Fuel guage smoked

andy · Post by **andy** » Sun May 27, 2007 7:52 pm

The smoke genie came out of my fuel guage this morning. I was pulling the SAV trailer, with a 24v hookup. I popped breaker # 12 last Monday pulling it, but the road was wet and I thought maybe I had gotten water into the connector. I just umpluged the trailer lights, reset the breaker and went on. Well today, it wasn't wet and I made it about 5 miles from the house when I noticed the fuel guage was reading below R. Then I saw # 12 had popped out, pushed it in, refocused on the fuel guage and saw a little puff of smoke in it at about the 1 o'clock position. Question, is the guage smoked forever or is there like a resistor or diode inside that can be replaced? I've looked at SAV parts and didn't find a guage and on the "new" VDO's I don't know if I need just a guage or the sending unit, or how to get to it and test it. Anything thoughts, comments, suggestions, would be greatly appreciated. Thanks.

Jim LaGuardia · Post by **Jim LaGuardia** » Mon May 28, 2007 2:43 am

#12 breaker is for trailer lights only, has nothing to do with the fuel guage.
The smoke most likely came from the breaker when it tripped, so you better find the short in the trailer lights.
As for testing the fuel guage, take the cover off to expose the fuel sender and ground the wire with a jumper to the chassis, it should peg the guage.
Search archives for more info as it is there.
Other than that look over the SWISS wire schematic(it is posted here if you don't have a copy) to trace the circuits.
Happy hunting

andy · Post by **andy** » Mon May 28, 2007 8:30 am

I think my problem on the trailer is where the wire connects to the frame of the trailer. I'm working on that. Now about taking the cover of the guage, this would be pulling the panel and getting at it from behind or does the ring come off of the front. I searhed archives yesterday and found info about replacing breakers, but so far nothing that I could easily identify as related to the fuel guage. Thank you for answering, I was hoping you would chime in. I appreciate you.

Jim LaGuardia · Post by **Jim LaGuardia** » Mon May 28, 2007 1:48 pm

I did not say to touch the guage, I said pull the cover off to access the SENDER. The access cover is in the bed just in front of the most forward rt jumpseat in the bed(4 screws hold down the plate).
Once you remove the cover, you will see the sender and can test the entire circuit.
Be advised the guage will show empty at 1/4 tank as the sender does not go to the bottom of the tank(normal).

andy · Post by **andy** » Mon May 28, 2007 4:13 pm

Jim, I found a llink to the SDP page that deals with the fuel guage. From there I figured out which cover you and they were talking about. I'm scrapping the previous owners' Line X off the access plate and will give it a shot. I did reach up behind the dash and found a knurled knob on what would be the right side of the guage as you are looking at it which was loose. There is also a connector of some sort on top of the guage that is discolored to a nicotine stain brown which I'm guessing goes to the fuel guage light. I suspect that is where the smoke may have come from. You have no idea how hard it is for my bifocaled eyes to try to trace wares on the schematic that was in the operators manual. Thanks again.

andy · Post by **andy** » Tue May 29, 2007 3:53 pm

Okay, I got the cover off and there is one wire attached to the sender. I jumpered it to the chassis and there was nothing going on with the fuel guage. It did not budge. I turned the truck on and there wasn't any voltage there or anything. On the resistance readings it was a short. Actually point 1 ohm on the 2 meg scale. I read the schematic and it appears that breaker # 7 is in the line. Breaker 7 did not need to be reset. I hope you have some other suggestions or ideas. BTW, it gets somewhat confusing comparing the schematic from the operation manual versus the SDP site. The wires are the same but the components are moved from one schematic to the other. Thanks again.

Jimm391730 · Post by **Jimm391730** » Tue May 29, 2007 10:52 pm

The sender is just a variable resistor, close to zero ohms when full and about 230 ohms empty. When you ground the sender wire the gauge should peg full (assuming the ignition is on).

Sounds like your gauge is truely smoked. Replace it. I'v seen gauges die several times, on both of my trucks and a few others, too.

The senders are much more dependable, but they do fail eventually from lack of connection on a riveted ground to the underside of the sender top. It can be fixed, but if you are going to go to that much trouble you should find an older (not sure of the year) Mercedes sender that is 3-4" longer and swap out the longer parts so you can measure all the way to the bottom of the tank.

Jim M.
712W and 710M

andy · Post by **andy** » Wed May 30, 2007 8:50 pm

Thanks for the reply. I guess I'm going to order a VDO guage from SAV. I'm probably going to try replacing the breakers too. Mr LaGuardia, is breaker number 12 the only 4 amp breaker in the set? From what I can make out on the schematic it appears to be. The actual number is worn off the breaker in my truck. Do you have any hints or suggestions on breakers? Also, when the book speaks about "fuse box" I am guessing they mean the circuit breaker panel, is that right? Thanks in advance. Regards.