Blown Fuse - PeachParts Mercedes-Benz Forum

sailor15015 · #1 10-30-2005, 11:18 PM

Hello all,
I had started this thread before the crash but alas it seems as if it was lost. To recap what it said my #14 fuse has been continually and randomly blowing. On my model car this runs the various acc accessories, horn, and the reverse lights. The things I have unplugged to varify that they aren't blowing the fuse are the aux water pump, acc monovave(that's the one on the outside that lets coolant in the heater core, right?), and the acc unit itself. I don't think the reverse lights are the problem because they will stay on and I have never observed the fuse to blow just by putting it in reverse. Also run off of this fuse is the seatbelt warning buzzer which quit working so I unplugged it, thinking it might have shorted internally and was causing the fuse to blow. This did the trick for about a week but the problem showed up again yesterday. The fuse box lid says that there is also some sort of "emmision control" device run off of this fuse. Does anyone know what this is or what I might be overlooking? Sorry for the long post I just wanted to be thorough. Thanks so much for the help.

sailor15015 · #2 10-31-2005, 09:10 PM

Bump, and thanks in advance.

sailor15015 · #3 11-09-2005, 11:57 PM

Ok. So I think my problem just might be my reverse lights. It blew on me twice tonight the moment I put it in reverse. Can anyone think of what might be shorting out? Switch at the shifter, trannie, bulbs themselves? I'd love to hear from anyone who has even a hint of a suggestion before I just start randomly replacing parts. I'd like to have a plan of attack. Thanks in advance.

Jim H · #4 11-10-2005, 06:59 AM

Quote:

Originally Posted by sailor15015

...It blew on me twice tonight the moment I put it in reverse. Can anyone think of what might be shorting out? Switch at the shifter, trannie, bulbs themselves? I'd love to hear from anyone who has even a hint of a suggestion before I just start randomly replacing parts. I'd like to have a plan of attack. Thanks in advance.

If the switch is in the positive leg, there is likely a short to ground in the wiring between the switch and the bulbs. Try removing both lamps and checking resistance of the socket terminals to ground. One may be grounded, but both should not be.

Best I can come up with at this time.

sailor15015 · #5 11-10-2005, 07:56 PM

Thanks Jim. Tried them both to a good ground and didn't get any response from the voltmeter, and yes, it was set on ohms. Could the bulbs themselves be drawing too much current, enough to blow the fuses? Where is the reverse indicator switch, on the shifter or in the trannie? Thanks

Jim H · #6 11-10-2005, 09:09 PM

Quote:

Originally Posted by sailor15015

Tried them both to a good ground and didn't get any response from the voltmeter, and yes, it was set on ohms.

That should be a fair test of the wiring so far...

Quote:

Originally Posted by sailor15015

Could the bulbs themselves be drawing too much current, enough to blow the fuses?

I wouldn't think so, but try removing them and sacrifice one more fuse. If it blows with the bulbs out, it's not the bulbs.

Quote:

Originally Posted by sailor15015

Where is the reverse indicator switch, on the shifter or in the trannie? Thanks

I think it is in/on the transmission...

sailor15015 · #7 11-11-2005, 12:13 AM

Ok guys get this. I was taking out my bulbs tonight to see if they might be causing the fuse to blow and I was out of 8 amp fuses so i stuck in a 16 amp fuse. I know I know but I just wanted to do a quick test. So I fire the car up, put it in reverse and hear this horrible grinding noise. Scared the crap out of me so I put it back in park, back into reverse and it did it again. Then I realized it sounded like the starter was engaging!! I turned the car off, left the key in the on position, put the shifter in reverse and it started the car!! That had to be the most rediculous thing I had ever heard of in my life. It had me completely stumped until I was rehashing the night's events with my parents. Then it hit me. Could this be the neutral safety switch with one heck of a short? If the reverse indicatior is wired into the nss then this would all make sense. I really hope this is it. Before I jump off and order the $46.55 part, does this sound like it to anyone else?

Dervman · #8 11-11-2005, 12:34 AM

There is certainly some logic to your diagnosis. What usually happens is that the Starter won't engage in Park and/or Neutral if the Neutral Safety Switch (NSS) is bad. My understanding is that the Starter Solenoid is energised from the START position of the key switch and that passes through the NSS so that you cannot start it in gear. I supose that if the NSS was shorting out you might get power from Reverse Light circuit to engage the starter solenoid. That would certainly blow an 8 amp fuse but maybe not a 16 amp.
By jove I think you have cracked it Holmes!!

sailor15015 · #9 11-11-2005, 11:43 AM

Thanks!!! I really hope this solves the problem. If the reverse indicator switch is integrated into the neutral safety switch then a short would explain it. The part's on order so I'll know in a few days.

sailor15015 · #10 11-17-2005, 11:13 PM

GRRRR!!! Ok so I got my neutral safety switch this after noon and spent three frickin hours installing it last night. I ran up to the store to get some more fuses, popped it in and it seemed to be working alright. Then when I was backing around in my driveway, the darn thing blew again!!! To recap, there is a short somewhere that is allowing the starter to draw current off of the reverse light circuit, blowing its fuse. I thought it might have been a short in the neutral safety switch but apparently its not. Does anyone know where I should go from here? On the plus side, at least I got both shifter bushings installed. Had to boil them and use one heck of a lot of elbow grease but finally got them in. Thanks in advance.

#11 11-17-2005, 11:59 PM

Did the fuse issue start after you worked on the shift linkage? Is it possible that a wire is being pinched by the linkage someplace and it only shorts out when the shifter is in a certain position (like reverse)? Have you checked for a ground short while having someone move the shift lever to various gears? Just a guess.

sailor15015 · #12 11-18-2005, 12:22 AM

The fuse issue started about two months ago. It just recently started showing itself as shorting to the starter. I used a 16A fuse instead of an 8A fuse because I was out of the latter the other day and before it blew it actually cranked the starter. I don't think its pinching a line because the linkage is above the line. I saw this while I was under there tonight. How would I check for a ground short? If I took it to a mechanic would they be able to find something like this?

sailor15015 · #13 11-18-2005, 09:03 PM

Bump

sailor15015 · #14 12-09-2005, 07:57 PM

Ok this makes two Mercedes that I've seen that are having this problem. My brother's '84 that I'm going to be buying in the next couple of days is also blowing the fuse that my '85 was. The things on it are reverse lights, monovalve and aux water pump, horn, and some kind of unidentified emission control. I replaced the neutral saftey switch on my '85 but I haven't had enought time around the '84. Is this just a coincidence that this is happening to two Mercedes of mine and its not happened to anyone else? Any ideas at all would be greatly appreciated.

Mjornir · #15 12-09-2005, 11:03 PM

Try unplugging the transmission kickdown switch in the '84 300D. A short in this circuit seems to be fairly common.

http://www.peachparts.com/shopforum/showthread.php?t=128500

Unfortunately, I think that it's unlikely to be the source of the problem in your '85 300D since you'd have to floor the accelerator to activate the kickdown switch. I guess that it's possible that the switch is stuck closed and the circuit is shorted, but that seems like a long shot.