KenRudd

Troubleshooting Intermittent ABS Failure from Road Bump, Impact, Shock

Symptoms: Occasionally ( but not always) after hitting firm bump in pavement, like a bridge expansion joint, or curbing / gators at the apex of a track corner, the ABS warning light will come on, and ABS will go into failure mode until the car is restarted. ABS seems to work fine other than this.

Background:
1. This “bump-fail” issue had been going on for about a year, and then ABS died entirely.
2. I replaced rear ABS sensors after determining one had failed without a doubt. I replaced both rears because two of the sensors became available, and they are very hard to get, so now I have one of the originals as a spare. This allowed ABS to generally function again, except after bumps. I had hoped this would fix the “bump-fail” issue too , but it did not.

So, its seems likely I had two separate ABS issues, the original “bump-fail” issue, and then a failed sensor. The ‘bump-fail” issue smells very much like a broken or loose wire or connector. But I understand looks can be decieving.

What I have done so far:

1. Looking at the last thing I touched, I tried testing the rears for intermittant connectivity issues. I pulled the main connector from the ABS control unit, put my meter across the appropriate pins for each sensor, and wiggled the s**t out of all the wiring I could, from the sensor in the wheel carrier, to where the wire disappeared into the body. Looked for blips on the meter. Found none. I have not yet tested front sensors in this way, but the all ohm out correctly when static;
a. Problem 1 : This may not accurately simulate the shock that the suspension feels when hitting bumps, but I can’t think of a way to accurately simulate that in my garage;
b. Problem 2: Digital Meter is kind of slow, a fast transient may not be clear on meter, but may be enough to annoy ABS controller. I do not have an O’scope;
2. Tried purposely hitting bumps to cause the failure, and to see if it seems to be front or rear suspension causing the issues;
a. Problem 1: There is lag from when bump is hit, to alert light, longer than the time delta between front and rear impact at highway speed, so not a good indcator of where issue lies;
b. Problem 2: The issue is intermittant. Often you hit severe bumps and no issue at all;
3. Sprayed DeOxit on rear sensor to harness connectors and on harness to ABS control unit sensor.
4. Reviewed the following threads:
a. https://rennlist.com/forums/928-foru...m-problem.html
b. https://rennlist.com/forums/928-foru...-included.html
c. https://rennlist.com/forums/928-foru...f-warning.html
d. https://rennlist.com/forums/928-foru...ing-light.html
e. https://rennlist.com/forums/928-foru...-a-86-0-a.html
f. https://rennlist.com/forums/928-foru...to-repair.html
g. https://rennlist.com/forums/928-foru...agnostics.html
h. https://rennlist.com/forums/928-foru...ed-please.html
i. https://rennlist.com/forums/928-foru...2-abs-ops.html

So here are my questions:
1. Has anybody seen a similar issue, where hiting bumps causes ABS to fail? If so where was the issue and what was the fix? I want to look at the obvious or know failure points first;
2. Barring a known likely culprit, can anybody suggest an improved testing methodology? Testing resistance across connector pins and wiggling wires may be a good start but:
a. Does not accurately simulate suspension shock;
b. Does not catch quick transient changes;
c. Does not catch any issue with ABS controller, ABS relays or ABS unit itself that may be glitching due to vibration/shock;
d. Does not catch sensor wire to body ground intermittant shorts ( like from a chafed wire hitting body).
3. As I am writing this, and reading other posts, how likely is this due to one of the “hiddden relays” in the wheel well?

Captain_Slow

My car does this. I'm leaning toward the ABS relay on the ABS pump inside the left fender. There's a relay on the CE panel, too. The relays are old and the sensor cable sheathings are crumbling apart. My plan is to replace the ABS relays, rebuild the sensor cables, apply deoxit to all the connection contacts, and hope that this fixes the intermittent ABS light. If not, then on to the sensors.

M. Requin

Have you had any odd ABS + "brake pad warning" issues? I haven't dug into it yet, but I have a suspicion that there is a short in the wiring harness between these two circuits that has been causing me similar issues.

GregBBRD

The most common issue (by far) is the short loom that runs from the inside of the engine compartment down to the front wheels.

Get the system to reset and then start gently moving the loom around down by the connector at the back of the spindle.

KenRudd

Unless they are insanely priced or NLA, I think I will replace all the ABS relays, including those under the fender:
928 618 930 00
928 618 931 00
As well as cleaning all related contacts/connectors in that area.

I can easily see mechanical shock causing electrical issue for those items.

KenRudd

Reset I understand, but then do you mean wiggle and move the loom while the car is running and after ABS self tests are complete, or am I modifying my test of hooking my meter to the appropriate pins in the ABS harness connector and looking for blips as I move the harness around ( with car off)?

KenRudd

No, but my pad sensors are tied off and not in use.

GregBBRD

Car running. Move the looms around gently while someone watches to see if the light comes on.

BTW....I've never had to replace one of those relays. I've substituted relays in, looking for problems, but never have found a bad one.

soontobered84

Greg,

You're talking about the "hidden" relays in the fender well on the ABS pump, right?

KenRudd

Just talked to Roger, part numbers and credit card in hand, and he also convinced me to hold off on the under-fender relay replacement until I was sure it wasn't something else.

Mongo

I had a frayed abs loom. Check to see if there are also any breaks in the wire sheathing.

Captain_Slow

Is there a simple test for the ABS relays? I'm imagining (perhaps a fantasy) removing the relay(s) and bench testing it by connecting wires from a battery to the prongs on the relay and listening for an audible sound that signals all is well...or not.

M. Requin

I tried the usual tests, energizing the relays and seeing if they conduct (in the right direction, where applicable), but I don't know if that is a sufficient test given the current these pass. Mine passed the bench test, and the remarks that these relays rarely fail encourage me to think that they are OK.

Captain_Slow

Gotta love it when Greg drops precious gems of advice into threads like this - all derived from incomparable decades of experience and knowledge. Others have pointed me to the relays in the fender well. I'm sure they do fail...problem is, these symptoms can be attributed to just about anything in the system. So...revising my process to start with removing the ABS sensor harnesses, inspecting, rebuilding (with the self-vulcanizing tape and a wear protecting wrap), and cleaning all the connections with Deoxit. Then hope for the best. I don't want to "fix" too many things at once so that I might actually identify what is causing the intermittent ABS light. I'm also removing all the fuses and relays from my CE panel and will give it a Deoxit bath.

crustychief

I had a bad relay in the CE panel. It was intermittent on bumps for a month or so then failed completely.