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Self diagnosis and fix for ABS/PSM lights on

deMort said:
Stuart Mackay said:
Fault code showed brake light switch, but that is now incorrect as I replaced that, If we hadn't cleared the code after fitting the new switch, would the abs/pasm lights still come on?

For a fault with the switch then clear the code , ign off then on and dont touch the brake pedal .. there should be no warning and no fault code generated .

Press the brake pedal or drive the car and the code should come back as before .

There is the wiring and the psm control unit that can be faulty .. not just the switch .. im not sure what code number you have or age of car / type of psm but i dug out some details that might help ..

But it is from an early car ..

Thanks DeMort, it is a 2002 996.2 C4 Cab.
All wheel sensors are showing continuity to ECU and we have spun the wheels and have been getting a pulse.
Car is fitted with an LED 3rd brake light and have disconnected that, but it is still showing an ABS/PASM light when driving.

The fault code is the same as in the checklist (brake light switch) I will however confirm when I put a tester on it again. Is the checklist ok for 2002 2 gen cars?
 
The test plan is from an early car .. i have limited data on the gen 2 but it will be similar .. i have a wiring diagram for the 02 which would help in identifing the correct pins for the brake light switch at the psm .. pm me an email addy for you and ill send it .
 
Thanks Iain, Richard Hamilton has sent me a pin diagram for the gen 2 and we used that today to test wiring between wheel sensors and ECU, which are all ok. Have pmed you on another subject, not wanting to hijack this thread.
 
Ive been thinking about this fault .. basicaly i went outside had a smoke then came back !

If i dont trust a fault code .. and the early cars the fault codes are a more general thing then i think then i would remove the psm plug and check everything from there .. scope the wheel sensors .. check earths , feeds and check the signal from the brake switch is correct .. if everything checks out then im only left with a fault in the psm control unit .

The car is saying there is a fault with the brake switch , that might not be the case but its what the car is saying .

If we test everything then we can ignore the code ... but a fault with a wheel sensor will normally trigger a code for that ... its obvious to the control unit .

Aftermarket brake switches are not the best in the world .. a resistance will cause a fault and a code .

Oh and have a good look at the psm pins .. corrosion will cause issues :D
 
My light only surprises me when I'm drivving

So my ABS / PSM warning lights come on about 5-10 mins into a drive, and not when I'm braking, mostly just randomly whilst cruising.... any bright ideas what this might be?
 
Brake light switch or dodgy sensor/wire/reluctor ring.
 
After reading the forum I decided to swap out my brake sensor as discussed, not a bad job, however it's not made any difference so will be looking at the wheel sensors next.

Any hints on best way to do this and what not to do

Thanks
 
Yeah here:

Alex said:
Next on to the wheel speed (ABS/PSM) sensors on each wheel.
Once the wheel is removed you can see on the hub assembly where the connectors are for the sensors and also the brake pad sensors. Simply pull the clip back that keeps them plugged in and then unplug the sensor. If you look into the sensor socket there are two sockets where the metal pins of the plug plug in to. Stick both prongs off your multimeter into the sockets and take a resistance reading.

w2fYJru.jpg



I don't know whether the reading is ohms or k-ohms but each sensor was reading around 1.6. If you spin the disc when taking the reading, it should fluctuate up and down slightly. All sensors were giving the same reading apart from the rear left which wasn't outputting anything at all.

Fitted a new sensor today and took a reading. Reads slightly higher at 1.7 but put everything back together and lights have no gone off. Problem fixed



:)

And only around £50 for a new sensor. Result!


psIz5cb.jpg
 
The Baron3 said:
After reading the forum I decided to swap out my brake sensor as discussed, not a bad job, however it's not made any difference so will be looking at the wheel sensors next.

Any hints on best way to do this and what not to do

Thanks

Hi, I had this and it caused me all sorts of frustration, much searching, many tests, more than a couple of discussions and messages with DeMort (Iain) and touch wood, a new battery sorted it. Interestingly, every time I disconnected the head unit when sorting out heating fan interference (wrong wiring adapter) the warning lights flashed on once but on and off with the ignition cleared it and now all good for months.
 
Alex said:
Yeah here:

Alex said:
Next on to the wheel speed (ABS/PSM) sensors on each wheel.
Once the wheel is removed you can see on the hub assembly where the connectors are for the sensors and also the brake pad sensors. Simply pull the clip back that keeps them plugged in and then unplug the sensor. If you look into the sensor socket there are two sockets where the metal pins of the plug plug in to. Stick both prongs off your multimeter into the sockets and take a resistance reading.

w2fYJru.jpg



I don't know whether the reading is ohms or k-ohms but each sensor was reading around 1.6. If you spin the disc when taking the reading, it should fluctuate up and down slightly. All sensors were giving the same reading apart from the rear left which wasn't outputting anything at all.

Fitted a new sensor today and took a reading. Reads slightly higher at 1.7 but put everything back together and lights have no gone off. Problem fixed



:)

And only around £50 for a new sensor. Result!


psIz5cb.jpg

Have you got a picture of the other end of he sensor? I have found he rear off side sensor not reading anything the others are around 1.6.

When I've looked on line at replacements it doesn't seem to look the same, the one that is failing has the same white connector has per your picture but the other end seems to be connected to the small push in brake wear sensor.

The ones on line seem to have a largish sensor. I suppose I could have pulled the wrong plug out are the the same? I couldn't see due to it being hidden behind the disc
 
I have worked it out, there are two identical sensors connectors one for the brakes and one for the speed sensors, just follow the wires to make sure you unplug and check the rights ones.
 
It turns out not to be the abs sensor, I've now checked all 4 sensors and replaced the brake sensor above the brake pedal and I'm still getting the abs and psm warning light come up after driving for a few mins. 


Any other ideas or hints I could check?
 
Haver you checked if any of the reluctor rings are broken?
 

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