Brakes

ABS Sensor Testing and Replacement

I finally got to the "fix the ABS system" on my list of things to do.

Apparentlly a previous owner had placed all four wheel ABS sensor leads in the "blank (no electrical connections)" position in the snap cover electrical connector at each wheel. (connector is for brake pad and ABS wheel speed sensor connections). This has the interesting effect of causing the ABS dash light to go out immediately when engine is started.

Perhaps this was done to hide an ABS problem or save money and not fix problem ????

Certainly something to watch for when doing a pre-purchase check.

Problem now: ABS light goes ON after car moves about 4 car lengths at about 10 mph. No ABS braking action.

I have a ABS test plan booklet but do not have either model of the ABS tester. After rigging up test leads I have confirmed that all relays, 3 hydraulic solenoids, power, stop switch inputs, and hydraulic pump, function.

The following checks were made using the output of each wheel sensor: (units are AC volts, rms, car run at 7 mph, unloaded, ABS CPU disconnected, measured at ABS CPU wire harness connector pins)

Front Right= 1.3 Front Left = 1.1
Rear Right= 0.9 Rear Left = 0.4

I removed, cleaned the magnetic particles off the RR sensor (tooth wheel was clean), but output only increased from .9 to 1.0.

The RL sensor is corroded in place. After 4 hours work it only moved .12". I'll have to destroy it to get it out (car was always in northern US). I've got a replacement coming in the mail but the question is this:

The ABS test plan does not list the units for the output specified for the sensor (at a wheel speed of 1 rev/second.) Just lists a ABS tester meter reading of 2.5

ABS sensor output goes up with wheel speed, but most references I checked indicate that there are several types on the market with significantly different outputs for the same tooth wheel rpm.

Anybody know the lower limit (or what is normal) for a 87' 928 ABS sensor at a specific wheel rpm ?

Anybody have any thoughts on the subject ?

So far it looks like either I have a low sensor(s) output or a ABS computer failure or something I haven't thought of !

Tom Robertson
87' 928 5spd. Venetian Blue, w/o ABS, Screeeeeech



In a previous post (928 ABS question) I described an ABS brake problem. And wondered about the normal ABS sensor output voltage magnitude.

I finally got the left rear ABS speed sensor out. It took 7 hours as the old sensor anti-sieze compound? had turned to stone. Had to destroy sensor to get it out. It took several minutes with a small Dremel Moto tool rotary brush to clean up ABS sensor mounting hole enough to get the used replacement ($50) installed.

It appears that an open-circuit voltage of about 900 millivolts rms, open circuit (at 7mph) is normal. The old
sensor had only 400mv output even though the coil resistance check OK at 940 ohms. ABS finally functions normally now !

Tom Robertson
87' 928 5spd. Venetian Blue