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alex yates
Brands Hatch
Brands Hatch


Joined: 06 Mar 2014
Posts: 13133
Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Wed May 02, 2018 6:56 pm    Post subject: Self diagnosis and fix for ABS/PSM lights on Reply with quote

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



Smile

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



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deMort
Reims


Joined: 21 Mar 2015
Posts: 4470
Location: Brighton


PostPosted: Wed May 02, 2018 7:41 pm    Post subject: Reply with quote

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 Very Happy

BTW .. your hands look just like mine Floor
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The return of Marty Wild
Österreich


Joined: 04 Nov 2016
Posts: 968



PostPosted: Thu May 03, 2018 5:35 am    Post subject: Reply with quote

Great write up, Alex.

And amazing that Iain has found his ‘identical hand twin’.
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alex yates
Brands Hatch
Brands Hatch


Joined: 06 Mar 2014
Posts: 13133
Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Thu May 03, 2018 6:12 am    Post subject: Reply with quote

Grin Thumb
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Andyvvc
Nürburgring


Joined: 20 Feb 2014
Posts: 410



PostPosted: Thu May 03, 2018 12:39 pm    Post subject: Reply with quote

"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 Rolling Eyes

Good write up Alex - definitely the first thing to check if the ABS/PSM light comes on Thumb
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alex yates
Brands Hatch
Brands Hatch


Joined: 06 Mar 2014
Posts: 13133
Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Thu May 03, 2018 1:16 pm    Post subject: Reply with quote

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
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aerokit
Newbie


Joined: 04 Mar 2017
Posts: 27



PostPosted: Sat May 05, 2018 6:56 pm    Post subject: Reply with quote

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
 
  
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alex yates
Brands Hatch
Brands Hatch


Joined: 06 Mar 2014
Posts: 13133
Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Sat May 05, 2018 7:02 pm    Post subject: Reply with quote

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F232348227027
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Stuart Mackay
Monza


Joined: 29 Apr 2013
Posts: 224
Location: Swaffham, Norfolk and Good


PostPosted: Wed May 16, 2018 4:00 pm    Post subject: Reply with quote

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?
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alex yates
Brands Hatch
Brands Hatch


Joined: 06 Mar 2014
Posts: 13133
Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Wed May 16, 2018 5:14 pm    Post subject: Reply with quote

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.
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deMort
Reims


Joined: 21 Mar 2015
Posts: 4470
Location: Brighton


PostPosted: Wed May 16, 2018 6:45 pm    Post subject: Reply with quote

Stuart Mackay wrote:
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 .
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Stuart Mackay
Monza


Joined: 29 Apr 2013
Posts: 224
Location: Swaffham, Norfolk and Good


PostPosted: Wed May 16, 2018 8:50 pm    Post subject: Reply with quote

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?
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mr gill
Monza


Joined: 21 Feb 2011
Posts: 214
Location: The Black Country


PostPosted: Thu May 17, 2018 9:53 am    Post subject: Reply with quote

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
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Pickse
Spa-Francorchamps


Joined: 20 Jul 2013
Posts: 314
Location: Fleet, Hants


PostPosted: Thu May 17, 2018 2:52 pm    Post subject: Reply with quote

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.
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deMort
Reims


Joined: 21 Mar 2015
Posts: 4470
Location: Brighton


PostPosted: Thu May 17, 2018 6:12 pm    Post subject: Reply with quote

Stuart Mackay wrote:
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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Stuart Mackay
Monza


Joined: 29 Apr 2013
Posts: 224
Location: Swaffham, Norfolk and Good


PostPosted: Thu May 17, 2018 6:55 pm    Post subject: Reply with quote

deMort wrote:
Stuart Mackay wrote:
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?
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deMort
Reims


Joined: 21 Mar 2015
Posts: 4470
Location: Brighton


PostPosted: Thu May 17, 2018 7:06 pm    Post subject: Reply with quote

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 .
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Stuart Mackay
Monza


Joined: 29 Apr 2013
Posts: 224
Location: Swaffham, Norfolk and Good


PostPosted: Thu May 17, 2018 7:14 pm    Post subject: Reply with quote

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.
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deMort
Reims


Joined: 21 Mar 2015
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Location: Brighton


PostPosted: Thu May 17, 2018 7:19 pm    Post subject: Reply with quote

Aghh ok .. ignore my last pm then Very Happy
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deMort
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Joined: 21 Mar 2015
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Location: Brighton


PostPosted: Thu May 17, 2018 7:55 pm    Post subject: Reply with quote

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 Very Happy
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