ABS Sensor Diagnosis Question

[88IXer]

I am trying to diagnosis which sensor in the ABS system has gone/is going bad. The car is a 1988 325iX with a manual transmission. The symptoms are as follows:

1. Turn key to on, ABS light lights up.
2. Start car. Light turns off and all appears well.
3. The first or second application of the brake pedal, even if the car is still stationary, will set off the ABS system and pedal pulses. If I take my foot off the brake pedal it will quit and the ABS light turns on. If I leave my foot on the pedal, the pump will activate indefinitely.
4. Sometimes the symptoms above do not occur and the light simply turns on after ~1-2 miles of driving.

Based on my reading this sounds like a bad sensor. I pulled each wheel sensor and my front driver's side looks rustier and somewhat twisted. All sensors have a resistance between 960 - 1000 ohms. The accelerometer sensor under the hood looks fine but I do not know how to check it. There is 0 ohms of resistance between the terminals. I have not been able to find a new (or guaranteed working) accelerometer so I am hoping it is the wheel sensor.

Any thoughts or insight in to fixing this issue is appreciated. The AWD works great so I would love to have the whole iX package working for the slick roads around here.

Thanks,
Brian

[TimeMachinE30]

ABS relay?

The silver relay underneath the steering wheel dash likely.

Either source a new one or open yours up and it likely has a bad solder. It can be redone per online tutorials and even a youtube video.

[ringmeister81]

Common problem that I also experienced in my iX. Unless it's the ABS relay, you most likely have a bad sensor. What I did was disconnect one sensor at a time and drive the car. Since the ABS light will come on when it sees the bad sensor, you will know which sensor is bad when the light does not turn on. If I remember correctly, the light would turn on in about a couple hundred feet or at the first stop sign in my neighborhood. You should be able to figure it out fairly easily. If this ends up not being the issue, then you should replace the ABS relay.


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[88IXer]

Just pulled the relay and the fuse/wire is still intact. Checked the resistance across the two solders to be sure and it registered 0 ohms. I assume this means that the relay is fine?

Since the iX has the additional accelerometer sensor, do I unplug it as well? What I am trying to ask do I unplug all 5 sensors, then one at a time plug one in and drive a few hundred feet? Will this work with the accelerometer? Most of the write ups I've found have not been iX specific so I wanted to double check. Thanks for all the input!

**Update**
When either front sensor was the only one plugged in, the light came on very quickly. When the accelerometer was the only sensor plugged in, it took much longer, but the light turned on. When either of the rear sensor was plugged in, I couldn't press the pedal without the ABS pump activating. For the rear driver sensor, the light never turned on, even after many stops and hundreds of feet of driving. The passenger rear sensor appeared to behave the same, except after a hard stop the light finally did turn on.

Does this mean both rear sensors are bad?

[DionK]

I'm having Exactly the same issue right now on my 88ix. Have not done anything other than research at this point to try and figure out what it could be. Previous owner told me "Regarding abs check magnetic pickup at each wheel for debris and clean as necessary. Also follow pickup wires to junction box where they all come together before going into car. Check connections for corrosion and connectivity there. After that the wires go to an abs computer but not likely the cause. I would check the plug in connector there just in case though"

Also the Bentley indicates that if its not the sensors it could be damaged pulse wheels. These wheels may be sending faulty speed signals back to the control unit.
Check that a crack has not made the pulse wheel loose. Check for dirt or worn wheel bearings that could reduce the clearance between the sensor and the wheel. Accumulated wheel dust has been known to cause problems. A faulty pulse wheel should be replaced.

[ringmeister81]

Unplug one sensor at a time. Leave the other three plugged in and drive. If the light comes on, the unplugged sensor is not the bad one. Plug it back in and unplug another. Repeat.


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[dubdub]

I have had this exact issue with my IX for many years. If it was the relay, the light comes on instantly, so it shouldn't be that.

I have done many of the same things... resistance of sensors is within spec. Replaced all (some) sensors with used ones off another IX. I have also replaced ABS ECU to no avail. The cleaning the debris off the magnetic sensors has helped and maybe the car would run fine for a few weeks, but it eventually comes back... to clean, I have just got the spray in there good or pulled them out when in the area. I have even thought it could be a cracked ring sensor, but when I replaced my front axles with new ones from BMW, the problem persisted.

RIght now I have pulled the light bulb. I will have to think whether I care enough to pull the cluster to put it in and do the unplugging test.

[88IXer]

I obviously misunderstood the procedure the first time... Last night I unplugged one sensor at a time (instead of having only one sensor plugged in at a time). However, the results are a bit confusing.

Front Driver unplugged: Light turned on pretty quick.
Front Passenger unplugged: ABS activated with every brake application. Light eventually turned on.
Rear Driver unplugged: ABS activated with every brake application. Light never turned on. Every now and then I could brake and not have ABS kick in, but at least 75% of the time, the ABS pump would run.
Rear Passenger unplugged: Same as rear driver. No light but ABS activated with every brake application.
Accelerometer unplugged: Light eventually turned on. ABS activated a few times before the light turned on.

When I say ABS activated, this is happening at nice slow stops from ~10mph to 0, so ABS should not be activating.

I really don't want to spend the $$ on rear sensors unless I am confident it will solve my problem. I just don't understand why ABS activates all the time when the sensors are unplugged. It was very difficult to get the car to come to a complete stop when either rear sensor was unplugged. Advice is welcome!

[barbierij]

when you drive the car, is it super hard to slow it down?

[barbierij]

this may help

http://www.bimmerforums.com/forum/showthread.php?1334167-1990-325iX-brakes-go-to-floor-sometimes

[88IXer]

The car slows down/brakes fine as long as the ABS doesn't inadvertently kick in... What happens is that you start the car and put your foot on the brake to shift into first and typically the ABS pump engages. Sometimes this doesn't happen and the brake pedal works normally. If this is the case, usually the light turns on after ~5 miles of driving.

I replaced one of the front sensors due to the metallic cylinder looking twisted and rusty, but it did not fix the issue. I have performed the diagnosis procedure as documented above and my only guess is that I need to replace (both) rear sensors. However, that would be a ~$200 gamble that I'm not convinced the odds are in my favor...

[ringmeister81]

That's really strange that the ABS is kicking itself on like that when a sensor is unplugged. It shouldn't work at all once the system doesn't get the speed reading from the wheel that has the sensor disconnected. I drove my car for a year with a bad sensor, ABS light always came on within a few hundred feet, and never had ABS feeling in the pedal at all. Your problem could be those rear sensors, or maybe the ABS system itself. Change the relay out, and put in some known working sensors if you can't buy new. You're on the right track.


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[barbierij]

I really don't think it is the ABS relay under the dash. I believe that relay just powers the ABS once the engine is running.

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[DionK]

This may also help.... See abs blurb in this link. Realoem puts my production date at 10/87. So I may be affected by this...I say maybe because my abs bulb in my cluster is burnt out.

http://home.earthlink.net/~hainesinutah/members/newsletters/iXchange22.html

http://home.earthlink.net/~hainesinutah/members/newsletters/iXchange7.html