ABS - quick second opinion please!

[simon-york]

I would appreciate someone else's thoughts on this if you will...

Earlier this year I took out my instrument cluster and found the ABS warning light bulb missing. "Oh yes, I thought, someone has taken this out to pass the MOT" (official inspection here in the UK). Sure enough, although the light went out once the engine started, it came on again as soon as the car moved forwards - or within a second or two. Everyone says that the ABS system should be ok, but that at least one sensor had gone - or was maybe clogged.

My mechanic agreed with this, but said he did not have the equipment required to check the system, and to go to the dealer for them to check it. Well, I have been busy doing other things so I have not done it and the annual MOT inspection is now due. So, I have tried something which I hope will prove a useful alternative and maybe help others.

My idea was if one of the sensors returns a fault as soon as the car rolls forwards, I would simulate this by jacking up the car and spinning one wheel at a time. Sure enough the offside front wheel turned on the warning light straight away - the nearside did not. I have not tried the rear wheels since I appear to have at least one faulty one. On this basis I am going to order a new offside sensor tomorrow.

What do other members think? Worth a try? Thanks in advance for your thoughts.

Simon Daubeney, York, England. 1989 530i SE M30 5 spd Bilsteins Alpinas

[tim]

wasn't there something about the seatbelt buckle mechanism?- there was a sensor or something that went bad and wasn't worth replacing- thus the many cars out there that have their abs lights removed.

My memory is failing here, but perhaps it's an avenue you can explore if the new sensor doesn't clear things up. It would also be instructive to run the codes-although you needn't go to a dealer.

[tim]

wasn't there something about the seatbelt buckle mechanism?- there was a sensor or something that went bad and wasn't worth replacing- thus the many cars out there that have their abs lights removed.


I take that back- I'm thinking of the SRS light. Sorry. Nothing to see here. Move along. Just a guy who smoked out too many brain cells.

[winfred]

if you have access to a lab scope you can read the waveform from the sensor, or with a regular multimeter you can read the ac voltage when you spin the wheel

[simon-york]

if you have access to a lab scope you can read the waveform from the sensor, or with a regular multimeter you can read the ac voltage when you spin the wheel

Thanks for that, but I have run out of time really. Given that one wheel turned the fault light on and the other didn't, it is surely likely that the offside sensor is faulty?

Simon Daubeney

[tim]

Yes, but wasn't that the diagnosis anyway?

Either an obstructed lens or a faulty sensor.

Have you looked at the diagram on the ETK?

http://rust.mine.nu/bmw/

Put it in a skid to see if the ABS pulses?

[ron525]

I would say you need the "nearside" sensor. Since the "offside" sensor reacted to that wheel spinning, while all other wheels are standing still, that sensor works. The "nearside" one is not working since nothing happened when you spun that wheel.

[632 Regal]

go, rippin it up, in a field, and try to lock the brakes, the tire that skids should be the offender. easier than checking manually and pretty much a guarantee to find the correct one.

[winfred]

one dies they are all shut down


go, rippin it up, in a field, and try to lock the brakes, the tire that skids should be the offender. easier than checking manually and pretty much a guarantee to find the correct one.

[Bellicose Right Winger]

Ditto on the "nearside" wheel/sensor. Perhaps you can check this by spinning the rear wheels. If the light comes on, and I think it will, because ABS ECU is seeing only (1) speed signal and it's expecting (4), this will confirm nearside.

After you locate the the wheel, make sure the slots in the chopper wheel behind the rotor aren't filles with rust and scale.

Paul Shovestul




I would say you need the "nearside" sensor. Since the "offside" sensor reacted to that wheel spinning, while all other wheels are standing still, that sensor works. The "nearside" one is not working since nothing happened when you spun that wheel.

[simon-york]

First of all thank you to everyone who offered their thoughts on this, I do appreciate you taking the time and trouble to offer their knowledge and experience.

I have done as you suggested and tested both rear wheels in the same manner - spinning them as fast as I could but neither turned the ABS light on, so it is only the front off-side which does. If I managed to get them spinning fast enough (and when the car is being driven it has to get up a small amount of speed before the light comes on) which wheel is likely to be showing up a fault - the one that does turn the ABS light on or the three that apparently do not?

I have ordered one sensor from my dealer this morning in readiness for the MOT test. Any further thoughts on which front wheel I ought to ask my mechanic to fit it to?

Thanks again
Simon

[632 Regal]

one dies they are all shut down

[winfred]

sinking someone elses jeep in a mud hole is more fun :D


well it sounded like fun

[Bellicose Right Winger]

I'm not prepared to change previous recommendation, but I'll propose a modified test.

I suspect the problem is the inability to spin rear wheels fast enough. There is a lot of drive train resistance making it difficult to get a reasonable wheel speed. I would repeat test with both rear wheels off the ground and car on jackstands. Start engine, put car in gear and accelerate to 15MPH and see if light comes on.

As a safety precaution do not leave drivers seat during during test. When you apply brakes to slow down rear wheels you "may" feel ABS pulsations through brake pedal. While I suspect Winfred is right and that ABS is shut down on any fault...seems to me the lawyers would want it this way, a first hand confirmation on E34's would be helpful. Perhaps you can tell us if you do more testing.

Assuming above test turns ABS light on, you may want to declare victory and simply change the front sensor that did not turn on ABS light.

Problem is you can't tell which rear sensor is providing signal, hence you could have one bad & one working rear sensor. But we can test this also.

Please do not attempt the following if you do not fully understand the risks involved. If above test turns ABS light on, you can test each rear wheel individually provided your car DOES NOT HAVE a limited slip differential (LSD). It is absolutely essential you DO NOT HAVE A LSD to do the following. Repeat the test above but use the wheel chock from the trunk to keep one of the rear wheels from turning. This forces the differential to do the same thing it does when you go around a corner. Make sure chock is positioned in front of tire so it can't be thrown out behind tire. Repeat on opposite side to check other sensor. Attempting this with a car that has a LSD will result in driving car off jackstands.

Paul Shovestul



First of all thank you to everyone who offered their thoughts on this, I do appreciate you taking the time and trouble to offer their knowledge and experience.

I have done as you suggested and tested both rear wheels in the same manner - spinning them as fast as I could but neither turned the ABS light on, so it is only the front off-side which does. If I managed to get them spinning fast enough (and when the car is being driven it has to get up a small amount of speed before the light comes on) which wheel is likely to be showing up a fault - the one that does turn the ABS light on or the three that apparently do not?

I have ordered one sensor from my dealer this morning in readiness for the MOT test. Any further thoughts on which front wheel I ought to ask my mechanic to fit it to?

Thanks again
Simon

[simon-york]

Paul

Many thanks indeed for that long and well thought-out reply, if I had not run out of spare time I would certainly try it - it sounds as if it would yield a good DIY diagnostic result. Sadly however since the MOT test with my mechanic is looming near I have booked the car in to my local dealer just for them to perform a test on the ABS tomorrow and I shall let you know their response. Hopefully they will confirm that one of the sensors or rings is clogged and which one!

Thanks again for your help - we should know soon whether my approach of spinning wheels seperately is a valid approach or not!

Simon, York, England.

[simon-york]

My mechanic wanted a dealer opinion before he fitted a new sensor and so this morning the 5er went in to my local dealer for what was supposed to be a quick, ten minute plug-in test. It didn't work out that way.

Becasuse the E34 is one of the older models they could not just plug in their diagnostic kit, so had to use an oscilloscope on each sensor individually. This sounds straightforward, but they were getting different readings as they tested and then re-tested each wheel- so much so that they began to suspect the ABS control unit was at fault. However after three hours at this they agreed the faulty sensor was the front near side - the one that did not turn on the ABS light when spun on the jack.

Their hourly rate is £103.66 or I think about $165 so I was getting rather unhappy as time went past, despitr their excellent free coffee, biscuits and being surrounded by the latest models - including the new 1 series and a grey 645i which looked stunning. To their credit though, they only charged me for 50 minutes labour which seemed very fair.

The moral of the story is - I already thought that this was the faulty sensor - thanks to the advice from members of this board, of course if you are able to do all your servicing yourself and subsitute parts, you wouldn't need to consult the dealer...

Thank-you to all those who contributed to the thread. I greatly appreciate it.