ABS Troubleshooting

[///M]

I think you English is brilliant. In school I learnt four languages and can only speak and understand one language. The confusion may be over the operation of the light.
What you are saying is correct in that it should go out on starting if it does not then it should go out when you drive off. If it comes on then there is a problem.
The point of overcomplication is that you are decorating the house before the bricks have been laid. Badly corroded ABS rings are a common problem on old e28 because of the small gaps so the signal can be lost. So the first thing I would do is to connect the sensor to an oscilloscope or a multimeter that has a frequency function.
So if there is no problem there then move on to the control unit. Simple things first and then on to the complicated problems.

[gadget73]

On all the stuff I've worked with that had older ABS systems, if there was an open circuit sensor the light would come on and stay on at startup. It didn't wait until there was motion to trip the light. When I've had the light go out, then come back on when moving were situations where the sensors were not open or shorted, but not generating the correct signal for whatever reason. Rusty or damaged tone rings is common, so is incorrect sensor gap on things that can be adjusted. I even once ran into a situation with mismatched tone rings. Somewhere along the line the tone ring on the front rotors changed from a stamped steel with holes to a cast steel with "teeth". Either could be used, but both had to be the same style.

[Hupa]

Good points, M & gadget!! Maybe I’m overcomplicating things. Well, I guess there’s no shorcut, but to start maticulous testing and ruling out stuff. Better get a case of beer and plenty of cigarrettes, and get to it. I let the sensor connectors soak in silicon over night, as they were pretty well stuck… maybe never opened in the car’s 35 years. I’ll try not to crack them.

Thanks again, guys.

P.S. gadget, you said open sensor circuit would trigger the light even without motion, but the folklore tells e28’s ecu would think all a-ok if all four sensors are disconnected?

[///M]

Do you know that you can check the sensors at the ABS control unit plug? By doing that and spinning the wheels if they are working then you should generate a voltage and frequency. Or drive the car at a set speed.
The reason for frequency is that the meter may not detect the low AC voltage at low speeds. But if you only have a multimeter with voltage then worth a try.
Write down the results and go from there. If all the sensors are working then you have tested them and the wiring to the control unit.
That is what I meant about the over complication. The diagram gives you the pin numbers for the sensors. No need to disconnect because if you are correct in your assumption that there is no fault with them then that test will prove/disprove your assumption.

[gadget73]

P.S. gadget, you said open sensor circuit would trigger the light even without motion, but the folklore tells e28’s ecu would think all a-ok if all four sensors are disconnected?

I don't know the E28 system. My old ABS experience is mostly ATE brake systems. If these use a Bosch system, that may behave differently. Usually the shop manuals have good troubleshooting info though. I'd be way more inclined to rely on what it says than me the random Lincoln guy who is here mostly for BMW diesel engine information.

[Hupa]

M, yes, I know the DMM procedure… but guess what, this morning I got to the garage… and decided to take the car for a spin before working on it… the start was a bit sluggish, so I metered the battery…12,0V (engine running)… gave it some revs, no change… what’s wrong, I thought, the car’s been running on several days for good periods and the battery is relatively new?! Then it hit me - the f##king cluster has the alternator light, of course it doesn’t charge and maybe the abs-ecu reads that signal as well. Plugged in the cluster… and the ABS-light works as intended: lights up at (electrical) power ON, shuts OFF upon engine start!!! Of course it lit up again when I picked up some speed, maybe 30km/h, but at least now it works as I thought it should, and maybe everything else besides sensors/pulse rings is OK!? But… for inspection, I need the light to stay OFF while at speed (I’ll replace the sensors/pulse rings off season), so to accomplish that (cheat), I A) disconnect all sensors, or B) power the ABS-bulb from the oil pressure- or the alternator bulb base. Do you guys know if the warning lights ”switch” is on the power- or the ground wire?

[///M]

You do puzzle me. You understood and sorted the issue with the car and yet you ask about the light. If you look at the diagram of the ABS you see a green wire to the warning light. The top of the diagram is the positive side of the circuit the lower part connected to the control unit is the negative or ground side. So the green wire is 12v+ in start and run of the key. The black/ grey is the negative side. You also seem to know the cheat of wiring it to come on and go off as if it is working.

[Hupa]

Apparently I can’t think straight as normal people do… and look for trouble where there’s none! Well… I’m not a qualified car mechanic, so I do need some reassurance. I think, I can figure out the light-part from the diagram (wonder why it never crossed my mind).

Anyway, it is nice that this site is still active, and there are ppl willing to help a newbie! Thank you, ///M, and others as well. I will most probably be popping in here for more questions as I work with the lovely, zinnober rot, car.

[///M]

Good luck with the work. Might be worth buying the rear ABS rings from https://www.e28specialist.com/product/b ... 411132486/ while they are available.
BMW used to sell the complete hub for the rear but they are NLA now. I have removed them before and de-rusted and replated them also have some new ones. They may also break as they are removed.
Remember " KISS", Keep It Simple Stupid. Look back at what I said from the start, why the bulb was removed, the most likely cause.
When I am making things to do some jobs I always come up with the most complicated idea and then allow time to come up with a simpler solution.

[Hupa]

Yes, I have new sensors for all corners and the pulse rings ( from Ivo) for the rears. The rear sensors are aftermarket cheapos (A.B.S.) from autodoc.. something like €50 a piece… hope they are ok, as the original ones go for smthg like €300 a pop. I also need the ”retainers”, or what they are, for the big rear nuts… If the front rings are bust, I need the new bearings as well, right?… hope they are fine.

Then there’s the leaky sunroof, blind rear & number plate light… oh well… why go to work if you’re not gonna spend the money!

[gadget73]

just confirm the output of the sensors is what it should be. If it is, it doesn't matter where they came from.

Something I found from messing with sensors on other things, there are only a few people actually making these things, and most the time the only difference is the price and name on the box. Usually whoever manufactured it will mold their logo or part number directly into the thing.

[///M]

gadget73
When the cars were new the way to save money was to buy from suppliers who made the parts as they were cheaper. No car manufacturer logo. So a ate slave cylinder was not labelled BMW. There was often the case that only the box only carried the label. On guy I used to know back then worked in a BMW/Volvo dealer garage. Would not be allowed now. He said that one of his first jobs was taking parts from on box without a label and putting them in a box with a Volvo label. They were genuine Oem so made by the manufacturer that supplied the exact part to the car manufacturer. That then changed to an Oe supplier so a manufacturer selling clutches to BMW sold a brake disc which was listed as Oem but only for clutch parts and nothing else.
As you said a lot of parts are now made in one place, most likely China. The difference is that back then the parts were Oem parts. So a suspension part was BMW or Lemforder, BMW or Sachs, BMW or TRW etc. Now you have so many brands and what they have in common is the country of origin.

[gadget73]

I was specifically meaning ABS sensors. Agree that a lot of stuff now is made who knows where, by who knows who, and to questionable quality standards. Thats an entirely different headache.

I dwell mostly in Ford land, so I'll just have to speak from there. The OEM Ford ABS sensors were made by ATE, and don't actually have a Ford logo or part number on them. I can order OE Ford parts, or I can get whatever the local parts store has on the shelf. Most of the time the cheap parts store box contains a part with an identical ATE logo, it just costs a lot less than buying the same part in a Ford box. If thats the case here, where buying a genuine BMW ABS sensor gets an identical part made by the same people as buying the aftermarket branded one, I just don't see any value in spending the extra coin on them.

[Hupa]

Hi guys. I ended up disconnecting all the abs-sensors (insulated the connectors with good amout of electric tape) in the summer, and the abs-light function was correct (i.e. desired) and the car passed inspection. Recently, however, the light started to come ON after initially turning off right after start. First, it went away and didn’t come back on if I just turned off the engine and restarted it… but now the light just comes back ON every time… some 10s after ignition… no amount of restarts/driving around turns the light off anymore. Does the ABS-box have some memory, or what the hell? Can I restore ”the desired function” by detaching the barrery ground for a couple of days? And yes, yes… I’m gonna fix the damned ABS when I have the time… just wondering about the reason the light started shining again.

[Hupa]

Just an update, disconnecting the battery (24hrs) did not change the behavior. Feels like it can’t be the open circuits, cause the light went away before. Interesting. I could, just out of curiosity, complete the circuits with resistors to see if that does something.

[Hupa]

Finally had some time to work on the car… took the new rear pulse rings and sensors to th carage… then decided to test one more thing before tearing the rear apart, as I had the ABS control unit unplugged. Had my 10yo son rotating the front wheels…Front Right measured from connectors 21/23 gives 1,0kOhm and roughly 0,5V AC (the boy could spin it maybe 10km/h).. and same numbers with Front Left from connectors 4/6. Hence, I figured the fronts are fine. I then jumped connectors 23-24, 21-26 to copy Right Front signal to Right Rear connectors (insulated the long 21-26 jumper wire) . Same thing to left: jumped 6-7, 4-9 (insulated this jumper wire) to copy Left Front signal to Left Rear connectors. Reconnected the ABS control unit, and measuring resistance from disconnected rear male connector pins gives the same resistance as fronts, approx. 1,0kOhm on both sides (didn’t have the boy spinning wheels so no voltage reading yet). Figured all is a-ok… power on -> ABS light comes ON, start engine -> ABS light goes OFF (wohoo)… but… after 10s, NOT driving the car the fuc*ing light comes back ON, exactly the same way it does with all sensors disconnected (for some reason the light worked ok for weeks like this). So… I figure there is something wrong beyond the sensors/pulse rings, since now all 4 sensor circuits are continuous with reasonable ohm-values and the light still comes on. I’m so frustrated! What to do/test next? Is there some way to test the control unit itself?

[Galahad]

Have you tried a different ABS computer? I don't know how expensive they are in Finland, they are very cheap over here

[Hupa]

Have you tried a different ABS computer? I don't know how expensive they are in Finland, they are very cheap over here

No I haven’t. I think, I’ve just f#cked something up. BTW, did some testing, and measuring from the open rear sensor connectors (now jumped to the fronts at control unit connector) one of the pins on both connectors shows zero resistance to the ground (car frame). Are the abs-sensors really grounded, or did I short them to ground (control unit casing) with my jumper wires? I did insulate them with electric tape but they run very close to edges of the big connector at the control unit…and the casing could shear the tape off when pushing the connector in. Hope my explanation is clear… damn, I wish I could attach a photo…

Inspecting the electric diagrams of the ABS system, I see no grounding for the ABS-sensors… who knows if they are still grounded INSIDE the control unit??? But how could this be? I mean the sensors produce (several) Volts AC voltage, and the sensor plugs go in either way… how do you ground 4 such signals to a common ground??? I’m really asking, cause I’m not really proficient in electronics.

[Galahad]

Have you tried a different ABS computer? I don't know how expensive they are in Finland, they are very cheap over here

No I haven’t. I think, I’ve just f#cked something up. BTW, did some testing, and measuring from the open rear sensor connectors (now jumped to the fronts at control unit connector) one of the pins on both connectors shows zero resistance to the ground (car frame). Are the abs-sensors really grounded, or did I short them to ground (control unit casing) with my jumper wires? I did insulate them with electric tape but they run very close to edges of the big connector at the control unit…and the casing could shear the tape off when pushing the connector in. Hope my explanation is clear… damn, I wish I could attach a photo…

Inspecting the electric diagrams of the ABS system, I see no grounding for the ABS-sensors… who knows if they are still grounded INSIDE the control unit??? But how could this be? I mean the sensors produce (several) Volts AC voltage, and the sensor plugs go in either way… how do you ground 4 such signals to a common ground??? I’m really asking, cause I’m not really proficient in electronics.

I don't know for certain, it is possible the sensors are grounded in the controller but they do not have to be. You should be able to probe the pins on the controller with a multimeter to check. The ABS sensors are variable reluctance sensors which are reversible if you only need speed information.

[Hupa]

Yes… I should find out if the sensors are grounded insede the control unit by measuring resistance over a known ground-pin on the control unit itself and going through the pins belonging to the sensors. If all are discontinuous, they are not grounded inside the unit. Then checking grounds from the unplugged front abs-plugs. If they are also ungrounded, it means my jump wires short to the ground (the control unit casing). This kinda makes sense, as now the ABS-light starts shining already before driving one inch.

[Hupa]

Did the above test. One of each of sensor pin pairs was grounded to the known ground-pins: 10,20,34. Also the sensor plugs go in the sockets only one way. This was a surprise: the sensors are grounded inside the ABS ECU (but why?) Well, this should help me go forward (likely my jump wires incorrectly connect rear ”live” pins to ”ground” forward pins, which ofcourse would be unacceptable to the ECU and switch the the ABS light on.,,