Hi all,
I just purchased a rebuilt climate control unit (switch unit) for my '85 W123 - the previous one nearly caught on fire sometime in the past due to the aux water pump seizing. I've replaced the water pump and my mono-valve tests OK.

One of the pre-installation tests involved checking resistance across the right side switch connector from pin 3 to pin 6, looking for > 35 (or 55 depending on which test document I look at) ohms. This is supposed to be testing the AC clutch relay. I'm getting zero continuity across those two pins. I am aware that infinite resistance is greater than 35 ohms, but I just want to double check. Is this acceptable?

Currently my system is discharged, waiting for me to finish installing the new controller and replace a few more things on the refrigerant circuit before I charge the system.

I do have an email in to tech support for the remanufacturing company, but I've got no idea when they will respond.

Thanks!

IIRC 1985 cars have a different AC clutch relay than the older cars, and it gets its power from the overvoltage protection relay mounted in the passenger footwell.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

The 1985 300D AC system uses a Klima relay which is mounted on the left fenderwell located under the hood.

Ah - That helps! It does look like that part is unique to '85 - so I wonder if the rebuilding company's instructions are not set up for the '85 Relay. The fan clutch worked before I disconnected it, so I suspect that the system is designed slightly differently.

Regardless, it wouldn't be a problem with the new push-button switch (CCU), but rather on the car wiring side. The test just insures that you won't have a short-circuit that could damage the new CCU. The CCU's job is to output 12 VDC when the clutch should actuate. What your car does with that is not the CCU's job. But, yes your 1985 car senses that signal in the Klima box, with minimal current draw, meaning a high-resistance load for the CCU.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Yeah, i am mostly just wanting to be sure i dont throw my money away by burning out the CCU. Thanks for the feedback

The Klima relay is unique to the 1985 W123. You also might want to consider an inline fuse for the auxillary pump (2amp). If the pump goes bad again it will save your push button unit. There was a fellow on this forum that was selling the in-line fuse for the pump but I do not remember his name.

I got word back from the rebuilder's tech support, they said to try it, so I will.
I already built the inline fuse holder for my aux water pump, though i misplaced the fuses so I haven't got it installed yet. If anyone is curious how to build one, I can do a writeup (I've made several custom wiring harnesses for my car to add stuff like heated seats without modifying the existing wiring, been meaning to write about it, but I'm just too busy lately)

Your system needs to have a slight freon charge before the compressor will engage. This is controlled by the low pressure binary switch located near the receiver/drier. You can also jump across the binary switch as you are charging the system which will allow the compressor to engage. The binary low pressure switch shuts the compressor down when the freon level is low.

For in-line fuses to components like the auxiliary water pump, I use a fuse holder used by General Motors made by Packard Electric. It is weathertight for underhood applications.
Search for Metri-pack 630 inline fuse holders. You can buy them with pigtails or loose terminals to crimp on your wires.
Also available to hold mini fuses.

Attached Thumbnails

   

I installed the new HVAC controller last night, everything works correctly so far. I still need to charge the system/jump the low pressure switch to make sure the AC clutch engages properly, but that's a job for another time. Actually I have a new binary switch & receiver/drier I will be installing before I try to charge the system anyway.

That's an interesting holder. I used one from the auto parts store that is fairly similar, but has the wires entering and leaving on opposite sides. I cut up a harness in a junkyard car to scavenge the male/female ends of the aux water pump harness, then soldered it all together to make a piece that can be installed without any modifications to my car's harness. Unfortunately I got the wire lengths slightly wrong, but it works just fine. I've used a similar method to tap power from the ash tray leads to switch a relay for heated seats as well.

Air-cooled VW's used an inline holder that fits german style fuses to run the backup lights. This could be used for this purpose as well, though I don't think there was ever a 2 amp fuse made in this style. It might be useful for running other accessories though, so you don't need to carry as many extras.

Attached Thumbnails

 

Be sure to have the system evacuated first before you charge the system with freon. The evacuation process involves putting a vacuum of about 25-30" of vacuum on the entire system. The purpose of this is twofold: to extract any residue/diret particles from the system and to determine if the system has any leaks before charging the system (the system should be able to hold a vacuum of 25-30" of vacuum for at least 30-45 minutes with the vacuum pump turned off). If the system will not hold this vacuum then you have a leak somewhere in the system. Hopefully not. If you do, there are remedies. You may need to have an AC shop do the evacuation for you.

Yup - i have a vacuum pump to do that myself, hopefully it works out well

The "aux pump" from both my 300D's is sitting on the shelf, replaced by a length of silicone heater hose. One was leaking slightly at the shaft, the other fine. When I first saw them, I scratched my head and thought "anal Germans", having survived several upper mid-West winters fine without such in my U.S. cars, and get plenty of heat in my 300D's without them. But, some here claim it is an essential part. Just don't let them start a dash fire, as has happened to others.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans