[Vern L Pool]

Hi. Brand new to the forum. I recently purchased a 1978 450 SL. I have no brake lights. Other lights are working. Fuse 10 was blown, but I replaced that. Switch seems to be working. I'm not sure what the resistance should be at the switch though. It seems I have power at the switch (6 volts). No power at the brake lights themselves. Anybody know which particular gremlin I should chase now?

 

[DiverRay]

6 volts at the switch sounds low to me. My '77 has 14V with the engine running, and 12V+ with just the battery for most of the "hot" wires.
For the brake lights, are you checking to body ground, or to the negative battery terminal? I've NOT looked into the wiring on my car, so I'm not sure if the switch acts as ground or hot. That will also make a difference on what to look for.

 

[Jyuma]

How do you know the switch is working? Where are you testing that you're getting 6 volts? There are no 6 volt sources in the car... only 12.

If I remember correctly the brake light switch is a normally closed switch that is held open by the brake pedal bracket when the brake pedal is in the at rest position which depresses the plunger on the switch opening the connection. When you press the the break pedal the switch plunger is allowed to extend to it's full out position which closes the switch and illuminates the brake lights.

I doubt that Mercedes is running two wires to the rear of the car for brakes (power and ground) so we can assume that the brake light switch is not grounding one side of the brake lights... which means the brake light switch is very likely supplying 12 volts to the brake lights. If that is the case (and it most likely is) then one terminal of the brake light switch should have 12 volts on it and the other terminal should have 12 volts only when the brake pedal is pressed.

If you truly are only getting 6 volts on the brake light switch then you need to determine why your getting 6 volts instead of 12. May I ask what ground point are you using when measuring the voltage on the switch?

 

[Vern L Pool]

Grounding at the door.
I think I might have a short in the left HB causing my problem.
I had a right tail light, but no left. Tried to track things down yesterday. Got right tail light, but no left (and other things off and on). Took out fuse to left HB today. Now I have all tail and brake lights. And no dash lights.

 

[Jyuma]

Grounding at the door.
I think I might have a short in the left HB causing my problem.
I had a right tail light, but no left. Tried to track things down yesterday. Got right tail light, but no left (and other things off and on). Took out fuse to left HB today. Now I have all tail and brake lights. And no dash lights.

Sorry... what is HB?

 

[Vern L Pool]

High Beam. I think I just need to check all the grounds

 

[Jyuma]

Does the vehicle have aluminum fuses or copper/brass fuses?

 

[Vern L Pool]

aluminum. Replacing them with copper as I go

 

[Jyuma]

aluminum. Replacing them with copper as I go

Replace them all before you go. All over eBay for dirt cheap.

 

[Vern L Pool]

Many problems, one source. I spent time today on the SL. The turn signals didn't work properly either. I figured that was the relay/switch (coupled with the hazards on my year). I found a used switch today, replaced it and ALL the troubles went away. No shorts anywhere. THE problem was the signal relay.

 

[Jyuma]

Many problems, one source. I spent time today on the SL. The turn signals didn't work properly either. I figured that was the relay/switch (coupled with the hazards on my year). I found a used switch today, replaced it and ALL the troubles went away. No shorts anywhere. THE problem was the signal relay.

Good job
Now replace all the aluminum fuses and save yourself a bunch of future headaches.

 

[Vern L Pool]

Already done

 

[Jyuma]

Already done