Weird instrument light electrical issue - PeachParts Mercedes-Benz Forum

ROLLGUY · #1 10-27-2015, 12:27 AM

I was replacing the gauge circuit board for a friend on his 300D, as the oil pressure gauge was leaking. The instrument lights worked just fine before replacing the unit with one from a wrecking yard. After replacing the board, the instrument lights do not work. I powered up the lights via the third pin (+) up on the left side (screwdriver pointing to the pin), and the lights glow like they should. I also powered up the bad board, and the lights work as well. When I plugged in the multi-socket connector, the lights do NOT light. I then checked the sockets with a meter, and the second socket has the power for the instrument lights (meter reads zero volts when light switch is turned off). What is up? Both the new and old board respond the same when power is applied to the third pin, so I would think that the third socket in the connector should be the one to supply power. I am not sure what to do, so I sent the guy home without working instrument lights.

cooljjay · #2 10-27-2015, 01:27 AM

The obvious thing....The rheostat? Did you try jumping the two pins? Also no mention if you removed and checked the fuse. Only two things I can think of...

ROLLGUY · #3 10-27-2015, 11:08 AM

Quote:

Originally Posted by cooljjay

The obvious thing....The rheostat? Did you try jumping the two pins? Also no mention if you removed and checked the fuse. Only two things I can think of...

Yes the rheostat already had the jumper, and I tested the lights with the board screwed in the instrument panel. The fuse is the same for other things that are all working as they should, so the fuse is good.

optimusprime · #4 10-27-2015, 11:45 AM

You talk about fuse .Is this the one that looks like a bulb holder on the rear of cluster .And can be mistaken for a bulb .. It could be that its gone..And the potentiometer dimmer can be mended easy .They fall to bits inside .Put the parts back in place .Repaired a few in my time ..

ROLLGUY · #5 10-27-2015, 03:11 PM

Quote:

Originally Posted by optimusprime

You talk about fuse .Is this the one that looks like a bulb holder on the rear of cluster .And can be mistaken for a bulb .. It could be that its gone..And the potentiometer dimmer can be mended easy .They fall to bits inside .Put the parts back in place .Repaired a few in my time ..

Read the above post, "Yes the rheostat already had the jumper, and I tested the lights with the board screwed in the instrument panel".
That means that the lights work when power was applied to the third pin (view photo).

cooljjay · #6 10-27-2015, 03:49 PM

Possible it is the connector.....Maybe(have seen it on other parts) that the female connector for pin 3 lost its tension, thus is unable to may contact with the male pin....May also be corrosion on the inside or you said he had a bad oil gauge, possible oil seeped inside......On my euro, I changed the female connector as the guide pin was broke and found out the hard way....one of the female pins slide out of the connector....

ROLLGUY · #7 10-27-2015, 11:01 PM

Quote:

Originally Posted by cooljjay

Possible it is the connector.....Maybe(have seen it on other parts) that the female connector for pin 3 lost its tension, thus is unable to may contact with the male pin....May also be corrosion on the inside or you said he had a bad oil gauge, possible oil seeped inside......On my euro, I changed the female connector as the guide pin was broke and found out the hard way....one of the female pins slide out of the connector....

Like I said in the first post, the lights worked with the board with the leaking oil gauge. I checked the power, and it is on the second socket, not the third. That is what is weird. I did not take apart the connector and switch the wires, so I have no idea why there is no power on the third socket, but on the second.

funola · #8 10-28-2015, 09:37 AM

Here is a post Instrument Cluster Repair that will help you figure it out. The lamps are on pin 12 (rheostat output) per diagram in the FSM. Pin 11 is +12v input to rheostat, if rheostat was jumped, pins 11 and 12 should read zero ohms. You did not mention ground, which is pin 15. A circuit must have a ground to work otherwise it's magic.

When checking for power, I find a test light is much more useful than a volt meter.

ROLLGUY · #9 01-03-2016, 06:05 PM

I am revisiting this issue, as it has popped up again. The gauges all work fine now, but there is no instrument lights. I have checked the obvious (grounds), but can't seem to find out why there is no instrument lights. There is power at the rheostat (bypassed anyway), so it should be powering up the lights. I am not sure what else I can do or check.....Rich

patbob · #10 01-03-2016, 06:32 PM

Cracked solder joint at the connector on the board maybe? Or maybe on the connector on the harness? Pushing the connector on pushes it out of place where it disconnects; pulling the connector off pulls it back into contact.

my123ca · #11 01-03-2016, 08:39 PM

Its either your fuse is busted or the copper trace is burnt. If your signal lights are not working then its a ground issue.

ROLLGUY · #12 01-04-2016, 12:42 AM

Quote:

Originally Posted by my123ca

Its either your fuse is busted or the copper trace is burnt. If your signal lights are not working then its a ground issue.

All the lights work except the dash lights, and it has power at the rheostat. I changed out the gauge board with a known good one, so I can't understand why it does not work in this car. The bulbs check out good too. Also, I can see no bad traces on the board.

patbob · #13 01-04-2016, 09:31 PM

Quote:

Originally Posted by ROLLGUY

All the lights work except the dash lights, and it has power at the rheostat. I changed out the gauge board with a known good one, so I can't understand why it does not work in this car. The bulbs check out good too. Also, I can see no bad traces on the board.

Check the connector on the harness. It's been a while since I've been behind my cluster, so I can't recall what that connector is like, but it might be like the big harmonicas that plug into the climate control unit. IIRC, those are pins soldered into a board, and its been known for the pins to have cracked solder joints that get pushed out of place and make intermittent contact when plugged into the CCU. In this case, it wouldn't be the pins that would be moving, but maybe the receptacle on the harness, or maybe the wire soldered to it.
Worst case, you could solder a fly wire to the trace that connects to the power pin so you could test if there's power getting to the board once its installed. Just don't leave that wire floating loose where it can short to something someday.
I suppose its possible one of the other connections to the board is sucking the power on that trace down, but I'd think it would blow a fuse, and it probably wouldn't have started when you replaced the cluster.