Eliminating a Cercuit
 

How does one test a single circuit when there is no access to either end?

I tried getting to the fuse box, but the vent started to crumble! :eek: So since N.L.A. I didn't dare try to access the rear of the fuse box.

As for the other end, haven't a clue where everything is on fuse 14 except the horns which have been disconnected for months, Klima relay, additional fan relay, and maybe one or two others.

Thank you all in advance! :D

Do what now? I don't understand what you're trying to do.

Eliminate a circuit, for example "warning monitor term. 8" so just that circuit could be tested rather than all those on 14.

I take it you've identified a problem on fuse 14 and are trying to isolate the problem. According to my wiring diagram*, that fuse only powers the neutral start switch and the backup lights. Does the fuse keep blowing? Does the car start? Do the reverse lights work?

Note that my book stops at the 1984 model year. I know some things changed for '85, but I don't know exactly what. Model year correct wiring diagrams are your friend.

Skippy, thank you so very much for your help! :D

As I have been saying over and over, but you probably haven't seen, put a fresh fuse in 14, turn the ignition, and BAM! the fuse blows. I have even tried a 25 amp and blows also fairly spectacularly. So there is a short and I would guess of some high amperage.

What has got me wondering is why fuse 14 blows when position one is energized?

Just thinking there is a hint in there.

What I am trying to do is separate out each of these:
backup lamps (Eliminated component from circuit.)
warning monitor terminal 8
emission control
horns (Eliminated components from circuit.)
switch-over valve overload protection
automatic transmission (Went through all them didn't see anything, Bud the "mechanic" didn't either and zip tied them.) (Eliminated component from circuit.)
relay additional fan terminal 86 (Eliminated component from circuit.)
control unit (Unplugged the E.C.U.)

There is no wiring diagram available on the internet or Mercedes except the climate control. On Mercedes, Tram who has all the volumes said there was nothing but the climate control, which so far doesn't seem to be the issue.

You can either go under the dash to reach the terminals on the underside of the fuse box (not easy or recommended) or you can remove the screws that hold the fuse box in place and pull it up into the engine compartment to try to turn it and get to the terminals on the bottom.

I'm going to regret answering. But here's what you do. Don't replace the fuse, or turn on the power. Locate every component in the circuit, and unplug each of them. Every one will have a 12V+ lead that comes from the fuse. Measure the resistance between this lead and ground. Map that back to the circuit diagram, and identify the shorted leg. If no ground is identifiable, then one of the components is internally shorted. Reinstall the fuse and turn the switch. The fuse shouldn't blow. Then reinstall each component. When you install the failed component, the fuse will blow.

If you have a dead short use an incandescent light bulb in place of the fuse. The bulb will glow bright if you have a short. This does 2 things:
- you stop wasting fuses
- the impedance of the bulb stops whatever was shorted from catching your car on fire

The fuse box is maintainable from inside. Take off the lower dash cover on the drivers side.
Disconnect the battery first, then undo the nut holding the fuse box frame into the firewall. Push the fuse box into the dashboard area, then go up under the dash and pull the fuse box down. The wire harness has enough length to lay the fuse box on the floor board.

So find a 12 V bulb socket with wire leads. Either from a wreck or go to the parts store and get a generic one. You want a single filament bulb like an 1156.

Get some alligator clips from Radio Shack and put them on the two leads from the bulb.

Disconnect your battery.

Drop the fuse box down on the floor board.

On the back of the fuse box look for the load side of fuse 14. Remove all the wires on the load side of the fuse. Note, this is the side of the fuse that has no power when there is no fuse present. Tuck the wires back so they don't touch anything.

Put the battery cable back on. You don't have to snug it up, just leave it loose for now. Clip your light onto the live side of fuse 14.

One by one, clip onto the wires that you took off of fuse 14. When you find the one that makes the light bulb glow bright, make a note of its color scheme. Probably black with some color stripe(s) since it's switched power.

Check all the other ones to make sure that is the only problem. If so, take the battery back off, hook up the remaining wires, and put the light bulb clips across where the fuse should go. Don't put a fuse in yet. Hook up the battery, turn on the key, and see if the bulb stays off or dimly lit. If so turn off the key, put in a fuse, and see if it holds.

Now to find what is failed. At this point, you can go down the list of devices on the factory fuse chart and see what is not working. But that may not be conclusive given your description above that you have things already disconnected or not working.

So the other option is to obtain the wiring diagram for your vehicle from startekinfo.com -> MB Workshop Resources -> Model 123 Service Data

From the wiring diagram see what wire this goes to and then find the problem.