ABS Testing Fault Finding

[Waylander]

When the ignition is switched on the ABS light on the dash should illuminate.
When the engine is started the light should extinguish
When the car reaches 3-5 MPH the system should carry out a sensor and valve check sometimes a noise can be heard from the front OS wheel arch.

If the ABS light stays on after engine start there could be a few reasons for this.

1. Brake light Switch Fault
2. ABS pump Relays
3. ABS relay G20 (944) G22 (968)
4. The ABS ECU Grounds
5. The ABS Wheel Sensor Grounds
6. The ABS Pump Ground
7. The ABS ECU
8. Low battery voltage

If the light goes out with engine start, then comes on when reaching 3-5 MPH this is caused by a malfunctioning wheel sensor, ground or the sensor.

Wheels sensors will only provide output to an oscilloscope for full diagnosis but it is possible to test them with a DVM.
When the hub is spun as fast as you can by hand (direction does not matter) the sensor should output a very small voltage AC, they should all be about the same output, for the same spin speed.

Checking the resistance will only show they are not open circuit.

Checks
1. Make sure the toothed section on the rear of the hubs is free from rust and metallic particles and dirt.
2. Make sure the head of the sensor is also clean and free of dirt.
3. Check cable continuity for the ABS sensor cables.

If one sensor puts out a significantly lower voltage than the rest replace it

The ABS sensors can be tested at the ECU cable connection.

Right Front ECU Plug Pins 21 and 11
Front Left ECU Plug Pins 6 and 4
Rear Right ECU Plug Pins 24 and 26
Rear Left ECU Plug Pins 8 and 9



.

if anyone has any more information or tests that would help please add them in

Pins for the ABS Relay
Image
Pin 15 Is the voltage sensor from the Alternator

ABS ECU Information

Two ABS ECU's can be used in our cars but don't be fooled by the part numbers, I have had this checked out by OPC in Polaris which is their parts history system.

The ECU that appears on eBay most often
928 618 119 05
This ECU was used between 1984 and 1991 for 928,944,944S2

928 618 118 00 is the later ECU fitted to 968, later 928
This is the correct one for our cars but both will work

The later ECU is now only available as an exchange part at the paltry sum of £744.00 plus £200 parts deposit plus VAT


Abs Pumps Modulators that can be cannibalised for parts.

Porsche 944S2, 968 Standard Abs Modulator
Bosch Pump 0265 200 038

Donor Pumps
Bosch Pump 0265 200 043 Mercedes W124 Chassis from 190 to 500SL
Bosch Pump 0265 200 042 BMW All sorts

 

[944Scott]

That's some very useful info.

 

[tref]

If the above diagnostics suggest it should be a wheel sensor fault, but the checks suggest all wheel sensors are ok,try also checking the resistance between the two sensor connectors and ground. A relatively common problem with them is to short out. Afraid I cannot now remember all the theory, but have determined a faulty sensor like this when otherwise all check out ok. I seem to recall it is necessary to check the resistance between both pins relative to ground, as otherwise the internal resistance can confound the issue, but at worst, check all four and compare, and if there is one odd one out, chances are that is the problem.