied with the rpm's of the engine. These were:

1,300 rpm - 12.9volts

2,000 rpm - 14.6volts

2,500 rpm - 14.8volts

3,200 rpm - 15.0volts

Therefore, it looks like the alternator is doing what it should. But the ignition warning light on the dash won't go off until the car is revved at over 2,500 rpm (the first time) and then over about 1,500 rpm thereafter. 

Until recently, it always went off immediately the engine fired up.

Any thoughts?

Robert

 

[RobH]

Yes it's obviously charging but actually those voltages sound a bit high - according to Lucas data an ACR alternator shouldn't really give more than 14.4 volts at 3200rpm and top out at 14.7v higher up the rev range.    Do you find you have to top-up the battery electrolyte frequently?

It's just possible  the multimeter is reading high I suppose but I think it more likely there may be a problem with the regulator in the alternator which is also affecting the ignition lamp since both are driven from the same point.

If you have a vehicle electricians locally, it would be worth getting them to test the alternator. 

[Robert Morrison]

Final update: A big thank you to everyone who offered advice.

A few weeks ago I asked my usual garage if they were able to test the alternator to determine whether or not the regulator was faulty or did I need to take it to a specialist? They assured me they could test it so the car was left with them 10 days ago. After 3/4 days they were able to tell me that the alternator was indeed faulty. But they couldn't determine which component was responsible....

At that point I had enough so just purchased a new alternator which was fitted on Tuesday. 

The ignition warning light now goes out immediately after starting the engine.

Result.