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Registered
1986 300SDL 311,500
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287 Posts
Discussion Starter · #1 ·
Okay, it's an 86 300sdl. Whenever I want air conditioning, I have to take the following steps.

We'll assume everything's off.

1. Start car (duh)
2. Push middle button, for air conditioning (though this doesn't matter)
3. Set temperature to minimum and fan to maximum.

At this point it should come on and I'm not an idiot, right?

Wrong.

I can drive 30 miles from this point and still have no air at the other end...

UNLESS

I tap the brake pedal.

It's true, once I turn the fan switch on, the fan does not start blowing until I have tapped the brake pedal. At that instant, the fan starts blowing at my desired speed. It happens too regularly to be a coincidence. I've tested it - when i tap my brake pedal, the fan definitely comes on (if it's supposed to have been on all along).

Any reason for this? or am I just haunted by crossed wires or something.

Please don't tell me it's a feature, not a bug.
 

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Registered
1991 300 SE
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18,534 Posts
It’s the ‘stop when you’re hot’ feature, all cars do that…

…j/k

To the best of my knowledge the brake system and ACC are separate systems. It is indeed a coincidence. The pedal tap may be causing something to connect (or disconnect.)

The ACC unit in the dash may be failing or the blower motor may be failing. I’d start with pulling the blower motor and testing it with a direct connect from the battery to see what it does.
 

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1990 300SE
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1,466 Posts
Very odd. Only way they are really connected is vacuum, and that's more for the vents and what not than the power for the blower motor. Very odd problem. I mean, this isn't exactly a Jaguar. It makes no sense in a w126.
 

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1986 300SDL 311,500
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287 Posts
Discussion Starter · #4 ·
There's a lot of things about my 126 that don't exactly add up. My cruise control, for example, works perfectly except that it won't SET - it will resume, speed up, slow down, go on and off, anything.. but it won't set, and if you hit resume before you do anything else it just floors it until you shut the cruise off again.

We think that it picked up some electronic gremlins under its previous owner, who would probably have just sort of halfway fixed things whenever they had to be fixed, instead of doing it right.
 

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Smoke Silver '87 560SEC, Black Pearl '87 560SEL
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4,583 Posts
A lot of these symptoms could be related to bad grounds. I think that pressing the brake pedal applies a ground to the brake light circuit, completing the circuit and illuminating the brake lights.

I'd check the grounds under the dash. On my 560SEL & 560SEC, there is a bolt under the dash (from memory - left side, above emergency brake pedal??) that connects a bundle of ground wiring together into the chassis. That one bolt (8mm) could be loose, creating lots of weirdness.

Is there another one behind the instrument cluster??
 

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Moderator
1991 500SEC Euro, 1987 560SEC Federal
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5,468 Posts
Hi Mike & a fellow dog lover,

I too, thought about a bad ground as I read this earlier. A lot of this stuff grounds at W1 (behind the dash) as you say. The brake lights are ground supplied at the rear. The B/Lamp switch provides power only.

As it appears haunted, (having not a clue here, save I agree with Mike & Gnds), here are a few diagrams....

Cheers, MBL
 

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