No warning lights, but no abs.........
12-22-2012, 10:31 AM
#1
Instructor
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Join Date: Mar 2007
Location: 1991 C4 20 mins from an Autobahn....YES
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No warning lights, but no abs.........
Stomping on the brakes on a wet road tells me so.
Anyone had a similar experience?
Any tips.
I do know that the brake pads are almost worn out(dealer warned me at last service)
Could that trigger it?
TIA
Anyone had a similar experience?
Any tips.
I do know that the brake pads are almost worn out(dealer warned me at last service)
Could that trigger it?
TIA
12-22-2012, 11:11 AM
#2
Nordschleife Master
Yes I had the same on my C2. The ABS hydraulic unit had failed (it's located next to the battery). I had a good used one from a breaker fitted (this was 350 GBP + fitting whereas a new one is over 2,000 GBP). The C4 brake system is more complicated because of the extra boost pump. Ideally you need to get a shop that has a Bosch ABS tester to check out what's gone wrong so that they can replace the correct part.
Worn out brake pads are not going to stop the ABS working.
Worn out brake pads are not going to stop the ABS working.
12-23-2012, 05:01 AM
#3
I got the same on my last drive this year.
Running with R compound normally I just might not have noticed it. But with winter tires it became quite clear and visible with 2 black stripes and a lot of smoke behind the car. Will deal with it when spring arrives,but please post your findings.
Running with R compound normally I just might not have noticed it. But with winter tires it became quite clear and visible with 2 black stripes and a lot of smoke behind the car. Will deal with it when spring arrives,but please post your findings.
12-23-2012, 11:41 AM
#5
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Rennlist Member
Some ABS trivia:
Research on the early ABS-equipped cars showed they had an unexpectedly higher accident rate. I do not know if the 964 was included. The report concluded with a theory that drivers were over-confident with ABS, and were driving closer to their limits. Later research showed the true reason: drivers had a momentary reaction to the pulsing of the ABS system, and lost critical milli-seconds. That was enough to increase the accident rate.
All instructions for ABS now say to expect the pulsing brake, and to get the brakes on full as quickly as possible.
A skilled driver can stop faster and handle better without ABS, but it does take practice on low-traction surfaces. I would not be concerned driving with ABS out, but I would find some low-traction surfaces in a safe area to practice braking and handling on a slick surface.
Research on the early ABS-equipped cars showed they had an unexpectedly higher accident rate. I do not know if the 964 was included. The report concluded with a theory that drivers were over-confident with ABS, and were driving closer to their limits. Later research showed the true reason: drivers had a momentary reaction to the pulsing of the ABS system, and lost critical milli-seconds. That was enough to increase the accident rate.
All instructions for ABS now say to expect the pulsing brake, and to get the brakes on full as quickly as possible.
A skilled driver can stop faster and handle better without ABS, but it does take practice on low-traction surfaces. I would not be concerned driving with ABS out, but I would find some low-traction surfaces in a safe area to practice braking and handling on a slick surface.
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01-15-2013, 05:32 PM
#8
Race Car
my abs working and I must say (contrary to the above re skilled driver etc..) that the stopping with abs in surprise/heavy braking necessity is amazing. I lock up the wheels much too easily at speed w/o the BAS and with it the entire experience is controlled and more effective than I imagined possible. I also have ABS on my cup but rarely engage it - maybe two way traffic, dogs and taxis less predictable 
Anyways thanks for all the abs tips here and in numerous other thread (also on 928 fiorum etc..)
What worked for me:
1. clean abs sensors with brake cleaner and blow out the toothed ring inside the hub
2. check and clean the 3 relays - 2 in frunk on left, 1 on fuse box
3. bridge fuse box relay 87-30 and see if light goes off/abs works ... it did, so problem solved for me (once i get a new relay)
I've cut and pasted some of the better/more detailed ideas here which helped me (a wrench noob!) solve this despite OPC not being able to after 3 hrs ...
background:
My ABS light comes on with ignition, not after car moves - thats important!
The ABS sensor is basically an electromagnet - an iron rod with many coils of a single wire wrapped within the housing - and the two ends exposed in the plug. Placing this coil/sensor next to a toothed, rotating gear (as machined on the hub) defines a generator when the hub turns .... generating an electrical waveform of the frequency of # of teeth passing the sensor/sec.
with this in mind...
- do a continuity check between the two leads in the plug ( should be several hundreds of ohms, as there are miles of fine wire wrapped around an iron core)
- check for continuity between the lead and ground ( yes = dead)
...
Is the abs/ecu receiving the 12v pos signal from the alternator 61 circuit ? This is what tells the ABS that the motor is running. If it doesn't get this signal it thinks that you merely have the key in the on position
...
If the sensor looks OK, then the fault is between the connector and the computer, or ..
- the sensor is not properly inserted in its bore
- the toothed wheel on the hub is packed with magnetic crap ( so the sensor does not 'see' the individual teeth - clean the gear
- the tip of the sensor is a mass of iron filings .... which can screw up the pulse generation.
The first thing to check is if you get an ABS warning before the car has moved - If you do the most likely culprit is the ABS relay on the CE panel. Fix this first before you bother doing any other checking. You can bridge the relay temporarily for testing purposes but don't leave it bridged as the ABS will be on all the time (keys in ignition or in your pocket) and wil flatten the battery.
Once that test is out of the way jack all 4 wheels off the ground and do the spin the wheels test - ign on (and relay bridged if you have to) spin one wheel at 4 mph or more - if it produces an error the sensor for that wheel is good. Then ignition off and relay unbridged then back on again to reset the ABS brain and repeat on another wheel etc. This is the quickest way to find which wheel has and ABS sensor faults.
Once you've narrowed it down to one wheel then start testing as described in Bertrands link to my earlier post.
...
If the warning light comes on at start up the unit is not getting power... three relays one on fuse panel two on the unit in the left front fender.
….
sensor is too far from the tone ring to see the edges coming by. Try taking all the sensors out, cleaning them up, and reinstalling them to the correct depth. They should sit just outside the tone ring, but not touching.
...
There should be continuity between the two sensor wires (about 1000 ohm) but none from each wire to ground.
If there there is continuity between one wire and ground, you have a bad sensor.
….
The easiest way to pinpoint failed sensors is to measure the resistance across each sensor (IIRC it's about 800-1000 ohms) starting at the ABS computer harness plug and then working your way nearer to the sensor itself to prove that it's not one of the connectors in the line that is the problem. You should also check the resistance between the sensor wires and ground - it should be infinite.
These sensors normally fail by shorting internally to the case of the sensor while still showing an acceptable resistance between the sensor wires. This will show up as a resistancebetween one sensor wire and ground = to X where X is between 0 and roughly 1000 ohms. The other wire on the sensor will measure roughly 1000-X ohms - all depnds upon where the short to ground is.
...
I had exactly the same problem as you and did the same tests, with the same result..
BUT... it wasnt the sensor... it was the sensor lead that goes back to the connector in the inner wing, which had rubbed and was earthing...
The only way you will find out if its that is either to change the sensor which MAY be perfectly good...or check continuity between the sensor plug at the sensor and the abs brain end...
I removed the sensor and have no continuity anywhere, even where the wires begin at the sensor. I did test each sensor wire individually and they tested fine. So the "break" must be in the sensor. Trust me, I was hoping for a chafed wire, less money and headache All the other sensors showed ~equal continuity at the pin connectors.
...
For some reason Porsche uses a completly separate power lead for the ABS system which is terminated piggy-back fashion onto the main connector at the positive battery terminal (this is true for the '90 C2 but I'm not sure whether it is the same on all 964s). There are at least two potential problems with this setup that can lead to erratic ABS operation.
1. The ABS power teminal is held on by a small nut which can loosen over time leading to high resistance or open circuit.
2. Corrosion can form under the terminal (stray battery acid etc.) leading to a high resistance connection.
In either case the ABS circuit eventually can't get the juice it needs to operate and indicates a fault condition.
Hope this keeps someone else from going crazy trying to find this problem!
Anyways thanks for all the abs tips here and in numerous other thread (also on 928 fiorum etc..)
What worked for me:
1. clean abs sensors with brake cleaner and blow out the toothed ring inside the hub
2. check and clean the 3 relays - 2 in frunk on left, 1 on fuse box
3. bridge fuse box relay 87-30 and see if light goes off/abs works ... it did, so problem solved for me (once i get a new relay)
I've cut and pasted some of the better/more detailed ideas here which helped me (a wrench noob!) solve this despite OPC not being able to after 3 hrs ...
background:
My ABS light comes on with ignition, not after car moves - thats important!
The ABS sensor is basically an electromagnet - an iron rod with many coils of a single wire wrapped within the housing - and the two ends exposed in the plug. Placing this coil/sensor next to a toothed, rotating gear (as machined on the hub) defines a generator when the hub turns .... generating an electrical waveform of the frequency of # of teeth passing the sensor/sec.
with this in mind...
- do a continuity check between the two leads in the plug ( should be several hundreds of ohms, as there are miles of fine wire wrapped around an iron core)
- check for continuity between the lead and ground ( yes = dead)
...
Is the abs/ecu receiving the 12v pos signal from the alternator 61 circuit ? This is what tells the ABS that the motor is running. If it doesn't get this signal it thinks that you merely have the key in the on position
...
If the sensor looks OK, then the fault is between the connector and the computer, or ..
- the sensor is not properly inserted in its bore
- the toothed wheel on the hub is packed with magnetic crap ( so the sensor does not 'see' the individual teeth - clean the gear
- the tip of the sensor is a mass of iron filings .... which can screw up the pulse generation.
The first thing to check is if you get an ABS warning before the car has moved - If you do the most likely culprit is the ABS relay on the CE panel. Fix this first before you bother doing any other checking. You can bridge the relay temporarily for testing purposes but don't leave it bridged as the ABS will be on all the time (keys in ignition or in your pocket) and wil flatten the battery.
Once that test is out of the way jack all 4 wheels off the ground and do the spin the wheels test - ign on (and relay bridged if you have to) spin one wheel at 4 mph or more - if it produces an error the sensor for that wheel is good. Then ignition off and relay unbridged then back on again to reset the ABS brain and repeat on another wheel etc. This is the quickest way to find which wheel has and ABS sensor faults.
Once you've narrowed it down to one wheel then start testing as described in Bertrands link to my earlier post.
...
If the warning light comes on at start up the unit is not getting power... three relays one on fuse panel two on the unit in the left front fender.
….
sensor is too far from the tone ring to see the edges coming by. Try taking all the sensors out, cleaning them up, and reinstalling them to the correct depth. They should sit just outside the tone ring, but not touching.
...
There should be continuity between the two sensor wires (about 1000 ohm) but none from each wire to ground.
If there there is continuity between one wire and ground, you have a bad sensor.
….
The easiest way to pinpoint failed sensors is to measure the resistance across each sensor (IIRC it's about 800-1000 ohms) starting at the ABS computer harness plug and then working your way nearer to the sensor itself to prove that it's not one of the connectors in the line that is the problem. You should also check the resistance between the sensor wires and ground - it should be infinite.
These sensors normally fail by shorting internally to the case of the sensor while still showing an acceptable resistance between the sensor wires. This will show up as a resistancebetween one sensor wire and ground = to X where X is between 0 and roughly 1000 ohms. The other wire on the sensor will measure roughly 1000-X ohms - all depnds upon where the short to ground is.
...
I had exactly the same problem as you and did the same tests, with the same result..
BUT... it wasnt the sensor... it was the sensor lead that goes back to the connector in the inner wing, which had rubbed and was earthing...
The only way you will find out if its that is either to change the sensor which MAY be perfectly good...or check continuity between the sensor plug at the sensor and the abs brain end...
I removed the sensor and have no continuity anywhere, even where the wires begin at the sensor. I did test each sensor wire individually and they tested fine. So the "break" must be in the sensor. Trust me, I was hoping for a chafed wire, less money and headache All the other sensors showed ~equal continuity at the pin connectors.
...
For some reason Porsche uses a completly separate power lead for the ABS system which is terminated piggy-back fashion onto the main connector at the positive battery terminal (this is true for the '90 C2 but I'm not sure whether it is the same on all 964s). There are at least two potential problems with this setup that can lead to erratic ABS operation.
1. The ABS power teminal is held on by a small nut which can loosen over time leading to high resistance or open circuit.
2. Corrosion can form under the terminal (stray battery acid etc.) leading to a high resistance connection.
In either case the ABS circuit eventually can't get the juice it needs to operate and indicates a fault condition.
Hope this keeps someone else from going crazy trying to find this problem!