Checking testing and fixing your ABS

[Waylander]

Hi All I have been trying to build a post to help anyone with ABS issues so if anyone has any thing to add to this it would be cool

So far....

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

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

It is also possible to bridge out the ABS relay G20 / G22 for testing
Bridge terminals 87 and 30 in the fuse box.

It is also possible to test the ABS pump Motor by applying 12v to one pin in the ECU plug and one pin in the fuse box With the ECU unplugged.

Apply 12v across pins 28 (ECU Plug) and terminal 87 (ABS Relay fusebox) the ABS pump should fire into life.
If you have no warning light after moving off and no ABS, I would suggest that the ABS pump is seized in the valve block and needs replacing.

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 Altenator

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


Pump rebuild pictures to follow
_________________
Cobalt Blue 968 Coupe 1992 K170 EDP

looking for a barn find RD400

"Dirty Jobs Done Dirt Cheap"

 

[Mr Don Seddon]

I had the ABS light coming on intermittently, one of the rear wheel sensors was dirty, cleaned up it cleared the problem. Don Seddon

 

[Waylander]

Rebuilding the ABS Pump the one on the left is the 968, and the right hand is a 190E one, the Mercedes one lives in the engine bay so stays nice and warm and dry unlike the daftest place ever in the 944/968 the OS wheel arch!

[/img] Basically the same just the brake pipes are all in different places. stripping the Merc one

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[/img] Undo these big Allen grub screws 10MM and they are very tight

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[/img] The Pistons you need for the rebuild

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[/img] All the bits you need removed, Motor , Pistons

[/img] My dead 968 pump

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[/img] All cleaned up and rebuilt even the relays on the top of the pump are the same bosch part number for both the 968 and the Mercedes pumps

[/img] The Mercedes ABS Modulator was £12.00 plus post from ebay, plus a few hours work and 2L of brake fluid, when bleeding the brakes start from the furthest point from the master cylinder

 

[Oily]

When bleeding the brakes, don't forget to also bleed the clutch at the slave cylinder.

 

[Waylander]

Not on a 968 you don't they are separate

 

[blade7]

Thought it was only LHD cars that shared brake and clutch fluid ?

 

[Waylander]

Ignore oily, he has these moments

 

[RDB964]

Hi,

In 2015, you posted an article on ‘Rebuilding the ABS Pump‘, in which there were originally lots of photos. Currently the pictures are not showing. Any chance you still have them to complete the article again.

I appreciate it was 5 years ago but, ever the optimist. Thanks

Richard

 

[Waylander]

You will find a pdf version on

[link=http://www.pdlibrary.co.uk]www.pdlibrary.co.uk[/link]

i take it you have a seized pump, as that was the doc was about

 

[RDB964]

Thank you so much for this help, it is really appreciated.

Well, I’m not sure what my issue with my ABS is yet, other than it doesn’t work. No lights or buzzers, just black lines down the road when having to avoid a car.

Ive started at the far end (ABS wheel sensors and am working my way back to the pump as I isolate issues. So in a sense, I’m getting ahead of myself in terms of preparation.

Found a thread on Rennlist that suggested a Merc ABS pump could be a donor and then saw the thread on PCGB which you majored on.

I’ve bought one off eBay, similar price to yours. If I get to doing what you did, I want the parts with me, as leaving the car disabled for any length of time isn’t my preference.

Seem to be chasing after faults, but like many people, have more time than expected and sharp focus, due to lack of distractions in recent months.

My pig is a 964 by the way, which I have owned for 11 years. Done some interesting things to it over that time, which has made it a much nicer office to be in.

Thanks again for your help and apologies for the rambling response.

Best Wishes

Richard

 

[Waylander]

If you are on the 968UK forum, there is a whole section of ABS trouble shooting

but I would think you have the pump motor seized issue, as when the drains get blocked from the front scuttle panel the water floods down on top of the pump, and over the last 30 years the rubber seals have gone hard and faded away

the big issue you will face is not snapping all the mounting bolts!!

 

[gavintonks]

Hi I don’t know if I’ve come through to the right person I’m new to all this I have a problem with my Porsche Carrera 911 996 on acceleration my abs locks up and stops the car PSM light comes on and so does the ABS light come on turn the ignition off wait for five minutes turn the ignition back on all lights have gone out I can creep forwards extremely slowly but apply the Excelerator and again the ABS locks up could you help me out here please I’ve replaced the sensor under the brake pedal I’ve replaced brake pads and brake fluid I’ve also changed the fuse to see if any of these would help nothing has helped me so far if you could help me in anyway that would be great kind regards Gavin

 

[Waylander]

Hi

this is the transaxle 968 part of the forum, but this sounds like an electronic s issue with the ABS,

I would personally get the car to a specialist or OPC as the one thing you don’t mess with, unless you really know what your doing is brakes