'83 300SD no power to A/C clutch, and draw on battery
 

I did a Sanden conversion on a friend's SD the other day, and had to hotwire the compressor clutch to charge the system. The pressure switch is new. There is no power at the switch or the compressor clutch. The blower is working. I believe there is NOT a Klima relay in this year, but is there a relay for the compressor? The paper on the fuse box lid does not show what each relay is for. I am not big on electrical, so any help in this area would be appreciated.
The second issue is a draw on the battery. Enough to completely drain it overnight. I pulled every fuse, and the draw is still there. I remember reading about the starter solenoid possibly being a problem, so I procured a new one. It is a major job to replace it, as the screws strip out, even if an impact driver is used. The heads had to be drilled out, and new screws sourced. I haven't got the starter put back together or in the car yet, so I don't know if that fixed the problem or not. If it turns out I still have a draw, any ideas where to look next?

You may have a bad diode in the alternator that will draw down a battery overnight. Good luck

That totally makes sense. It would be easy to pull the plug on the alternator to check. Thanks!

Remember this trick for the solenoid Phillips head screws:

They are loctited from the factory. Apply heat with a propane torch to where the threads are before putting an impact screwdriver to it. Heat it to approx 500 F, that's the temp where loctite softens. They will come out easily w/o stripping.

I will remember that for the NEXT time. It is too late, I already stripped two of the three screws getting them out. I had to drill out the other two. Also, after drilling the heads, the stubs came right out with my fingers. No loctite was evident.

Don't know about 83 300SD, but my 84 300D has a relay for the AC clutch. It is under the plastic cover on the L inner fender. The other relay there is for the radiator fan. My 85 300D has just the fan relay since a Klima box controls the AC clutch. I know SD's have a relay box inside the wiper tray. In many cars, the lid of the relay box documents each.

Why not check battery draw-down w/ the starter out? Unlikely it would be a draw since the solenoid is a pretty robust on or off connection. I agree that the alternator could be a draw since all that prevents reverse current flow is the diodes, which always have some leak (usually negligible). Main suspects are always the glove-box lamp and trunk lamp. Insure those switches close when the lids are closed.

I read a while ago that newer cars (say 2005+) have many more parasitic loads. So much so, that when people park for 2 weeks, their car won't start. That keeps airport parking lots busy giving jumps. But, there are now small lithium jumper batteries which fit in a glovebox. They will start a gas V-8, but don't know about an old diesel.

Did was to pull all the fuses to be sure that anything powered by a fuse was not causing the problem. That leaves only three things not having a fuse. The starter, alternator, and the doorlock motor. I suspect the door lock motor, as I still had a draw with the starter and alternator disconnected. As for the A/C, there is no listing of the relays in the fuse box, so I don't know which one of the six or so relays is responsible for the compressor clutch.

Compressor is relay D. Its right next to fuses 15 and c. Page 103 in the wiring diagram has the fusebox layout.

Thank you so much I'll check it out.

OK, I disconnected the starter, alternator, lock motor, and clock, and still have a draw on the system. I also removed every fuse one at a time with my meter on the battery post and cable, but the draw does not go away. It still has a draw enough to drain the battery overnight. I also checked the compressor relay, and it is fine (checked for power to the coil=yes). It still has no power to the compressor wires. I even switched around th relays, still the same. I am at a loss with these problems. I am not sure what to do next. This customer will probably take it to another shop. Hopefully they will find the problem.....Rich

After more work, still the same
 

Well I found out that all the relays are good, and the compressor relay does energize the blue brown wire at the relay terminal (87). I assume that wire goes to the pressure switch and then to the compressor clutch, but there is still no power there. I have a W123 schematic, but I am not sure that will help.

The car still has a significant draw on the battery, and I am not sure what to do other than give up.

What is significant draw? Is there an alarm system? Radio disconnected? Any other after market accessories?

I would assume the W123 wiring diag for the AC is similar to the SD so just follow it if a diag for the SD is not available.

The compressor clutch only draws 1 to 2 amps max as I recall. Why do you think the compressor clutch has anything to do with the power draw? When the ign is off, there should be no draw from the climate control system.

With key on. did you try shorting out the pressure switch and did that actuate the compressor clutch?

You should probably read the entire thread, there are two problems. The draw on the battery is enough to kill it overnight. The problem with the AC, is there is no power to the pressure switch. I do know that the pushbutton unit is energizing the compressor relay like it should.

Looking at the 123 diagram, the power goes from the PBU to the coil terminals of the relay. When the relays energizes, power goes from there to the pressure switch and then to the compressor clutch. Again, that is for a 123, the 126 may be different. If they are the same, I may just run a wire directly from terminal 87 to the pressure switch and call it good.

On the W126, power for the relay comes from 2 separate fuses. F18 powers the relay coil through the Pushbutton Controller and Pressure Switch. F11 then goes to the relay contacts and powers the clutch itself.

I tried to provide a link to the W126 diagram but they must be still having issues, or it is seeing heavy use at the moment.