Well,one of the issues I started to try and work out on my '88 VDP is solving the ABS Failure warning light. I've already removed the wheels and cleaned all four ABS sensors with Brake Cleaner. I'm still getting the ABS Failure Warning so my next step would be to either try and find a new Overvolt Relay or possibly fix one of my existing ones. I have one on the car and a spare but neither seem to fix the problem. Does anyone know of a way to test the Overvolt Relay to see if it's working properly. I understand there's an internal fuse that tends to go bad on these. What are my options?

 

The Fuse often is BLOWN. You need to check that FIRST.
The circuit board usually has cracked' cold-solder' joints so running a soldering iron across the board and 're-flowing' the solder will usually cure the faults.

bob

 

How do I go about testing the fuse aside from opening up the relay. Can it be tested like a typical five terminal relay without disassembling it?

 

If it's not a potted (sealed) relay you should be able to pry the cover off and have a look inside?

 

Here's the schematic for the overvoltage relay from the Electrical Guide. Applying 12 volts from your car battery (or maybe even 9 volts from a standard transistor radio battery) to terminals 31B and 86 should result in continuity across terminals 30 and 30A. If there is no continuity then either the fuse is blown as Bob suggests or the relay contacts are not closing properly. The Zener diode is used in a "crowbar" circuit. When the "Zener voltage" is exceeded, the diode allows reverse current to flow to ground, creating a short that blows the fuse. I don't know the Zener voltage selected for this circuit, but I would guess it is something like 18 volts (it has to be higher than typical alternator charging voltages or the fuse would blow under normal operating conditions).



I assume you can open the relay since Bob mentioned cold solder joints on the circuit board.

Another test you can perform at the ABS Module electrical connector is to measure the resistance of each wheel speed sensor and its wiring harness. Based on an X100 I recently measured, I would expect DC resistance to be be in the range of 1,000 ohms to around 2,000 ohms. The Electrical Guide shows which pins in the connector apply to each speed sensor. If you find the resistance of one of the circuits is much lower than the others, it is probably shorted. If you find one with infinite resistance, the sensor is dead, a harness wire is broken, or there's a problem with the electrical connector.

If I recall correctly, another relatively common issue is either poor solder joints in, or water ingress into, the ABS Module.

Cheers,

Don

 

The fuse is a GLASS fuse with a white foam around the metal link so the fuse-link is NOT visible. If you do not know how to test continuity of a fuse, then the diagnosis is best left to someone familiar with 'electrics'.

Soldering is not difficult but requires a soldering iron and solder.

bob

 

I managed to test my Overvolt Relays today. One tested with good continuity and the other one had nothing. I managed to open up the faulty one to see what I'm assuming was a bad fuse. Although I didn't see an actual fuse I did see where there was a gap of what had looked like a strand of wire going across two terminals. I'll resolder a piece of copper wire strand across the gap so I can use this as a future spare. I've already cleaned all four wheel sensors and know I've got a good Overvolt relay installed so my next step will be checking for proper resistance of the wheel sensors as Don suggested.

 

Curious,but is there a reset for the ABS module or a system reset on the '88 models to possibly delete the fault code or do I just clear the dash VCM?

 

Rather than a piece of copper wire, which probably would not burn through if an overvoltage condition occurred and leave you with a working relay but a fried ABS Module, you could consider using the fusible link from a low-amperage mini fuse by breaking away the plastic case and using wire cutters to remove the the curved link and solder that across the terminals in the relay. At least that way you would know that it would do the right thing and break if an overvoltage condition occurred. I don't know the interrupt rating or breaking capacity of the original fuse in the relay, but in a crowbar circuit the fuse usually has a low breaking current. Maybe the fusible link from a 2.5 amp fuse would be a good start.

EDIT: Another thought is that you could use the wire from a small glass-envelop buss fuse by applying tape to the glass to keep small shards from flying everywhere, carefully breaking the glass and extracting the wire. Depending on how much length of the original wire will fit across the terminals in the relay, the actual breaking current may be altered, so you may have to estimate the change in order to choose a fuse from which to extract the wire.

EDIT: Somehow I didn't see Bob's post #6 about the fuse in the relay being a glass-envelope fuse. Guess it would be best to just try to find the correct size and rating?

Cheers,

Don

 

As far as I know, the only way to clear the ABS warning lamp is to cure the fault.

 

I believe the original fuse is rated at 8 amps. It looks as if it may have been replaced beforehand but I can't verify that as I've never had to deal with an Overvolt Relay before. It looks as if someone just had a thin piece of silver stranded wire going across the gap,which doesn't look to be more than about 3/4" across. If someone has a pick of what an original fuse link might have looked like it may be helpful. I did think of using a strand from a glass automotive fuse but not sure if it would withstand being soldered without breaking the gap and melting.

 

95 leaper


Don has the professional schematic....mine is a sketch...both the same
please see my pics in my album titled OVERVOLTAGE RELAY.
I have an 89 XJ ...Same as your 88


Hope it helps you. its an 8 amp fuse.
I rebuilt my relay with a new TVS/Zener Diode. and bulked up the solder trace some in places where it was corroded away.


The ABS fault will go away the next time you start the car and there are no faults after the Initial Test when you get above 3 mph. (you will hear it)


My ABS fault was the diode in the OV Relay.

 

I managed to use a good spare that I had laying around but plan on fixing the damaged one. Now,as for the ABS fault,here is my situation. First,I cleared all faults off the VCM to start from scratch. When I first start the car I get an ABS fault showing but it then goes away after about ten seconds. But if I shut the car off,wait about 10-15 seconds and turn the key to the "on" position to read the codes,there is an ABS fault code appearing but once the car is started and runs again,it disappears. Is this just the computer doing an ABS test mode?

 

95 leaper


Yes...that is the INITIAL TEST of the system ( call it the" non -moving test")
then ...you start down the road and the system check again , this time it is testing the dynamic portion (SENSORS) of the ABS. if all is well no fault.
the system periodically tests itself while driving also.


this will happen every time you start and drive the car.

 

 

Hi Ordinaryboy1986,

Welcome to the Jaguar Forums! It's great to have you with us.

The low brake pressure light at startup is typical of XJ40s. It can take the pump up to 30 seconds or so to pressurize the hydraulic brake boost system, especially if the accumulator is no longer holding pressure as well as it should.

I don't think the missing overvoltage relay is related to the warning lamp, but it won't hurt to replace the relay.

Please visit the New Member Area - Intro a MUST and post a required introduction so we can learn something about you and your Jaguar and give you a proper welcome.

Cheers,

Don