I am working on a 300SE/M189 with idle troubles. It took a new idle solenoid and two new pressure switches along with endless fiddling with the throttle linkage to get it to idle at 850 rpm in gear and in Park/Neutral. Painful, because on this car, Mercedes decided to put the solenoid below and to the rear of the injection pump.
But the following applies to any model with the solenoid controlled by pressure switches on the transmission.
The pressure switch on the right side of the transmission controls the solenoid in forward gear, left is reverse. I learned this the hard way.
So there were three challenges.
Determine if the solenoid was working, and had enough force to move the linkage
Determine if the pressure switches were working
Adjusting the throttle “lift” for the correct idle speed increase
By grounding the solenoid manually, I could see that it actually worked about 25% of the time and sometimes it was too weak to move the linkage. I took it out and bench-tested it and found that when it engaged I could easily push it back with my finger. So Gernold sold me a good used solenoid, which worked all the time, and was much stronger. New ones are just now available from the Classic Center for $1680.
I installed the new solenoid and found that it worked well with manual grounding, but not while driving. So to check this out further, I made a little harness with leads long enough to reach into the cabin (attached) with a light that would show when the solenoid received ground from the pressure switches. I added a switch that would manually ground the solenoid. Driving the car, the reverse switch provided ground but the forward switch did not, so I replaced it. That got the solenoid working in forward gear.
With the transmission in Park, I used the switch to get a 100 rpm bump in idle speed and ended up with 850 rpm in both Park and 4.
With the switch and light in place, I took a test drive that answered the question, does the solenoid stay engaged while driving? The answer is, NO. On this car the pressure switch stopped sending ground when the transmission shifted from 2 to 3. It re-engaged when the car slowed to 4-5 mph.
Cheers,
CT
