1993 300TE ABS light on constantly - PeachParts Mercedes-Benz Forum

JensenHealey · #1 05-08-2020, 10:31 PM

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 Kohms for one front wheel sensor and 0.997 Kohms for the other. The rear sensor gave a resistance reading of 0.67 Kohms. 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 front wheel sensor gave a slightly higher reading in this test than the other. I have also tested the rear speed sensor in the diff using this same AC volt test and got a similar reading from that sensor.

I have pulled each front wheel sensor out of the hubs and cleaned the buildup of metal fillings off of them and looked at the toothed wheel inside. After reassembling them the problem was still the same.

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?

brooktre · #2 05-11-2020, 10:50 PM

I had a 91 w124 with the m103 engine that the ABS light was on. It turned out that a new alternator was needed.

Usaguy · #3 05-11-2020, 11:41 PM

exactly what battery voltage do you have when running?

porkface · #4 05-12-2020, 11:29 AM

i have fixed a lot of abs or srs and/or lousy running by replacing the ovp relay. yours is REALLY old because the 2 fuse relays have been around for 20 yrs. replace the relay with oe from mb. the kaehlers and uros that are out there cause problems out of the box.

when checking abs sensors, you need to scope them while the wheel is turning. resistance doesn't imply correct voltage signal. given your light never goes out, i'm betting it's not the sensors. replace the relay and cross your fingers. good luck, chuck.

URO Parts Support · #5 05-13-2020, 02:39 PM

Quote:

Originally Posted by porkface

the kaehlers and uros that are out there cause problems out of the box.

Hi guys, quick FYI: After receiving complaints (hat tip to Mr. O'Brien on B.W. for his detailed feedback), in 2018 we made several design changes to URO Overload Protection Relay relay 201 540 3745. We made the circuit board traces wider to handle heat better, and added plastic insulators to prevent PCB-to-metal-case shorts.

In fact, if you check the verified buyer reviews on that big "jungle" online retailer, we received all 5-star reviews in 2019 and 2020 for our updated OVP relay 2015403745.

JensenHealey · #6 05-15-2020, 07:31 PM

Quote:

Originally Posted by porkface

i have fixed a lot of abs or srs and/or lousy running by replacing the ovp relay. yours is REALLY old because the 2 fuse relays have been around for 20 yrs. replace the relay with oe from mb. the kaehlers and uros that are out there cause problems out of the box.

when checking abs sensors, you need to scope them while the wheel is turning. resistance doesn't imply correct voltage signal. given your light never goes out, i'm betting it's not the sensors. replace the relay and cross your fingers. good luck, chuck.

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.

I did do both a resistance test (ohms) and a AC voltage (while spinning the rotor) test on each sensor. I was told in another thread that if you just start the car without moving it the ABC computer does do a check on each sensor without the car moving and in theory if everything is up to spec then the ABS light should go out without actually moving the car.

porkface · #7 05-17-2020, 10:02 PM

you got a kae, kaehler, relay. i'll say it again, i've had trouble with them brand new.

JensenHealey · #8 05-19-2020, 11:43 PM

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!!!!

Usaguy · #9 05-20-2020, 03:50 AM

Quote:

Originally Posted by JensenHealey

I will quiz them for any additional details or suggestions when I pick the car up.
ARGH!!!!

This:

Quote:

Originally Posted by christuna

exactly what battery voltage do you have when running?

Quote:

Originally Posted by brooktre

I had a 91 w124 with the m103 engine that the ABS light was on. It turned out that a new alternator was needed.

porkface · #10 05-20-2020, 10:13 AM

Quote:

Originally Posted by JensenHealey

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!!!!

yeah, that's too bad. i would think they would pursue it further as there's not much else to go wrong. if it is the alt, it could be overcharging or be bleeding too much ac voltage through a bad diode, very easy to check. i never put an abs ecu in ANY bosch abs car, but it's been 20 yrs since i've worked on that generation mb. only thing i've done is replace the front sensors or the ovp relay. COULD be their KNOWN good relay wasn't.

JensenHealey · #11 05-20-2020, 10:49 AM

Just a FYI, the alternator and battery in the car are brand new.

Folks on other MB forums have indeed suggested acquiring another ABS ECU to try due to issues with cold solder joints and etc.

porkface · #12 05-20-2020, 11:00 AM

new or not, the alt. is easy enough to check for overcharging or ac voltage leakage. in your case, everything has to be tested.

JensenHealey · #13 05-27-2020, 01:13 PM

Status update:
I received a replacement ABS ECU and plugged it into the car in place of the original one (located behind the battery) and no change.
The ABS light is still illuminated on the dash cluster after starting the car.
I am at a complete loss as to what to do next.