1991 300d Dead tach and A/C
 

I have searched for info on the location of the tach amp for a w124 but all I find is info on W123s. I have a dead (started out intermittent) tach and now that it's dead the AC compressor won't engage. There is no speed signal at the clutch control relay behind the battery. I am getting a speed signal from the engine RPM sensor. (poked a pin through the wires coming from the sensor and used a scope to see the signals there)

I am able to make the compressor start by bypassing the relay with a jumper wire.

I have a CD of the service manual but I find that it has many errors and incomplete schematics.

A/C Speed Sensor
 

I have 1993 300d w124 with a speed sensor problem.
the A/C works when I am up to Hwy speeds, but stops working at idle. I have a couple of questions about the speed sensor:

1) Is it integral with the clutch (I can see that the clutch is sold separately).
2) Can only the speed sensor be replaced?
3) Changing the clutch should not involve recharging the A/C freon, correct?

Any comments here would be helpful.

I replaced just the speed sensor. It was a few years ago and I don't remember where I bought it. It was an easy job from what I remember. Now the tach is dead but it's not the sensor fault since the AC still works.

Was it the W124 model and what year?

1991 300D turbo which is a W124.

Source for Speed Sensor?
 

Do you remember when you changed that speed sensor:

1) Did you need to discharge/recharge the freon?
2) Did you have to remove the compressor before changing the sensor?

I couldn't find the speed sensor online anywhere, but I could buy a used compressor (on EBAY) that still had the speed sensor. I found an instruction sheet online (83-532.pdf) which seemed to imply that I need to remove the compressor.

Any advice you could give we would be appreciated!:o

The speed sensor that failed on my car was the one that senses the speed of the flywheel not the compressor speed sensor. I didn't have to do anything to the air conditioning system. If I remember correctly it is mounted on the bell housing of the transmission.
If the compressor sensor is bad is does not effect the tachometer but the engine speed sensor effects both air conditioning and tachometer.
The speed sensor on the the compressor is a safety feature. It compares the speed of the engine with the compressor and disengages the compressor of something is slipping.