Ian928

Copied from 944 Turbo/s forum, hope its okay there are so many 944 forums?

Hope someone can help with our ABS diagnose... Car is a 1989 951

We did drive this car without a ABS relay for 1,5 years, so it is quite possible that more that one sensor has failed during that time. Without the relay, the ABS warning light never went out, after installing the relay the light comes on after the speed reaches 5mph or something like that.

I have read a lot of threads about testing sensors and I have tested resistance between pins in the connector to the brain, measured VAC between pins in the connector to the brain while driving and measured resistance between ground and all associated pins.

Please help me assess these results:

RESISTANCE OVER SENSORS
FR pin 11 and 21: 960Ohms
FL pin 4 and 6: 1020Ohms
RR pin 24 and 26: 890Ohms
RL pin 8 and 9: 1014Ohms

VAC WHILE DRIVING 10mph and 50mph
FR pin 11 and 21: 0,2 - 1,6VAC
FL pin 4 and 6: 0,2 - 1,0VAC
RR pin 24 and 26: 0,1 - 0,2VAC
RL pin 8 and 9: 0,1 - 0,2VAC

OHM BETWEEN PIN FROM SENSOR AND GROUND (measured with the multimeter set to 2000KOhm)
4 no connection
6 no connection
8 001
9 000
11 no connection
21 no connection
24 000
26 001

I am starting to suspect that both rear sensors are faulty. Do people agree?

I did try to remove one of the rear sensors but it was stuck to the degree I almost damaged it without it moving. Can it be turned around to help release them or do they have to come straight up? And what do I do to break them loose? Heat?

968workaholic

Since the resistance at the sensors seems ok, I would think there is a harness issue that's causing them to ground out, not the actual sensor itself being faulty

Van

I can't really offer much advice, but a few years ago I had an ABS sensor die on my 996. Fortunately, I was able to look at live readings with the Durametric tool, so it was easy to isolate to the right rear sensor.

Because of corrosion between the aluminum control arm and the metal part of the sensor, there was no way to remove it without destroying it. I imagine you'll find the same issue on your car.

It might be worthwhile to check and make sure that there are no clumps of iron filings stuck to the sensors (look inside the end of the control arm with a flash light), and also make sure that the rear axle flanges have the ABS ring teeth... Maybe the PO changed wheel bearings and put the wrong pieces back?

Also, be aware that the 944 runs a 3-channel ABS system. The computer/pump controls both rear wheels on the same circuit - if one wheel locks up, the brake pressure in both is released. I don't know if that type of system would run on a different set of parameters than the front.

Ian928

I was not being clear here, I removed the multiplug from the brain and did all the measurements from there. So a problem in the wiring should surely show up?

Ian928

Is it possible to look into the control arm? I need to check. This is a M030 car if this matters. And, the ABS has been working before in our ownership.

Also, nice information about the rear wheels beeing controlled on the same channel. It may very well be that the numbers for the rear are correct.

marc abrams

I'm not familiar with Porsche ABS systems but from my experience with WABCO, Meritor, and Bendix ABS systems, a ABS light coming on after the vehicle is moving is generally too much of a gap between the sensor and tone wheel or filings as Van said. A ABS light coming right on after being powered up would more likely be a sensor out of range , short or open in the harness, etc. Your ohms look good, the rear voltage is low. Sounds more mechanical then electrical.

Ian928

So, no one has measured the voltage on all ABS sensors on a 944 so I can compare values? Is it too rude to ask someone sit a functioning ABS if the could measure their values? It is pretty easy and can be done i side the coupe. I modified a couple of blade connectors to use in the multiplug, but if your multimeter has slim probes it probably can be done without connectors.

marc abrams

Actually you already did. From the trucks I've worked on the ohm's and voltage are generally the same front to rear.

968workaholic

You need to address why the rear sensors have continuity to ground as step 1

marc abrams

With a ohmmeter set to the 2000K range you're going to get all sorts of erroneous readings. You can measure the resistance of the air at 2000K.

Ian928

I used 2000K to try to get a number. I tested with lower values also. Then I had no continuity (ohmmeter show 1) on the fronts and ohmmeter showing 000 on the rears. It seems very low, it also frels strange that both rear wheels measure the same.

marc abrams

Again, in the 2000K range you can spit on the sidewalk and the ohmmeter will go nuts. It's likely the longer run is why you are seeing some continuity. My money is on the gap being too large.

Ian928

I ordered both rear sensors today.

If the problem is to big gap, then what? I can not see a way to adjust gap?

marc abrams

I see your point. You need to see my point too. Your ohm readings look good so the electrical end seem okay. You're just not generating voltage from the rear sensors. The ABS Module See's the front wheels are turning but not the back. This is probably why the ABS light comes on after you start moving. Can you read codes from the ABS system because that would help. When I was working, only two possible codes from each sensor circuit you could get. Electrical circuit out range (shorted or open sensor, wiring) or Not reading MPH (gap to larger). 95% of the time the ABS light was on from slightly loose wheel bearings. A slight amount of play was all it took. I know l'm comparing apples to oranges here. Just trying to make a point here on ABS systems. Replacing the sensors may not be a bad idea.

Ian928

Thank you Marc, I see your point no problem! I will start by changing the sensors and cleaning the tone wheels. It is interesting what you are saying about loose wheel bearings, that is certainly possible. I see that loose bearings would move the tone wheel away from the sensor when driving.

By the way, I did a search and realised that I was wrong about where the sensor is, I did not realise that the tone wheel is on the driveshaft, I thought the tone wheel was inside with the bearings. This probably means you can check and clean them from outside? Or is it unacessible like so many things on this car?