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Self diagnosis and fix for ABS/PSM lights on

Alex

Well-known member
Joined
6 Mar 2014
Messages
22,076
This topic comes up quite regularly, especially now these cars are knocking on a bit so thought I'd do a little write-up that may help folk.

My ABS/PSM lights both came on last week so I had a go at fixing the problem myself. First thing I checked was the brake light switch. Easy enough to disconnect, twist and pull out of it's location. The switch easily comes apart and is really simple to check for continuity. There are 4 spades coming out the switch. Continuity is tested across each opposite pair. Put a voltmeter across both spades and actuate the button to ensure the circuit is closed then open. One pair was a bit ropy so I opened the switch up to see.

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Once apart it was visible the contacts were caked in cr4p.

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The spade connectors simply slide out and can be easily cleaned.

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Cleaned up, all back together, continuity test passed and fit back in the car. Fault lights still illuminated so fault not cured. 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.

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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!


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Reading is 1700 ohms or 1.7 Kohms .. 1.6 to 1.7 Kohms is about correct for these sensors .. open circuit is its broken.

To be technical you scope these and look at the pattern over time and voltage but the resistance check is one i often use .

Brake pedal switch is a 2 stage switch , one for brake lights and the other for PSM .. the fault is usually a delay in the car seeing both .


Excellent job young man :thumb:

Im off to retire in my sunny holiday villa now as im no longer wanted here :D

BTW .. your hands look just like mine :floor:
 
Great write up, Alex.

And amazing that Iain has found his 'identical hand twin'.
 
:grin: :thumb:
 
"Easy enough to disconnect, twist and pull out of it's location"

It might be - but getting into a position to actually do that was the challenge i had lol! I got as far as managing to disconnect one side. Couldn't get the other side off doh! But clipping it back in fixed my issue for a month or two. Long enough for me to order a new sensor from Porsche Notts - it wasn't too pricey either.

If i had realised the plates come out and can be cleaned i would have persevered :roll:

Good write up Alex - definitely the first thing to check if the ABS/PSM light comes on :thumb:
 
Its a doddle in a cab with the roof down, although folk wonder what's going on when all they can see is a pair of legs sticking 5ft in the air :grin:
 
Excellent - thank you - this is the issue I seem to be having.

Where did you order the sensor from if you don't mind me asking please? Do you have a link or a part number?

Cheers and thanks again for the write up :thumb:
 
So Tech experts, on our C4, I have replaced the brake switch with new and all four wheel sensors are reading 1.6 kOhms, any recommendations for what to check next?
 
Reluctor rings. There's a whole in the opposite side of the hub housing so the ring can be inspected. On older cars they tend to split. Once they do, they play havoc with the ABS/PSM and chuck up the fault lights.
 
Stuart Mackay said:
So Tech experts, on our C4, I have replaced the brake switch with new and all four wheel sensors are reading 1.6 kOhms, any recommendations for what to check next?

Put a tester on it .. read out the fault codes and see what the problem is .
 
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?
 
I had this problem on mine last year, and as Alex mentioned it was the splined ring on the end of the driveshaft, it was cracked... replaced it and it hasn't come back since
 
In my case a broken wiring loom to the right front.
Error code reads as 'Right Front Sensor Implausible".

ABS/PSM lights occasionally triggered. Normally cleared and engine stop/start.
Normally happened after hitting a bump with the front right.

My Indy changed the sensor but it kept happening so I fitted the loom repair kit.
Job done.
 
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 ..
 

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