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W126 1990 500SE (ASR) & W126 1991 300SE
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Discussion Starter #1
Hello all,

My cruise control stopped working after I got my car back from a garage. I am hoping they just left the unit unplugged, and would like to check this. But I can't seem to find the unit! Can someone please show me where it is on my car?

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I guess it has to be mechanically connected to the throttle linkages, so I found this old photo of the linkages, and hope it is pictured somewhere there?

Thanks in advance!
 

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1986/1990 W126
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13,659 Posts
Hmm not sure on the 6.

What did they do under the bonnet?

Sorry to say the highest liklihood is the module under the dash on the drivers side. A silver box, they go shonky with age.
 

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W126 1990 500SE (ASR) & W126 1991 300SE
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86 Posts
Discussion Starter #3
Hi Ian,

I have a warm start issue, and they said they had adjusted the mixture at idle. I don't remember whether they said it was too rich or too lean, but it hasn't fixed the problem. I assumed they had adjusted that screw that requires a very long allen key, near the air intake.

I have a 1990 500SE, on which the cruise control works perfectly. If I can find that module under the dash, can I switch them over as a test? Or do different 126s have different modules?

Thank you!


Martin
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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4,283 Posts
Hey Martin,

The hot start issue needs a bit more description, but generally speaking, this condition is caused by a loss of fuel system pressure since the car was shut off.
Cranks and won't start.

MBL
 

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W126 1990 500SE (ASR) & W126 1991 300SE
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Discussion Starter #5
Thanks for the reply. Basically the symptoms are that it starts very quickly from cold, but requires a few seconds of cranking when hot.

To be honest, I had given up trying to solve the hot start issue, as I have literally tried everything, except the cold start valve. If that's dripping, or delivering petrol when the car is hot, perhaps that could be an issue. But the part is £500 from Mercedes! So, if that's the cause, I'll learn to live with the 2-3 seconds of cranking!

At the moment, I just want to find that cruise control unit and make sure it's plugged in. I'm hoping for the cheapest fix ever! :)
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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The color start valve is energized ONLY as the starter motor is used, and then after certain conditions apply, such as engine coolant temperature for example.

The accumulator [back at the fuel pump package] has a small reservoir of fuel acted on by a spring loaded diaphragm. Its designed to retain fuel pressure within the [at rest] system for 30-40ish minutes or so. If there is a leak somehow and the fuel pressure is released.. good luck getting it going at all.. til it cools.

Ian is correct that the most common failure of our cruise controls is in the cruise amplifier. The solder joints can fail on the printed circuit board and the electrolytic capacitors fail over time.

That unit is a silver aluminum box attached to a steel steady mount for the steering column. Its near the brake pedal up under the dash.

Good luck man.

MBL
 

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W126 1990 500SE (ASR) & W126 1991 300SE
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Discussion Starter #7
That unit is a silver aluminum box attached to a steel steady mount for the steering column. Its near the brake pedal up under the dash.
Thanks MBL. You guys are probably right about the cause of the cruise control. But, obviously making sure it's plugged in will be the first step.

Here's an old photo of under the steering column on my car:

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Is the amplifier in shot?

Thanks for the accumulator suggestion - I will give it some thought.
 
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