A/C Compressor Trouble Code
 

I have a 1995 SL500 with OBDI that has an A/C compressor trouble code. BASE MODULE - CODE 16 :: A/C Compressor Clutch Shorted.
Replaced the compressor w/clutch along with a new drier. While the wires were disconnected from the compressor, I checked for shorts and none were found.
I can reset the code and all is well untill I start the engine - the code comes back. If I jumper the clutch it engages and I get an output from the compressor speed sensor.
Any suggestions on what to do next ???

Interesting that you're getting code 16 and not code 6 - which must mean something - what, I'm not sure. Are you getting any codes on any other pins - like rpm signal faulty on the DM?

No other codes. Looking at the schematic, I can't see how the Base Module can determine "a shorted clutch" since only the compressor speed sensor goes to the Base Module. The clutch is driven directly by the A/C
Module in the dash.
I'm just guessing but I think your right. The speed sensor and the rpm sensor
determine error conditions:
+ speed sensor equals rpm sensor ---> OK
+ speed sensor less than rpm sensor ----> BELT SLIPPING
+ no speed sensor with normal rpm sensor ----> CLUTCH OPEN
+ ???????????? ----> CLUTCH SHORTED
Does the A/C Module communicate with the Base Module any other way ???

The Base Module does turn on the compressor.

So either a short OR open in the wiring between the base module & the compressor clutch.

That would make sense but I'm looking at a schematic that shows the clutch wire going to X85-pin 2 then to N22-pin 9 (A/C Module). This is for LH (hot wire ?) and HFM engines with a N16/1 Base Module. The schematic does show alternate wiring for CFI (continuous fuel injection ?) engines with the clutch wire going directly to N16 (Base Module). I'm sure I have the LH engine but this would not be the first time I misread a schematic. I guess I will have to verify the wiring to make sure. Thanks.

You have an incorrect wiring diagram! Base module DOES control the compressor on all SL's from 1993-1996

Check for a broken wire at the compressor.

If you have your wire colors you could try jumping from the base module to the clutch.

I'm still not able to resolve this problem. I have verified the wiring and I am able to energize the A/C compressor from the Base Module. I had access to another AC control box (N22) and replaced it. Same problem. I did try this:
I disconnected the wire going to the A/C compressor clutch and put an analog meter on the line going back to the Base Module - when the AC is on (automatic) the engine idle rpm rises slowly and the voltage rises at the same time. When the idle drops, the voltage drops. The idle increases then decreases at about a 2-3 second rate. When the AC is off, the engine idles normally and the voltage drops to zero. The voltage never gets near the required 11-14 volts which is probably why the code 16 is triggered.
The TD signal brought out to the 38 pin connector-pin 13 is >3VAC per the shop manual. I can't find the location of the transducer that produces this signal since my documentation has errors. I checked all the fuses and holders for opens and the common funky contacts. (At least the ones that show up on my questionable schematic)
Summary: New engine wiring harness about 2 years ago
No codes other than Base Module - Code 16 (clutch shorted)
No AC Control Module (N22) codes
New AC compressor/clutch
Still need more suggestions --- Thanks

After extensive troubleshooting, I found a bad ground behind the left headlight. From schematics - it looks like this is the ground for many AC related sensors and controls that go to N22 (AC Dash control module). It is not the ground for the AC compressor but cleaning the chassis, bolt and lugs seems to have done the trick.