Another Car, Another Climate Issue. AC This Time. Topic is solved

V50M66 · Post by **V50M66** » 02 Jun 2025, 13:34

About 8 months ago I picked up a new-to-me '98 V70 NA manual that I've been fixing up and modifying. Now that it's finally getting warm out, I've started to dig into the non-functional AC. For reference, this car has the simple MCC system, with manual everything and cable-driven blend doors.

When first filling the system, the compressor was cycling normally. It then stopped cycling a minute or so later, despite the system being nowhere near full. Here's what I've tested so far:

- High-side pressure switch tests as it should, and jumpering the connector does nothing
- The AC fuse tests good
- The AC relay bench tests good. I also swapped it for a known-good relay out of my other '98 with no change.
- If I jumper the relay socket in the fuse box with some wire, the compressor clicks on just fine and sounds good
- Testing for activation voltage at the relay in the fuse box, I only see 0.55V

I'm at a loss for what the problem could be. Anyone have any ideas?

Post by **scot850** » 02 Jun 2025, 15:53

Have you checked the clutch gap? There are specs for it and what is called the zip-tie/bread clip repair to shim it into spec.

I can't recall, but do MCC cars have the cabin temp monitors above the grab handles?

Neil.

V50M66 · Post by **V50M66** » 02 Jun 2025, 18:42

Haven't checked the clutch gap, but that likely wouldn't be an issue here, as the compressor flat out refuses to click on under any circumstances. I'm not getting activation voltage at the relay.

It'll click on just fine if I jumper the relay to force 12v to the compressor, but refuses to do anything else otherwise since it's not sending 12v to click the relay on.

No temp sensors for MCC

Post by **abscate** » 02 Jun 2025, 20:56

I believe on the 1998 the AC command control comes from the MCC, in 1999 it comes from the ECu. There might be a chassis break change in the model year….gulp. Post up last six of your VIn and build date

You’ve. Only got a month of summer left, anyway.

V50M66 · Post by **V50M66** » 02 Jun 2025, 21:29

abscate wrote: ↑02 Jun 2025, 20:56 I believe on the 1998 the AC command control comes from the MCC, in 1999 it comes from the ECu. There might be a chassis break change in the model year….gulp. Post up last six of your VIn and build date

You’ve. Only got a month of summer left, anyway.

Funny enough, I actually started down this road because I was going to delete the AC. Got the delete belt and everything. Went to remove hardware, double checked there was no refrigerant in it first, and it was almost half full, so I figured there might be hope.

This is a very early ‘98. Built in early ‘97, and has the external slave, single-mass M56 in it. I believe the change to ECU-actuated AC compressors was for the changeover to ME7 engine management for the ‘99 model year.

V50M66 · Post by **V50M66** » 03 Jun 2025, 15:47

Replaced both high and low side pressure switches today with no change in behavior.

Tested for 12V at the high-side connector and it’s doing what it should, so no issues there.

Not sure where to proceed from here.

xanthefin · Post by **xanthefin** » 04 Jun 2025, 02:37

Motronic M4.4 does know little bit of A/C status too in theory.

Post by **abscate** » 04 Jun 2025, 04:03

Is the 12 volt command coming from the MCC module to the compressor clutch?

V50M66 · Post by **V50M66** » 04 Jun 2025, 08:35

abscate wrote: ↑04 Jun 2025, 04:03 Is the 12 volt command coming from the MCC module to the compressor clutch?

I'm not sure if there's a command coming from the MCC - I have no way of reading data from that module (the app only supports ECC, though VIDA makes no mention of being able to read it even with the VIS so perhaps MCC really is that archaic).

I do know I'm not seeing activation voltage at the relay, so something is preventing the signal from reaching the relay. It did work for about 30 seconds when first filling the system, but after that, nothing.

V50M66 · Post by **V50M66** » 05 Jun 2025, 19:09

Replaced the MCC today, with no change. Not sure where to go from here. I did find the wiring diagram for the MCC system:

: Screenshot 2025-06-05 204902.png (485.12 KiB) Viewed 862 times

...though I'm unsure what exactly I should be testing, if it should be seeing voltage, etc. Any electrical nerds have any ideas?