Help with ABS problem please

[Mikes2]

it can't be a sensor if the light comes on before the car moves

[gunman]

it can't be a sensor if the light comes on before the car moves

On my 80 tdi, some bright spark had broken a wheel sensor and removed the bulb from the dash (the car went trough an MOT like this , this was just before I bought it ), when I put the bulb back in the ABS light came on and stayed on all the time until I replaced the sensor, car didn't have to move for the light to show.

[chiefmechanic]

Abs

On these cars the light comes on with the ignition and battery voltage should put it out even without the engine started providing you have 12 volts at the battery unless there is a defect. The car does not need to move. If the light comes on after the car moves then there is another fault.

[gunman]

OK, thanks fro that. It's a brand new battery (but wasn't when my wife last drove it). When the car is started the voltage is sitting at 14v.

[twoqu]

Are you trying to communicate with the ABS with the ign on or car running. Try both!
Apparently diagnosis can only be accomplished with the car stationary. Whether something like a faulty speed signal is implying the car is moving or not I cannot be sure. This can be accessed in the measuring blocks of the engine ecu I think. Again its unlikely but might be worth checking. Oddly I think it should display something like 4km/h even when stationary.
Do the brake lights work correctly?
Other than that if the controller does not respond it is suggested that its a power or earth issue. They also recommend removing the connectors from the K line post one at a time and then trying to connect in case there is an issue there. Might be worth inspecting them for corrosion.
If you can find the appropriate wiring schematic that might help determine where to check the fundamentals, power and earth, at the ABS controller.
Bit pointless having diagnostic capability then it locks you out!

[twoqu]

Sorry the info I have is all over the place!
In the Bosch instructions for its ABS tester it also mentions to check the ignition switch terminal 15. If thats flakey it might explain the power issue. If it was low voltage it can recover once the car gets up to full power and should extinguish the light.

[twoqu]

Last thing to try before my brain clocks off is to try and subsitute those relay at the ABS modulator. I don't think they are anything special. If the light went out after you disturbed them it might indicate an issue.
Shame you were not nearer as I have the Vag1710 which I think would diagnose the issue without interrogating the controller if the ABS controller is a 35 pin type!

[gunman]

Sorry the info I have is all over the place!
In the Bosch instructions for its ABS tester it also mentions to check the ignition switch terminal 15. If thats flakey it might explain the power issue. If it was low voltage it can recover once the car gets up to full power and should extinguish the light.

That's interesting, there was a code relating to the imobiliser that the voltage was too low.

[twoqu]

As I said previously, if the car has been standing fully charge the battery. Although the voltmeter suggest its charging correctly it has probably been depleted during the lay up. If its not got 12.6V at the battery terminals its not fully charged. Easily checked with a multimeter. Once you are sure its ok, you can start looking at the other issues.

[gunman]

Ok, ill check the voltage, but the battery hasn't been layed up, its new and was only used in another car for a week.

[twoqu]

Well the first rule of any suspected electrical problem is to start with a fully charged battery!
The light being on without the car moving, the lack of response from the controller, and the other low voltage code all suggest either a power or earth problem. Thats why its worth checking the basics before pulling other things apart.
Might be worth cleaning any ABS related earths you can see.
That speed display I mentioned earlier can be found in group 4 display zone 3 in the engine ecu. It should show 4 km/h with the vechicle stationary. Don't ask me why!

[gunman]

I had a brief chance to look at this today, Battery is fine, I checked power to both sides of fuse 32 which is grand. Also tested for power at the connection on the ABS pump, there is power getting to it, when testing one of the pins I could hear the pump running (must have the the resistance of the tester). I also checked the earth to the pump and the ecu which was also fine.

Still has the same issues tho, not sure what to do now short of start replacing bits, starting with the ECU.

I didn't get the chance to bleed the system as rain has stopped play but I drove it and the brakes feel much better now, there is still a bit of travel but there always was. The brakes do stop you now and don't feel dangerous anymore.

[gunman]

Nearly forgot, When I bought my 80 tdi and put the abs bulb back in I'm nearly 100% sure the light never went out until I replaced the smashed sensor. If a sensor was duf to the exgtent where it was shorting out could the light stay on even before the car moves? I had thought of disconnecting each sensor and seeing if the light went out until the car moves, anyone reckon this is worth the time? Or anyone got a cheap ABS ECU form an avant? I have one here from a tdi avant.

[twoqu]

Do you mean fuse 31?
I would still try to communicate with the ecu by removing the K lines for the other ecus one at a time or remove the ecu connector and trace back to the K line for the ABS ecu. Whilst you are there you can check for power at the ecu connector. If you have power and earth and the K line is still connected and you still don't get a response then maybe its time to start replacing stuff.
However there is nothing more frustrating than randomly replacing stuff only for the fault to remain!
I might be able to give you some clue about what pins to check at the ecu connector if you are interested in doing it that way. Unfortunately the later ABS does not seem well documented.

[K Simmonds]

I have an ABS control unit you can have for the cost of postage, to be honest they very rarely go wrong so I never sell them.