W210 1997 E420 Electrical problem
 

I am having an electrical problem that is effecting many things, the following is a description of it. Turn the key to run and everything is pretty normal. Then I start the engine and release the key back to run. Then engine starts and runs normally. But, with the engine running, the radio shuts off, the windows do not move, and the transmission will engage only 1st gear but will not shift up. No codes set. It is like the power is not coming back on after dropping out to start the engine. After the engine is turned off and then the ignition back on, without starting the engine everything is working again. Could it be the ignition switch? Any ideas out there?

You need to find a wiring diagram and see what rail those items are on just to make sure things are not going through a relay.

With the engine running, slightly turn the key CCW and see if the items come back on. Is so, it is an ignition switch problem.

Thanks for the input 97 SL320! I messed around with the ignition switch earlier, just like you were suggesting also. No Luck. I also found parts of the wiring diagram online and could not find a common thread. A proper and complete wiring diagram is needed. But this morning things seem to be back to normal. The car was outside in a heavy rain on Sunday afternoon and I am now thinking that I have a moisture or corrosion problem, either in the CAN box or the main fuse box. I will pull the covers off this weekend!

Update: When starting from an overnight cold start condition everything is fine. As the car is driven to a fully warmed up condition weird stuff would start to happen such as the radio cutting off, no transmission shifts, digital display messes up, etc. What I found is that the charging system voltage during the trouble is 18.8 volts! WOW! So I am in the process of replacing the alternator as the battery tests good. I thought about doing the regulator only but I don't want to do this twice. I am hoping that nothing got damaged by the over voltage. If there is damage..... well let's not go there right now. New alternator is due to arrive tomorrow.

you didn't mention your findings after examining the pit under the rear seat
which is a target rich environment for water intrusion (fuses, relays, PSE,
battery etc)

The battery and the surrounding area was quite clean. There was no rust on the floor and I could not find any corroded electrical connections, power side or ground. The PSE was still in its foam sound insulation which was undamaged. After seeing the 18.8 volts on the battery when the car was running I was expecting a low acid level in the battery but it is up where it should be.

After installing the new alternator everything is back to normal. Charging voltage is good and it seems that nothing was damaged by the over voltage condition!

yay !

The K40/4 has over voltage protection for the electronics. Similar concept to the OVP relay found in the earlier models. This likely shut everything down and protected it from the gross overvoltage.