I was driving my 79 300sd (W116) home from work on Friday when suddenly the tachometer, temperature gauge and gas gauge all went dead. I also lost my turn signals. Unsure of what had happened, I popped the hood and checked the fuse box...sure enough, fuse #4, an 8amp fuse that controls the following: "glove compartment light, stop lamp, turn-signal lamp, back-up lamp, instrument cluster (automatic transmission) (tachometer) leveling system indicator lamp (cruise control)", had blown. I figured no big deal and replaced the fuse, but it blew again as soon as I started the car. I'm not sure how to begin figuring out how to fix this issue...I assume that there is a short somewhere? I'm pretty mechanically inclined, but don't have much experience with cars...any help or advice you could give would be much appreciated!

Unfortunately when there is a bunch of items connected to the same Fuse you are stuck having to check each item.

If there are any Electrical Connectors in the Circuit of the that list of components connected to that fuse you can disconnect it (do one by one) and see if it has an effect on the fuse.

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84 300D, 82 Volvo 244Gl Diesel

Ok so heres the update: I spent most of this afternoon attempting to chase down the electrical issue. First I removed the instrument cluster, unplugging all electrical conections, popped in a fresh fuse, started her up...and the fuse still blew. Next I tackled the glove compartment lamp. I disconnected the wires from the pressure activated switch, tried a new fuse, and it blew again. Onto the rear lights (stop, backup, turn signal). There is a large plug that connects the car's wiring to all of the rear lights, I unplugged both sets of rear lights and the fuse blew again. Next I headed up to the front turn signal lights. Again, there was a large plug connecting the cars wiring to the lightbulb wiring, I disconnected these plugs on both sides and the fuse blew again. I'm getting to the end of my ideas here, so I check all plugs and wires that I've already exposed for any signs of wear or corrosion/blackening indicating a short, but to no avail. I checked underneath the fuse panel to see it there were and burns or damage and couldn't find anything suspicious. The only component that I didn't check was the "leveling system indicator lamp (cruise control)" because I didn't know what it was or where to look. Other than that I'm really at a complete loss as to what to do next. meanwhile I still have to drive the car and I feel like I'm taking my life into my hands cruising around with no brake lights or turn signals! If someone could tell me what to look for to find the cruise control componant above I'd appreciate it...and and help with possible next steps would be appreciated too! Also, I'd like to see a wiring diagram for the car if anyone has one or can tell me where to look for one, and I'm thinking it wouldn't be a bad idea to confirm that the fuse box key that I have is accurate too. Anyone else with a '79 300SD want to post the components listed under fuse 4? Thanks a lot for any help or advice!

Have you tried disconnecting everything and working backwards? Disconnect all, put the fuse in and see what happens. If it holds, hook up the brake lights and sully forth with a degree of safety as you seek the source at your leisure.
You don't have a leveling system so the cruise control must be hooked up somewhere. Disconnect the cruise control as much as possible.

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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

Thanks for the reply. What does the cruise control look like and where are the components located- under the dash, under the hood? The cruise doesn't work anyway so disconnecting it is probably a good idea. Thanks

There's an amp under the dash somewhere and components in the glove compartment. Check to see if other fuses cover the components to make sure this bad fuse isn't just hooked into the light.

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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

Hi
If all else fails, they sell a Short Finder at some auto parts store for about $30. What it is - is an automatic reset circuit breaker that you hook up to the fuse holder of the fuse that's blowing. Then you have a meter that reads magnitism. The circuit breaker will keep triping and resetting while you run the meter along the harness wires. While the line is hot it will produce a magnetic field which is read by the meter. But once you get to the short - the magnetic reading will stop since the current is not flowing past that point. It is especially helpful where you have a number of wires bundled together and wrapped in the harness - since it shows you where to dig in to look for damaged wires.

But try everything else first

Good Luck,

Joseph

Thanks for your description on how the Short Detector works works as I have always been curious about that and never taken the time to look it up.

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84 300D, 82 Volvo 244Gl Diesel

I'm not sure but does the kick down switch under the throttle (go) pedal live on that fuse? If so you might check under the car for the wire that comes out under the transmission tunnel to see if it has rubbed on the frame or somewhere and chaffed until it shorts out. I spent a year trouble shooting that on my old 123 coupe.

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Gary
OBK #37

Another update - ok, so I got a copy of the wiring diagram for the car from w116.org and there is a ton of stuff on fuse 4...of course. So I unhooked everything that I could locate which includes everything on the fuse except for the following which I either couldn't find, or didn't know what they were: change over valve relay; stop lamp switch; back-up lamp switch; switch over valve; overload protection device switching unit; seat belt warning unit; driver belt sw; diagnostic plug (TDC sensor/adaptor). On the back-up switch and stop lamp switch, the bulb assemblies are unhooked, but I couldn't find the actual switches. If anyone could help me to locate these additional items so that I can unhook them I'd appreciate it. If you want, I can e-mail you a pdf of the wiring diagram (too big to post here I think). Oh, as you've probably figured out, the fuse is still blowing... Thanks for any help and suggestions!

The stop lamp switch is activated when you depress the Brake Pedal; so it is down there by where the Brake Pedal and the Linkage for the Brake Pedal is.

I am not sure on your model but on my year and model with an Automatic Transmission the Back Up Light Switch is combined togetether with the Neutral Safety Switch. (Go to one of the online part sellers and look up the part for your year and model and see if it tells you Back Up Light and Neutral Safety Switch; and you can also see what it looks like.)

One of the other members will have to tell you the location.

But, it is either hooked in to the linkage of the Transmission Gear Selector or on the Transmission itself down where the levers come out of the side of the Transmission.

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84 300D, 82 Volvo 244Gl Diesel

Is the kick-down switch connected through this fuse? If it is then a faulty kick-down solenoid located on the transmission would be very suspect. The solenoid is in a location subject to a lot of contamination. I am not near our W116 model car so I can't check out if ours is similar to yours but will check later today if you still have a problem.

Jim W.

Jim W - indeed the kickdown switch is attached through fuse 4. If you could check on the location of the solenoid on your model and let me know what to look for I'd appreciate it. Thanks!

Here is what I found: In 1998 we had these notes on our service order: Check why kickdown blows fuses. Check wires and solenoid. Found solenoid shorted. Replaced kickdown solenoid. The kickdown solenoid is located under the accelerator pedal and the replacement is a do-it-yourself job. The part number for our 1978 300SD is 1 000-304-11-90, yours probably is the same but I would check first. They can be ordered for about $50.00.

A good place to check for part numbers, etc., is the Classic Center in Calif. They are very helpful. 1-949-598-4850. Have your VIN number ready for any inquiries.

Good luck, keep us posted.

Jim W.

Oh great! I am having the exact same issue with my w116 300SD fuse #4 keeps blowing when I start the car. Any Updates ? Where you able to find the culprit?