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Discussion Starter #1
Hello all
I have a 1993 Mercedes 300TE with the 3.2L DOHC M104 engine. It is a 2wd base model, not the 4Matic type of car. I have been working on the car for some time to revive it from an extended dry storage by the previous owner.
I have pretty much everything done except for a problem with the ABS light on the dash being illuminated 100% of the time once the car is started.
Without moving the car or pressing the brake pedal, once the car is started the ABS light comes on and never goes out.
I have done a lot of research and reading on this issue. The car has the 16 port trouble code breakout box in front of the battery but does not have the light and button built into this box to allow the ability to see the pulse or blink codes without any additional equipment.

I have looked at the over voltage protection relay and ran various tests as outlined below that are applicable to a 9 pin single fuse OVP relay:

Pin 1 should be connected to the positive battery terminal (12v), and Pin 5 should be connected to the negative battery terminal. Check for voltage at all the remaining pins. ONLY Pin 9 should have voltage (12v).
With Pins 1 and 5 connected as above, connect Pin 3 to the positive battery terminal. This is like turning the ignition key. You should hear the click of the relay. Now you should have 12v at Pins 9,8,7 and 2. No voltage should be at Pins 4 and 6. However, Pins 4 and 6 should have continuity now.


The OVP relay passed these tests just fine. I also opened up the hydraulic unit by removing the plastic cover held down by the single screw and looked at the two relays under that cover. I tested those two relays as well and they seem to be operating properly.

I used a Volt/Ohm meter and checked the resistance readings of the three speed sensors (one in each front wheel and one in the differential) and got 0.970 ohms for one front wheel sensor and 0.997 ohms for the other. The rear sensor gave a resistance reading of 0.67 ohms. I also checked the two front wheel sensors using the AC volts setting on the meter while spinning each wheel. I got a signal from each sensor when doing this test. One sensor gave a slightly higher reading in this test than the other. I have not yet been able to test the rear speed sensor in the diff using this same AC volt test but I will do that later today.

I am also going to try pulling each front wheel sensor out of the hubs and cleaning them and looking at the toothed wheel inside.

I built a blink code box using an LED and a momentary push button circuit to see if I could pull any pulse codes out of the car but this did not produce any codes related to the ABS system.

Anyone have any other suggestions about things I should be looking at?
 

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Current, 90 300CE, 92 400E (Sold 95 E320,70 250C, 91 190 2.6, 91 420 SEL, 95 300D, 87 TD)
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Cleaning the wheel sensor is a good start. When you get that done let us know.

Sometimes the brakes were metal to metal at one point and little fragments of metal stick on the sensor magnets.
 

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Discussion Starter #3
OK, there was indeed a significant amount of metal fillings built up on each of the front wheel sensors.
However after cleaning them and reinstalling, I reconnected the battery and no change.
After starting the car, I still had the ABS light on the dash illuminated.
Is there something other than just disconnecting the battery overnight that I should be doing to reset the system?

2631996
2631997
 

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Discussion Starter #4
BTW, I also had a friend help me test the rear sensor in the diff as well.
I disconnected the wiring plug under the carpet under the rear seat and hooked up a volt/ohm meter set on AC volts to the sensor plug while my friend started the car.
He put the car in gear (rear of car up on jack stands) and I did get a signal from the rear sensor plug on my meter.
 

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Current, 90 300CE, 92 400E (Sold 95 E320,70 250C, 91 190 2.6, 91 420 SEL, 95 300D, 87 TD)
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You could try this I don't know if W124s cars have this reset, but try it

To reset the lights, start up your Mercedes-Benz and turn the steering wheel all the way to the right, then all the way to the left. Do this a couple of times. Turn off the engine. Restart
 

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Current, 90 300CE, 92 400E (Sold 95 E320,70 250C, 91 190 2.6, 91 420 SEL, 95 300D, 87 TD)
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Also check the fuses. Wiggle them all around especially the ABS ones. make sure they're all copper ceramic fuses not the aluminum/ plastic ones.

The aluminum fuses are known to cause electrolysis between the copper and aluminum contact point usually a white film/ crust.
 

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Discussion Starter #7
I pulled the cover off of the main fuse box under the hood (driver's side of the car, sort of near the brake reservoir) and carefully looked at the list of fuse inside the cover.
I did not see any fuses listed that that made any reference to the ABS or brake system.
Can you provide any pointers to which fuses would be supplying power to the ABS system?
 

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Current, 90 300CE, 92 400E (Sold 95 E320,70 250C, 91 190 2.6, 91 420 SEL, 95 300D, 87 TD)
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In the fuse box there should be a manila fuse chart on they back of the lid. One should be labeled ABS.
 

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Discussion Starter #9
Nope, As I said I carefully read through that whole listing on the inside of the fuse box lid TWICE :) and there was nothing there referencing ABS or brakes.
 

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Just for fun, have you taken the car out on a sandy piece of tarmac and tried to lock up the wheels? What happens? As for the OVP relay, I understand that you tested it but what is the date code on it? They are a very common cause of an ABS light and they cause other problems too so its not wasted money putting in a fresh one.
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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A few random thoughts . . .
  • The ABS system is powered directly from the OVP relay and the fuse that is on it - there is no separate fused circuit. No power (due to a bad OVP or bad OVP fuse) will cause an immediate ABS fault light, triggered by a diode "OR" circuit that detects lack of power to the ABS relays. Even though you have "checked" the OVP, you may want to check the fuse on it or consider replacing it. The OVP is a very common failure item.
  • A set of contacts on the stop light switch (powered from Fuse 5) are part of the ABS system; make sure the switch is good.
  • The system does not check for sensor voltage output until the car reaches 3 MPH. Therefore, dirt/metal on the sensors will not cause an immediate ABS light, only a light after driving.
  • The ABS controller monitors battery voltage - make sure your charging system/battery are functioning properly. System voltage less than 10.5 volts will cause an immediate ABS light.
  • When you measured the resistance of the sensors at "0.970 ohms", "0.997 ohms" and "0.67 ohms", I assume (and hope) you mean K ohms (e.g., 970 ohms, 997 ohms, and 670 ohms). This would be normal, otherwise, they are shorted which not likely for all three.
 

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Discussion Starter #12
Can anyone tell me what pins on the big wiring connector that plugs on the ABS control computer behind the battery go to the contacts on the stop light switch so I can check that without pulling the whole dash apart?

Or better yet a wiring diagram for the ABS system?

BTW, I have checked the fuse on top of the OVP relay. It's fine.

I will double check fuse 5 if that is critical to the ABS system getting the right signal from the stop light switch.
 

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Take fuse 5 out and really scrutinize it. I've had a fuse that looked good in the holder but when I took it out it fell into 2 pieces. Sometimes it's a very fine crack.
 

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Yes, when someone says "check" a fuse, they don't mean look at it. I'm not trying to be glib here. The process involves confirming continuity across the entire fuse and the posts it is captured in.
 

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04 odyssey (265k) 1995 E320 wagon (295k)
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After wheel sensor, I vote OVP or alternator.
These cars don't like voltage being out of spec.
I know there are threads here where ovp or new brushes fixed the problem. Spend your covacation searching these threads. It should help pinpoint.
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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A couple of additional thoughts to add . .

If the car starts, fuse 5 is good. It also supplies power to the starting relay/solenoid.

Make sure your brake lights are working correctly. The ABS controller senses the voltage on the lamps so if this part of the circuit is "floating" (e.g., burnt out brake lights, an open circuit in the wiring), the ABS controller can behave in strange ways, including turning on the ABS light.
 

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Discussion Starter #18
Just got back from testing the brake lights and determined that they seem to function just fine.
 

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Discussion Starter #19
FYI, I received a new KAE OVP relay today and installed it this evening. After installing the new relay, I started the car and I still have the ABS light illuminated and a 1400 rpm idle.

And after that I did pull the NEW fuse out of the new OVP relay and checked it with an OHM meter. It was fine.
 

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Discussion Starter #20
Just a progress report here on my ABS light on constantly problem.
I took the car to a local Mercedes specialist (not a dealer) and they put the car on their Mercedes W124 ABS diagnostic tool.
This tool plugs into the factory harness in place of the ABS ECU behind the battery.
I believe this is a factory diagnostic tool.

They confirmed that the wheel / diff sensors are all within spec.
They did think there was an intermittent issue with the silver 6 pin relay inside the pump unit.
They said they "cleared any codes"
They tried a known good silver relay and that did NOT solve the problem.
Finally late in the afternoon they called me to say "they give up" and they are NOT able to isolate or repair the issue.
So tomorrow morning I am going to pick the car up pay a small bill of $80 plus tax for their diagnostic time and take the car back with the damn ABS light still on.
I will quiz them for any additional details or suggestions when I pick the car up.
ARGH!!!!
 
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