Just to make sure, you can run to the junkyard and cut a few terminals out like it is shown here, then connect the oscilloscope directly to the sensor on the compressor to rule out issues with the wiring. I would fall through the ground if two totally different parts (let alone that one of them is new) could have one and the same flaw, considering that the compressor was replaced and it never fixed the issue. You can even try using a regular multimeter to eliminate the possibility of a faulty or misset oscilloscope.

What you can also try doing is disconnect the pigtail going to the compressor, then de-pin the power pin for the clutch and connect it back separately to see if the behavior would change or the Control Unit would throw a code.

Hope this helps and best of luck!

 

Thanks for the helpful jury-rigging guide!

You're right about the unlikelihood of the rpm sensor failing on both compressors; however, I transferred the old rpm sensor to the new compressor, which makes this all seem a lot more probable. At this point, more or less every other component has been swapped, including the climate control unit itself. I must've dropped or damaged the RPM sensor during the repair process, or maybe it was already on its last legs and giving bad or incomplete data, and finally kicked the bucket when everything got hot and the resistance went up

I still can't figure out why it didn't trigger the diagnostic fault code. It also remains a mystery why the system didn't completely fail until the car got hot in the bright sun, but everything else seems to line up behind the idea that I need a new rpm sensor.

 

Sorry for the delay..

All that can still lead to a faulty wiring.. Considering that you already have an oscilloscope and it doesn't take much to de-pin connectors, it would still make sense to do some small checks before committing to replacing the sensor..

 

quick update:

I purchased the 1996 specific manual and the A/C diagnostic procedure includes a section on the code 22, compressor lock sensor. The 1993 Manual I was using doesn't have this. When using the complete procedure, my (recently junkyard pulled) climate control unit returned code 22, compressor lock sensor.



Additionally, the solar sensor was giving a "Bad" reading, based on the A/C manual for a 1996. When I followed the instructions for testing, I was reading -4.15 volts instead of 4.15 volts. This appears to be a misprint. The 1993 Manual instructs you to place the positive lead on the "S5" pin, while the 1996 Manual instructs you to place the positive lead on the "TS" pin.



1996 compressor lock sensor diagnosis

1993 solar sensor diagnosis

1996 solar sensor diagnosis

 

Oh wow, that pretty much explains everything.. I frankly didn't expect a 1996 to be so different from 1993, even though a post-facelift was redesigned quite a bit.. Sounds like a time for me to go ahead and invest in one of those manuals for my own collection as well..

At any rate, thanks for an update! Hope you don't feel too bad about coming full circle on this issue..

P.S. Those massage seat coverings, haven't seen those since forever..

 

Excellent thread

Arsenii providing solid inputs toward solving the issue. MrReeves99 actively learning, testing & adjusting strategy, forging ahead, ultimately discovering the cause & solution to the problem

 

Final update: the A/C is working now.

The real culprit here was multiple part failures obscuring each other. The final solution was to replace the rpm sensor, compressor and (eventually) climate controller.

I went ahead and got an OEM denso compressor with sensor and clutch wire assembly pre-installed. Sent back the UAC compressor I had previously purchased. Thought the internal magnets may have failed after one day of use due to poor manufacturing. Still not a terrible guess, but I may have been testing the sensor improperly. My memory of the beginning of this job feels faint already. I can't recall with certainty that I jumped the compressor clutch before testing the signal with my oscilloscope. The climate controller continued to flash the AC light and failed to engage the compressor.

In any case, with the OEM compressor assembly installed, I was able to get the appropriate signal from the rpm sensor, but the a/c button light was still flashing, and the climate controller was still refusing to engage the compressor. The A/C service guy who purged and refilled my system said that he wasn't seeing ground on the magnetic clutch relay coil, and that he thought it might be the issue. Forcing this wire to ground would engage the compressor clutch, and the light would stop blinking.

From previous reading in the service manual, I knew that closing the ground to the MGC wire was the "compressor on" condition, which meant that, because the compressor did not engage at all, the climate controller was probably just malfunctioning, and that the wiring was fine. However, for the symptom of a flashing AC power button light, the service manual does not point to any sensors other than the "compressor lock" sensor, aka the rpm sensor, which compares the compressor speed to the engine speed, to determine whether the compressor is locking up, and shuts off power to the magnetic clutch if the compressor slows by more than 20% relative to engine speed, to prevent it from damaging the system via excessive pressure. After confirming that the tachometer signal was being received properly as well, I went and got a new climate controller from the junk yard, which solved the problem.

I think this project probably began with a bad expansion valve or evaporator, and ended up with two additional broken parts. Very difficult job! Had I known better, I would have looked at the evaporator and expansion valve first, as freezing could potentially explain the time delay between air conditioner failures. When I removed the original evaporator, it was wet and covered and dirt and leaves, from the time period where no cabin air filter was installed. Apparently, some came without it for no particular reason.

An interesting note: the solar sensor seems to be there to supercharge the ac in extremely bright sun. It "pulls up" on resistors in the air mix servo motors, and only reacts a very tiny bit, a few tenths of a volt, when a very very bright light shines on it.

I'm somewhat of a beginner with electrical work so some of my terminology may be off here. At one point, I accidentally jumped the relay coil instead of the switch, which may have fed too much voltage to the climate control unit and fried it.

Always replace the evaporator if you're replacing the compressor, which generally means replacing the expansion valve, anyway, which is a lot of work to reach under the dash. I did most of it upside down hanging from the chair by my legs.

 

Thanks for the update. Seems like all your problems started when you had a new but faulty compressor.