I took my 89 S4 ROW in to have the ABS warning light issue examined in a non invasive way. They tested for power to the sensors and confirmed they all had power. ABS warning light came on each time I drove the car. A year later I took it into another mechanic and now it’s intermittent but they can’t figure out what’s wrong. I’m thinking they cleaned something and it went from always on to intermittent? I’ve included some context and the mechanic’s diagnosis below what could they test or may have missed? thank you!!!

- ABS comes on only when the car is in drive and is driven

- it does not come on if the car is turned on, left in park, or put into drive with the brake or handbrake applied

- It doesn’t come on if put in reverse and driven

- if the warning light is off, the ABS works. If it’s on it doesn’t

- it’s intermittently on now

- when it comes on, it stays on while the car is being driven. If I turn it off it may or may not come on

mechanic’s diagnosis

Checked all 4 wheel speed sensors for resistance
and continuity to ground. All sensors are within
specifications.

ABS light not illuminating on start-up

Disconnected all wheel speed sensors and checked for wave form. All wave forms look good.

Removed interior panel, Tire pressure module, Power windows module and bracket to
access ABS control module. Disconnected harness from module.

Checked for continuity on wheel speed sensor circuits. All circuits test ok.

Checked for power and ground at module and
both were ok.

Cover on Module was corroded. Removed case
from module and looked for any signs of failure on the circuitry. No visible signs of failure or damage.

Cleaned all connections at ABS module and wheel speed sensors.

Re-installed ABS module. Re-assembled interior. Road tested vehicle. ABS light no longer illuminates.

ABS function tested and functions normally. If light comes back on suspect intermittently sticking ABS relay. Relay is functioning at this time.

I took the car back a week later when it came back on

Started vehicle and noted ABS warning light on.

Checked fuse. Checked ABS relay on fuse panel again. No issues found.

Removed panelling and accessed ABS control module.

Checked for power and ground at ABS module.

Module has power and ground.

Lifted vehicle and removed left front inner
fender.

Removed cover for ABS pump module.

Removed relays and checked relay function.
Checked diodes on relays. No defects found.

Applied power and activated ABS pump. ABS
pump function is normal.

Re-assembled vehicle and started engine. ABS light is now out again. Road tested vehicle and
ABS is functioning normally. Suspect Module could be "freezing up", de-powering and re-powering system appears to rectify faults for period of time but intermittent fault will likely return.

Unable to find source of fault. Drove vehicle 12km. No faults apparent. Started vehicle multiple times
and could not re-create fault.

 

I would put D100 on every electrical connection as well as the ground wires at each wheel ,
do the connectors in the spare tire well and under the hood and the computer connectors,
and the relay/ fuse at the CE panel.
Make sure the wheel bearings are properly adjusted.
also do a battery disconnect . wait for 5 mins then road test

 

When my ABS light came on, an 87, I swapped an ABS relay from another 928, and light goes out.

I bought a new ABS relay for about $100, installed it, and no troubles since.

 

The abs system has two self-test cycles. First is when ignition is turned on and the second is when driving above about 5 mph. The first checks electric system (power, relay, ground, wires), and the second monitors the wheel sensor signals. Every time the ignition is cycled the system resets.

 

The vast majority of intermittent ABS faults are a broken wire in one of the front looms that go from the wheels to the engine compartment.
Difficult to isolate, even with a Bosch ABS tester hooked up while test driving the car, until the problem occurs.

 

I’m with Zeus+ and Greg. I believe if it only comes on after your moving it is the wheel sensor or as Greg said the wiring going to it. Each corner has this horrible cylinder shaped connector that typically just crumbles away over time.

 

Thanks a mil!

 

I have a Bosch ABS 2-LED-Tester. Let me know if you want to borrow it.

Colin

 

Yes, those cylinder shaped connectors have become a high failure item.
When they are a problem, we cut them off and use a Deutsch 6 pole connector, which just happens to fit inside the stock black plastic clamshell, so it all looks stock.
Note that when this occurs, it is simple to build new connecting looms from the chassis to the sensors.