Electrical troubleshooting on 85 W123 - PeachParts Mercedes-Benz Forum

Graham · #1 04-23-2013, 08:28 PM

I had my floorpans and some other rust work done last fall. Car parked since. But now, I find there is a short somewhere. Fuse #1 blows immediately.

Looking at wiring diagram, it seems this fuse serves RH rear tail light and RH front parking light. It also serves some other lights (license plate, warning signal, rear markers and it seems the instruments cluster. Maybe some other things too. But cluster lighting and front/rear right side lights are out too (not surprising since fuse has blown1)

It could be that something was disturbed when getting at the door sills (where some wires run) or under the dash somewhere (lower dash on both sides removed). I still have to put the dash panels back, so everything is still open.

Question is - how to troubleshoot this?

jay_bob · #2 04-23-2013, 09:08 PM

My favorite troubleshooting trick. I have done this trick for shorts in control panels at my job many times.

Get your (incandescent) glove box or dome light bulb out of the socket (this works for 124s or older with the bullet fuses, newer cars with the ATC-fuses you have to make a little rig out of a bulb socket and some quick connects)
Put it in place of Fuse #1
Turn on the marker lights
If the bulb is glowing you have a short
When the bulb goes out you have found the short

The bulb gives a safe series impedance to help you find the fault without hurting things.

jay_bob · #3 04-23-2013, 09:57 PM

The wiring diagram for the car is on startekinfo.com, workshop resources, go in a couple pages to find the 123 chassis entry in the table.

Not much going on electrically with a 123 so they are easy to troubleshoot with a set of prints. No computers or any kind of can-bus vodoo magic, everything is a straight wired circuit.

I would disconnect the battery then loosen the nut on the face of the fuse box. Then drop the fuse box behind the dash. The wire pack has enough slack to let the fuse box lay on the floor board for testing purposes.

On the back of fuse 1 there will most likely be several wires. Disconnect one of them and leave the others connected. Hook the battery back up, see if the short went away. If not try lifting another one of the wires until you find it. Then go tracing down that wire to see if there is any damage.

Its all detective work from here...

Graham · #4 04-24-2013, 02:20 PM

OK Thanks. I didn't know the fuse box could be moved like that.

jay_bob · #5 04-24-2013, 02:27 PM

Yep just be sure to disconnect the battery first.
And double check it is in the hole correctly when reinstalling and nothing touching what shouldn't be.

The old 123s are scary on the overcurrent protection, the hot feed from the battery goes to the starter, then to the light switch, then the key switch, before hitting ANY of the fuses. There is no way I can get away with something like that in my world (UL 508A industrial controls).

Graham · #6 04-28-2013, 09:39 PM

Actually the glove box bulbs don't fit the fuseholders - at least not on an 85 W123

But I have made some progress. What I did, is remove the cluster and unplug the multi-pin plug on the rear.

Checked #1 fuse holder with meter, and it seemed short had gone. Put in a fuse and front/rear running lights, warning buzzer, shift bulb all worked.

Checked cluster lighting circuit on bench for a short, but it seemed fine.

my123ca · #7 04-29-2013, 06:47 AM

There is a blue wire from the instrument cluster that runs thru the floorpan and ends in rear door switches. It provides illumination to the switches. Remove the cover on the drivers knee rest(?), you will find where it is connected. If you disconnect the blue wire there and the fuse does not blow, you found your problem.

whunter · #8 04-29-2013, 06:56 AM

Quote:

Originally Posted by my123ca

There is a blue wire from the instrument cluster that runs thru the floorpan and ends in rear door switches. It provides illumination to the switches. Remove the cover on the drivers knee rest(?), you will find where it is connected. If you disconnect the blue wire there and the fuse does not blow, you found your problem.

Good call.
With floor / rocker welding, it is easy to melt or burn a wire.

.

Graham · #9 04-29-2013, 10:27 AM

Quote:

Originally Posted by my123ca

There is a blue wire from the instrument cluster that runs thru the floorpan and ends in rear door switches. It provides illumination to the switches. Remove the cover on the drivers knee rest(?), you will find where it is connected. If you disconnect the blue wire there and the fuse does not blow, you found your problem.

That makes sense.

I still have the lower dash off on both sides, the cluster out and the centre console open (CC/radio out). Floorpans are still uncovered. (Car is still in process of rust treatment and restoration - see my other threads!)

According to the wiring diagram, the console, window sw, hazard sw lighting comes off a terminal c124, but I have never been able to find it!

I don't recall seeing a blue wire on the driver side floorpan. But will go take another look. Maybe it runs above tunnel?

Graham

Graham · #10 04-29-2013, 02:18 PM

Quote:

Originally Posted by my123ca

There is a blue wire from the instrument cluster that runs thru the floorpan and ends in rear door switches. It provides illumination to the switches. Remove the cover on the drivers knee rest(?), you will find where it is connected. If you disconnect the blue wire there and the fuse does not blow, you found your problem.

So far as I could see, the cables from the cluster plug are wrapped and then enter a larger harness that runs under dash toward center console. But it does not appear there. There are several blue wires, one of which connects to the CC illumination and the light tube spider. The other two disappear again and presumably go to the hazard switch and the window switch lighting.

I checked the resistance between pins 12 and 15 on the cluster plug. That should measure resistance of all connected bulbs in center console, window switches etc. Meter registers 0.0000 but then gradually settles on about 4.7ohms. The cluster illumination measures 3ohms across pins 12 and 15 - it has 9 bulbs so about in proportion. Total 7.7 ohms. That should draw about 1.6 amps.

So maybe short is somewhere else?? But with cluster out, running lights, shift light and warning system work and fuse does not blow.

I need to look at those other blue wires. I seem to recall that one used to go to the old radio illumination. But new radio does not need that. So where does wire go now? Need to do some more detective work!

Graham · #11 05-04-2013, 10:46 AM

Well, I have almost everything working. I didn't cut any wires and #1 fuse no longer blows!

What did I do? I wish I knew!

I dismantled the cluster, the centre console and I already had the lower dash panels out. There was one blue wire from aftermarket radio that went to antenna switch (which i don't use) I removed that. But radio is not on fuse #1, so that could not have solved problem. Otherwise, I tidied up some wires, checked that there was no direct short at fuse holder, used new brass fuse and voila, it works.

One thing does not work - that is front courtesy light. It works manually, but not automatically. Back courtesy light does work when doors are opened.

The car at one time had an aftermarket alarm system. It could be activated/deactivated using a remote via the radio antenna! There are a some black wires that come from the door switches but go nowhere and appear to be broken. I have just coiled them up. Perhaps they were part of that system?

There are also two black wires that appear to come from high up far right hand side of dash above the EGR computer (Middle thumbnail below). They also go nowhere. Also now coiled up. Any ideas what those may be?

I can live without the front courtesy lights, so maybe that is it for now!