Anyone successfully revive an ABS sensor ?
 

I recently discovered the scam that Bosch has got going called... ABS sensors. $200 a pop, and I got 3 on one of my cars. I am determined to beat this extortion. They could have easily designed 'em with plug-in wires.

Regarding an '84 500SL Euro, WDB 107 046 1A 00 3640, I have determined through continuity testing that apparently the plug end and wires are OK but seemingly where the wires enter the sensor 'head' itself is where there's a break.

Has anyone dug into this,... a material resembling hard rubber... to determine what, if anything, other than simple connections to the sensor therein reside ?

If yes, what'd you see and could you repair it and if so how, and how'd you reclose it epoxy, etc. ?

How'd you reconnect the wires, solder, etc. ? Or was that necessary ?

Am I missing something here in assuming the sensor is nothing more than a magnetic inductor with a couple of attached wires ?

Please help me escape the monopolistic schemes of Bosch the author of these Garden of Electronic Delights http://www.peachparts.com/shopforum/...cons/icon8.gif

If I succeed I'll post the resolution.http://www.peachparts.com/shopforum/...ons/icon14.gif

Appreciatively,
Courtney

I think one of mine is intermittent and I'm going to look into this myself soon. Yes, it's nothing more than an inductor encased in a waterproof jacket with a cable. I will say that if you replace the wiring you need to get high-strand, high-flex wire to replace it. Ordinary wire with only a few strands (regardless of gauge) won't be able to take the constant flexing over time.

I believe it is just a VRM type sensor which is a coil of wire wrapped around a magnet (basically). So if you put an ohm meter on it you should read maybe 100 or 200 ohms? I just put a meter on a VRM sensor we had laying around and it read 260 ohms but it is a little bigger and made for sensing teeth on a gear box but should be similar. Obviously if you read "OL" then there is a wire broken. If the break is at the entry point of the wire then you should be able to wiggle the wire and get intermittent contact. Otherwise the break is inside although I don't see how it could break. I would think a junk yard would be a decent source.

Edit: Well I just looked at a picture of one for a 84 500 SEL (I don't know if that is the same). Anyway it looks pretty strange. It doesn't look like any VRM I have ever seen with the little thingy sticking out the end. Does it seem like there is a magnet in it?

Reply by OP...
 

Thanks for the tip on the wire. Suggestion as to source ?

Yes, there is a magnet in it. Is your example a single bolt mount as opposed to the later with a metal attached mounting bracket with two holes ?

Thanks for the responses and if the poster that also plans on this pursuit would let me know about success and how it was accomplished, I'd be most grateful.

Courtney

I looked it up on Fastlane on this site. So you can look and see if is similar. I did some web searching and it appears they are not all the same (surprise). Some people are saying they are Hall type and others say you can do an ohm check like I said. So I don't know. I did see one site where the guy was working on a Range Rover and he said he resoldered his and added strain relief with a tie wrap. I think I Googled "ABS sensor ohm check". Good luck!

No way a part of that era would be Hall Effect.

I'd disconnect the end from the ABS, jack the wheel up, put the meter in ohms (I assume you have a digital meter) and spin the wheel; if your reading jumps (hopefully not from or to 0 ohms or infinity), heh heh, it's probably good. The amount of current this thing puts out is so small I don't think it could hurt the meter.

I have never had a bad abs sensor so I have not taken one apart (I do dissect almost everything that fails before actually replacing). My experience with other sensors with coils in them is that if you are lucky the break is at the ending of the winding so that you can unwind enough to make a new connection. If it is at the start of the winding then it may be impossible to get enough wire to make a connection.
As for a source of fine strand flexible wire, I have used the leads of a multimeter.

10/22/08 update:

I began questioning my continuity testing on the right front ABS sensor and therefore removed/inspected the left front sensor as well, which upon removal appeared in worse shape than the right.

Cleaned it up, tested for continutity, resinstalled and voila ! all is now working but I'm concerned that this may not last as I assume I could still have broken wiring and just have serendipitously reinstalled so as to render temporary functionality.

Couple of caveats: use the metal shield that installs over the sensor end of the ABS rig and be especially careful to avoid putting either the ABS or brake pad sensors in a strain on wheel turning. I had a helper sit in the car and turn the steering wheel while I observed all four wiring cables to asssure that none would be under strain or chafe under the most severe right and left turns. I'd remove the brake pad and ABS sensors together as it makes the ensuing work simpler. I used tie wraps for strain relief though assume there's a better way.

I conclude by requesting anyone who knows how to properly test [continuity & otherwise] an ABS sensor to let me know so that false assumptions are avoided.

I'd also appreciate guidance as to reinsulating the wire pair on each sensor and a recommendation as to reconstituting the plastic plug end's shield/shroud as time and heat had caused utter disintegration so that the bare metal plug with wire connections are all that remain though I can plug it in and it does function.

Thank you..... to all contributors in this endeavor.

I had some wire insulation fall apart once on my FIAT's electronic ignition pickup. I "rebuilt" it with high temp RTV. I used the red kind but the gray should work as well.

Reply to Strife
 

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I did as you say here and got responses from the digital [ohm] meter
but am uncertain as to proper interpretation, of course. If as you say I didn't get 0 or infinity, it's working though how properly is unknown.

I next plan to replace the ABS unit itself as I lucked up and got a new one for less than one sensor. It'll be a while before I get it installed though since it's 14 degrees outside now :-)

Will report back on results.

Thanks to all respondents.

Courtney

Testing with a digital meter in this type of circuit is inherently problematic if you are trying to see voltage spikes as you turn the wheel.

You really need an analog (needle) meter to see this type of transient, and that would probably only work if the meter was very sensitive as you'd be measuring voltage not resistance.

I wouldn't expect to see ANY change in resistance as you turn the rotor, because that's not how the circuit works. You might see the readings change because voltage was induced into the circuit, but that's a false reading not reflecting resistance. It would indicate that it was working, however.

It's an inductive sensor so no change in resistance would occur. Now, impedance WOULD change as you spun the rotor, but not DC resistance. Remember, (or learn for the first time) that impedance is a function of frequency while resistance refers to DC. So if one were to put a test frequency through the sensor with a current meter in series, then you may very well see spikes as the rotor turns. Actually, I don't know squat about this particular circuit, but it's doing one of two things: Either measuring inductively generated pulses, or detecting spikes in a provided AC signal through changing impedance. Either way, the only thing an ohm meter will tell you is if the coil has continuity. I would suspect that it uses the latter approach, since you would have no variance in the size of the spike with respect to wheel speed. Inductively generated pulses would get bigger in voltage and smaller in duration as wheel speed increased. But, either method would work......

If you were to hook up an oscilloscope set on high sensitivity, you should certainly see spikes as you spin the rotor. If not, supply the sensor with a 1K test tone, and meter the current swings across the circuit as you spin the rotor.


As far as fixing on of these, they are nothing more than a coil of wire wrapped around either a ferric core or a magnet. I would suspect it's a ferric core and not a magnet, but I've never opened one before. I can't see the need for a magnet. If you can get it apart and repair the broken wire, and get it back in one piece, it should be fine.

-edit-

Actually, I could see the need for a magnet, if it were using inductively generated pulses rather than AC impedance swings. Somthing would have to be magnetized for that approach to work, either the sensor or the little ABS studs on the rotor....

-edit-


-tp

If it is a VRM type then basically it is a generator. So when the teeth go by, it makes an AC waveform. So there would be 2 tests:

1) Static resistance: With wheel not moving measure DC resistance. I would guess it should be in the range between 100 ohms and 500 ohms but that is just a guess. 0 and infinity would be bad.

2) Dynamic test: Set meter to AC volts and spin wheel. Assuming it passed the first test it should produce some voltage and the voltage should be higher as you spin faster. If you set the meter for DC volts you will see basically nothing. So make sure it is set on AC volts for that test.

Sensor R is approx 1000 ohm
Output will read between .25-2.0 AC volts at 60 RPM
So , you simply spin the wheel 1 turn per sec with the meter set on 5V/AC scale ..

R factor has nothing to do with output measuement and remains constant....meaning, you do not use ohms scale to test an ABS sensors output...R only verifies sensors coil Resistance for open or short.
You will get a Different reading on OHMS scale , but that is a false , voltage induced reading.

What you really want is a scope sinewave so you can tell each and every reluctor trigger on the sine, but a AC reading will give you an output reading, just not as accurate/complete as the actual signal the ABS module is looking for ..it wants to compare frequency waveshapes b/c that is what the module sees.

I want to point out (just so I don't look like a complete idiot, whether or not I am actually one) that I wasn't thinking that resistance itself would actually change with proper operation of the sensor. Ohmmeters (obviously) have to put out some kind of current in order to determine resistance (a very tiny current), and changes in that voltage induced would make the meter jump because it would believe that more or less current was flowing. The resulting measurement value is meaningless, but if it does jump, the sensor probably works, but the wiring of course could be intermittent with wheel position, etc.

I wasn't familiar with the construction of this particular inductive sensor but a long time ago in a galaxy far away we tested inductive positioning sensors on printers for go/no-go this way in the field. The voltages and currents were so small that ordinary cheap (non-VTVM) analog meters wouldn't move the needle much, if at all, and you really needed to scope it back at the shop.

The current that sensor could produce by hand-spinning a wheel couldn't possibly hurt a DVM (not even my $2.99 harbor Freight special).

I agree...the point being made is the sensors output is Voltage and being a reluctor wheel design makes it an AC Voltage. Resistance and Current are not the measures of Sensor OUTPUT , The ABS module looks for Voltage and measures the Frequency...nothing else. The Htz determines the wheel speed, the V is simply output and that is a go/no go for the Module.
So, to test each Cycle , a scope is needed to see the A/C sine wave...only then can one see the results the reluctor wheels sectors have on each passing of the sensor.
A Voltage output test by hand turning the wheel 60 rpm simply verifies that the sensor does in fact have the capacity to generate a voltage signal. It is a DIYer test only and that is all it tell you. But we do not expect a diyer to have a scope, so this at least lets one get a sig strength test and one can then also make the same test on another wheel [ at the same 1-turn/sec] for a rough comparison. V output should be close on comparisons.
This 60 RPM/AC Voltage reading measure procedure is standard sensor testing for ABS sensors and is a poor mans scope.....but be aware that many ABS faults are caused by rusted and broken teeth on the reluctor wheel, metal fillings on the sensor, and large air gaps... not the sensors.