"wheels are rolling " --> seems that your ABS module is perfoming the "active" built in equipement when the car pass 5mph ( just upon startup ,the PDAS/ABS computer process a "static" built in test equipement ).

I would clean the 4 ABS sensor ( bad or implausible signal ?.. ) and check the wire ..

 

Having a difficult time identifying the rear ABS sensor wires that come up through the wiring harness. I unwrapped the main harness (as shown in the photo) and found four yellow wires (the color for the RR ABS sensor signal I am tracking) and found one wire that should be the 0.75 mm^2 cross section wire, but it does not show continuity to either ABS connector at the ends of the car. I suspect that I am not in the right harness, but there are not too many choices. Since there are no connectors in the harness in this area, the manual gives no clues.

Has anyone reading this thread been in this area and could offer some advice as to where the rear ABS sensor wires come up through the car?

 

Progress ... Flamefront I took your advice and started at the beginning. Pins 44 and 45, LR ABS sensor may just be my issue. I did the DVM check on the connector as you had on page 1. I get c1020 on all but LR. I just get infinity '1' so I guess I don't have a connected ABS sensor, either through the plug or a problem in the harness somewhere, does that sound about right?

 

Sounds like you're right on track - and you'll have to find the discontinuity somewhere just as I'm doing.

Check at the front firewall connector first - the male pin for the LR ABS sensor has a white wire on the female connector - see if the male pin that matches this position has continuity to the ABS module pin first - if it does, then your problem is "upstream" on the way to the ABS sensor itself. You can check that end of the harness at at the wheel. First see if your sensor reads ~1kOhm and move downstream. I hope you don't get caught in the forest of wires in the passenger compartment like I am...

 

When I worked on jet fighters we used a special machine to find bad connections. It sent a signal up the wire and the bad connection reflected the signal back. There was a distance scale so we knew how far the break was. Is there a similar civilian machine?

 

Found a bit of time this afternoon to explore my issue.

Recap - No LR ABS sensor being seen at the ABS control plug. I proved the continuity to the T2 plug on the RHS wing.

I then unplugged the ABS sensor and I had the expected c1020omhs at the sensor.

I found what I thought was the problem, a bad earth (broke off when i touched it - circled) onto the body from the double plug ended cable which is attached to the wishbone. I replaced the earth but still nothing seen at the ABS controller.

I started to buzz out that cable (Anyone know the part number?? ) and found I had no continuity at all, either on the ABS or brake pad connections. I by passed it completely and viola! I now see the sensor all the way to the ABS control plug.

Flamefront, I can only assume this is replicated on the RR ABS sensor, gotta be worth checking? Mark 1 = into the tunnel, 2 = short cable across the wishbone to the plug for ABS sensor.

 

Nice work, NateSaJack. Glad you found your root cause, and maybe mine as well.

I may need a bit more clarification on what exactly you bypassed: Do you mean the earth wire, or did you go around the large connector, or some other connection?

I'm inferring that "buzz out" means to use the diode check feature on a DVM to check for continuity...not familiar with that term on this side of the pond.

 

Hi, I haven't got time for a decent drive tonight to prove beyond doubt and my 'fix' won't stand up to the roads round here. I'll try fix properly tomorrow as it doesn't look like the part is readily available.

So, the part I bypassed is that large yellow-white ish cylinder. This is where the ABS and Pad wear sensors plug into on mine. It is about 14 inches long and runs to another cylinder. The pic above is of the cylinder closest to the tunnel which hangs free. Both cylinders have a black clasp around them which keeps the plugs in place. Both cylinders have an earth lead, one runs to the body and the other onto the wish bone.

I unplugged the plugs (ABS and Wear - both similar looking but you can't connect them up wrong) going into it from the tunnel and had continuity from the ABS controller plug to that point (I bridge the LR ABS ones as per your image on page one). So I was confident that the harness through the car was ok.

I then looked for continuity across this 'Cylinder-wire-Cylinder' part and got nothing at all. Which makes sense as I had continuity through the car and Omhs at the sensor. It also ties in with the pot hole I hit on that side.

and yes, I used the Diode to show the continuity through the harness etc and the ohmmeter to prove I was seeing the ABS sensor.

I have searched long and hard for the part number. It doesn't appear on PET and this Link suggests it doesn't have a part number after all.

I have spoken to a mate who suggested I take it out, cut off the cylinders, check them for continuity and replace the wire part with new.

 

flamefront,
i have been on this same mission with an '89 C4. i actually have a complete C4 cab for parts that i was hoping to use parts off of it to fix the 89. i found that the wire underneath the car going to the left rear sensor was cut and i'm sure that is what was setting off my alarm.
point being that i removed the cable from the cab all the way from the sensor almost up to the firewall so i could help you identify which one of those wires on the side of the passenger seat is the one that goes to the connector by the firewall. if i remember correctly it is a thicker black wire which actually has the yellow and brown wires inside. now, there should be two of those, one for each rear sensor so distinguishing them apart might be an issue. i'll take a look when i get over to the shop

 

heres the part thats causing me the issue. I have just wired round it for now.

1 = Earth to wish bone, 2 = the earth to body which broke off, 3 = where this cable runs through a mount ( i cut it here as i couldn't get it out)

 

I confirmed that along the passenger seat the two wires coming from the rear sensors are indeed two thick black cables which house the sensor cables. Unfortunately they are both the same for left and right and i could not see a way to tell them apart. At least on my car the sensor wires are not loose and exposed as on your earlier pic

 

Thanks to everyone for posting some helpful tips -

NateSaJack: Looks like you found your culprit. Unfortunately, my short "cyl-to-cyl" harness is OK, and my problem appears to be downstream.

drola: Thanks for the tip - I had not dug into the large black wires (or carefully looked for them - I was keen on finding a yellow wire per the schematic) in the passenger cabin main harness, so I'll work in that direction and report back.

Maddening behavior update: I drove the car after buttoning up all I had disassembled and just to taunt me, no warning lights came on for ten miles. I did not trust the condition (as pleasant as it was) since I knew that I did not have that RR ABS sensor reading to the module, or at least not when parked, so mine must be intermittent. Upon about the sixth brake application, the familiar lights all came back on at around twelve miles in, and a new friend - the brake pad wear light came on.

I think I have the gremlins surrounded and they know it....

 

Pics to follow but I have conducted open heart surgery on the lead.

Its made up of 5 wires, earth, Pad wear x2 and the ABS which are brown-blue and brown-white.

The brown-blue shows signs of water ingress and yep, when I open it all up I found a break caused by rust most. probably.

There is also a small nick in the sheath at the right spot to so I'm now pretty sure I have the culprit.

 

FF, bearing in mind yours is intermitant is it worth bridging the Abs at the wheel end of the 'cyl-cyl', connecting diode to abs controller end and wiggling away at anything you can get near? If it stops or starts buzzing you'll know.

As you can see above mine had deteriorated from a break of the sheath and water rusting it until there is no connection left. Had I tested this a few weeks ago I might not have found it at all.

 

NateSaJack, you have inspired me to again troubleshoot my issues....here's the background for those keeping tabs.

1991 964 C4

-I've only had my car for a year, it has passed thru a few owners recently.....when I got it, I found some lamps were missing from the clock display.

-I've installed a duck-tail, but recently I followed the on-line guides and installed a jumper to extinguish the spoiler warning light. BUT the rear spoiler light is still on.

-I've corrected a few electrical gremlins that must have been in the car a long time (the fans for the AC are new, but weren't doing much because the resistor pack/final drive unit was bad

-I've installed an OBC display tach, it works, in fact, the outside temp works without me needing to fit the outside temp sensor....so the mirror was replaced or the car previously had an OBC, or something else. The OBC ONLY reports outside temp, all other displays are incorrect or '0'

-I have the scan tool software and recently found a bosch hammer.... it tells me the LF wheel speed sensor is not reporting. I've heard that the LF wheel speed is tied to the spoiler, could it be a problem tied to the spoiler light on the dash?

-I've tried a new LF speed sensor, no difference, I've checked the wiring and resistance all the way to the ABS ECU, and it shows the same (slightly higher than 1000 ohm) resistance throughout.

-I bought the blue relay for the OBC, I've checked and swapped the various relays in the relay/fuse box that work with the OBC and ABS.

now, I'm short on ideas, unless I can find someone local who is willing to let me try swapping some parts around to see what is causing my issues.

looking for some guidance, sorry to throw this thread off on a tangent.