Self diagnosis and fix for ABS/PSM lights on

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. Once apart it was visible the contacts were caked in cr4p. The spade connectors simply slide out and can be easily cleaned. 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. 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!

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 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:

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/232348227027

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.

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.

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.

Aghh ok .. ignore my last pm then

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

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:

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.

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.

Looking at he price of these on design 911 the sensors seem to be around £170 but I've found the below site that has what seems to be the same ones for £20, anyone used them? https://www.buycarparts.co.uk/ridex...NRkk7kMaiuDI5W9nizGynu371ym-ikJRoCyPoQAvD_BwE

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?

Steering angle sensor?

Not sure how to do that, I presume being up on a ramp would help, so maybe this is one for my indie. I'm going the NeC at the weekend and looking to buy a Icarsoft POR 2.0 so hopefully can get some diagnostic info. Will also check out the steering sensor that the previous poster suggested.

Richard is an absolute legend.

...

This is a great thread thanks for the info Alex etc Just to add more info to help others, should they encounter the same conundrum as me. I replaced the brake light in the spoiler with an LED one (The previous one was also an aftermarket LED light but a few of the LED's were out.). I scanned the car (Foxwell NT530) and saw that it had a 'Brake pedal position sensor' fault. As we want to retrofit cruise control, I thought I'd try to fix it to prevent problems with that. Note, the brake lights functioned normally. Naturally, I thought the problem was with the switch on the brake pedal, so I bought a new aftermarket switch which looked identical to the OEM switch (apart from colour). I drove the car and got the ABS & PSM lights coming on after about 1 minute of driving. This was repeatable, i.e. it happened every time after restarting the car & driving. However, this time when I scanned the car the fault had changed to 'Implausible signal Brake switch circuit A'. As the switch was brand new, I thought the faulty circuit may be caused by the LED light. I had read that the LED brake light could cause problems, so I experimented with putting a load on the LED light wiring (I used a 21w bulb in parallel with the LED light). The ABS & PSM lights kept coming on. After much fiddling with the light and double checking the function of the new brake switch with a multimeter (it tested perfectly), I decided to do Alex's first post and clean up and re-install the OEM switch. Result ! - the ABS & PSM lights stayed off. I removed the temporary 21w bulb and the original 'Brake pedal position sensor' fault came back. This fault therefore was not being caused by the switch in the first place but by the LED brake light. I've got a 6 Ohm 50 Watt load resistor on order which I'll wire in parallel with the LED light to hopefully cure this fault. So, if you've not been able to follow the above, here are the conclusions: 1. An aftermarket LED brake light can cause a fault of 'Brake pedal position sensor'. It needs a load resistor to let the car know current is flowing. 2. An aftermarket brake pedal switch can cause a fault of 'Implausible signal, Brake switch circuit A'. So I recommend always using an OEM switch.

Just asa warning, I'd did all of this, even changed my brake swtich out twice both times with new OEM parts. Checked the sensors on the wheels. Everything I then had a brain wave, as some people have had issues with brake light clusters which has thrown this fault out. When I checked I was getting no brake lights when I pressed the brake... I checked the fuse to find it was hanging out, I popped it back in and hey presto no abs/psm light on dash. It turn s out when I had a Terraclean done a while back that fuse had been removed along with a few others and that had not been put back. Simple things

Had my 2 orange lights popping up on the dash (ABS & PSM) on a cold start for the last couple of weeks. Restart the car and they go. Anyway, on Friday they were on permanent so Friday afternoon I thought I'd have a snoop round. Decided to check the front sensors first as you can easily do those without even lifting the car. Just put the full lock on the steering and whichever way the wheels are pointing, check the sensor on that side. You can just unplug the double socket clamp (where the wheel speed sensor & brake sensor plug in on the hub carrier) and check the resistance with a multi-meter (as seen on the first post in this thread). Drivers side 1.6Kohms, passenger side.......off the scale with seriously high resistance.........which translates to F**ked! Ordered a new sensor, then spent around 2 hours drilling the old sensor out and removing the snapped cap head bolt holding it in. I find the easiest way is snap the head off the sensor, then chain drill around the periphery of it (on the inside of the diameter, not in the aluminium hub carrier) until everything can be pulled out. There's a big 8mm diameter magnet in the middle so you can't drill straight in in the centre, you have to drill round it. Once the sensor was out, I then had to tackle the snapped bolt. The bolt has been snapped in there for 10 years but I decided not to tackle it until the sensor failed. Fortunately I have one of these sensor bolt drilling jigs MisterCorn designed which made the job a doddle (see below video). Finshed cleaning everything up on Friday and just had the new sensor delivered today. 20 minutes jacking the car up, wheel off, fit new sensor, fit new Stainless Steel cap head bolt dowsed in copper grease, start the car to ensure lights are out, wheel back on and Bob's your uncle. I reckon 99% of outfits would've fit a new sensor and a new hub carrier at a cost of around £2000 all in. New senosr - £20: https://www.ebay.co.uk/itm/18523740...teid=0&campid=&customid=&toolid=10001&mkevt=1