Fuse #14 blows as soon as it is put in. - PeachParts Mercedes-Benz Forum

gregp1962 · #1 08-24-2018, 11:16 PM

Which is the most likely component in circuit #14 that is likely to blow this 16a fuse?

back-up lamp
- warning monitor term. 8
- emission control,
- horn,
- overload protection (autom. transmission, relay auxiliary fan term. 86, control unit/water pump/heating water valve for climate control system)

Diseasel300 · #2 08-24-2018, 11:18 PM

Sure helps to know what car...

jsp300D · #3 08-25-2018, 02:37 AM

I guess its a w123. Check the horn wiring. there maybe a piece of open wire.

TurboRedkneck · #4 08-25-2018, 08:20 PM

Had a similar problem with this (I think) circuit on my 83 300d. I remember that it killed the horn. Thanks to this forum I learned that the kick down on the transmission can short to the case and kill this circuit. You did however say it blows immediately. As soon as the car is turned on or after a little driving? In my case, it blew as soon as I hit the kick down switch because the single wire running from the throttle down the drivers side of the transmission to a solenoid on the rear can get it’s insulation rubbed off and ground to the transmission case (or other metal it touches). A little electrical tape fixed it for me.

gregp1962 · #5 08-26-2018, 12:16 AM

Sorry, yes, it's a 84 300CD US. I'll check the horn wiring and kicj down. The horn isn't working. So, you're probably right about that one.

I can't even get the fuse in. It blows when the fuse touches the contacts.

Father Of Giants · #6 08-26-2018, 01:01 AM

Sounds like a short to ground or a short to power.

Basically either it's an open wire touching the body/any ground source or an open wire touching another open wire.

TurboRedkneck · #7 08-26-2018, 09:31 AM

Are you putting the fuse in with the ignition on? If not, there is definitely some weirdness going on as, at least for me, none of that stuff should be live with the key off.

Alec300SD · #8 08-26-2018, 11:06 AM

Quote:

Originally Posted by gregp1962

Which is the most likely component in circuit #14 that is likely to blow this 16a fuse?

back-up lamp
- warning monitor term. 8
- emission control,
- horn,
- overload protection (autom. transmission, relay auxiliary fan term. 86, control unit/water pump/heating water valve for climate control system)

It could be arced over/pitted contacts on the reverse relay located inside the neutral safety switch.

The reverse relay gets activated every time the car is running (when you shift into or out of park).
The contacts wear and eventually lead to failure of the # 14 fuse.

To test, simply remove the backup up bulbs and see if cures the issue.
If that cures the issue then replace the neutral safety switch, or continue driving without backup lights..

jay_bob · #9 08-27-2018, 11:41 AM

Here is how to find the device causing the problem.

First you need the wiring diagram for the vehicle. The 123 wiring diagrams are available online from startekinfo.com and various other sources.

Next you need to disconnect the battery then dismount the fuse box. The fuse box is wired such that there is enough slack to bring the fuse block back inside the vehicle interior and lay it down on the floorboard. You have to go in from below the dashboard.

Once you have done this you need to remove the wires landed on the load side of fuse 14.

Now you want to find a spare tail light bulb and socket (from any car) preferably with some wire leads on it. Take one lead from the light bulb and connect it to the load side fuse terminal for fuse 14.

You want to now reinstall the fuse 14 and reconnect the battery and turn on the ignition. Be careful there is nothing lying on the floorboard that can short the terminals on the fuse box. The fuse should hold since there is nothing connected.

Now take the free lead from the bulb socket and touch each of the disconnected leads from the fuse box. If you find a lead that lights up the bulb bright the thing causing the short is down the wire from here. Look at your wiring diagram and see which wire this is based on the color stripes. You can now follow down this circuit and locate the short. When the light no longer glows brightly you have found the problem.

gregp1962 · #10 09-06-2018, 10:14 PM

Jay bob, There are way too many wires and other junk in the way to be able to bring the fusebox into the car. Did you seriously do this without taking apart most of the under dash components? When I get more time, I'll check the various circuits at their sources. Time is my problem.

jay_bob · #11 09-07-2018, 07:25 AM

I did this on the 83 240D when the climate blower fuse position 8 melted and I repurposed slot 5 (otherwise unused on a 240D) to run the blower.

You have to remove the lower cover under the drivers side dash. Then go around to under the hood and remove the two screws that hold the fuse panel to the firewall. Then go back under the dash. I ended up lying on the floorboard, shoulders on the floor, butt on the seat backwards of the normal orientation, legs up over the seat back (I reclined the seat as far as possible first). Very awkward but doable.

Once you get in that position you can see and reach all the way up to the top. Then maneuver the fuse box down to the floor. They built the wiring harness so that it has enough slack to bring the fuse box to the floor.

Can’t say it enough, be sure to disconnect the battery first.

gregp1962 · #12 09-08-2018, 04:44 PM

Maybe the 240D has way fewer things in the way to do that. I'll have to test the different components directly. I'll look for the schematic for the 1984 300CD US now.