928 ABS Issues brain trust

[R3wir3d]

Hi all , after a bit of insight for fault finding.
Listed below are the issues on a 1987 928 s4 .
Any thoughts would be awesome.

ABS not working
- ABS light goes off after car starts but goes on as soon as you go over about 5 miles an hour. ABS won't function.
- When running the car on axle stands with the car in gear the light remains off but if you spin either of the front wheels by hand it will activate the warning light.
- l have tested the ABS relay on the main fuse board and the two on the ABS pump and they appear to be ok.
- resistance test on sensors as follows
FR ohms974 volts 0.8
FL ohms995 volts 0.3
New sensor on FL ohms1671 volts 0.6
BL ohms1022 volts 0.1
BR ohms1008 volts 0.1
NB: volts were read while spinning hubs by hand.
Have checked continuity on all wires from senor to ABS control unit. All OK.
Checked and cleaned all earth points.

Thanks .
Seth

[dr bob]

The favorite weak/failure spots in the system start with the power feed connection just forward of the PS reservoir. Old wiring, poor connection, broken terminal block, loose nut.

Then the wheel speed sensors, and the wiring/connections for them close to the wheels. Test the resistance from the ABS controller plug, so you can find if there's something wrong between the controller and each sensor. Damaged wiring and crispy connectors are too common as the cars age. Be sure to check for continuity to ground from the signal conductors -- there should be none.

Do you have the system wiring diagrams? Page 9 has ABS on the right side of the sheet. Note that there are two separate connectors in each sensor circuit, pointing to the economy of checking each circuit first from the connector at the ABS control unit. (under the driver's dead pedal...)

And as always the general question -- What work was last done on the car? Anything anywhere near the ABS sensors or wiring? Some of those (fronts) are in the engine bay at the spring tower. Rear are in the spare wheel well.


I can still hear Jim Bailey's mantra "relay relay relay" as he proposed diagnostic steps for electrical system problems. I'll add cleaning ground connections to the list. The controller grounds land at MP-IV at the left front. The hydraulic unit has its own ground strap. Each of the wheel sensors has a cable shield that's grounded near the sensor. Points are listed on the wiring diagram. The grid map of ground points is on the bottom left of that same sheet 9. There's a 3-axis ground point and connector diagram in the set for my 1989 car, on sheet 12.

[R3wir3d]

The favorite weak/failure spots in the system start with the power feed connection just forward of the PS reservoir. Old wiring, poor connection, broken terminal block, loose nut.

Then the wheel speed sensors, and the wiring/connections for them close to the wheels. Test the resistance from the ABS controller plug, so you can find if there's something wrong between the controller and each sensor. Damaged wiring and crispy connectors are too common as the cars age. Be sure to check for continuity to ground from the signal conductors -- there should be none.

Do you have the system wiring diagrams? Page 9 has ABS on the right side of the sheet. Note that there are two separate connectors in each sensor circuit, pointing to the economy of checking each circuit first from the connector at the ABS control unit. (under the driver's dead pedal...)

And as always the general question -- What work was last done on the car? Anything anywhere near the ABS sensors or wiring? Some of those (fronts) are in the engine bay at the spring tower. Rear are in the spare wheel well.


I can still hear Jim Bailey's mantra "relay relay relay" as he proposed diagnostic steps for electrical system problems. I'll add cleaning ground connections to the list. The controller grounds land at MP-IV at the left front. The hydraulic unit has its own ground strap. Each of the wheel sensors has a cable shield that's grounded near the sensor. Points are listed on the wiring diagram. The grid map of ground points is on the bottom left of that same sheet 9. There's a 3-axis ground point and connector diagram in the set for my 1989 car, on sheet 12.

Thank you for your input Bob , I have passed your insight on (trying to help remote diagnose for my dad) .
As far as what has been done its more a case of what hasn't been done on the car (mostly just gearbox and torque tube at this point)

[dr bob]

I will share that my knowledge and there for the advice is limited to left-hand drive cars, so some of the descriptions, like the ABS power connection may not be immediately forward of the power steering reservoir on cars that have both the steering and brake master cylinder on the "other side". Likely the PS reservoir is still on the left side since the PS pump is still on the left side of the car. Is the ABS unit on the right side? The starter and alternator are on the left side and therefore the battery to engine power and front-of-engine harness might run up the left side, but the central electrics panel is under the front left (passenger) on LHD cars sold on NZ.

Why part of the world needs the car on the other side of the steering wheel sometimes escapes me...