How to diagnose the ABS system

Hi everyone

I’m still getting the ABS light quite a lot these days.
Sometimes right after starting the car up the ABS light stays on after driving.
Somtimes there is no light at startup and it turns on after driving between 1-10 Minutes.
I also did a test brake today and the ABS did NOT work at all.

I really want to fix that issue but last time i tried to diagnose it, i blew an ABS fuse when shorting the two ABS diagnose wires in the trunk. ( a fuse under the driver side dashboard)

Now im not sure what i did wrong. Can anyone link me a detailed procedure on how to set up the ABS diagnosing in the dashboard?

Also what usually fails in the ABS system? I got a 1990 XJ40 4.0 Daimler


In my case, the abs light went on if I started the engine with the brake pressed without pressing the brake the light was not on but turned on while driving when I pushed hard the brake (also the brake pads light went on) I changed the abs accumulator (the round ball over the abs pump) and no more abs light and system working good.

Seems like yours was a different issue then.

I drove some more today (200km) and started the car a few times inbetween. The ABS light was contantly on this time.

I also touched the relays under the driver side dash.

All relays except the left black one ( green circle around it) produced a click when turning the ignition key (I could feel it easily with my fingers, however the black did not react like that)
Also after driving for 2 hours, the black relay was really hot. (approximately 50-60c right where the coul would be) All the other relays which clicked were cool to the touch.

Maybe this could lead me to the issue? Or is it normal that one of those relays is hot and does not click?

I’d appreciate any intel here :slight_smile:

Cheers Marco

Have you tried abs when you experienced light off ? Did it work? If sometimes the light is off maybe you have a bad abs sensor or a bad contact of a cable. My mech told me that sometimes he solved abs problems cleaning and reseating the main connector of the abs ecu with a contact cleaner

as i mentioned in the first post the abs doesnt work. the weel locks up and it screeches.

Whats weird is that 1 month ago i did the MOT and the testbrake was perfect. No ABS issues.

The only thing that changed was i let a shop install new rims and wheels. But the ABS issues did not appear IMMEDAITELY after the change in tires and rims. only a few hundres km’s later

Marmo …

The first thing I would do is pull the ABS overvoltage relay and bench check it. A VERY VERY VERY common reason for the ABS light. The solder joints crack and trigger the light (sometimes on and off and sometimes constant). The relay is very expensive to replace and very simple to repair. How to check it …

Screenshot 2023-05-27 at 7.04.17 PM

Apply 12v to 31B and 86 (a couple of small wire off the battery works nicely) Now check for continuity between 30 and 30a using a multimeter. If you don’t have one you can buy one for under $10. NOTE … when you apply the 12v you may hear the solenoid click, but this does not mean it’s working properly. However If you do have continuity between 30 and 30A the relay is working properly … congratulation now move on to the harder troubleshooting items.

But if you don’t have continuity first check the internal fuse (yes there is one). Older relays actually have a glass fuse and newer ones use a thin wire. If that’s not the problem then reflow the internal solder joints until your relay passes the test.

Hi Grooveman

Thanks for your input. The Overvoltage relay is in the trunk right?


-I just did the blink test and the code is 21

-Also there was no relay in the trunk, just a empty slot (I’ve read somwehere that the 90’s models dont have a overvoltage protection relay. Not sure if thats true)

Also, i took the black relay which didnt click and got hot out. I tested it with a 12v source and it clicked strongly. also the resistance when energized was zero, however it also got rather warm after leaving it energized in testing. (maybe thats a feint…)

Marmo …

You could be right, the '90 models may not have a ABS relay labeled “overvoltage”, at least I couldn’t find a reference to one other than on the '89 and earlier XJ40s.

Sometimes I think there should be a different forum for XJ40s before '90 and later :confounded:

I believe you are correct about there being no ABS Overvolt Relay on 1990 and later XJ40s. The EPC does not show a VIN range but they sell extremely slowly now and my sales history of hundreds of them doesn’t show any retail customers I recognize as having anything later than 1989.
That’s an awesome explanation on how to fix them. I’m saving a copy for my customers. Thanks!

MY abs is working again. No light for 5 startups 5 days and about 150km of driving.

I think its just a temporary fix since i just unplugged the relays under the dashboard and put them back in. Then i could hear the abs pump pumping again and the error went away


Clean all the connectors… it can be a bad contact.

Will do that if the abs light comes up again. thanks all for inputs