richardpope50

My Sierra ABS ECU has just started to fail the self-test and I cannot work out why. It all appears to work correctly up until that point.

I have gone through all tests listed in this post
https://passionford.com/forum/technical-help-q-and-a/374177-abs-light-2wd-cossie.html
especially those detailed by jon@work and all pass but my two dashboard ABS lights remain on - my red handbrake light does not come on at all.

The self-test appears to test:

• Low fluid in the reservoir – mine checks out OK and circuit completes
• Low pressure in the pump accumulator – Mine pumps up and the pump turns off correctly completing the circuit
• Valve block resistance – mine is within ohms range
• The four wheel ABS sensors – mine check out OK within ohms range

Does anyone know what else the self-test checks or can suggest any more checks as it seems to be that the ECU is not switching on relay 3 (green one) although that relay all checks out (when jumped)?

FYI. The Sierra diagnostic socket cannot interrogate ECU error codes as none of the wires go to the ECU. What they do test, all checks out anyway. I have also tried a proper (Crypton) ABS diagnosis loaned by a friendly garage and it fails to register any codes.

Any clue would be appreciated. Yes, the ECU itself could have failed but how does one test that out?

Davebrannigan

Hi All, new to the forum but not the car. I’ve had my XR4x4 for 29 years and she was 10 months old when I bought her. We have history!

Always maintained myself.

For or several years I’ve heard an odd noise coming from the bulkhead like a wah wah wah but not always and I kind of ignored it as it seemed harmless. It may still be however...

I recently went out in the car (which only does about 500 miles a year) and noticed that when applying the brake the orange abs light lit and relays clicked behind the glove box.

Both these relays operate simultaneously when pressing the pedal. Interestingly they are both purple not one not one green and one purple as most suggest they will be. I’ve never changed one. I Opened them both up and operated them manually. Good contact across both sets.

Removed the ecu and did all. The resistance checks. Found as others have sail the 9&10 are only closed when system up to pressure as suggested by Haynes book.

During my testing that buzzing noise noise started and I found it is coming from the chamber on which the pressure switch is mounted. When the switch was disconnected it stopped presumably because the ecu detected a fault and ... well what I don’t know but it stopped. I assume this switch isn’t anything special is it?

Im wondering if what I’m hearing is pressure bleeding past a valve seat causing it to bounce and buzz?

Where red to go from here? I’m pretty sure that in the past I could press the pedal several times before the pump kicked in so I’m wondering if the accumulator has failed and is not storing energy? Could it be the pressure switch is bad causing it to stop pumping at loo low a pressure?

Is is there a way to measure the pressure. I’d see that as a good way forward. Any advice appreciated. BTW I swapped out. The abs ecu with one I picked up years ago for trouble shooting and it made no difference.

thanks all

Dave

christos

Probably accumulator has failed.