86 W126 sudden rough low idle/harsh shifting

[Michaelpat]

All was well. Parked 1986 W126 on slight upward incline. Left sitting, engine off, for 30 minutes, give or take. Came out, started right up. When rpm dropped below 1000, immediately the idle was very lumpy: chugging like a steam engine. Above 1000 rpm, all smooths out. Worse when cold, slightly better at operating temperature. Power is fine, except now the shifting is very harsh. Otherwise no other apparent problems like unusual temperature, missing functions, etc. I have tried to inspect the vacuum lines out of transmission, but am having difficulty following their courses. Inspection of the other vacuum lines give no clues. Door locking system operates as usual. I have not put a vacuum guage on any line yet. No other apparent problem in the enngine compartment.

Any and all ideas, suggestions or recommendations would be welcomed.

Michaelpat
michaelpat@bwig.net

[sur5er]

Just a SWAG, but check the Over Voltage Protection Relay and Fuse.

[GeneB]

I have a similar problem with a '85 500 SEC...Idles up to 1500, next time about right, next ime 700, 1200, really all over the map. I also have a bad mono-valve I think which has it hot all the time, which may be playing havoc with the A/C compressor clutch and possibly idle control. Should I also suspect the Voltage control??

[Robby Ackerman]

Since you've got a rough idle and a hard shifting transmission I'd look for a vacuum problem.

[Michaelpat]

Indeed it was the vacuum. Of course, located in a most difficult spot. The back side of the #4 intake runner at a vacuum point where the offending connection was impossible to see. Found it by running my hand around the area after tracing the vacuum line up from the transmission. Smoothed out immediately upon connection. FYI - this is the area where the crossover between exhaust manifolds runs. Very warm and uncomfortable to the touch!

Thanks to all who threw in for me.

Mike