I have a 1980 300SD w116 that has all of a sudden started burning out fuse #4 the moment I start the car. Unfortunately for me this fuse is connected to several critical components in the car that are as follows: Glove compartment light, Stop Lamp, Turn-signal Lamps, Back-up lights, Instrument cluster (automatic transmission) (tachometer) leveling system indicator lamp (cruise control) overload protection device switching unit, seat belt warning unit, driver belt sw........The day before this happend I changed the engine V belts and fixed an exhaust leak at the flex pipe down the turbo, next day drove to work everything was fine but when I started the car again all hell broke loose.... I read a post here on the forum some time ago about another W116 300sd having the same issue of burning fuse #4 when the car is started but the author never posted back if he was able to fix it, so it makes me think this might be a common issue on the w116 300sd. If anyome could point me in the right direction of where to even start diagnosing this problem I'll be eternally in your debt! This is my daily driver and I feel a little anxious driving around with no brake lights, turn signals, fuel and temperature readings! Scary!

This is not much help, but it seems to me that the most likely place to have a shorted wire is under the driver's seat. Try unplugging the wire to the seat belt warning system and see what happens. Or, alternately, try starting it without sitting in the seat and see if the fuse holds.
Do you have a levelling system in that 116? Haven't heard of that before.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

You will need to disconnect each load on the circuit until it doesn't blow the fuse. You may be able to measure current flow by putting an ammeter in line between the battery post and cable. Then disconnect loads until the drain goes away. Lots of detailed procedure posts available.

Common areas on other vehicles have been rusty/damaged light sockets. Check areas near where you were working. You'll need to start at 1 end of the circuit & move systematically toward the other if you don't luck into it easily.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Now that you mentioned the seatbelt relay thing....I been having the red seatbelt warning light turn on sporadically for some time while driving even though I had the seatbelt on all nice and tight.....I did not even know there was a wire under there, I will check it once I get home. By the way do I need to remove the seat in order to access it? Or is it easy to see/reach by just moving the seat back/forward? I don't think I ever seen this plug or wire before, maybe I didn't look closer. Thanks for the suggestions you guys are awesome!

I disconnected the seatbelt wire from under the drivers seat but no luck. I notice that now it burns the fuse just by cycling the ignition to the glowplug position Any thoughts?

There is a louvered cover on the wall where the passengers rest their feet in the front of the car just below the fan. Remove that cover and you'll see an electronic box with a big plug coming out of the bottom of the box. My #4 fuse stopped burning out when I disconnected this switch. The switch puts the transmission in first gear when you take your foot of the brake. I've had two diesel w116 and both had a problem with this tranny switch.

By the way... if you did resolve your problem, it would help the rest of us if you posted your solution. Thanks.

What did you find out? I am having the same issue. Before it would blow the #4 fuse randomly. This happened 3 or 4 times. Now, it blows as soon as the key is turned to start. I have so far disconnected the brake light switch, cruise control plug, turn signal plug, the part behind the louvered cover on right side, plug under driver seat for seat belt. Any ideas? I haven't touched the instrument cluster as I doubt this is the problem.

Got it figured out about a week ago. A wire for the glove box light was grounding out. Simple fix. Just took a little digging. It was grounding out about 2-3 inches before the light. Luck I was able to pull enough through the opening to see it. Otherwise I might be still chasing that demon. Now, I think it is time to sell the car. I like it but I think it would be better served by someone who can truly appreciate it. I will post an add within the coming weeks once the rear seat comes back from upholstery. Thanks everyone for the help. This is an awesome forum to be apart of!!!