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BenzWorld W126 Host
NOW: '90 560SEL WALD & SEC front Conversion. EOL:83/500SEC 5,6l 86/300SE 86/420SE 87/420SEL 87/260SE
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Discussion Starter · #1 ·
Fanboy posted a thread 07-15-2013

We have Identical ABS problems. The difference is that I have preformed a huge amount of parts change. This is a re-listing of out posts, as we are hoping someone will spread some new input on how to further attack the issue:

The problem with the ABS is Identical on my w126 300SE '86 with 210k km
and Fanboy has a 1990 300SEL with 60k miles.

ABS Self test is ok, then when you start off driving, or 3-5 mph, the ABS light comes on, and stays on.

I have done Major part switches, and have NOT reached a solution, although I have reached the end of my knowledge!

Please read, and report back with All and Any suggestions no matter how Labor Intensive it is so I can help find and solve the problem!

Parts I have changed and looked over are in this order:

First step, measured and cleaned front sensors. Ohm was measured ok, but wires were rotten, so I changed them. The acceptable variable I know to be 1300-2100 ohm, mine were 1800.

Second step: Removed Rear Sensor from the Differential. Not difficult, total 15 min job. I looked at the robotics inside the diff with a telescope Cam. It looked in order (like new) Then I measured the Ohm for the rear sensor, it was within the Norm. My stepfather is an MB Tech in the US, and he assures me it shouldn't be the rear sensor. I am skeptical as it's the last part I haven't changed, but he assures me.

Third step, replaced both Relays in the ABS pump. no help.

Forth step, Replaced the ABS Pump. Yeah, I did... still nothing!

Fifth step, I removed the front brakes to check the ABS RING itself. For this the Front brakes must be completely removed. On the right side, there were hammer marks in the ABS Rings teeth, likely from some idiot trying to hammer in the ball-bearing not understanding it has a certain position to enter and exit it's placing. The teeth I straightened to what I consider to be a fairly good fix with a flat screwdriver. After this I was sure I had solved it, but alas.

Sixth and final non helping step, today I received a new ABS control module that sits on the Firewall. A Simple 30 second switch. this also failed to help :-(

At this moment its driving me insane, Because, in Europe every second year we need a EU tech control. If something is faulty, the car is illegal to drive! So I HAVE to fix the ABS problem, or the car is trash! Literally! Please please please, someone??? suggestions?

The funny thing is, I find similarity in the stories concerning both mine and Fanboy's cars history. He mentioned that his car had been sitting hot & humid, and that the some rubbers lifespan was compromised, and that perhaps the wiring for the ABS was defect somewhere throughout.

My car shows signs of an engine fire at some point. Not recent! The integrity of my wiring is also compromised. Although needed components at the time were surely changed, I can't help but wonder as I see traces of fire exposure between the ABS pump and the Fuse box.

I am not a mechanic. I don't have extended knowledge. I have no clue of to what wires or parts to diagnose or change from this point on. The car isn't worth anything, so all diagnostics and parts-changes I'll need to preform myself.

Please, someone of competence, take the time to help me get my beloved w126 back on the road, before it meets the crusher! And further help the future people in distress, as our w126 cars elder, as so will their wiring. With that said, this is a problem sure to evolve in magnitude for w126 owners!

Englevakten

This is Fanboy's post:
http://www.benzworld.org/forums/w12...ht-still-even-after-cleaning.html#post5873801
 

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84 500SEL AMG, 90 560SEC AMG, 85 500SEC AMG Widebody
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Well, you have replaced the two front ABS sensors (new ones I assume), the ABS pump and the ABS computer. The only thing left is the rear sensor. If that doesn't fix the ABS issue, remove the dash light until you finally find the issue. There is no OBD port to scan from a bi-annual certification standpoint.

There are electrical tests a dealer can run to diagnose the issue and source of the warning light that are more comprehensive.

Al
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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Two comments:

1. My ABS used to activate on the left front wheel when doing a very low speed turn. I don't remember if the ABS light would come on in that situation.

I found some of the sensor teeth in the hub were shiny, like the sensor had been rubbing on them. I also noticed a couple were bent. I did my best to straighten them and also experimented with shimming the sensor slightly. The situation improved but long term fix was to replace that sensor with new.

I guess my point here would be to ensure the wiring you replaced is good.

2. On long road trips on straight roads, the ABS light will occasionally come on. Wiggling the steering wheel back and forth a couple times usually clears it. I have removed the steering wheel and cleaned the SRS rings but it still happens occasionally. Maybe something is going on inside your steering wheel?

Best of luck!

Mike
 

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the fact that the light comes on only when you are driving indicate a speed signal issue .if the sterring wheel position sensor had a problem , the light would come on by just turning the wheels without moving .the main problem is usually bad wires in front sensors , but if you put new ones , this is no longer a issue . use a a/c voltmeter , and test the front sensors by jacking car up and spin each wheels .[ a scope would be better if you have damaged teeth ] .do it at rear also , just start engine and spin .[ connection here is under back seat ] . also ,a bad voltage protection relay will keep light on . also , tire sizes , they have to be similar .
 

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91 560SEC Beryl/Palomino 91 420SEL Black Pearl/Mushroom 89 420SEL Ice Blue/Navy 88 420SEL 94 E320
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A bad brake switch will also trip the ABS system.

Part isn't expensive, but a bit hard to get to removing and replacing it.

Fixed my 84's problem after doing all you have done, exception being the ABS computer. I swapped one ABS brain out and had the ABS light still.

No ABS light after the brake pedal switch was replaced.

Good luck
 

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BenzWorld W126 Host
NOW: '90 560SEL WALD & SEC front Conversion. EOL:83/500SEC 5,6l 86/300SE 86/420SE 87/420SEL 87/260SE
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Discussion Starter · #6 ·
Well, you have replaced the two front ABS sensors (new ones I assume), the ABS pump and the ABS computer. The only thing left is the rear sensor. If that doesn't fix the ABS issue, remove the dash light until you finally find the issue. There is no OBD port to scan from a bi-annual certification standpoint.

There are electrical tests a dealer can run to diagnose the issue and source of the warning light that are more comprehensive.

Al
I have of course installed new sensors, and measured them to be correct. It is NOT an option to remove the dash light, as this was done for the EU control. They of course tested the system on a brake bench, as as long as it's registered with ABS from dealer, it must work, or EU control FAILED!

At this moment people are mentioning using ovo/voltmeters and measuring the ac readout from all the sensors. They should be in the same ball park, a little less at the rear. I'l see what I get out of this, but I'm still waiting for ingenious feedback from you brilliant guys out there!

In two weeks I have an appointment with a MB Tech, if I haven't worked it out withing then with Your help, I'll hope his knowledge is sufficient. Anyways, please continue to post feedback, and when I advance to the next step I'll keep you posted. Probably after the MB Tech visit in a couple of weeks.

Englevakten
 

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1990 300SEL
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In two weeks I have an appointment with a MB Tech, if I haven't worked it out withing then with Your help, I'll hope his knowledge is sufficient. Anyways, please continue to post feedback, and when I advance to the next step I'll keep you posted. Probably after the MB Tech visit in a couple of weeks.
Eight years later -- any update on this?

Brand new front ABS sensors on mine did not fix the problem. (I initially thought it did fix the problem, but it turned out that the mechanic hadn't reattached the ABS warning light, so I thought it wasn't coming on.)
 

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1986/1990 W126
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Doh...

I had a similar thing, my indie recently cleaned the rear sensor while it was there, made a difference for a bit but now it's doing it again so maybe it was only psychological!
Luckily for me the light isn't on for the annual certification test, it only comes on the motorway sometimes.
Wasn't the front sensors.
 

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1989 300SE 240k miles
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Ok. I’ll chime in. My instinct for the OP is the ABS rings. And if the light comes on at 5mph, that’s a problem during the diagnostic of the ABS system. It happens with movement after every restart.

In my car, when I bought it, my ABS light came on at 5mph every time I drove it. Then, I had the front wheel bearings replaced and the sensors cleaned. And the light went away. Then, once I hit 239k miles or so, my ABS light came back on occasionally. But it would come on at higher speeds (usually 45+ mph), or in rain, or over speed bump. So not during the diagnostic signal.

I then did the usual cleanings and such again. No change. A month ago I replaced the right front sensor. My ABS light did not come back on until last week. Both times last week the car was parked on a slight (upward) incline. Both times,I used the parking brake over night. Both times, the ABS light was on before I backed out of the driveway. I’ve also parked here overnight without the parking brake and had no ABS light the next morning.

I’m wondering if there’s some connection to the parking brake? Or resulting from the parking brake? Via the rear differential and rear ABS sensor.
 

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1990 300SEL
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No certification test worries here, and I don't even drive it when it's raining, so really it's just a psychological thing for me -- at least the warning light is turned off (disconnected) --

Is there any way to buy the tone rings? Or is there any way to buy the wheel hub? I don't see either one listed for sale anywhere.

We did see voltage from both front sensors when spinning the wheels, but we don't have an oscilloscope, so can't see how nice the wave is. (Although scope prices are coming down -- I might go for one if I can get it for under $100.)
 

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Don't V8s have two rear sensors? I thought it's only the 6s that have a single rear sensor on the differential. That is a good possibility though.
I understand the w126 ABS is 3 sensors for all of them. Regardless the engine.
 

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1990 300SEL
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Interestingly, here's a thread about hacking (much cheaper) Volvo 740 ABS sensors to fit our cars:

 

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84 500SEL AMG, 90 560SEC AMG, 85 500SEC AMG Widebody
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Note on REAR ABS Sensors

Gen I W126 rear ABS sensors are different from Gen II rear ABS sensors and are NOT interchangeable. Rear Gen I W126 ABS sensors are NLA and I have never found a suitable replacement from a different car make - pre-1986 R107s (380SL or 500SL) might fit, but ABS was an option on a lot of pre-1986 cars. Rear Gen II ABS sensors are still available. Cleaning the FRONT hubs help with ABS warning lights, but at some point the ABS sensors have to be replaced - the wiring cracks and water gets into the wiring. ABS gear rings that are press fit onto pinion gear shafts in the diff are NLA as well.

If you own a Gen I W126 and need a rear ABS sensor, check your local junk yards and go see any R107 or W126 that is pre-1986 and see if the car has ABS. Just need a 5mm hex bit remove the bolt holding the rear sensor in the diff (clean out the head of the bolt before trying to remove!). Then, remove a couple rubber grommets under the car and remove the back seat area to find the two-wire electrical connector and remove the two screws. Feed the wire lead through the body to drop the sensor to the ground under the car. Take a multimeter and check the resistance of the ABS sensor (ohms). Needs to be rock solid steady between 0.6 and 1.6 ohms to be in working condition. I strongly suggest you install a length of quality heat shrink over the two wires and any remaining wire insulation to help lengthen the life of the sensor.
 
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