1997 AC Compressor Clutch - Diode?

[as400jockey]

I have a '97 850. AC compressor clutch wasn't kicking in.

System was charged. Various cut-out switches were all working OK. 12v was showing up at the pigtail leading to the clutch. So the electrical worked OK.

The line from the clutch electrical connector runs to what I believe is a diode or maybe a voltage / current limiter. See photo below with the red circle.

THIS is what has failed. The juice isn't passing through this.

I have found very little about this. I cut it out of the line and wire-nutted the wires together and the clutch works OK. But I'm hearing that this may be a diode to prevent voltage spikes running backwards and frying the relays and/or ECM, etc.

AC Compressor.jpg (81.54 KiB) Viewed 11075 times

So - what is this thing? If it's "just a diode" then what spec should I be looking at? GM uses a diode in their compressor setup, but it's wired in parallel to the clutch. This thing is wired in series. So the juice to run the clutch passes thru this diode. "A big fat diode" should be good enough.

But if it's a voltage regulator.. then that's a different story. Maybe it's passing only 10V to the actual clutch... Hmmmm....

Volvo knows zero about this. But then they get to sell a $600 compressor when a 50 cent diode is the culprit...

I contacted a compressor remanufacturer to see if they will share any knowledge. But I'm hoping that this list has some info...

Thanks!

[jblackburn]

That is a thermal cutout switch if the compressor gets too hot.

Bypass it - common failure point in the Volvo AC system. The compressor shouldn't overheat if the low-pressure and high-pressure switches are working correctly.

[cn90]

1+

Yep,Thermal sensor, known to be a problem in Denso compressors, but not Zexel (which is Volvo OEM).
After bypass, just make sure the R134a amount/pressure is correct.

[as400jockey]

Thanks to cn90 and jblackburn for the info. Curious - this thermal sensor is mounted to the compressor, but it isn't slapped up flush against the compressor itself. So it must be measuring the temp of the air that it's getting from the neighborhood of the radiator fan. So I guess you're more likely to have trouble running the AC on a really hot day, eh?? Seems counterintuitive...

If they're worried about the engine overheating, I'd think they'd use the coolant temp as the canary in the coal mine...

Thanks again for the info. Save hundreds of $$ and lots of aggravation!

[jimmy57]

That is a diode like you suspected. The temp cutout is screwed into back of compressor if it is fitted. Usually the diode is integrated into coil but can be external on replacement brands.
on 97 and down the spike is taken by pressure cycling switch and fed back to a monitoring pin on ECM.
They used the diode pictured on most B230F compressors but their more common failure was to short.

[abscate]

The thermal sensor protects against compressor overheating , not coolant and engine overheating.

[as400jockey]

Jimmy57 - thanks for the additional information. So - there is both a diode and a thermal sensor. Your comment makes sense - the thermal switch is inside the compressor housing. The power from the wiring harness goes into a connector, the other side goes to a hole in the back of the compressor, another wire comes out, goes to what it seems is the diode, and from the diode to the clutch.

I was wondering what was in there... I dreaded pulling it out... or even getting at it for a good grip.

So - sounds like I should plumb in a diode before I fry something... maybe I'll pry off the existing one and carve into the Lucite or silicone that's hiding the little bugger and see if I can get some specs...

This could have all been caused by the lead from the pigtail into the diode - it detached from the diode pretty easily and showed some corrosion, so after 17 years it probably cracked and failed...

I'll post what I find so that the next guy can avoid spending $900....

Thanks again to all for the help.

[jimmy57]

The spike feedback could cause an issue. How long and how certain it is to happen are unknowns.
The thermal cutout switch on back of compressor was deleted on later volvos and on replacement compressors for almost all 95 and later 850's. The high pressure sensor does the same task.

[as400jockey]

I don't think it's a diode...

I was all set to string in a diode when I decided to remove the square thingy and carve it open. See attached photo.

The fat wire at the bottom left of the photo - that is the wire from the clutch. It's inserted into the left hand side of a barrel crimp that was molded into this thingy. What had been coming out the right side was the wire from the wiring harness that has the 12v coming into this contraption. As an FYI, I think that this was where my trouble was - I think the crimp failed on the wiring harness side.

Also crimped into the left side is a very small wire- you can see what's left of it sticking out of the left side of the barrel crimp. That proceeded into this electronic component. This component appears more likely to be a capacitor. It's full of what might be dielectric, and some of it appears to be maybe glass. The round area in the upper right of the photo has a rivet underneath it. That rivet connects it to the metal bracket from my first photo. So that would be ground. So - one side is connected to the hot side - 12v... the other side is ground... and what appears to be dielectric in between. Hence - I'm smelling capacitor, not diode...

When I put an ohmmeter on what's left of this (from the stub if the tiny wire that disappears into the dielectric, it reads 1.2Mohm, quickly goes to 1.9Mohm and then to "infinite". Wait 2 seconds, repeat... 1.2... 1.9... infinite.

Smells like a cap.

So I need a capacitor to ground. Now the question is - what cap should I put in here? Something like the old cap in a mechanical points ignition?

It's probably there to absorb the spike at on/off or brush noise while the clutch is engaged...

Any thoughts?? My feeling is that I'd be safe putting an ignition cap in there...

[patsaison]

I have a 1998 V90. In 2004, on a very hot day, stuck for a long time in bumper-to-bumper traffic. my AC failed. At that incident, the Volvo dealer replaced the compressor--what did I know then. Now, in 2018, 14 years later, same conditions--very hot day, stuck for a long time in bumper-to-bumper traffic, my AC failed. This time, I checked the fuses. The AC compressor's 15 Amp fuse had blown. I went to AutoZone and bought some fuses. Each time I put in a new fuse, it would blow within a minute.

If this square thing is a thermal sensor, that would make sense, and it shorts out. Of course the dealer wants too much to replace the compressor, and the honest dealer I have been going to closed shop. So I am trying to do it myself. Some one mentioned if the fuse keeps on blowing, it is either the solenoid (assume he meant the clutch coil) or a wire. If it is the clutch, then if I can just replace the clutch; I would not need to evacuate the system.

I bought an Zexel compressor on eBay. If I just replace the clutch assembly, I don't know which wires I would have to remove along with the clutch solenoid--do I remove and replace the square thing along with the clutch assembly? I don't know for a fact that the existing (installed and non-working compressor) is a Zexel, since the compressor had been replaced in 2004 by the Volvo dealer. I am just assuming it is an Zexel. I have not gotten down there to take a look since I need to use the car--it is my daily driver.

Since I have the replacement compressor (New Old Stock) it would make sense to replace the compressor--14 years old--but then I have to deal with evacuating the existing freon, and vacuum and recharging--never done it before, a bit apprehensive about screwing up. If I can just replace the clutch, then I would not need to deal with all that freon work.

What do you guys think?