[xj-s] Over Charging?

Hi All,

I recently replaced the alternator on my 88 XJS (75amp). Now I
have the overcharge indicator (lightning bolt) on at all times
while the engine is running.

I put a multimeter on the back and it’s showing almost 18 volts.
The guage in the car is showing about 13-ish.

Any ideas? I didn’t replace the silver connector on the back… is
this the load dump module? Think that would solve the problem?
Any other ideas?

Thanks!!!

Chris
88 XJS
Charlotte NC–
cmcclenn
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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cmcclenn wrote:

I recently replaced the alternator on my 88 XJS (75amp).

I put a multimeter on the back and it’s showing almost 18 volts.

Your new alternator is defective. Take it back.

– Kirbert

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At 08:08 AM 10/02/2008, you wrote:>Hi All,

I recently replaced the alternator on my 88 XJS (75amp). Now I
have the overcharge indicator (lightning bolt) on at all times
while the engine is running.

I put a multimeter on the back and it’s showing almost 18 volts.
The guage in the car is showing about 13-ish.

Any ideas? I didn’t replace the silver connector on the back… is
this the load dump module? Think that would solve the problem?
Any other ideas?

Thanks!!!

Chris


Something does not add up.

If you measure voltage on the big tab connector at the rear of the
alternator that should be the same as your battery voltage within 0.5V.
This is because they are connected together with a big chunk of brown cable.

Normally this is around 14V with the engine running at 1,000 RPM or more.
The dashboard gauge should read about 14V or halfway in the Normal range.

18V is mighty high on a battery.
What do you measure at the battery ?

The lightning bolt is a lamp generally connected between battery
voltage ( with ignition switched on ) and a point in the alternator
which has no voltage until the engine starts up and generates 14V or
so. Then the lamp goes out. There can be a few variations on this.

You can try connecting the silver thing again, very likely that will
get your lamp off because it has a vital function to do with the alternator.
If not, report back. There is a small chance the alternator is dud.

Richard Dowling, 6/60 South Parade, Blackburn, Vic. 3130, Australia.
Ph: +61 03 9894 4937.

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In reply to a message from Richard Dowling sent Sun 10 Feb 2008:

Hi Richard,

The silver thing is already attached… plugged in or not, it’s
putting out 18V. I checked at both the battery and at the back of
the alternator.

Is the silver thing (I believe it’s the load damper) perhaps the
culprit? I did reuse an older one…

Thanks!

Chris–
The original message included these comments:

You can try connecting the silver thing again, very likely that will
get your lamp off because it has a vital function to do with the alternator.


cmcclenn
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

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The silver thing is already attached… plugged in or not, it’s
putting out 18V. I checked at both the battery and at the back of
the alternator.

Is the silver thing (I believe it’s the load damper) perhaps the
culprit? I did reuse an older one…

Thanks!

Chris---------------------------------------------------------

18V is very bad for the battery, and other things too.

The small tab connector on the alternator should connect to the load
dump module.
It should also have a wire, brown with a black stripe I think, which
goes back to the lamp Jaguar calls " Ignition Warning ".
That is probably the lightning bolt lamp you mentioned.

No matter what voltage you have, if the battery voltage is the same as
the alternator voltage the lamp should be off.

The load dump module normally has no influence on the alternator
voltage output.
If it shorts to ground through wiring fault or internal fault it can
keep your lamp on and maybe mess up the regulator inside the alternator.

If everything is o.k. the voltage on that small tab should be close to
battery voltage.

Maybe as Kirby mentioned the alternator is dud, but they are so simple
a rebuild should not be faulty.
It would take a fully documented moron to rebuild and test one and
not notice it is kaput.
Yeah, well I guess that could explain it.

Richard Dowling, Melbourne, Australia.
1979 XJ-S V12 coupe, 1988 XJ-S V12 convertible, 1985 XJ6.

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