'98 V70 MCC Air Conditioning Relay Not Engaging

[bmdubya1198]

Trying to diagnose an AC issue with this '98 V70 base I've been working on. I've looked at the diagrams, and from what I can tell, the MCC unit sends 12v to the ECU, then the ECU grounds the AC compressor relay, sending power to the compressor clutch. I manually activated the relay, and the clutch engages. So everything there seems to be fine.
The low pressure switch is closed, so the charge and switch shouldn't be an issue. When I tested for voltage at that connector however, I got about 10v on my multimeter, and nothing on my test light. Interesting, I just had the same problem with a '93 940. That system is MUCH simpler to diagnose, though.

Now if my thought process is right, the MCC unit should be the problem here. However most failures of the MCC include lots of thing acting up. The unit itself works perfectly fine... AC light turns on and off when I press it, same with recirc, and the blower works perfectly at all speeds. The only problem is that I don't have power (or enough power) at the low pressure switch connector.
Obviously there are multiple modes of failure, but I wanted to get others' input on this, and that I'm understanding the MCC AC system correctly.

I repaired the 940 by re-soldering the joints on the relay within the MCC unit. Maybe it's a similar issue? I told the owner I would likely have to source a replacement, and as you all probably know by now, they're not easy to find. Shoulda grabbed the ones I saw at the JY when I could! Every now and again they'll get a couple cars in with MCC.

[abscate]

Grab those MCC control units, they aren’t easy to find.

I’ll have a peek at the circuit and see if I can offer advice

[bmdubya1198]

Yeah, I’ll definitely start grabbing them. Not easy to find at all.

That would be awesome, thanks!

[abscate]

This is from a 1999 but IF it’s DENSO it is probably the same

1, MCC sendssignal to ECM from MCC pin B22

2 It has to makeup through the closed low pressure switch , jumping this switch won’t turn on the compressor like your Uncle Buick, it just enables the ECM to turn in the compressor. A well meaning friend putting voltage here can cost you a lot of money.

3 the ECM received the signal on pin B32

4 if software agrees,it grounds the compressor relay at pin B44

5 compressor relay powers ACcompressor

6 the compressor makes cold molecules and sends them to you in the cabin

Very Swedish

[abscate]

So your troubleshooting has been perfect

Try key position I, then pushing the recirc twice to read the little flashing lights on the MCC CONTROL UNIT and see if you get 5-1-2 low voltage

[bmdubya1198]

I'll take a look for errors on there today, thanks!
Although jumping the connector at the switch won't trigger the compressor clutch directly, it should still provide the ECU with the required signal to ground the compressor relay. That signal should be 12v, right? Because that's not what I'm seeing there.

Should be the same though looking at that diagram, although 1998 NAs are M4.4.

I love the bottom illustration.

[bmdubya1198]

It seems the ‘98 may not be able to read MCC codes that way. Every time I try I get nothing.

[abscate]

Yes, the 1998 might be different

The signal through the low pressure switch could be 12v or 5v logic , since it’s actually controller to controller signal

10V doesn’t sound Right though

I’ll check Abbys after CHE beats ARS this morning

[bmdubya1198]

I thought it could be a 5v reference, but the fact that my meter showed 10v led me to believe it should be 12v.

[abscate]

What’s the part number on the MCC controller?

With kpII I turned on blower and AC, and the command voltage at the low pressure switch went from 0mto 3.5V

That’s probably a TTL signal from MCC controller to ECM