Lost my dash lights and the CCU lights... - PeachParts Mercedes-Benz Forum

MercFan · #1 10-05-2009, 12:03 AM

I jumped in the car earlier tonight and noticed that the entire instrument cluster on the dash is dark along with the climate control - all dark as the night.

I thought it was maybe one of the fuses - I've checked them and re-checked them with the ohm meter - all good. I can't remember if it was the dimmer switch next to the dome light (by the rear view mirror) that controlled the instrument cluster lights?! If so, id doesn't do anything to them now...

Any other checks I could do to troubleshoot this before I have to tear into the dash?!

Thanks,
james

P.S.
First the coolant leak and now no dash lights... great - must not be my week.

Jeremy5848 · #2 10-05-2009, 12:23 AM

An ohmmeter check is not always sufficient. A fuse has to pass only a tiny amount of current to satisfy an ohmmeter . . . but that tiny current isn't enough to operate anything. To be sure, you need to remove the correct fuse, which in your car should be #1, a white 8 Amp fuse. Then you need to be sure it doesn't have a hairline crack cutting it almost in two or corroded ends. If the fuse is good, check the terminals in the fuse box, make sure they are clean and shiny. Finally, put the fuse back, turn on the parking lights and check the right taillight. If the right taillight works, fuse #1 is good.

If the fuse is good, the next suspect is the dimmer rheostat in the instrument cluster. If you can't get the lights to work by rotating it back and forth a few times, you will have to pull the cluster and clean, replace, or bypass the rheostat. All the dash lights are dimmed and brightened by that one rheostat.

Jeremy

Biodiesel300TD · #3 10-05-2009, 01:05 AM

Jiggle the dimmer knob back and forth a couple times. Sometimes the get bumped or dirty.

MercFan · #4 10-05-2009, 11:11 AM

Quote:

Originally Posted by Biodiesel300TD

Jiggle the dimmer knob back and forth a couple times. Sometimes the get bumped or dirty.

The fuse is good - all the lights operate properly (head lights, high beams, both tail lights, brake lights, blinkers, emergency flashers, dome light, radio - all good). I tried jiggling and turning the dimmer knob - to no avail.

I'm going to pull out the dash tonight and bypass the rehostat - also I read on another thread that if BOTH dash lights and climate control lights are out it maybe that the circuit board is cracked somewhere - is that next thing to investigate?!

James

toomany MBZ · #5 10-05-2009, 11:18 AM

My bet is on the rheostat.

MercFan · #6 10-05-2009, 11:43 AM

Quote:

Originally Posted by toomany MBZ

My bet is on the rheostat.

I'm going to put a jumper accross the rheostat and then see what happens - I'll report back.

Jeremy5848 · #7 10-05-2009, 12:28 PM

A crack in one of the PC boards or a broken jumper wire is possible but check the rheostat first. There are one or two Phillips head screws to remove and then the rheostat can be carefully pried off of the board with small screwdrivers. The Rheostat sits on two pins and the connection is tight in most cars. First time for me I was sure something was gonna break but the rheostat eventually slid off of the pins. Then wrap a wire around the two pins and try the lights again.

Yak · #8 10-05-2009, 07:24 PM

Quote:

Originally Posted by MercFan

I can't remember if it was the dimmer switch next to the dome light (by the rear view mirror) that controlled the instrument cluster lights?! If so, id doesn't do anything to them now...

Dome light?

You do mean the knob on the instrument console, right? Did you happen to reset your odometer recently? It's that button/knob.

If it's bad, excellent DIY thread here: Refurbing the Dash Light Rheostat (Caution: For The Purists Out There)

MercFan · #9 10-05-2009, 08:03 PM

Quote:

Originally Posted by Yak

Dome light? You do mean the knob on the instrument console, right?

Yeah... I was originally confused and thought the rheostat was on the dome light but I later caught on that it's the left knob in the the instrument cluster itself. Thx for the DIY link -

MercFan · #10 10-05-2009, 10:20 PM

I pulled out the dash and unhooked all of the components behind it - I now have the rheostat in my hand. It appears to be shiny and new. Measuring its resistance as I turn the little shaft yields the following measures:

CCW all the way - 18.5 ohms
CW all the way - 2.5 ohms

... so it would appear that the rheostat is working fine. I did not disassemble it any further - don't see a need for it as it really appears to be in good shape.... I'm wondering if it's worth putting a jumper across it?!

EDIT: I did put a jumper across the two terminals. It's too dark to work on it tonight. I'm going to re-assemble the cluster tomorrow. But I doubt this is what's causing my problem...

Jeremy5848 · #11 10-06-2009, 12:47 AM

The resistance numbers sound reasonable. I just checked a spare rheostat from a W124 instrument cluster; resistance varied from 1 Ohm to 16 Ohms.

If shorting the terminals doesn't restore the instrument lights then there is a broken wire or bad connection somewhere.

MercFan · #12 10-06-2009, 01:45 PM

Found the problem - I ended up disassembling the cluster to have a look a the circuit board under the rheostat(pot) and sure enough right away I saw two of the small copper lines printed on the circuit board were smoked.

It's in a pretty tight spot under the components that control the oil pressure, engine temps and fuel level - wouldn't wanna take these off the board, but I think I may have a chance to solder in a pair of jumper wires to 'bridge' the blown out lines - any particular size/type of wire that I should use for that?!

In retrospect, I do remember that I recently replaced on of the white fuses because I lost both front passenger and rear driver window controls. Replacing this fuse cured the problem with windows... I remember wondering what else could have been burned out - I guess I know now. Interesting that the circuit board went before (or with) the fuse... hmmm.

Graplr · #13 10-06-2009, 02:08 PM

Quote:

Originally Posted by MercFan

any particular size/type of wire that I should use for that?!

The smaller the better. If you have some on hand, just use what you have. If you are buying, I would buy around 20 AWG. And buy stranded, it solders much better.

I've had to do the same thing on my cluster.

MercFan · #14 10-07-2009, 12:01 AM

Since I have the entire cluster apart I'd like to paint the needles in a flourescent paint (Yellow or orange). Does anyone have suggestions on where to get this paint - Hobby Lobby?! and what kind to get?!

Thanks,
James

Jeremy5848 · #15 10-07-2009, 12:35 AM

Painting the needles was a popular topic here a year or two ago. The search function should bring up lots of suggestions.