'ANTILOCK' light why me

[marky123]

Owned my 944 now for only a few months and only done about 300 miles. Yesterday the Antilock warning light came on a few seconds after i pulled away. I have checked the fuse for the ABS and it is fine. I noticed too that there is a relay for the ABS. Where should I start?

Thanks in advance, Mark.

 

[Diver944]

Search is your friend []

http://www.porscheclubgbforum.com/tm.asp?m=301689&mpage=1&key=abs&#302100

Ignore the wandering off topic at the end, it's what we do [8|]

 

[Neil Haughey]

So is the consensus then to start nearest to the wheels i.e. sensors and work inboard from there? Since my car has been driven so little this year and is kept outside corrosion down there seems like a good bet. I remember back when I first posted this the ABS failed a few times, about 6 weeks ago I drove the car every day for 3 weeks and the light never came on once.

 

[Riverside]

ORIGINAL: marky123

Owned my 944 now for only a few months and only done about 300 miles. Yesterday the Antilock warning light came on a few seconds after i pulled away. I have checked the fuse for the ABS and it is fine. I noticed too that there is a relay for the ABS. Where should I start?

Thanks in advance, Mark.

Mine came on on Thursday last week & now has the same symptoms as yours. I start the car & the ABS light goes out as normal, then as I pull off the driveway at the point where I would normally hear the abs pump whirr briefly there is no noise & the abs light (and central warning light) come on, and stay on until I switch off the car.

On Friday evening it started working normally again, then on Saturday morning about 5 miles into a trip the light came back on again & it's back to the above, always coming back on as I move off.

A bit of research over the weekend found lots of threads similar to the one Paul quotes above, where people have this problem but no solution is found, but fortunately a couple where the eventual fix was reported too []

Perry Mahoney's post to Titanic on 2/4/05 seems to give a good place to start with the fault diagnosis:

From my very, very basic understanding, the ABS system does 3 checks.

Check 1 - on first starting the car's engine. If the ABS light goes out once the engine has fired, then the ABS ECU is happy with everything, which does not involve the 4 wheel sensors. If the ABS light doesn't go out, then it is is unhappy with something, not involving the 4 wheel sensors.

Check 2 - when first on the move, the ABS does a quick check on the pump and 4 wheel sensors at around 6mph. We often hear this brief check taking place as the pump is behind the off-side wheel arch liner. If the lights stay out, then the system is happy with the readings from each of the 4 wheel sensors.

Check 3 - whilst on the move, if any wheel sensor suddenly stops reading the wheel speed (i.e. break in the wire) then the light comes on.

With my previous 944 - an S2 - it was a break in one of the front sensor wires which was the culprit and the car would always pass check 1 above, but would occasionally fail check 2 and always, after a couple of miles, check 3.

I too changed my pump (a well known independent told me, incorrectly as it turned out, that this was the fault!!) but the problem persisted. By chance, whilst on a past '944 Euro GT' in Germany, I put my hand behind the wheel and moved the ABS sensor wire. The problem disappeared for about 50 miles or so, before returning. Another waggle and it would go away again.
A quick phone call to Simon Butterworth at 'Porsche-apart' after I returned had 2 used front sensors on their way to me (I figured the other front sensor wire was probably weak, too) and problem solved!

Hope this helps

Perry
1991 951

My car has a bit of play in the OSF wheel bearing at the moment. I haven't looked to see where the abs sensor & rotor are mounted but I'm wondering if the play in the wheel bearing has affected the abs sensor gap? Apparently the gap isn't adjustable, the 944 maintenance manual describes this as a 'feature' (meaning it's not designed to be adjusted).

Bearing in mind that mine started working again briefly & then failed again I'm hoping it's just a poor connection to one of the wheel sensors & that waggling the wire to each wheel (one by one) might be a useful place to start the process of elimination. I'll adjust the wheel bearing too & if needed get it replaced, if none of that gives any clues I think I'll have to start throwing money at it.

btw my abs definately worked before & doesn't now, that was the first thing I tried []

 

[sawood12]

ORIGINAL: Neil Haughey

So is the consensus then to start nearest to the wheels i.e. sensors and work inboard from there? Since my car has been driven so little this year and is kept outside corrosion down there seems like a good bet. I remember back when I first posted this the ABS failed a few times, about 6 weeks ago I drove the car every day for 3 weeks and the light never came on once.

That is exactly what I found with mine. My previous owner didn't clock up that many miles in it and when I bought the car and started to use if every day I got this symptom intermittently. After a few months it stopped happening and 2yrs down the line it's not cropped it's head up since.

 

[marky123]

OK guys, i have had every wheel off and checked every sensor and they are all around 970ohms and the resistance changes when i rotate the wheels so i believe these are working. Next to check is the connections to the pump / control unit. Anyone know whereabouts these parts are located as i cant find em. Also worth pointing out that there is a ABS relay, is this a common fail point?

Thanks in advance, Mark.

 

[Riverside]

The pump is under the right wing behind the osf wheel, the abs ecu is under the dash just ahead of the door, it;s described in the manual as under the passenger side but this may be for a lhd car. I haven't looked for the ecu yet but I'd guess you'll need the part number or similar to identify which one it is. The wiring diagram should help. The relay in the fuse box is easy to find.

 

[944cabby]

ORIGINAL: marky123

Owned my 944 now for only a few months and only done about 300 miles. Yesterday the Antilock warning light came on a few seconds after i pulled away. I have checked the fuse for the ABS and it is fine. I noticed too that there is a relay for the ABS. Where should I start?

Thanks in advance, Mark.

Mine did this yesterday actually - it's happened before on occassion but it's always when i use the car early in the morning when it's very cold outside or after there's been a frost.

 

[hotblack944]

Hi Mark,

ORIGINAL: marky123
Also worth pointing out that there is a ABS relay, is this a common fail point?

Don't know about common, but on my S2, the ABS relay G20 failed causing the ABS dash warning light to come on. It was about £70 to replace IIRC.

HTH,

Jim.

 

[marky123]

**UPDATE**
It seems the ABS warning light has inexplicably gone off. For the time being everything is ok. This stinks of a bad connection. Question for 'hotblack944' - how did you find out that it was the relay that was faulty? Just change it and hope for the best?

 

[sawood12]

My ABS light hasn't come on for the last 3 years so you might be lucky.

 

[marky123]

Hey, fingers crossed Scott, just hope it was a bit of moisture in the relay case or the like.

Mark.

 

[hotblack944]

I can't honestly remember why I convinced myself it was the G20 relay - combination of internet research and talking to Jon Mitchell I think. I've got a feeling waggling the relay about caused the light to go out for a few weeks, then it came back on and waggling didn't help after that. I'm no electrical guru, but I bench tested it and thought that it wasn't working properly, then destroyed the case taking it apart to see if it was a dry solder joint inside. There was no turning back from that point, so I bought another from the OPC and that fixed the problem.

Not much help, sorry []

Jim.

 

[marky123]

This morning my ABS light was on and this afternoon it had gone off again! Well, tonight i bench tested the relay, quite straight forward really, and it works fine. There are another two relays in the ABS pump behind one of the front wheels, it might be that i have a look at these too.

Do you think the order is...

1. Engine is started.
2. At 6mph the ECU does a check to ensure sensors are ok.
3. ECU then primes pump.

I'm thinking if i can find out if the relay is being asked to energise at 6mph it could well half the circuit.

Another point was someone said that the wheel/tyre size was crucial as the rolling radius of the wheels effects the sensors. So i have cups on my car. Could tyre pressure differences make this happen??

Food for thought ... ;0)

 

[sawood12]

I think it is the ABS pump or accumilator that primes at 6mph because you can hear it as you pull away. Not sure if this is helpfull, but back when I was getting my intermittent problems sometimes the ABS system would be fine when I started up and set off then at some point during the jouney (not necessarliy at a regular time or milage) I would hear a buzzing noise that sounded like it came from behind the dashboard at the right hand side. This buzzing would sound for a few seconds before the ABS light coming on. Again I could never isolate this sound as it was intermittent and only happened about three times, but i definately heard it and it was the same every time it happened. The buzzing noise could have come from either the accumilator or the ABS unit in the wheel arch, or maybe from something behind the dash. The other times it happened the light would be on from startup.

 

[marky123]

Armed with an electrical cleaner spray, i intend to find the ECU and remove the multi plug, clean up the connection with my super-special spray and a miniature brush and then try driving the car again. If not fixed then its onto the pump armed with my spray. I think it's just a connection issue and it pleases itself when the light comes on and when it doesnt.

 

[944Turbo]

The ABS ECU is behind the carpet in the drivers footwell, I bet its a wire breaking down and becoming intermittent where they flex,
Tony

 

[morris944s2john]

The ABS sensor can be found when you remove the brake disc. It consists of a magnetic probe with a wire from it pointing at a square tooth cog wheel that sits around the wheel bearing behind the brake disk. As it all gets pretty rusty and grotty behind there, its perfectly possible that a bit of rust fell off and stuck to the magnetic sensor giving an error. I'm guessing that the ECU likes to see a square wave signal from each sensor and when a wheel is about to lock, it slows down, the frequency of the square wave becomes a lot lower and this triggers the pump to pulse the brake fluid pressure to that calliper thus preventing a wheel locking up.

Someone please tell me if my guess is right!

 

[marky123]

Well, firstly thanks for the advice. Today i fixed the ABS.

After locating the ABS ECU and removing and cleaning and reassembling the huge connector (to no avail) i turned my attentions to the ABS pump. After removing the wheel arch liner the problem was right in front of me. There is a black earth lead that runs from the back of the pump housing to the chassis just forward of the pump. It was absoloutely rotten. I cut the wire, put on a new ring crimp and drilled a new hole. Job done.

Next - onto my squeeky and rattley exhaust...

Cheers, Mark

 

[Neil Haughey]

Cheers Mark,
Looks like I can save myself a lot of time and go straight to the pump. I have had to renew / rebond several earth connections already to cure other problems so it wouldn't surprise me if I have the same problem. I find that my antilock light is on all the time now which would seem to imply that it isn't a problem with the sensors and I never here the pump-accumalator anymore when pulling away.