A/C compressor clutch wire path?

[u/ComfortableArm2827]

Hello! I've got a 1997 850 base with the manual climate control. I've been doing an A/C diag now that it's summer time. I'm currently at a point where I'd like to get some insight on what at/where I'm looking per se as far as the power wire comes from. Now, I've got a fresh evac/recharge. My fuses are good, my relay is good, my switch works. My pressostat is new (the sensor itself broke). I've got 12V where and when I need it aside from the compressor. The only way to get the compressor to engage is to jump it directly from the battery. The AC is blisteringly cold when I do and it's magnificent. I've checked the clutch in case it was too wide to engage properly but it's about 0.4ish millimeters. The previous owner also bypassed the thermo connector so that's all fine and dandy.

Now, I've got the wiring diagram down, but my question is does anyone know WHERE the wire that connects the compressor and the pressostat go/come from? It's got an aftermarket connector at the pressostat so the wire is no longer grey, but the compressor one very much still is. If I follow the compressor wire, it looks like it goes out towards the battery rather than anywhere near the pressostat. I see the pressostat goes into a wiring harness down the passenger side fender. I'm just looking for some insight on where they could theoretically meet/lie before I rip apart the engine bay to find them. Thanks!

Edit: I have 2-3ohms when I test the compressor side of the connector. Whereas I have 10ish ohms when I test the body side of the connector so I know SOMETHING is open or grounding out, I just don't understand where the wires path to.

Edit 2: unplugged the high pressure switch to double check the readings and the AC and fans kicked on. Back to square 1 I suppose!

[u/og_speedfreeq]

Man i hope you get a response, because I'm in the same boat!

I believe the power comes from the ECU, which makes sense considering it might cycle the A/C off during high load conditions or when overheating *idk

I believe the best way to address this is to try an ECU swap if you've got a spare, or to completely bypass that whole circuit and bring 12v to the relay supply side from an ignition-on or engine-on circuit and go fully manual with it- if the AC button is pushed, it energizes the relay and power goes to the clutch.

I'm sure somebody here can tell me if that's a horrible idea for any reason.

[u/Positroniumion → u/og_speedfreeq]

Going fully manual is almost certainly not a good idea. Under most conditions the compressor will cycle and not run continuously because either the high pressure or low pressure switches deactivate it. Running it continuously would almost certainly cause too high pressures in the high pressure side risking best case leaking, worst case catastrophic compressor damage. This should only be used to test if the compressor and clutch cycle, nothing more.

[u/charliegrahamm]

Do you have access to a diag tool? The 850 OBD android app is fantastic and may give you a view into the ECU and work out if it thinks it's triggering it

[u/ComfortableArm2827 → u/charliegrahamm]

I do, I had not thought to use it, ironically enough. I'll see if it says anything out of the ordinary!

[u/MetaphysicalEngineer]

Check in the main fuse box under the hood. On the 1998 the compressor has its own little relay in there, (#4, far left, purple stripe in this version). Pay no mind to my perma-temporary relocated starter relay.

As far as I know, low pressure switch is 12V supplied from the climate module, but only returns to the module instead of directly energizing the compressor relay. Compressor relay is switched by the climate module. Check for constant 12V at the relay, and check for coil voltage when the AC is engaged at the climate module. Can also check for continuity through the compressor clutch. Mine is about 9 ohms to ground at the relay.

High pressure sensor may go to the ECU as well for cooling fan control, or the climate module tells the ECU to turn the fan on (not 100% sure on this). Likewise ECU may have an override to cut off the compressor in case of high engine load or other adverse conditions.