Pushing break pedal turns off radio

[CDNWOLFBBQ]

Hey everyone. I have a weird issue that has just started. It sounds like a short but I have no idea where to start. Here are the symptoms:  when I press the break pedal I loose the gas gauge and radio. Same if I try to use the turn signals. But the lights turn on and function normally (at least until I press the break). I have checked the tail lights and from what I can tell everything is functioning. Perhaps something under the dash?  I’m sorry for the youthful nature of the post. I’ve only just inherited this 65 and I’m new to Mercedes too. Thank you for any assistance!

[roymil]

suggest you take a look at the schematic diagram in the technical manual and then start checking the wiring with a meter.   Since you are not blowing fuses I'd start looking for loose or broken ground return wires someplace.

[Todd]

As roymil states, you have a failed ground connection.  So what's happening is that when you apply more load to the circuit (brake light bulb elements and turn signal elements inside the bulbs, use more current, which in turn, requires more current flow to ground.....and so you lose the radio and fuel gauge, which must be sharing those ground connections.

Take a peak under the dash with a bright, LED, pen light, and look for all the brown ground wires.  You may find them in groups, connecting to one location.  Loosen, clean until shiny, and re-tighten, all ground locations.  Do the same thing with the ground from the transmission to the body, and from the battery ground, to the body (that's 4 separate connections).  There are also grounds on the back of the gauges.  Many of these will be tough to get to.

[roymil]

I can think of seeing two main ground braids to the body but I didn't know there were 4.  Can some one point out in a picture or describe where all 4 are?   I'd like to inspect the ones I haven't looked at yet.  thanks   

[Todd]

I meant "four points of contact."

[CDNWOLFBBQ]

Thanks everyone!  My initial thought was a bad ground.  Its weird that it just started all of a sudden.  But off I go to inspect all the wires.  Thanks again for all the input.  My initial thought is under the dash too.  I have inspected the tail light wiring - everything seems in order there.  Engine bay ground was previously replaced and was cleaned when I got the car (needed to replace the battery to get her going).

[Todd]

Typically "looking" at the connections in that mode of, "does it look ok, or does it look bad," won't result in solving an electrical ground related issue.  I don't mean to be rude or be "trolling" you by saying that.  The way to test a ground connection is to use a good quality digital ohm meter.  Touch one of the meter probes to the eyelet connector on the wire, and then touch the other to the nut that secures it.  See what the meter says.  You're looking for a very low reading, like .2 or .3ish.  Then with the probe still touching the eyelet connector, touch an unpainted part of body, or find a shiny screw to touch, that's threaded into the body.  See what that reading looks like.  You'll have to play around with how you have the probes touching, and where you touch, because there may be a haze of oxidation, like a layer, and that will throw off your reading.  Same with touching that shiny screw....maybe it's not all shiny and nice where it threads in, and that will give you a bad reading.  (If you're inspecting a spade connection, you just touch the connector on the wire, and what it's connected to, again, watching out for the pitfall of whether or not there's a haze of oxidation or corrosion that may give the probes a false "bad" reading.)

You may want to unbolt the ground wires at the tail light wiring, where they attach to the trunk, scrape or sand the eyelets so they are shiny, do the same thing to the little bolts and nuts (or replace them), and then clean the bolt hole surfaces, so that you have the best metal-to-metal-to-metal contact (eyelet-to-bolt&nut, bolt&nut-to-body of car).   My coupe has more ground connections further forward on either side of the trunk floor.  I'm not sure of the SL's have that, because I've never worked on one.  So you might look for those, or ask the actual SL owners here. 

Same rules of engagement will apply to the battery cable ends, and the 2 ends of the ground at the transmission, as well as anything under the dash.  There is also a ground wire at the back of my tachometer, with the eyelet going over the stud that also mounts the U shaped bracket that holds the tach in place.  Tough to get to in my car.....literally it's lay on my back and blindly snake my arm up inside the dash, and feel for the knurled knob that holds that eyelet and the U shaped bracket.  I don't know if that's a ground that would be involved in your issue though.

Hope you find the issue easily.  I've seen it in Volkswagens, often, but not had to chase it down in a Mercedes.

[lpeterssen]

Dear friends

Agree that the problem could be related to ground post for accessories that reside on the trunk.

Brake light is fed from fuse 5, and fuel indicator from fuse 2.  So those two accessories are not related directly on the positive side.

But as indicated by Todd, they could be sharing the same ground on the rear wiring harness.

Originally, if harness is not modified, brake lights, turn signals and all consumers which reside on the tail lights, have their own ground post next to each taillight.   You have to check that it is that way. A ground post for each tail light next to it.  Should be clean clean and shining each of those grounding surfaces and the “o” terminals itself.  Buy some conductive carbon grease on Amazon.

The fuel gauge signals come from the fuel sender unit at the trunk, which basically sends back to instrument cluster ground signals in different intensities.  The fuel sender unit has a grounding post inside the trunk next to wheel arch shared also with the fuel pump.  Check that ground post as well.

Do same procedure of cleaning, sanding polishing and applying a small amount of conductive carbon grease to that terminal.

Best regards
L.Peterssen
 

[lpeterssen]

Dear friends

Re thinking the issue it sounds better that we have some melted wires to the blue/black cable coming to fuel indicator. 

Fuel indicator receives a positive from fuse 2.  The other end of the indicator feeds A variable intensity ground signal.  If that cable has compromised insulation jacket at some point and its touching the brake positive wire inside the rear harness tubing with also compromised insulation, then a positive signal will be transferred to that blue black cable.  Then gas gauge needs a positive signal on one end and a negative in the other, as it is receiving two positive signals it will mark nothing.

After checking the grounds as suggested by Todd I will go to the rear harness itself.

Best regards
Lpeterssen

[Todd]

I've never heard of carbon grease.  Thanks for telling me about it!  Just ordered a tube off Amazon.

[Garry]

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