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Premium Member
1995 W124 E320, 1982 W123 240D, 1991 W126 560SEL, 1993 W201 190E, 1978 C107 450SLC, 1992 W140 300SD
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468 Posts
Discussion Starter #1
Hi, I'd like to start a thread documenting my saga tracing a peculiar electrical short in my W126 "Pegasus". The car is 88 560 SEL with 282K miles. I'm the only owner that matters ;)

The short started couple months ago with a faint and intermittent "Battery" light on the dash when the key was out of ignition. The car ran fine with this. However, the situation relatively soon progressed to a constant bright light and an engaged fuel gauge/sender. This drained the battery relatively fast.

I've measured the drain to be about 3amps. The short was also such that when running and turning on AC at full power would cut off the radio and cause an erratic run with all kinds of fault codes thrown.

The pictures are the symptom of the fault. The wire going to #6 hole is hot. I suspect the short is somewhere near the fuze box as it's getting power from "always on" line. I've briefly examined the underside of the fuse box but didn't see anything obvious.

On a related note, couple months ago I burned out a seat adjustment relay somehow. Didn't discover it until recently - perhaps that is what caused the short. But so far I don't consider these two to be related.

I'm going to remove the dashboard anyways since I need to troubleshoot one vacuum element where there is no air going to the driver foot area - getting ready for the winter.

The dash removal is not that hard - I have done it on other models and the principle is the same - don't break any plastic pieces :) I'll post the pictures of dashboard removal later.
 

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1989 560 SEL, 1993 400 SEL, 1986 420 SEL, 1977 450 SL, 1984 300D, 1981 380 SL, 1983 500 SL
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606 Posts
I had a similar problem with my fuel guage sending unit. Turned out that the antenna shorted out and had melted the wires from the trunk to inside under the back power seat. I repaired the wire harness and all is fine. I would check that harness first. It's amazing what one short can screw up in these cars.
 

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Premium Member
1995 W124 E320, 1982 W123 240D, 1991 W126 560SEL, 1993 W201 190E, 1978 C107 450SLC, 1992 W140 300SD
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468 Posts
Discussion Starter #3
I will try to follow that lead as well. Interestingly enough, when the short was "weak" I could make it go away by pressing the brake pedal couple times. Whenever the brake lights engaged the gauge would return to normal and the "low battery" light would turn off.
 

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1989 560 SEL, 1993 400 SEL, 1986 420 SEL, 1977 450 SL, 1984 300D, 1981 380 SL, 1983 500 SL
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606 Posts
I wish I would have taken pictures of my problem. I'm sure that my problem is the same one that you are experiencing. My evolved so much so that I could turn on the car but I couldn't turn it off with the key. I had to disconnect the battery and pull the big relay in the fuse box to shut it off. All of my problems resulted from the burnt wiring harness that went from the inside of the trunk on the drivers side to the inside of the car under the rear seat. hopefully this will help you out.
 

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Premium Member
1995 W124 E320, 1982 W123 240D, 1991 W126 560SEL, 1993 W201 190E, 1978 C107 450SLC, 1992 W140 300SD
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468 Posts
Discussion Starter #5
I've been beating my head against it for quite some time now. I've learned several tricks now and have started to understand the system - reading wiring diagrams is quite different from actually seeing the implementation.

I'm going to start back again to the "hot" wire going to the instrument cluster which is not supposed to be hot I believe. Hopefully, that will lead me somewhere.
 

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2012 CL550 4MATIC Coupe
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10,001 Posts
If you wish, I will teach you how to read the schematics, show you how to read where what is what, and help you T/S the Electrical system.

First we need to find what circuit the drain is on. Put the ammeter on the line so you can see the drain. Then, remove the fuses one by one until the drain stops. Then we will know what circuit to look at.

I just got done troubleshooting a bad short myself. Actually a couple. The strange one happened when I stepped on the brakes, the Tach and gauges shorted out. It kept popping fuse #6 which was the Alarm which worked fine before, the ABS and the ASR and the Cruise Control. All three systems I had removed or replaced.

I started tracing the circuits and the ABS and the C/C were both fine. The short was coming from the alarm circuit. So I traced it out and when I unplugged the alarm, no more short. I did some more tracing back and found out that there were three wires for the hood pin switch, and I had chosen one of the wrong three for the hot side. Don't know why or what that other wire is, but it was bleeding voltage into the line and shorting it out. Once I swapped the wires, no more shorts!

I would NEVER have thought that a switch so far removed out of the circuit would cause a short, but it did.

So the first step is to find which line is causing it.
 

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1995 W124 E320, 1982 W123 240D, 1991 W126 560SEL, 1993 W201 190E, 1978 C107 450SLC, 1992 W140 300SD
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468 Posts
Discussion Starter #7
mclare, thank your for you help. I will definitely contact you when I reach a situation where I cannot move forward. I'm a "troubleshooter" by trade although in a Computer Science field. But I've always wanted to understand electrical circuits (analog and digital). So I'm teaching myself right now. I'm also able to understand the diagrams from AllDataDIY which I highly recommend.

I have disassembled the fuse box and removed all fuzes - but the short is still present. I think I've made theoretical progress right now after reading schematic of the charging system and understanding how the alternator works and how the "warning" light works.

On the Charging System schematic below the "BU (Blue)" line is not supposed to be hot in the "off" position of the key. The only time it's supposed to be hot is when car is running to "counter balance" the voltage coming from the battery on terminal 7 of the X1 connector (15 pin).

Second, the light should only light up when the voltage from terminal 7 on X1 is greater than voltage from terminal 9 on X1. The only time there could be voltage on terminal 7 is in run position. However, if that wire is compromised and is touching ground, and there is voltage in wire on terminal 9, then the lamp would light up. (Warning Indicators Diagram)

Another interesting point is that there is voltage being supplied to fuel sensor, oil pressure sensor and temperature sensor. From the diagram "Warning Indicators/Gauges/Clock/Tachometer" this leads me to believe that wire "BK/PK" on terminal 12 of X1 is somehow touching always hot wire from the supply.
 

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2012 CL550 4MATIC Coupe
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You are correct. Your grasp of the schematics is most impressive! I too am a Computer Engineer as well as an E/E.

One correction in your Nomenclature however: You had me confused for a minute when you called out X1. It is not X1, but x1. There is x1, x2, x3 and x4. x1 is the 15 pole round plug connector and x2 is the 8 pin on the back of the speedo. x3 is the one on the Tach side and x4 is the single one in the back of the speedo.

The upper case X denotes a Terminal Block (either screw or mounted) and the lower case denotes one of those four plugs. I was looking all over for X1 going "I've never heard of Block X1, but it sounds familiar". Then I traced back the BU wire.

Do you have the paper version of the ETM? Or have you seen the beginning part of the ETM where it details all of the Nomenclature with regards to the Electrical system? PM me your email and I will send it to you. I think once you read through that it will really open your eyes up as how MB labels everything and I am sure it will all fall in place quickly with your skills.

I agree completely with your assessment and suggest you proceed with ringing out the lines and check for short to grounds and opens. Since putting my car back together, I have sound several of both and have been addressing 20+ year old bad wiring. The fact of life is that the insulation is hardening daily and are becoming more and more brittle. That leads to corrosion at least, then an open.

When I had just gotten the engine back in, there was no power to the FPR where there had been power last year before it died. So I rang out the wires and found that the +12 from X26 Pin 1 (The big one behind the Fusebox) was open and not feeding the FPR. It was fine just last year.

Good luck and keep me posted. LMK if you need anything.

M
 

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1995 W124 E320, 1982 W123 240D, 1991 W126 560SEL, 1993 W201 190E, 1978 C107 450SLC, 1992 W140 300SD
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468 Posts
Discussion Starter #9
Well this saga ends with a disappointing culmination as the short turned out to be a bad alternator diodes. The alternator was leaking current through it to the "field" wire. That somehow was energizing the other gauges. I guess there are no diodes in the instrument cluster since there should not be current going through that line in the off position.

But in this saga I've learned many valuable lessons. One of these being that the x1 connector description on All Data DIY is apparently incorrect (I was looking at the MY as of 86). I was fortunate to have removed another instrument cluster from a MY 88 prior and had cut the wires. So I put the x1 back together from that.

I've replaced the alternator with Bosch reman unit and also installed Bosch battery. So far no leak symptoms show up.

But now I need to troubleshoot the potentially bad seat control module since I'm still having trouble with overheating relay. There is a 0.5 V on the "control" terminal in the off position. Which I think energizes the relay for a significant period of time causing the overheating. I don't think it's a short since the voltage is not near 12.6 volts of the main supply.

Thank you,
Dmitry
 

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85 300SD, 89 300SEL, 87 300SDL
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You guys seem to have a good grasp on the charging system. Any idea why the battery light on my 89 300sel never turns on? I've checked the bulb and it's good. The alternator isn't putting out voltage but it is getting exciter voltage (from the small white and 2 blue wires). The volt regulator looks good too (at least 1/2 inch of brushes). Any help would be appreciated, Thanks
 

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1995 W124 E320, 1982 W123 240D, 1991 W126 560SEL, 1993 W201 190E, 1978 C107 450SLC, 1992 W140 300SD
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468 Posts
Discussion Starter #11
Does it not turn on when the key is in the "run" position before the car is started?
 

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Premium Member
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Hi, I'd like to start a thread documenting my saga tracing a peculiar electrical short in my W126 "Pegasus". The car is 88 560 SEL with 282K miles. I'm the only owner that matters ;)

The short started couple months ago with a faint and intermittent "Battery" light on the dash when the key was out of ignition. The car ran fine with this. However, the situation relatively soon progressed to a constant bright light and an engaged fuel gauge/sender. This drained the battery relatively fast.

I've measured the drain to be about 3amps. The short was also such that when running and turning on AC at full power would cut off the radio and cause an erratic run with all kinds of fault codes thrown.

The pictures are the symptom of the fault. The wire going to #6 hole is hot. I suspect the short is somewhere near the fuze box as it's getting power from "always on" line. I've briefly examined the underside of the fuse box but didn't see anything obvious.

On a related note, couple months ago I burned out a seat adjustment relay somehow. Didn't discover it until recently - perhaps that is what caused the short. But so far I don't consider these two to be related.

I'm going to remove the dashboard anyways since I need to troubleshoot one vacuum element where there is no air going to the driver foot area - getting ready for the winter.

The dash removal is not that hard - I have done it on other models and the principle is the same - don't break any plastic pieces :) I'll post the pictures of dashboard removal later.


Was this ever resolved?


.
 

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Thank you, russianwol, and my apologies for the lack of clarity. I was referring to this, specifically...


On a related note, couple months ago I burned out a seat adjustment relay somehow. Didn't discover it until recently - perhaps that is what caused the short. But so far I don't consider these two to be related.





Thanks again.


.
 

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Premium Member
1995 W124 E320, 1982 W123 240D, 1991 W126 560SEL, 1993 W201 190E, 1978 C107 450SLC, 1992 W140 300SD
Joined
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468 Posts
Discussion Starter #15
Ah, I haven't had a chance to review/verify this issue. I've changed the bases on my seats for MY 91 parts and haven't actually driven the car since then. It's undergoing engine rebuild right now.

They also apparently changed the design of how seats operate without a key in ignition but I'm not 100% sure. I guess I'll find out next summer when the car is put back together.
 

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.

Thanks. I've been tracking down a power seats problem and thought you had found something.


.
 
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