Ignition Warning Light Suddenly On - Alternator?

1988 Jaguar XJS V12.

While driving to work today, idling at a traffic light, suddenly the Ignition Warning Light came on (Looks like lightning bolt). At the same time, I saw the Volt gauge fall from 14 to 12.

The rest of the drive, the Volt gauge remained at 14V, but the Ignition light remained on, very dim at idle, and a bit brighter when accelerating.

I did have the headlights on, which I usually never do. After switching them off, the warning light did not go away.

I’ve read archives, and there seems to be confusion as to battery light vs lightning bolt light. The consensus (and my manual agrees) is that the battery light is Overcharge, and the lightning bolt is Alternator? So does this simply mean my alternator is not putting out enough juice?

What to check? Belt is newish and tight. My Alternator is original Lucas manufactured in 1987. Maybe it’s time?


Assuming Alternator is going out, I am all for getting the GM CS130. It was part of my rolling restoration.

I removed smog pump and A/C compressor from the car years ago. I’d like to fit the GM Alternator where the smog pump used to be. But it will be a couple years before I’m ready to redo the entire A/C system, if ever. So can I just run a smaller belt from crank to Alternator? Or do I need a compressor there to attach belt to alternator? If so, I could temporarily put GM Alternator back down where OEM used to be? Or go ahead and buy a new compressor, but don’t hook it up yet?


Lacking the need for a tensioner for the A/C compressor belt, I’d suggest you drop the GM alt idea and instead go for the Land Rover alternator in the original location.

I thought of that. But once I decide to put in an A/C Compressor on day, can I move the Land Rover up to where the smog pump was?

Researching Land Rover Alternator, looks like in the US, I’ve located a Bosch rebuilt AL9346X which seems to be a direct fit for the 95 3.9L V8 in the LandRover. 100 amp. No pulley though. Will the Lucas pulley move over?

Got home, battery was getting 13.7V at idle, then it dropped to 12.6V. Revved a bit, got it back to 13.7V, but within about 10 secs at idle it dropped again. Could just be regulator, but like many have said, may as well replace entire alternator since it is the original Lucas.

Maybe one (or more) of the alternator diodes is going bad.

Nope! Don’t think so, anyhow. Probably gotta find a pulley. This is a car with a V belt driving the alt, right? You’da had to find a pulley anyhow, those modern alts come with a ribbed belt pulley.

Look for Aristides post from Apr.2018- 86V12 Alternator Upgrade alternatives. Could be helpful.

Here it is:

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Thanks, Aristides…, But where can someone actually buy one, here in the US? I searched around and got nowhere.

Thanks, I did look at Aristides thread, and managed to find one using the Land Rover YLE10100 OEM number.

What I found and liked is a Bosch rebuilt one (Bosch AL9346X). 100Amp.
Says it fits Land Rover Defender 110 1993, Defender 90 1994-95, Range Rover 93-95.

But yeah, it was either with serpentine pulley, or no pulley. So that will be fun to source. Wouldn’t it be more important to get correct diameter based on new alternator, not old one?
Bosch’s website states the pulley to be 58mm (2.3") for this one. (but is that inside groove, or outside edge?)


Greg, I don’t follow the “diameter” question?? BTW, have you looked at Rock Auto? Good selection there.

yes, I saw RockAuto. I decided to go Bosch Reman from another vendor I use. Bosch is probably the highest quality. They even offer 2 year Road Assistance if I break down due to alternator! $180 + core (Hopefully they’ll not check that I send them back a Lucas?)

Diameter of pulley - the alternator is designed to put out 100amps at a certain max RPM. If I buy too small a pulley, I overcharge. Too large, and I undercharge.

Aristides, any chance you could easily measure your pulley diameter?

Ah, ok. The RPM will be changing constantly I would think, and hopefully the regulator would cope with that. I think I would try to get as close to OE as possible, then you won’t be messing around changing belts. (IMHO)

OK, will do. I’ll measure my OE pulley and go from there. Gotta also make sure it’s the right thickness for the belt, and especially the pulley shaft. Can’t find anywhere Land Rover or Bosch states the diameter of shaft, I guess I could measure it myself. May be smartest to go to auto electrician shop who rebuilds Alternators and see if they have a used pulley for me.

Yes, the advantage to living close to a big city- an auto elec.shop. I’ll bet you’ll be glad when the car is “finished.”

Ha, yes and no. I envisioned it as a 5 year restoration. I do enjoy driving it, but lotta 32+ year old parts still to replace/upgrade. But it’s getting there! When there’s nothing left to do on it, I may get bored. :slight_smile:

That’s true. Just taking photos to bore people with, and sitting in the man-cave cleaning trophies gets old pretty quick. (Joke. Joke. Not serious.)

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Sorry Greg, no can do, car is still at the body shop.
The pulley diameter is a compromise, like anything in this world, of best charging at idle and less drag at high revs.
The regulator will take care of overcharging.

The one I got had the same pulley diameter, so I had not to worry about that.
You might be lucky and the Lucas pulley could fit on the Bosch one? But I’m sure you will be able to find one the correct size.

As for pulleys specs:
The Outer diameter (OD)
The Pitch diameter which is smaller and corresponds at the diameter of the center of the belt when it’s in the groove. That’s the diameter being used to calculate transmission ratios.
The Inside diameter which is the hole for the Alt shaft.


Thanks. I will outline in a separate thread once I get everything sorted. Could be helpful, like yours, especially for us in the USA.

Reading the archives, it seems the GM CS130 is the choice to replace the weak Lucas, but it either has to be mounted in smog pump location, or an expensive/unique mount must be purchased if put back in original position.

Hopefully your findings and mine can be another good choice that many here can use. BTW, do you know who the OEM manufacture of the Land Rover alternator is?