Antilock warning light - ABS Pump anyone?
- Thread starter Tom
- Start date
Tom
New member
Does anyone know if the ABS pump is operated after the wheels sensors are tested or the other way round?
I've done a bit more research and have found out the following generic information about ABS self test. It would be great if someone could confirm at what point the pump is tested (if it is tested).
Ignition turned on: Anti-lock light lit (along with all warning lights).
Engine started: Apparently most systems carry out a check of the ABS solenoid valves at this point. They are tested by monitoring the current through each solenoid when energising the valve for a short time. This can detect most problems such as a stuck valve, shorted wiring or open circuit conditions. If tests pass, anti-lock light clears. The solenoid valves are part of the pump assembly on our cars.
Car moving: The pump is actuated to prime the accumulator (also part of the pump assembly) at about 3mph. Mine used to operate when I reversed out of the garage, so this is definitely done at a very low speed. The signals from the wheel sensors are also checked but I'm not sure at what speed this is done.
With my car, the pump no longer groans when I first move off. As soon as I get up to a higher speed (6 mph?), the anti-lock light comes on.
So the big question is - Is the pump not groaning because it is tested at 3mph and fails (if so, why doesn't the anti-lock come on at that speed?) or is it that the wheel sensors are failing so the pump is never actuated? The only other thing I can think of is that the pump is switched on at 3mph and maybe there is a check done at the higher speed to check it has stopped, reached normal pressure etc.
I've done a bit more research and have found out the following generic information about ABS self test. It would be great if someone could confirm at what point the pump is tested (if it is tested).
Ignition turned on: Anti-lock light lit (along with all warning lights).
Engine started: Apparently most systems carry out a check of the ABS solenoid valves at this point. They are tested by monitoring the current through each solenoid when energising the valve for a short time. This can detect most problems such as a stuck valve, shorted wiring or open circuit conditions. If tests pass, anti-lock light clears. The solenoid valves are part of the pump assembly on our cars.
Car moving: The pump is actuated to prime the accumulator (also part of the pump assembly) at about 3mph. Mine used to operate when I reversed out of the garage, so this is definitely done at a very low speed. The signals from the wheel sensors are also checked but I'm not sure at what speed this is done.
With my car, the pump no longer groans when I first move off. As soon as I get up to a higher speed (6 mph?), the anti-lock light comes on.
So the big question is - Is the pump not groaning because it is tested at 3mph and fails (if so, why doesn't the anti-lock come on at that speed?) or is it that the wheel sensors are failing so the pump is never actuated? The only other thing I can think of is that the pump is switched on at 3mph and maybe there is a check done at the higher speed to check it has stopped, reached normal pressure etc.
Tom
New member
Posting to this thread after 10 months as I've finally made some progress and thought it might be helpful to others out there with ABS problems:
I decided to do some diagnostics myself, since the garages that I have used have all failed to fix the problem, I've already had a replacement ABS pump fitted (against my instincts and advice from people on this forum) and I didn't want to throw any more money at it. What I've discovered is that one of the connections to the rear left wheel sensor is shorted to ground. For the record, I tested the wiring as follows and it is very easy to get to:
1) Pulled out boot release switch and removed carpet on right hand side of drivers footwell - it is velcroed along the bottom and the end furthest from the door.
2) Disconnected connector from the ABS ECU. There is a spring clip at one end only: Once this is released, you pull that end away as far as possible and then the other end unhooks from its retainer.
Note: When referring to the wiring diagram (944 Turbo, 944 S Model 90 Sheet 13, Page 97) the pins on the connector at the end of the wiring loom are numbered top to bottom, right to left , so Pin 1 of the connector socket is at top right when you bend it round to face you.
3) Made two cables with small spade terminals on one end to fit in the ABS ECU connector socket.
4) Tested resistance between each pair of sensor connectors - (11 & 21, 4 & 6, 8 & 9, 24 & 26). All came it at just under 1k ohm (lowest 955 ohms, highest 986 ohms).
5) Jacked each side of car up, attached oscilloscope to each pair of sensor connectors, spun wheels and checked trace. All produce a sine wave with similar voltage.
6) Tested resistance between each sensor connector and ground. Rear left sensor pin 8 shorted to ground, pin 9 has 952 ohms to ground (about the same as the sensor resistance).
So I'm now pretty certain that the short to chassis is the cause of the problem and fits with the original symptoms which began with the anti-lock warning light coming on occassionally when I reached about 10mph.
If you've read this far, thanks!
Anyone got any idea where the short is likely to be? I'd like to disconnect the sensor cable at the rear trailing arm and see which side (sensor or ECU end) has the short but not sure how to get it apart (don't want to break it).
I decided to do some diagnostics myself, since the garages that I have used have all failed to fix the problem, I've already had a replacement ABS pump fitted (against my instincts and advice from people on this forum) and I didn't want to throw any more money at it. What I've discovered is that one of the connections to the rear left wheel sensor is shorted to ground. For the record, I tested the wiring as follows and it is very easy to get to:
1) Pulled out boot release switch and removed carpet on right hand side of drivers footwell - it is velcroed along the bottom and the end furthest from the door.
2) Disconnected connector from the ABS ECU. There is a spring clip at one end only: Once this is released, you pull that end away as far as possible and then the other end unhooks from its retainer.
Note: When referring to the wiring diagram (944 Turbo, 944 S Model 90 Sheet 13, Page 97) the pins on the connector at the end of the wiring loom are numbered top to bottom, right to left , so Pin 1 of the connector socket is at top right when you bend it round to face you.
3) Made two cables with small spade terminals on one end to fit in the ABS ECU connector socket.
4) Tested resistance between each pair of sensor connectors - (11 & 21, 4 & 6, 8 & 9, 24 & 26). All came it at just under 1k ohm (lowest 955 ohms, highest 986 ohms).
5) Jacked each side of car up, attached oscilloscope to each pair of sensor connectors, spun wheels and checked trace. All produce a sine wave with similar voltage.
6) Tested resistance between each sensor connector and ground. Rear left sensor pin 8 shorted to ground, pin 9 has 952 ohms to ground (about the same as the sensor resistance).
So I'm now pretty certain that the short to chassis is the cause of the problem and fits with the original symptoms which began with the anti-lock warning light coming on occassionally when I reached about 10mph.
If you've read this far, thanks!
Anyone got any idea where the short is likely to be? I'd like to disconnect the sensor cable at the rear trailing arm and see which side (sensor or ECU end) has the short but not sure how to get it apart (don't want to break it).
bertelli_1
New member
Good progress! To get at the connectors - locate the black plastic 'barrels', the 2 halves are just clipped together, you will see the clip in the centre. Then wiggle out the white plastic part, the connectors just pull out. There are 2 connectors at the, one about 6" from the abs sensor and another above the axle - then the wiring disappears into the cabin.
Thats great! and ties in with what I've been told by a specialist which is even better!
I've found two earths not connecting so far, as I understand it the caliper should be directly earthed, but still have got the warning light and checked the sensors on those wheels too, but undoing the "barrels" as Bertelli says, so I think I'm on the right track!
I've found two earths not connecting so far, as I understand it the caliper should be directly earthed, but still have got the warning light and checked the sensors on those wheels too, but undoing the "barrels" as Bertelli says, so I think I'm on the right track!
Tom
New member
Thanks Bertelli - I was able to pull the plug out near the wheel and confirmed that one end of the sensor is earthed. My assumption, based on the wiring diagram and the fact that the 3 other sensors are NOT earthed is that this sensor is faulty. Can anyone confirm that the earthed sensor is a fault and so I should look at replacing it? Anyone else had this? Seems like a strange failure - I'd expected a chaffed wire shorting to the bodywork or something.
bertelli_1
New member
The sensors are 'variable reluctance' - the same type as the crank sensor - they create a voltage as the toothed wheel passes. They are susceptible to interference so the wiring is shielded - this should be earthed (the short brown wires next to the 'barrels'). The 2 wires from the sensor should NOT be earthed. I think you've found the problem Tom... You may find that the sensor is so badly corroded in the hub that it has caused a short.
Tom
New member
Rear passenger side sensor replaced and.....
It's FIXED !!!!!
No more nasty warning lights.
So my advice to anyone else with "Anti-lock warning light comes on at 6mph" problem is to not only check the sensor connections for resistance and signal but also for a short to the chassis.
It's FIXED !!!!!
No more nasty warning lights.
So my advice to anyone else with "Anti-lock warning light comes on at 6mph" problem is to not only check the sensor connections for resistance and signal but also for a short to the chassis.
bertelli_1
New member
Result! Was there any evidence of the cause of the short?
Tom,
On my car the ABS and main warning lights illuminated once the car was moving.
The ABS light was off after the car was started but before it moved away.
My Independent dealer checked everything and the problem was the pump.
I source a replacement pump and the dealer cleaned, rebuilt and fitted it.
Where the pump is fitted seems to collect a load of crud which does not help.
Mike
White 2.7 automatic
On my car the ABS and main warning lights illuminated once the car was moving.
The ABS light was off after the car was started but before it moved away.
My Independent dealer checked everything and the problem was the pump.
I source a replacement pump and the dealer cleaned, rebuilt and fitted it.
Where the pump is fitted seems to collect a load of crud which does not help.
Mike
White 2.7 automatic
That's one that can be tested yourself if you doubt it. Disconnect a pad wear sensor (ie break the circuit) & if the dash light comes on then you know an unworn sensor completes the circuit. If the light stays off with the sensor disconnected you know it doesn't. The sensor is as I described, it's just a loop of wire in a plastic block.
Neil Haughey
New member
BTW just to add a note, I have had a pump problem on my race car. The light never ever comes on whilst driving but sometimes the ABS just wasn't working as in occasionally a front wheel would lock up. It seems though that using the car more has freed up the pump and fingers crossed its OK now, I have road tested the car a good few times including after the end of season races at Silverstone and it has been fine.
One or two ppl posted similar experiences over in the 964 section.
One or two ppl posted similar experiences over in the 964 section.
Determined the way the pins are numbered by looking at the factory manual - all the pins that should be there are only there in one numbering permutation! I made it numbered from the loom end of the connectr, 1-18 on the longer row, 19-35 on the shorter row. In my case, all sensors have the correct resistance, but one side of the left rear is shorting to earth.
Thanks Tom and all for posting a useful thread.
Thanks Tom and all for posting a useful thread.
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