wozza64

Hi folks
just recommissioning an 87 928 s4
turn the ignition on abs light comes on.
start the car light goes out.
move the car and the light comes back on.
anyone know where I should be looking and how I can test any suspicious parts.
thanks Warren

mj1pate

all the dash lights may come on with the key in on-but-not-start position.
the ABS system undergoes a test during your brief, initial foreword motion, and if a cable/ component fails, the light will come on and ABS is disabled.
unless your MY has an OBD2 port, I doubt there is a way to get actual diagnostics.
Be VERY careful about the power bus to the ABS
Hydraulic unit on the passenger driver firewall. That bus Carrys enough current to weld and is reportedly unfused.
Usual culprits include wheel hub ABS sensors that are covered in metal flakes, wire harnesses between wheels that are frayed.

545svk

Both times I has this issue it was frayed wires between the hub and the spare wheel well.

Mrmerlin

at this point you should service all 4 of the ABS sensors,
remove them and clean and reinstall them use DC111 on them so they wont corrode,
and inspect the respective grounds at each wheel hub ,
and the wire connections under the hood and in the spare tire well.
Also add fresh wire sheathing so the wires dont get damaged,
the ABS sensors harnesses were notorious for shedding their black insulation.

Based on your report you have either a broken wire or a bad sensor,
this fails the trigger test, once the car starts rolling over 2.5 MPH.

mj1pate

As Stan has mentioned, we’ve all been thru this with ABS.
When the retainers that secure the rear ABS harness to chassis or subframe disintegrate, the harness can hang over the spinning rear axle half shafts and get beat to death. If the harness wires haven’t been irreversibly destroyed, then the wires can be resoldered with heat shrink.
Fitting new insulation around the twisted pair ABS harness can be a challenge, but many of us including me have successfully used self fusing insulation wrap around the harness. Be SURE and resecure the harness to the chassis/subframe using retainers from PET. Roger has them usually in stock.