E38 728i ASC/ABS Light on - Tricky Diagnosis
Discussion
I have a 1999 E38 728i which I picked up a while back as it was in lovely condition and low mileage, but had been stood a while and in need of some light recommissioning.
It runs and drives lovely now it’s had almost everything it needs sorted, but the ABS and ASC lights are permanently on. I ran diagnostics on it and it came up with a fault code for a rear ABS sensor. So assuming an easy fix I changed this sensor (changed both rears in fact). Light stayed on, so cleared the code, light still on. Went for a drive round, light still wouldn’t go out, but read the codes again and still clear, but light still on.
Decided to go into every module on the car to try and spot something related and under Automatic Transmission, it came up with an error code 95 CAN Wheel Speeds. This code is the only code on the car that won’t clear (apart from a fault on the airbag system and the rear PDC which are next on the list) and is obviously the root of the issue. After playing for a little longer, the data stream which shows the actual readings from the wheel speed sensors actually shows 3 out of the 4 sensors reading 2kph when the car is stationary and the other one at 0kph, which should be correct. The only problem is as this can only be read via the 20 Pin OBD port under the bonnet, I can’t really look at the readings whilst the car is being driven.
Any ideas? Maybe worth checking reluctor rings, but if that was the case surely the code would clear and then return when the car was driven and wouldn’t be picked up on a stationary vehicle?
It runs and drives lovely now it’s had almost everything it needs sorted, but the ABS and ASC lights are permanently on. I ran diagnostics on it and it came up with a fault code for a rear ABS sensor. So assuming an easy fix I changed this sensor (changed both rears in fact). Light stayed on, so cleared the code, light still on. Went for a drive round, light still wouldn’t go out, but read the codes again and still clear, but light still on.
Decided to go into every module on the car to try and spot something related and under Automatic Transmission, it came up with an error code 95 CAN Wheel Speeds. This code is the only code on the car that won’t clear (apart from a fault on the airbag system and the rear PDC which are next on the list) and is obviously the root of the issue. After playing for a little longer, the data stream which shows the actual readings from the wheel speed sensors actually shows 3 out of the 4 sensors reading 2kph when the car is stationary and the other one at 0kph, which should be correct. The only problem is as this can only be read via the 20 Pin OBD port under the bonnet, I can’t really look at the readings whilst the car is being driven.
Any ideas? Maybe worth checking reluctor rings, but if that was the case surely the code would clear and then return when the car was driven and wouldn’t be picked up on a stationary vehicle?
Sensors are OEM. Haven’t changed the fronts.
Diagnostic machine is a full fat Launch, but the problem with trying to run anything inside the car is that the old BMW 20 Pin connector is notoriously awkward and it can keep disconnecting at the best of times even when you’re doing it under the bonnet, so can’t see it working, but I can give it a try.
I’ll try manually testing out the ABS. It was a bit tricky yesterday as it was hammering it down all day.
Diagnostic machine is a full fat Launch, but the problem with trying to run anything inside the car is that the old BMW 20 Pin connector is notoriously awkward and it can keep disconnecting at the best of times even when you’re doing it under the bonnet, so can’t see it working, but I can give it a try.
I’ll try manually testing out the ABS. It was a bit tricky yesterday as it was hammering it down all day.
Those sensors reading 2kph at static, is certainly an oddity.
That said, I’d be inclined to use those Three as a reference point. Might be worth checking the supply voltages to all of the sensors. Look for a pos voltage, look for a ground, and if there’s neither of them look for resistance to ground (You’re seeing if there’s anything at the end of the wires with that one, in this case, hopefully the ABS module)
If you’re happy with that new ABS sensor and wiring, I think you’re going to be looking at a replacement hub.
That said, I’d be inclined to use those Three as a reference point. Might be worth checking the supply voltages to all of the sensors. Look for a pos voltage, look for a ground, and if there’s neither of them look for resistance to ground (You’re seeing if there’s anything at the end of the wires with that one, in this case, hopefully the ABS module)
If you’re happy with that new ABS sensor and wiring, I think you’re going to be looking at a replacement hub.
A non clearing rear ABS sensor fault code is 99.99999% of the time a dead ABS ECU. This is for post 9/98 build cars with the later ASC system. The plastic cover will have the Bosch part number ending in 001. If it has DSC (rare on a 728i) it will end in 003. They are both notorious for failure.
You can try a good used unit (just change the plastic ECU not the whole thing) and the ASC type doesn't need coding - it codes itself as soon as you start the engine and drive down the road. DSC types need coding as the steering wheel slip ring (measuring steering angle) does however.
You can get new Bosch ECU's from Andrew Page for far, far less than the official BMW pisstake price.
You can try a good used unit (just change the plastic ECU not the whole thing) and the ASC type doesn't need coding - it codes itself as soon as you start the engine and drive down the road. DSC types need coding as the steering wheel slip ring (measuring steering angle) does however.
You can get new Bosch ECU's from Andrew Page for far, far less than the official BMW pisstake price.
I was aware they were prone to failure, but was didn’t know the best way to make a positive diagnosis for that. That was also my next question, can you just change it for a good second hand unit?
Is the ECU attached to the ABS unit under the bonnet? I don’t have the car nearby today for easy reference!
Is the ECU attached to the ABS unit under the bonnet? I don’t have the car nearby today for easy reference!
Tried INPA and it's exactly the same. I was 99.9% sure it would be as on the very odd occasion that the Launch can't read a very OEM-Specific code, it still throws up a "nonsense" code that can only be read by OEM-Specific machinery, but worth a try.
I guess if it doesn't read a code then that again points to the ECU, so I've ordered a second hand unit with all the different part numbers on it matching and found one that was offered with a 3 month warranty.
Will try that and hope for the best!
I guess if it doesn't read a code then that again points to the ECU, so I've ordered a second hand unit with all the different part numbers on it matching and found one that was offered with a 3 month warranty.
Will try that and hope for the best!
mbwoy84 said:
I was aware they were prone to failure, but was didn’t know the best way to make a positive diagnosis for that. That was also my next question, can you just change it for a good second hand unit?
Is the ECU attached to the ABS unit under the bonnet? I don’t have the car nearby today for easy reference!
Just undo the small Torx screws, remove the plastic ABS ECU from the pump, fit another one, done. 25 minutes at most. Is the ECU attached to the ABS unit under the bonnet? I don’t have the car nearby today for easy reference!
Mind you, we've no idea now old the car is.
I swapped the ABS ECU over and it's still exactly the same.
Fault code points to Rear Right ABS Sensor. I can clear the code, but the lights stay on and the code will return after driving it and re-reading.
The wheel speed sensors still all read 2kph whilst stationary apart from one of the front ones (think it's front left, but not 100% sure), which reads 0kph.
Not sure where to go next!
Fault code points to Rear Right ABS Sensor. I can clear the code, but the lights stay on and the code will return after driving it and re-reading.
The wheel speed sensors still all read 2kph whilst stationary apart from one of the front ones (think it's front left, but not 100% sure), which reads 0kph.
Not sure where to go next!
A 2 mph differential WILL cause ABS faults & will only clear when the speed sensors read the same.
Several years ago, I had an e39 (as you say, pretty much same system) I bought the wrong size tyres for the rear & as soon as I got above 30mph (2 MPH speed difference front/rear reading live data) the Warning light came on.
As soon as you get rid of that differential it will stop thinking it has wheel-spin and ditch the light.
Several years ago, I had an e39 (as you say, pretty much same system) I bought the wrong size tyres for the rear & as soon as I got above 30mph (2 MPH speed difference front/rear reading live data) the Warning light came on.
As soon as you get rid of that differential it will stop thinking it has wheel-spin and ditch the light.
It possible to get the car off the ground, and see what speed sensor readings you’re getting, whilst the wheels are moving ?
If there’s an inline connector before the speed sensors, i’d be inclined to disconnect it, and see what your diagnostic shows as a base figure. Wether it be a “0 mph” reading or a “- mph”
If there’s an inline connector before the speed sensors, i’d be inclined to disconnect it, and see what your diagnostic shows as a base figure. Wether it be a “0 mph” reading or a “- mph”
Edited by RUSSELLM on Thursday 14th November 13:26
mbwoy84 said:
I swapped the ABS ECU over and it's still exactly the same.
Fault code points to Rear Right ABS Sensor. I can clear the code, but the lights stay on and the code will return after driving it and re-reading.
The wheel speed sensors still all read 2kph whilst stationary apart from one of the front ones (think it's front left, but not 100% sure), which reads 0kph.
Not sure where to go next!
Swap the rear axle sensors and see if the error moves to the left wheel or stays at the right. If it does you need another sensor (try reusing the old ones there is a chance that one of them is OK). If it stays it may be that the rings are in bad condition or dirty. Fault code points to Rear Right ABS Sensor. I can clear the code, but the lights stay on and the code will return after driving it and re-reading.
The wheel speed sensors still all read 2kph whilst stationary apart from one of the front ones (think it's front left, but not 100% sure), which reads 0kph.
Not sure where to go next!
RUSSELLM said:
It possible to get the car off the ground, and see what speed sensor readings you’re getting, whilst the wheels are moving ?
I’ve got a ramp, so yes stupidly I never thought of that! But anyway, it’s all sorted now fingers crossed. I did check and clean the connection prior to fitting the sensor, but had a closer look at the wiring today and found an iffy looking piece of wiring, which I’ve now repaired and for the first time the light has gone out!
So fingers crossed that part of the puzzle is sorted now.
I’ve also found that two of the rear parking sensors are faulty and removed the passengers’ seat to get the part number off the seatbelt pretensioner, which appears to be the cause of the airbag light. That and a temperamental heater blower and that’s all the issues sorted touch wood!
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