95 E300D BLOWING FUSE #3 - PeachParts Mercedes-Benz Forum

COLDSTART · #1 10-21-2013, 05:50 PM

I am blowing fuse # 3 which controls the dash lights and the right hand side marker lights I just went outone night to drive it and there were no interior lights ,I put in a new fuse and it blew as soon as it made contact . Any ideas where to begin would be great thank you

jay_bob · #2 10-21-2013, 08:45 PM

I'm an electrical controls engineer, so this is my life, running down faults in control panels.

I am assuming you have a copy of the relevant wiring diagrams for your car. These are available at startekinfo.com or w124-zone.com. Also you will need a decent multimeter. The cheapie/freebie from Harbor Freight will do in a pinch but if you can afford a few tens of dollars you will thank yourself.

Also obtain a standard 1156 brake/backup/turn bulb and a generic replacement socket with wire leads from the auto parts store. Then get a bag of alligator clip test leads from Radio Shack.

Using the bulb and socket, and the clip leads, attach the bulb across where the fuse terminals. Unfortunately in the 124 this is easier said than done as one of the terminals is recessed. The carcass of a blown fuse and a folded piece of aluminum foil will help you get connected to the recessed terminal. (No don't use the foil as a fuse

, just use it to extend the terminal out where you can grab it with the clip lead.)

The reason you want to do this is so that you can locate the fault. The bulb provides a load in the circuit so that you don't keep burning fuses trying to locate the problem. If the light is glowing brightly, then you have a short to ground. When you have removed the short, the light will stop glowing, or only glow very dimly.

There is a bunch of stuff on that fuse #3. Starting from the load side of fuse #3 you will have to separate and break down the circuit until you find the problem. Fortunately the fuse block is easy to work on. Remove the 6 screws around the perimeter of the fuse assembly and lift up. You will need to disconnect the battery before raising up the fuse carrier, or else you will end up shorting it out on something under the hood.

From here it is good old detective work.

Jeremy5848 · #3 10-22-2013, 02:11 AM

Jay Bob wisely points out that electrical troubleshooting is not simple or easy. It's a chess game of move and counter-move. It is possible, however, to use the history of others to simplify the chase.

One thing we need to know is more on your statement "no interior lights." Do you mean the dome lamp or the instrument cluster? Does Fuse 3 blow even when the head/tail/instrument lights are turned off?

If Fuse 3 blows when you turn on the lights, there are a lot of light bulbs to check and bulbs usually don't create a short circuit when they fail. I recommend first checking the dimming rheostat in the instrument cluster. With the cluster still in the car, turn the rheostat fully counter-clockwise so that the instrument lamps would be off if they still worked. Now turn on the headlights or at least the parking lights. Does the fuse blow? In the "off" position of the rheostat, the instrument lamps should all be out of the circuit. If the fuse still blows then there may be a problem with the rheostat or the wires connecting to it.

Next, remove the instrument cluster and unplug the round 15-pin connector. Now try replacing Fuse 3. If it now does not blow, you know the problem is in the cluster or after it, in the circuits for the console illumination.

Next, after removing two screws, pull the rheostat out of the instrument cluster. Plug the 15-pin connector back into the cluster and try Fuse 3 again. That will tell you if the problem is before or after the rheostat or in the rheostat itself.

One item on Fuse 3 that can fail and then blow the fuse is the electronic relay that allows the instrument lamp rheostat to control all of the lamps without itself burning up. The relay handles the current of the lamps in the console, removing some of the load from the rheostat. If there is a short in one of the lamps or sockets on this relay, Fuse 9 should blow. The relay itself can blow either Fuse 3 or Fuse 9 if it fails internally. This electronic relay is called N40, the "Instrument Illumination Control Unit," and it is miserably located behind the instrument cluster, bolted to a bracket and electrically connected to a 4-pin plug. To test N40, remove its 4-pin plug. Replace the rheostat and the 15 pin plug. Now see if Fuse 3 still blows. If it does not, the problem may be in N40. Replacing it with a good one is about the only way to know.

This picture shows the area in the dash where N40 lives, a little to the right of the speedo cable. It is the only picture I can find quickly. Under the screwdriver in the picture is a rectangular connector with green wires. Under it are two relays with plugs on them. One of the relays has a 4-pin plug and that is K40. Sorry I don't remember if it is left or right. The two relays are attached to a bracket that is screwed to the dash in two places. The front screw is fairly easy to reach; the rear screw is a PITA -- the picture shows how to remove it with a magnet to catch it when it comes loose. You can remove the 4-pin plug to test the relay without having to remove the relay.

There are other items on Fuse 3 as you said, but those are the ones most likely to fail and make Fuse 3 blow. As you can see, it's a long process, like Jay Bob says.

COLDSTART · #4 10-22-2013, 06:12 PM

thanks ,Interior lights are the dash lights, I will start the process on the weekend I appreciated your advice and will see if I can figure it out and I will be patience because it is a very nice car .

jay_bob · #5 10-22-2013, 07:40 PM

My best friend when working on my 124 is my ipad and a link to w124-zone.com.

It has the complete service manual online, which will give you the steps for removal of the instrument cluster and center console, which you will probably need to do in order to isolate the problem.

When you retrieve the pdf files, hit the little redirect arrow in the web browser, that will give you the chance to open the pdf file in iBooks. When you do that the file is stored in the memory of your iPad and you do not need to be online to retrieve it later. Very handy if your garage is too far from the router.

COLDSTART · #6 10-24-2013, 12:17 PM

so far the fuse blows when the lights are turned on even with the rheostat turned down nextI pulled out the cluster and unplugged the 15 pin connector I replaced the fuse turned on the car and when i put the lights on the fuse blew again so I guess that tells me there is no issue with the cluster, just a few other things even my front park lights have not been working for a while and also my out door temp display has been blank not sure if this is any help. I am considering replacing the bulb failure control with a known good one to see if that has anything to do with the front marker lights , thanks again