brake light operation reversed
 

W201 190d, and brake lights are always on unless i step on the brakes. Any ideas?

My guess is a momentary off switch replaced the momentary on switch. Did this just happen?

Sixto
83 300SD

The operation is the same with the switch from the parts car. Stepping on the brakes opens the switch. I wonder if there's a stuck relay someplace. I've had a couple ppl point it out to me in the last week or so.

If the car has a bulb warning system, I'd start with that relay. In 124s and 126s it's the very long delay in the fuse box. If there's no bulb warning system, maybe the cruise control module is messing with the the signal. I don't see how though since there's a direct link between the brake switch and lights.

Sixto
83 300SD

Swapped relay, no fix

The brakes switch on W210 operates by breaking the circuit as opposed to making one, bit odd but it does seem to play havoc with ABS and other warning systems when faulty.

Sounds as though you may have a short somewhere, wire frayed, have you tested for parasitic drain?

In a 126 with bulb warning relay, cruise control, ABS and anti-theft tap into the brake pedal signal. Any of those could be back feeding +12V to the brake lights. But why then would the pedal break the circuit? When electrical things don't make sense, check the OVP relay/fuse and the voltage regulator.

Sixto
83 300SD

I don't use the cruise control, can I just pull the module out?

I think you need to check the fuse. My guess is that if the fuse is out. The stop lights are probably drawing power from the cruise control circuit, which bridges the switch. If you pull out the cruise module, I bet you don't have any stop lights at all. As a quick check, are your backup lights working? They're on the same fuse.

Actually, electrically it is working now. The pedal is not ever opening the switch which turns the brake lights off, the MC is stopping the travel before it opens the switch. I took the cover off of the 3rd brake light so I could mess with the pedal, the switch and see what was happening. Brake lights were on all the time, going off if I got my finger in there to operate the switch.

Until I figure out where the missing 3/16 of an inch is, I've JB welded an acorn nut onto the plunger of the switch to extend it.

I swapped the pedal assy, including brake switch from the 84 parts car to 87 driver because the input shaft to the clutch master broke a month or 2 back, the missing distance may be in there someplace, but assy's look the same.

Thanks all for the schematic poking and suggestions.

On the '98 210 switch, it self adjusts when fitting. Not sure of the same as yours but basically you pull the plunger all the way out then fit the switch, apply brake pedal to auto adjust.

The original post said

But it was discovered that the lights were on all the time and never went out.

The above is one reason that shops "fail" in the eyes of a customer. While the symptoms would be apparent to the tech, sometimes it isn't that simple.

I have a dealer work order from a parts car.

Customer states: " Car won't start "

Techs findings: Can't duplicate problem car starts X times.

A few days later, Customer states: " Key won't turn in ignition "

Techs findings: Replaced lock cylinder and column locking mechanism.

Yes, I didn't see what was really happening till I took the cover off of the 3rd brake light to see it for myself. I thought that it somehow reversed because a helper saw the lights go on/off when I hooked up the spare switch and cycled it, and because the lights were steady on otherwise.