[xj40] Load Dump Module, it can easily be fixed!

Hi Guys, this hopefully will save some of you some money in the
future and sa few of those charging problems

The Load Dump Module was responsible for a charging problem, where
the alternator had been checked and was OK, and the battery was
brand new. The car would run for about 3 hours and then stop,
battery dead, no charge from the alternaotor, battery charge
warning light on throughout the whole event. Charge battery
overnight, car would start and run again for about 3 hours on the
battery until dead again.

After a lot of searching the archives and checking connections
etc.etc it was time to open up the load dump module. It basically
contains 2 resistors, 2 diodes and a capacitor. The main purpose is
to protect the electrics from overvoltage spikes, but it also
supplies a battery monitor voltage to the alternator and controls
the warning light. It’s 12v supply is taken from Fuse 1 in the
centre console ( shared with some light modules ) which when
removed or fails stops the charging and puts up the warning light.

Anyway, in my case one of the diodes in the module had failed and
gone short circuit, pulling down the battery monitor feed back to
the alternator and switching off the regulator in the alternator,
causing the symptoms mentioned above.

The whole module can be rebuilt or repaired easily with parts from
your local electronics store for about $10. Jaguar charge $400 for
this module !! It’s a small single sided circuit board and easy to
work on.

A word of warning though. The Haines manual shows a schematic of
the load dump module which is not accurate in section 5. They have
a short circuit shown accross the 2 diodes which should not be
there. ( it wouldn’t work with the short circuit added anyway )

Also the circuit diagram in section 12 is wrong. It shows 2 wires
going to the alternator, when actually one of them goes to ground,
and the diode shown in the feed from the fuse is back to front.

I have seen threads to charging problems, and maybe this will help
some of the ‘lost souls’ who have just about given up.

Good luck guys.

89 Soevereign 3.6.–
steve marsden
avoca beach, Australia
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I just repaired the load dump module on my 1989 XJ6/xj40. I found one of the zenier diodes was shorted. The original Lucas diode was ZP5028S. I found a cross reference chart which showed that STMicoelectronics number LDP24A was a direct replacement for ZP5027S, ZP5028S, and ZP5030S Diodes. The LDP24A dumps at 24Volts while the lucas dumps at 27v, 28v and 30v respectivly. I figure 24v is enough. Its working well so far. Be sure to replace both diodes with the same exact type. You don’t want them to have different dump voltages. They were about $5 US each.

Can one of you please post a photo of this “load dump module” ? I have never heard of this and would like to know what it looks like and exactly where it’s located.


My daily driver is a 1994 but I am working right now on a friend’s 1988 car.

oops!, I was talking about the overvolt relay not the dump module - (I just deleted that post)

here’s a 16 year old post by Bryan_N:


Aug '04

In reply to a message from Butch Schneider sent Thu 12 Aug 2004:


It’s buried beneath the air intake/filter box. Follow the lead from
the alternator to it.–
The original message included these comments:

Just checked the archives and there doesn’t seem to be a reference to the
physical location of the load dump module for an 88 xj40. Just where is it
attached at?