Constant Speed Solenoid stops working when engine warm

[gcw206]

My Constant Speed Solenoid (#000-072-01-00) works great before the car warms up, but after that it will not work.  This situation just started last week; prior to that all was well.  Both the Forward and the Reverse pressure switches on the auto transmission are under a year old, and the trans fluid and filter are about 1000m old.  When the CSS stops working, I can jump from the ground terminal on the CSS to ground, and then the magnet will energize, slightly move the linkage forward, and the idle will return to 800 rpm (w/o the CSS, the idle is just above 500 rpm).  Since the CSS magnet does work when jumped, and both the pressure switches are new(er), that leaves the wiring in between, or a low pressure situation in the transmission (not enough psi to engage the F and R switches)?  Is it possible for the transmission to have good pressure at start up, but a lower pressure when warm? 

[Pawel66]

I am not sure if the reason for malfunction is one of the two components you mentioned, but to help on these two:

The wiring is simple: red/green wire goes from the right switch over the transmission to connector plate and another red/green wire from the left switch to the same connector plate (same plate that is for the transmission solenoid wires) and then from the connector plate red/green wire goes to CSS. They provide ground.

If the occurance of the issue is related to warm up, I would have thought it indicates more towards oil pressure - but someone more experienced may provide another cause or deny it.

But I think the higher temperature the higher pressure...

Anyway - oil pressure check and adjustment will not harm anything...

Oil pressure adjustment is quite easy. You just need a gauge with banjo bolt connection and a bit filed screwdriver. You will find procedure here on the forum or in the techmanual. Be careful not to mix up the pressure units. I did it myself so anyone can do it.

[gcw206]

I solved the problem:  The Pressure Sending Switch on the Drive side was bought new and installed a few weeks ago, and I assumed that it was good.  After some additional trouble shooting, I decided to switch it out with the Reverse Pressure Sending Switch, and the problem changed from the Drive side to the Reverse side.  The "new" Switch is defective, and I will return it for another. 

[Pawel66]

As someone wrote here: NEW means Never Ever Worked...

[Shvegel]

I read the words “Both pressure switches...under a year old.”  and scolled down to add one the seven  tag lines (New means Never Ever Worked.) From a talk on electrical diagnostics I used to give to students when I was a BMW trainer and someone beat me to it.  My legacy is complete. I can die now.

It’s only happened to me a few times in 45 years of fixing stuff but when it does it can be very annoying.  Nice job on swapping the switches.  That was a good idea.