ABS warning light comes on
09-22-2009, 07:30 AM
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ABS warning light comes on
Hi Everyone,
Last week my ABS warning light started coming on shortly after I start to drive my 1988 928 S4, auto. Then this week I noticed at startup I had a stiff or "wooden" brake pedal, which goes away after I start driving a block or two and use the brakes.
BTW, the brakes work just fine. The brake pedal is just stiff at startup.
Looked all over the forums and found plenty of information and I may have a bad ABS relay, No. XVII. I just wish I had good one to try before I drop $90-$110 for a replacement relay.
Things to know:
1) ECU power relay: The PO had his shop install a custom relay at the battery to cut power to the ECU when the engine was turned off. This was to stop battery drain when the car was garaged, which was most of the time when he owned it. IE, the ECU is reset at every engine startup because it is disconnected when the engine is turned off.
2) Fuse: I checked the ABS fuse, #16, which is supposed to be 10A per the charts, my #16 fuse is 15A. I left it as is for now, but if someone knows something about this that I don't, please clue me in. Once this is sorted out, I will replace the fuse the a 10A one, unless I hear otherwise.
3) Master Cylinder/Booster: Visual inspection, it looks ok. No leaking, all connections tight, reservoir is filled and fluid is clean. With engine off, I pumped the brakes to bleed off the vacuum booster, then held the brake down and started the engine and the pedal sunk down when the vacuum booster started working. So my guess the booster is fine.
4) Corrosion: All the work I have done on the car so far, I have not found any loose or corroded connectors. This car seems to have been well cared for. Checked the reciepts for the PO and no work has ever been done on the ABS system.
I didn't check the wheel sensors or the relays that people say are up inside the fender.
Does anyone know where I can buy an inexpensive ABS relay? Or let me borrow a good one to try, before I drop $100 on a new replacement?
Thanks,
Joe Taylor
Last week my ABS warning light started coming on shortly after I start to drive my 1988 928 S4, auto. Then this week I noticed at startup I had a stiff or "wooden" brake pedal, which goes away after I start driving a block or two and use the brakes.
BTW, the brakes work just fine. The brake pedal is just stiff at startup.
Looked all over the forums and found plenty of information and I may have a bad ABS relay, No. XVII. I just wish I had good one to try before I drop $90-$110 for a replacement relay.
Things to know:
1) ECU power relay: The PO had his shop install a custom relay at the battery to cut power to the ECU when the engine was turned off. This was to stop battery drain when the car was garaged, which was most of the time when he owned it. IE, the ECU is reset at every engine startup because it is disconnected when the engine is turned off.
2) Fuse: I checked the ABS fuse, #16, which is supposed to be 10A per the charts, my #16 fuse is 15A. I left it as is for now, but if someone knows something about this that I don't, please clue me in. Once this is sorted out, I will replace the fuse the a 10A one, unless I hear otherwise.
3) Master Cylinder/Booster: Visual inspection, it looks ok. No leaking, all connections tight, reservoir is filled and fluid is clean. With engine off, I pumped the brakes to bleed off the vacuum booster, then held the brake down and started the engine and the pedal sunk down when the vacuum booster started working. So my guess the booster is fine.
4) Corrosion: All the work I have done on the car so far, I have not found any loose or corroded connectors. This car seems to have been well cared for. Checked the reciepts for the PO and no work has ever been done on the ABS system.
I didn't check the wheel sensors or the relays that people say are up inside the fender.
Does anyone know where I can buy an inexpensive ABS relay? Or let me borrow a good one to try, before I drop $100 on a new replacement?
Thanks,
Joe Taylor
09-22-2009, 01:23 PM
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Hi Matt,
Yes, I crank up the car and when I drive away from the house or shortly thereafter the light comes on. Remember that the ECU is reset each time I turn the car off with the custom relay.
VTY,
Joe
Yes, I crank up the car and when I drive away from the house or shortly thereafter the light comes on. Remember that the ECU is reset each time I turn the car off with the custom relay.
VTY,
Joe
09-22-2009, 01:40 PM
#4
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I had a very similar issue.
After much work the cause was a 'sticking' ABS relay..........the one on the main fuse/relay board.
The sticking relay also caused a large current drain from the battery..........I'll see if I can find my thread.
After much work the cause was a 'sticking' ABS relay..........the one on the main fuse/relay board.
The sticking relay also caused a large current drain from the battery..........I'll see if I can find my thread.
09-22-2009, 01:55 PM
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Hi Malcolm,
Thanks, I did find and read your posts in my research on the forums here - the one about your relay and there where a few others who had similiar posts.
Thats what gave me the idea to check the no. XVII relay, IE I want to swap out mine for a relay that is known to work and see if that clears up the problem before I buy a replacement.
VTY,
Joe
Thanks, I did find and read your posts in my research on the forums here - the one about your relay and there where a few others who had similiar posts.
Thats what gave me the idea to check the no. XVII relay, IE I want to swap out mine for a relay that is known to work and see if that clears up the problem before I buy a replacement.
VTY,
Joe
09-22-2009, 02:00 PM
#6
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Brakes feel wooden. As in hard to push in brake pedal and decreased stopping power?
Sign of vacuum boost system not holding vacuum for brake assist?
If ABS light comes on after driving around and it completes it's self-check, then it's sensing a malfunction.
From bad relays (one on the relay-fuse panel) to the 2 tall relays by the ABS pump (Buried behind splash panel in front of front left wheel) to the speed sensor/wiring to each wheel.
Magnetic tipped sensor can pick up rust crud falling off the brake discs and foul up the sensor reading.
Sign of vacuum boost system not holding vacuum for brake assist?
If ABS light comes on after driving around and it completes it's self-check, then it's sensing a malfunction.
From bad relays (one on the relay-fuse panel) to the 2 tall relays by the ABS pump (Buried behind splash panel in front of front left wheel) to the speed sensor/wiring to each wheel.
Magnetic tipped sensor can pick up rust crud falling off the brake discs and foul up the sensor reading.
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09-22-2009, 04:11 PM
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Hi Joe 
Its not a relay....or the ABS ECU.... its a sensor or in my case sensor wire...
Precisely the same symptoms as yours...light ONLY came on after moving a few yards, brake pedal hard sometimes...
Took ages to find mine...tested the resistance to all 4 sensors at the abs control unit and found one reading very very slightly different....
So took out the sensor...front left.... cleaned and put back....didn't cure it....so removed the sensor AND ITS WIRE that went across the suspension and through the inner wing....replaced with a new sensor/wire and all cured
Took a VERY close look at old sensor and all seemed fine until I examined the wire itself, it had rubbed on the suspension where it was MEANT to be held by small plastic clips which had broken off years ago....there was a slight split in the sensor wire , which when i peeled it back revealed a totally corroded inner wire that had obviously been shorting, which caused the abs control unit to turn the abs off....
So I seriously doubt its the relay...or control unit...
Hope this helps
All the best Brett
Its not a relay....or the ABS ECU.... its a sensor or in my case sensor wire...
Precisely the same symptoms as yours...light ONLY came on after moving a few yards, brake pedal hard sometimes...
Took ages to find mine...tested the resistance to all 4 sensors at the abs control unit and found one reading very very slightly different....
So took out the sensor...front left.... cleaned and put back....didn't cure it....so removed the sensor AND ITS WIRE that went across the suspension and through the inner wing....replaced with a new sensor/wire and all cured
Took a VERY close look at old sensor and all seemed fine until I examined the wire itself, it had rubbed on the suspension where it was MEANT to be held by small plastic clips which had broken off years ago....there was a slight split in the sensor wire , which when i peeled it back revealed a totally corroded inner wire that had obviously been shorting, which caused the abs control unit to turn the abs off....
So I seriously doubt its the relay...or control unit...
Hope this helps
All the best Brett
09-22-2009, 04:41 PM
#9
Race Car
My 85 would fire the light when started it was a bad relay. If your driving and it comes on it is most likely one of the sensors. After the relay was replaced the lamp would come on after driving and found the rear ABS sensor bad. Sometimes you can get a ton of crap on the sensor wheels so you may want to clean those first, but bets are on a wheels sensor. There is a way to check them while rotating the wheels with a meter look at the manual, thats how I found mine.
09-22-2009, 06:40 PM
#10
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As Tampa said.
If ABS warning comes on before you move the car most likely cause is the ABS relay on the CE Panel (Not the ones on top of the ABS Unit under the front left fender)
If ABS warning comes on after driving 10-20 yards and exceeding 5mph (? IIRC) it is most likely a failing or failed sensor (probably a rear one) or the wiring to the sensors as Brett said.
The stiff/wooden (perfect description by the way) is an alarming side effect of the ABS being confused by an imperfect signal from a sensor. I have the same issue with an intermittent ABS warning on my SE (Doesn't always happen after first movement - can be after an hour of driving). If you happen to be braking just as a sensor throws a fault The ABS brain first sees the fault as one wheel skidding ie not going round and activates the ABS - but when that has no effect it throws a fault and turns off the ABS. The wooden feeling is I think due to the way the ABS unit operates - it closes a valve in the line from the MC to the appropriate caliper (or rear calipers) and opens a valve from the caliper into a reservoir in the ABS body releasing the pressure momentarily from that Caliper. At that point pressing the brake pedal is pressing against a closed valve hence the wooden or nothing happening feeling. Once the wheel is sensed as turning again it undoes the above valve openings/closings. With a sensor fault I think it takes a little longer to recognise the fault than normal operation thus the pedal feels wooden for longer.
If ABS warning comes on before you move the car most likely cause is the ABS relay on the CE Panel (Not the ones on top of the ABS Unit under the front left fender)
If ABS warning comes on after driving 10-20 yards and exceeding 5mph (? IIRC) it is most likely a failing or failed sensor (probably a rear one) or the wiring to the sensors as Brett said.
The stiff/wooden (perfect description by the way) is an alarming side effect of the ABS being confused by an imperfect signal from a sensor. I have the same issue with an intermittent ABS warning on my SE (Doesn't always happen after first movement - can be after an hour of driving). If you happen to be braking just as a sensor throws a fault The ABS brain first sees the fault as one wheel skidding ie not going round and activates the ABS - but when that has no effect it throws a fault and turns off the ABS. The wooden feeling is I think due to the way the ABS unit operates - it closes a valve in the line from the MC to the appropriate caliper (or rear calipers) and opens a valve from the caliper into a reservoir in the ABS body releasing the pressure momentarily from that Caliper. At that point pressing the brake pedal is pressing against a closed valve hence the wooden or nothing happening feeling. Once the wheel is sensed as turning again it undoes the above valve openings/closings. With a sensor fault I think it takes a little longer to recognise the fault than normal operation thus the pedal feels wooden for longer.
09-22-2009, 06:48 PM
#11
Race Car
Hi Joe
Its not a relay....or the ABS ECU.... its a sensor or in my case sensor wire...
Precisely the same symptoms as yours...light ONLY came on after moving a few yards, brake pedal hard sometimes...
Took ages to find mine...tested the resistance to all 4 sensors at the abs control unit and found one reading very very slightly different....
So took out the sensor...front left.... cleaned and put back....didn't cure it....so removed the sensor AND ITS WIRE that went across the suspension and through the inner wing....replaced with a new sensor/wire and all cured
Took a VERY close look at old sensor and all seemed fine until I examined the wire itself, it had rubbed on the suspension where it was MEANT to be held by small plastic clips which had broken off years ago....there was a slight split in the sensor wire , which when i peeled it back revealed a totally corroded inner wire that had obviously been shorting, which caused the abs control unit to turn the abs off....
So I seriously doubt its the relay...or control unit...
Hope this helps
All the best Brett
Its not a relay....or the ABS ECU.... its a sensor or in my case sensor wire...
Precisely the same symptoms as yours...light ONLY came on after moving a few yards, brake pedal hard sometimes...
Took ages to find mine...tested the resistance to all 4 sensors at the abs control unit and found one reading very very slightly different....
So took out the sensor...front left.... cleaned and put back....didn't cure it....so removed the sensor AND ITS WIRE that went across the suspension and through the inner wing....replaced with a new sensor/wire and all cured
Took a VERY close look at old sensor and all seemed fine until I examined the wire itself, it had rubbed on the suspension where it was MEANT to be held by small plastic clips which had broken off years ago....there was a slight split in the sensor wire , which when i peeled it back revealed a totally corroded inner wire that had obviously been shorting, which caused the abs control unit to turn the abs off....
So I seriously doubt its the relay...or control unit...
Hope this helps
All the best Brett
As Tampa said.
If ABS warning comes on before you move the car most likely cause is the ABS relay on the CE Panel (Not the ones on top of the ABS Unit under the front left fender)
If ABS warning comes on after driving 10-20 yards and exceeding 5mph (? IIRC) it is most likely a failing or failed sensor (probably a rear one) or the wiring to the sensors as Brett said.
The stiff/wooden (perfect description by the way) is an alarming side effect of the ABS being confused by an imperfect signal from a sensor. I have the same issue with an intermittent ABS warning on my SE (Doesn't always happen after first movement - can be after an hour of driving). If you happen to be braking just as a sensor throws a fault The ABS brain first sees the fault as one wheel skidding ie not going round and activates the ABS - but when that has no effect it throws a fault and turns off the ABS. The wooden feeling is I think due to the way the ABS unit operates - it closes a valve in the line from the MC to the appropriate caliper (or rear calipers) and opens a valve from the caliper into a reservoir in the ABS body releasing the pressure momentarily from that Caliper. At that point pressing the brake pedal is pressing against a closed valve hence the wooden or nothing happening feeling. Once the wheel is sensed as turning again it undoes the above valve openings/closings. With a sensor fault I think it takes a little longer to recognise the fault than normal operation thus the pedal feels wooden for longer.
If ABS warning comes on before you move the car most likely cause is the ABS relay on the CE Panel (Not the ones on top of the ABS Unit under the front left fender)
If ABS warning comes on after driving 10-20 yards and exceeding 5mph (? IIRC) it is most likely a failing or failed sensor (probably a rear one) or the wiring to the sensors as Brett said.
The stiff/wooden (perfect description by the way) is an alarming side effect of the ABS being confused by an imperfect signal from a sensor. I have the same issue with an intermittent ABS warning on my SE (Doesn't always happen after first movement - can be after an hour of driving). If you happen to be braking just as a sensor throws a fault The ABS brain first sees the fault as one wheel skidding ie not going round and activates the ABS - but when that has no effect it throws a fault and turns off the ABS. The wooden feeling is I think due to the way the ABS unit operates - it closes a valve in the line from the MC to the appropriate caliper (or rear calipers) and opens a valve from the caliper into a reservoir in the ABS body releasing the pressure momentarily from that Caliper. At that point pressing the brake pedal is pressing against a closed valve hence the wooden or nothing happening feeling. Once the wheel is sensed as turning again it undoes the above valve openings/closings. With a sensor fault I think it takes a little longer to recognise the fault than normal operation thus the pedal feels wooden for longer.
I had the same problem one of my front sensor wires had been damaged, I opted to replace the sensor, since I had one I got on ebay cheap. Swapped out the sensor and hasn't been a problem since.
Do clean those sensors, I couldn't believe the amount of crap on the sensors on my car. 20+ years and 140k miles of brake metal particles.
09-22-2009, 07:02 PM
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Hi Matt, Brett, Tampa and Jon,
Does anyone have a photo of the wheel sensor in situ? What it looks like?
It would be nice to know what to look for, before jacking up the car and pulling a wheel off.
Thanks,
Joe
Does anyone have a photo of the wheel sensor in situ? What it looks like?
It would be nice to know what to look for, before jacking up the car and pulling a wheel off.
Thanks,
Joe
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