I searched the forum, but didn't find anything on this.

I've been getting a new warning light on my '88. It's the lightning bolt looking one next to the minor fault warning light. I looked it up in the owner's manual and it says it's the ignition warning light. However, there was nothing in there about what condition causes it to remain illuminated while driving. The car runs the same as it did before the light came on. What's going on here?

 

It's been years since I had my '88 XJS but I recall the lightening bolt meaning 'over charge' .
Do you have a voltmeter to check battery voltage?

Cheers
DD

 

13.1 engine off. 14.3 at idle. The warning light decided to go out. I'll drive it to work today and see what happens.

 

Ignition system warning light is back on. The car is running the same as before. Hot idle rough, Makes good power under WOT. What condition causes this light to go on?

 

Robert,

I may be wrong, but I believe that there are 2 elements on your car, that help to protect the various circuits from charging nuances.

The first is a load dump module attached to the alternator that helps to protect circuits from overcharge output from teh alternator, either because of diode failure within teh alternator, or even the battery being disconnected when the engine / alternator are running with the potential for excess current.

The second element is an Overvoltage indicator unit. This is a 3-wire module that measures the alternator output and, if sensing excess voltage, signals the lightening bolt bulb on the dash.

The implication of the light illuminating is that there's a problem somewhere in either the voltage output of the alternator, the ability to discharge it or the circuit and module which is measuring it and then sending the signal to the light.

I would start by measuring voltage at the various points - direct at alternator, at battery, at load dump module and at Overvoltage indicator unit and then drawing a conclusion. If you determined that you're absolutely happy with your alternator, battery etc, you might want to bypass the Overvoltage indicator Unit which, froma VERY quick perusal of teh wiring diagram, you could do with 2 of the 3-wires (one seems to have the need to stay connected as it's ignition switch connected).

Hope that helps a little bit.

Paul

 

Dump module versus over-voltage unit:

Near as I can tell a car will have one or the other, not both, with the change being 1988-89. Cars with the over-voltage unit have the lightening bolt warning icon. Cars with the dump module don't.

Agreed on bypassing the over-voltage unit. Looks like the red/black wire is the one that actually goes to the warning light. Not sure if the wires are individual or in a molded connector. If the latter, I don't see why the unit can't simply be unplugged.

Aside: the Series III sedans of similar vintage and V12 engine never had the over-voltage warning circuit nor, as far as I can tell, the dump module. I've oft wondered why Jaguar deemed these things necessary on an XJS but not a sedan. The electrical architecture is virtually identical between the two.

Cheers
DD

 

Nothin' wrong with those readings

Cheers
DD

 

The only pics I can find of the Overvoltage Indicator Unit seems to show 3 separate spade connectors, so the output wire(s) to the instrument cluster could be removed and tidied up, I think.
Paul

 

Tks for that, Doug. I guess that would make sense.

I wonder if Jaguar fitted these because of some strange concerns on the capacity and output cycles of those early alternators on XJSs? I thought if anything the early alternators were probably marginal in terms of a decent output, but maybe there was some logic in their thinking?

Paul

 

Oh, I'm sure there was some logic. But what was it?

The Lucas alternators, while as good as any other alternator, were under-spec'd for the cars, yes. And as you imply, why would a lower output alternator create a harmful over-voltage situation?

And doesn't the voltage regulator keep voltage within safe limits? Did Jaguar have some reason to think the voltage regulators wouldn't work as designed?

And why just the XJS and not the V12 sedans?

So many questions!

Cheers
DD

 

Maybe Jaguar had remarkable foresight and anticipated problems with remote bulkhead battery posts, and difficulties with elongated cable current flow to their first implementation of a boot-based battery for the XJS?!

Actually, there might even be some logic in that??

Paul

 

Once again by the time I got a meter to it, the warning light was off again. 12.47 engine off, 14.18 with it running.

 

Alternator most likely on the way out. The light illuminates when battery voltage is supplying field current via the bulb (which is why you can't replace with an LED) to the alternator instead of it supplying it itself via the internal field diode pack. This current is usually only used to fire the alternator up when the engine starts.

 

To be clear, the (rather oddball) "overcharge' circuit and warning light (lightning bolt) is not the same as the (conventional) battery/ignition light/exciter circuit. The overcharge light is trigged by the "overcharge unit".

But, yes, the alternator could be failing. More specifically, the internal regulator would be failing, allowing system voltage to rise too high....thus triggering the overcharge warning light.

Cheers
DD

 

If the voltage is being measured at 14.18v (not sure if that's at at battery or at alternator output?) might that imply it's not the alternator but more likely a faulty overcharge unit?

Paul

 

Yes, as 14.18v isn't excessive

Cheers
DD

 

Is the 88 different from the 89 in this respect?

 

Circa 1989 is when Jaguar introduced the alternator dump module. If my minor research, via wiring schematics, is correct, the dump module replaced the overcharge warning light and controller.

Cheers
DD