One of these days I'd like to make my ABS work. When I bought the car last summer, the PO said that the warning light is sometimes on and sometimes isn't, but since I've owned it it's been always on except for about a week in January when it was off. My mechanic is convinced that the culprit is probably the ABS relay in the fuse box, even after I replaced it with a used relay from 928 International (the mechanic says I should have spent the $80 for a new one). Are there any other common failure points? For what it's worth, the light comes on as soon as I start the car, regardless of whether my foot is on the brake pedal. Is there a straightforward troubleshooting procedure in the shop manual,

or posted on the web somewhere?

Thanks,
Joe Elliott
'88 S4
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If ABS yellow light comes on at startup, before moving, then it's probably power to the ABS system related. There's 1 relay on the main relay/fuse board.
2 relays under the left (driver's) front splash shield. The big silver relay is different from 1987 to 88 model year.

One of the listers posted on the Web Forum that wheel sensor check is done at 40'ish mph. Not too sure.
Probably need a Oscilloscope to test each wheel sensor.
Don't think a DMM will be able to pick up the square wave.

The wheel sensors are magnetic and will pick up all the ferrous metal flakes (junk).
Think cleaning off the metal flakes will help the sensor.

And on my 88, the ABS controller is next to the left knee above the hood release.

EW
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Ernest,
I seem to recall a self test that is done at initial startup of the ABS system, and if the ABS controller finds a fault signal it will cause the alarm and ABS lamp to light up. So more things that just the power relay might be involved.

You can check the internal resistance and power flow.
Disconnect control unit plug and test with an ohmmeter connected between following plug terminals:

• Speed sensor, front, left between term. 4 and 6
• Speed sensor, front, right between term. 21 and 23
• Speed sensor, rear, left between term. 8 and 9
• Speed sensor, rear, right between term. 24 and 26

Later models with a ABS/PSD ecu:

• Speed sensor, front, left between term. 48 and 50
• Speed sensor, front, right between term. 46 and 47
• Speed sensor, rear, left between term. 44 and 45
• Speed sensor, rear, right between term. 42 and 43

Per the Porsche 'ABS Test Program' manual, with your multi meter (DVM), you should see...

800 - 1,800 ohms between the two speed sensor terminals (disconnected) of each sensor, and  >20,000 ohms between ground and any one terminal (also disconnected).

Next step would be testing the output which would require an oscilloscope. Turn the wheel 1 rev per second to test it.
 

It appears that an open-circuit voltage of about 900 millivolts rms, open circuit (at 7mph) is normal. a bad or old old sensor has only 400mv output even though the coil resistance check OK at 940 ohms. Also check for shorts against chassis ground.

Regards,
Theo
'88 928s4 cherry red
The Netherlands
http://928s4.no-ip.info

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The way these wheel sensors usually fail is that corrosion of the surrounding ally squashes them, and the case shorts to the internal coil, so, when testing for internal continuity also check between each wire from each sensor and earth, there should be no continuity (infinite resistance).

This sort of failure is 'visible' to the control unit before the wheels turn.

Smiffy

Locating the sensor connection to the main wiring may be a little difficult. For the rear side you will find the waterproof connections just behind the upper arm against the body, for the front side they go through the chassis (via a rubber grommet) into the engine bay.

Theo