ABS Troubleshooting

[Click]

Since I've gotten my e28 the ABS has not worked. Light goes on with the ignition, turns off when started and comes back on shortly after rolling away.

I have replaced both front sensors with genuine BMW units to no avail.


The rear pulse wheels are pretty shot so I have the rear sensors unplugged to take them out of the equation. I plan on purchasing a new set of rings from http://www.reluctorrings.com/cars/bmw shortly.



Anywho the front pulse wheels are in good shape and the sensors are brand spankin new. The connectors on the body side have crumbled a bit but they make fine electrical contact.



My next suspicion was the ABS unit. I purchased one from a fellow member here and it purportedly came off a car with working ABS.....and it still didn't solve the problem.

I know in my E30 as soon as you roll away it does a quick little self test once you start driving and you can feel/hear the ABS pump. So the ABS pump was my next target.

First thing I did was test the relays. There was continuity between terminals 30 and 87a on the valve relay and once 12v was applied to terminals 86 and 85 the relay clicked and terminal 30 disconnected from 87a and connected to 87. Valve relay checks out. Did the same test for the pump relay and it checked out fine. Checked to see if power was getting to the relays. Power was getting to terminals 86 and 30 for the pump relay. That's odd, it should be 86 and 87 according to the ETM. I checked continuity between the abs harness plug prongs and the female relay socket connections. For the pump relay 30 was continuous with 13, 87 with 10, and 86 with 11. According to the ETM it should be 30 with 10 and 13 with 87. There is no continuity between 13 and 87 so I think this is just labeling boo boo in the ETM. I checked a spare ABS pump and it was the same way. I jumped terminals 87 and 30 with a short length of wire and the pump buzzed to life....so that works.
Back to the valves. Resistance of the valves was tested by probing connector 30 of the relay socket and terminals 2, 4, 6, 8 of the ABS harness plug prongs, Each read 2 ohms. Valves were tested by applying power to terminal 30 the of valve relay socket and selectively grounding terminals 2, 4, 6, 8 of the ABS harness plug prongs. All 4 clicked. Everything seems ok with the valves. The pump itself seems fine.
I then continued to test continuity of the wiring between the pump and the ABS module connector...all 8 wires checked out.
Next thing I checked was if the ABS module was getting power, pin 1 had 13.4V and pin 15 had 14V (1.6V before starting car). Odd that there is a 0.6V discrepancy but they're getting power. Pin 25, the brake input, gets 12.6V with the brake applied, none with no application. Checked continuity of pins 10, 20, 34 (grounding pins) with some metal part of the dash support. All three read 0 ohms demonstrating a good ground for all. Other than some minor voltage discrepancies, the ABS module plug seems to be getting all the correct grounds/power/signals etc.
So I unplugged the front driver's sensor so we're only playing with the front right sensor. Tested the resistance between module connector and the connector for the front right abs sensor, 0.2 ohms for both leads, pretty low. Tested the resistance of the sensor itself 19.9 kiloohms or 19900 ohms. I couldn't find a spec for e28 sensors but I have heard e30 sensors are 1-2K ohms. I know that e30s have different pulse wheels and such and is a different design however. So that curious. I should test the other sensor as well.

I filled in the ETM with my findings and realized I missed to test a couple things (like continuity between 86 and 12 for the motor relay) but it seems like everything is in working order and maybe I have another bum ABS module. Anyone have any input?

[demetk]

The rear pulse wheels are pretty shot so I have the rear sensors unplugged to take them out of the equation.

Are you expecting the system to work without the rears connected? When you do that, the ABS will take the whole ABS out of the equation.

[Click]

I know the e30 system analyzes each input independently. You can troubleshoot the system by unplugging all 4 then plugging one in, going for a quick spin to see if it light up, then unplug it and plug in the next sensor. I'd assume the e28 system is the same. So yes I would expect the light to not come on with only 1 sensor plugged in.

[demetk]

Then I have no more to add to this. Good luck.

[danc]

I can't speak about the E30 system, but as demetk said, the E28 system will be disabled with ANY sensor, or sensors, bad or disconnected.
Measure AC voltage at the sensors while you spin the wheels. Fronts should be similar to each other, rears should be similar to each other. Fronts vs rears? I can't help with that.

[Click]

Hmm, if that's so I better order those rings, install them and plug everything in...

[Click]

Replaced the rear inner hubs, plugged everything in and it works! Old rings were hammered and giving a poor/dirty signal.

It's interesting to see that the hub design has changed. The ABS wheel on the new one sticks out whereas on the old one it was flush.

[JohnH]

I know the e30 system analyzes each input independently. You can troubleshoot the system by unplugging all 4 then plugging one in, going for a quick spin to see if it light up, then unplug it and plug in the next sensor. I'd assume the e28 system is the same. So yes I would expect the light to not come on with only 1 sensor plugged in.

The system is quite happy with all four speed sensors detached. When you reattach one of them (with no other sensor attached) and a signal is returned then the system detects an imbalance and triggers the light. If you attached one (with no other sensor attached ) and the light doesnt come on, then you know that you have no signal being returned. This doesnt prove that the sensor is at fault, rather that there is a break in the sensor line.

A better way to test these sensors/reluctor rings is at the detached ecm connector hook up a multimeter reading AC volts and drive the car at say 20mph and check for peak volts for each sensor. The one that is noticable higher/lower that the others is the problem area.

Heres an example taken from my own car


Like yours, both rear reluctor rings were at fault.

An oscilloscope reading shows the problem. This for the FL



and this for the RL



Pleased to see you sorted your problem, I havent got round to fixing mine yet.

Edit; fixed pics.

[tig]

I'm struggling with an ABS issue on Minerva. This thread seems helpful and I've decided to bump it as I dive in deeper. Finding good resources with search has not been fruitful. Any additional links would be appreciated.

To recap my situation: This is a Euro 85 chassis & harness, but all ABS components are from other US cars. I rebuilt the ABS pump myself (it was never fully tested). The ABS computer is from a donor car (never fully tested). All four sensors are brand new (and I've replaced both front ones with even more brand new units). All hubs (ABS gear rings) are brand new.

When ABS light is off (see below) ABS works when hard-stopping at speed. All four brakes bzzzt/pulse as expected.

For a while I was seeing two problems:

1) ABS light would either come on at start and stay on or come on after driving a bit and stay on.
2) When braking at slow speeds ABS engages (bzzzt/pulse) when it shouldn't.

After replacing both new front sensors (from Ivo) with newer front sensors (from Ivo) (because a test had indicated one was bad) I can't get the ABS light to come on anymore. But problem #2 persists.

I'm about to dive in starting at the ABS computer, trying this suggestion from above:

A better way to test these sensors/reluctor rings is at the detached ecm connector hook up a multimeter reading AC volts and drive the car at say 20mph and check for peak volts for each sensor. The one that is noticable higher/lower that the others is the problem area.

Wish me luck and please post any links to more diagnostic info if you have... Thanks.

[tig]

I started with popping the ABS ECU and studying the ETM for pin-outs. I labeled the pins for the 4 sensors on the connector with a sharpie so I could quickly swap on the side of the road. Since my multimeter does Hz, I decided to do measure using that.



A quick test drive (30mph) showed:

LF (pins 4 & 6) - ~650hz
RF (pins 21 & 23) - ~680hz
LR (pins 7 & 9) - 0hz
RR (pins 24 & 26) - ~650hz

I didn't expect it to be a rear sensor.

Verified sensor and gears looked tits.


I got ~1k ohms on all four sensors, including the LR. I pulled the LR sensor out and verified it got 1k ohms on the bench. It did. I tried generating voltage by spinning the hub with it attached. Nothing. (And I proved I could get a small voltage spike by doing the same on the RR).

I had another old, used, LR sensor in my pile so I popped it on and tested it. Worked great.

LF (pins 4 & 6) - ~650hz
RF (pins 21 & 23) - ~680hz
LR (pins 7 & 9) - ~670hz
RR (pins 24 & 26) - ~650hz

A bunch of test driving later and I'm convinced ABS is finally working correctly.

So the diagnosis is done. A brand new (from Ivo) ABS sensor ohms out ok, but does not generate AC voltage.

DO NOT ASSUME, JUST BECAUSE A SENSOR OHMS OUT OK, THAT IT WORKS.

[tig]

BTW, some good info can be had here:

https://www.r3vlimited.com/board/forum/ ... ost6384143

[Click]

Just logged in here for the first time in a while and saw this. That's a cool way to check with the Hz setting. A couple years after I made this thread the ABS was again acting wonky, that time one of the rear senders ended up being the issue. I picked up a 4 channel Oscilloscope off of ebay and used it for troubleshooting which was pretty cool. My peak heights and wave forms were not the same between all sensors (which may have been due to my crappy old O scope) but the frequencies were identical which is what you found.



Haven't driven my e28 since late 2018 when the G265 started making a loud whining sound. That happened shortly after I bought my first house (and a big fixer upper at that) and am hoping to have some time to pull it soon and see about splitting it and replacing whatever bearing has gone bad. I really miss driving it (and also dislike DDing my nicer lower mileage vehicles, e28 has ~350k). House is mostly done labor wise at the moment. I'm currently saving for new windows, carpet and kitchen cabinets/appliances now. Current projects in the way are installing heat in the garage, do a cooling system refresh on the F150 I bought last year, and take care of the couple tiny rust spots I found just starting on my 325is while I had the driveshaft pulled for output shaft seal replacement and center support replacement...

[ovide]

For those with broken ABS rings, we have replacement rings.



BMW never made/sold these as a separate part, you had to buy the whole drive-flange, but these have become NLA now (and extraordinary expensive as well)

We sell them as a set of two: https://www.e28.nl/en/product/bmw-reluc ... s-fitting/

[tn535i]

One of the best movie lines ever in The Martian was 'I'm going to have to science the shit out of this"

This brings that to mind and also how similar it is to things you might do with audio. So...

If you don't happen to have a DMM that measures Hz you can probably use a laptop computer and a simple RTA that you download to look at waveform from the sensor just like an oscilloscope. I would first look at the voltage from the sensor and make sure it does not exceed the line level input for your Laptop or device which is probably about 1-2v. Just spinning the wheel by hand I imagine you would be safe directly connecting the sensor to your line or microphone input on computer. If you have means to isolate/attenuate signals with an external device you can turn an ordinary laptop into a fine oscilloscope.

I've been using TrueRTA for years and long ago paid for their higher resolution version as I use it for sound system work I do. Easy to use and very powerful and I'm sure there are others similar but the company is here in TN and has been kind to answer my questions a time or two.

https://www.trueaudio.com/rta_abt1.htm

I have on occasion run a line to devices like sensors and brought the signal into my laptop, O2 sensor for example self generates about 1v. Fun stuff I've done with my son is put small magnets on a yo yo and use a small coil to measure it's speed as it slept or getting the rate of fire on an airsoft AEG.

STSOOT

[gadget73]

tachometers also work as a frequency meter if the output voltage is high enough.

[Hupa]

Hi,

my e28 is missing the abs indicator light. Anyone know the specs / part number? Maybe even where to get one. Is the bulb itself different from the others on that row of indicators? At least the socket/holder what’s it called is different.

Thanks!

[///M]

It could be missing the indicator light because the ABS is faulty. If the light does not go off on starting the car then it will not pass the car test under EU law. If the light never comes on then there is no problem! (it is a car without ABS)
The bulb should be a 12v 1.2w , the other bulbs should fit. The difference in the holder for the ABS bulb is that it has two wires connected to it, 12v+ and 12v-. It is a push fit in the rear. The other holders are a twist fit.

[Hupa]

Yes, I suspect the light has been removed since the abs is not working… the abs rings are so rusty you almost can’t make out the teeth. I found two instrument cluster bulbs at Schmiedmann

62111368299

this is the 1,2W but seems to be with the holder. And then the other is

07119905358

This is the 3W, is a simple glass base, and comes without the socket. Without other knowledge, i’d try to put this one in the abs. But from your text, it should be the first one? But what to do with the ready socket? Away with it?

I also looked at
https://www.wallothnesch.com/en/catalog ... egory/274/
there’s the whole set: 15 of what seem to be 62111368299 and 3 that seem to be 07119905358. The latter ones (3) are the ones with glass base.

I’m a bit confused… so I get the 1,2W (62111368299) remove the plastic (twist) connector and place it in the ABS connector? If so, what are the (3) glass base 3W bulbs in the set?

Thanks.

[///M]

The bulb in the holder can be removed. https://www.ipconlinestore.com/dual-fil ... -watt.html
This is the bulb in the holder. Can be purchased any where with or without the holder sometimes called a "grain of wheat bulb" or "wheat bulb as in the link.
The larger one is for the illumination in the instrument cluster and fits the larger hexagonal holders at the top. So buy the set or just search for what you want. If you have "motor factors" in Finland places that sell motor parts they might sell them.

[Hupa]

Thanks, mate!

[Hupa]

Ok, got the abs-bulb replaced… it was an easy one, picked up a pair of 1,2W glass bulbs at the auto parts store for dizzying €1,49 (~the same in$’s).

Well, the light shines continuously when the power and/or engine is on. Expected, as someone took the bulb out at one point.. so there is a real problem… most probably the rear rings/sensors are shot. But… before commencing the repairs, I’d like to verify a comment from JohnH above ”The system is quite happy with all four speed sensors detached.” Is this really so? Disconnect ALL sensors, and everything looks to be ok (the abs-light works as if the system is in perfect condition)? I’d like to get the car inspected, but at my skill level the summer is over before the car is fixed! So, I’d be happy to buy some time for preparing the repairs & gathering the parts… and still enjoy some motorvating with the beautiful car.

Does the behaviour of the abs-light give any further hints (it is continuously on, once the power in on… i.e. doesn’t go off when starting the engine, it goes off only with power off)?

All advise is welcome, of course! Have a nice summer, everyone!

[///M]

I did wonder what you were doing wanting to put a bulb back when it was obvious to me why it was removed. Now that you have a working ABS light disconnect all four sensor wires at their plugs but use a cable tie to hold the wires together not connected. Drive the car and see if the light goes out!

It is very unlikely that will happen. The light signifies a problem so with all of the sensors not working you will be expecting the warning light to function as if the system has no problems. Let me know how it goes.
The 96 tooth ABS ring was not used after the e28. The e30 had a 48 tooth ABS ring which did not have the same corrosion issues. I have recently seen corrosion that caused the ABS ring to split so a common problem unless you live in a dry climate.

[Hupa]

I put the bulb back in to start finding out what is wrong with the system (can’t do that without the light…). Having consulted a couple of e28 dudes here in Finland, it seems that there are further issues besides broken sensors/corroded rings, since the abs-light is CONSTANTLY ON, i.e., does not go off when the engine is started. So, I think the sensor-disconnect won’t do anything (i’ll try it, of course). Malfunction might be in the pump unit… need to check the relays, if they are ok or even there (i saw a thread here, where one of the relay was removed to disable abs, and when the light didn’t go off, the bulb was also removed). I suspect the same might be the case in my car. Then to check/test the abs-ecu. Once (if) the problem is just a broken/missing relay and I replace it, I think the correct operation of the abs-light would be: turns on with power, turns off starting the engine, turns back on while driving (if the sensors/rings are bust). Only then, disconnecting the sensors would make the ABS appear to be intact, and the light would stay off while driving. Certainly, I can just rewire the abs-light, so it functions as if the system works, but eventually, I want to fix it.

Do you agree?

Edit: here’s the missing relay story:
viewtopic.php?t=104584

[///M]

I think you are over complicating the issue as I do not follow the logic.
ABS light comes on when the car is started. It goes off when the car drives. That is the check of the system. If the light does not go off then there is a problem. What is the problem? The control unit is not picking up the correct signal. That is why it is a failure in most countries.
Corroded ABS rings= fault
Faulty sensor =fault
Poor electrical connection on 12v+ or 12- = fault
Fault in wiring to control unit = fault
Any number of faults can cause the problem of the light staying on. A good place to start is to check if the ABS control unit is getting a signal and go from there.

[Hupa]

What I’ve been told (several sources) that if there is a mere sensor related problem, the light should still go off when the engine is started, and lights back on when you start driving the car, and the system picks up corrupted/ uneven signals from the sensors.

So, I think there are problems beyond sensors/rings, because my abs-light does not go off upon engine start. I think we just have mixed expectations of how the abs-light should behave, right?

It makes sense to me that if everything else (than sensors) are ok, the light should go off when engine starts…since if you are not moving, all sensors have 0V & 0Hz anyhow, so the system can only detect a faulty sensor when you drive and sensors give out voltage readings that the system can cross reference. Unless… the system performs a continuity check, which would give out a broken/open circuit (I doudt the system does this, but don’t know).

So, you disagree with the expected behaviour of the light, and say it stays illuminated all the time.. from turning the key to ON, through START , no matter what the fault might be?

I’ll test disconnecting the sensors first, so we’ll see if that makes a difference. Thanks for your comments & support…///M, it’s nice to get feedback before doing something stupid on my own.

Edit: to be clear… all r.h.s bottom row warning lights come on as the key is turned to ON (engine still not running), when the engine is started, all the lights go OFF, unless there is a fault of course. If only an ABS sensor/ring is broken, the ABS light should still go off, but come back on when you start driving and the ecu gets (corrupted) sensor signals other than 0. Right? But my light never goes off and thus I think there is another fault besides sensors. Man, this is hard to explain with my rally-english…