That's the low pressure sensor that triggers the relay that engages the clutch on the compressor.

You need this switch for the system to function correctly. For troubleshooting purposes, you can short the two pins together inside the connector for a couple seconds and see if the compressor engages. If it does - get a switch. If not - get a switch but keep hunting through the circuit until you figure out why the compressor doesn't engage.

 

I did do some jerry rigged backyard bullshit and was able to get the clutch to engage with a jumper going from the relay box to the switch on the compressor...

can you confirm that the switch to engage the clutch and the AC compressor transducer switch that connects into the filter/dryer are in the same wire harness together?

 

Re-reading what you wrote here...and looking at the PCM schematic (not posted)

The switch to engage the AC Clutch (via the PCM) is the switch that connects to the filter/dryer. The pressure transducer connects to the high side (small line) down by the coolant reservoir if I recall correctly. I don't believe the pressure transducer needs to be working for the PCM to engage the A/C clutch. I think it's just a safety so if the head pressure gets too high, it will disengage the A/C and/or turn on the 2nd cooling fan.

The HVAC unit in the dash provides a very weak ~12v signal that goes through the normally open low pressure switch on the filter/dryer which then goes to the PCM which grounds the A/C Compressor Clutch relay which then powers the coil and engages the clutch.

If the low side pressure is high enough (above ~30 psi) it will engage the clutch and let the A/C run. As the low side drops below ~30 PSI (which correlates to the freezing of water on the evaporator), it opens the switch which tells the PCM to drop the compressor out and let the system warm up again. Rinse/lather/repeat - this is how a cycling clutch orifice tube system runs.

Kind of make sense?

 

I don't know if they're in the same harness or not.

The clutch / system schematic is below...for a 96 Roadmaster Wagon, but reasonably sure they're similar for all B bodies

 

That’s a nice schematic, but from what I tested ewrlier. My clutch coil switch attached to the compressor wasn’t getting any power. I’m also leading to believe either A. The wire has a short and switch is shot, B.) that switch isn’t even powered unless/until the transducer switch on the filter/dryer receives good input to send power via the relay?
Or C.) not having the pressure switch on the filter dryer is the root of all my problems.

 

These are the lines that connect to the compressor. One goes to the condenser, the other goes to the filter dryer I believe. The small adapters on the lines... are those for a electrical connector or are those just servicing points? From what I understand... the only electrical sensor that is in all the lines is only the one attached to the filter/dryer.

 

The three wire pressure transducer goes on that pressure hose yes.
That is how the PCM knows the actual high pressure side psi .
The cycling SWITCH is on the drier.

You , as stated above, need the cycling switch on the drier as open circuit is off.

Pretty sure you also need the transducer
Edit:
The AC will not function and a hidden code 66 should set if transducer voltage is above or below spec .
Unplugged would be below spec voltage

 

You’re the man! Just ordered a cycling switch and the transducer. Appreciate your help. Hopefully this all goes together well.

 

My AC only comes on if I rev the engine, and then it cuts out at the worst possible times. My mechanic consultant suggested that the clutch clearance might need to be adjusted. It's so danged hard to get to the compressor on this engine that I didn't want to mess around until I had more info. Help! Yes, I have a 95 wagon also.

 

Fuel pump jumper wire
For testing
It goes to the fuel pump relay.
When the relay is OFF this wire is connected to the fuel pump.
When the relay is energized, this wire is disconnected and goes nowhere.

Switches and external controls,
Yes, 98% sure.
The PCM uses rpm, throttle angle, air temp, some other stuff but I think if any of those are defunct, you would already know.

The Impala does not have a separate evap temp sensor like F bodies , but the calibration just ignores that.

I only mention that because some tuners in the old days based their stuff on a Camaro calibration and AC didnt work till that was sorted.

 

So like I said before, everything was stripped on the car. In the engine bay, the two connection points for the evaporator were exposed for a long time(years). Could debris entering those 2 holes cause issues with my system? Such as.... after I just serviced the system, the clutch will engage for 2 seconds, then disengage for 2 seconds and this is happening repeatedly. I’m only able to service to roughly 15 PSI of 134 for it to not kick off and it’s blowing warm air the entire time. Optimal service psi is like 30-70 I think. Im not able to put enough Freon inside the system that I’m required to do so without the clutch kicking off. have yet to feel cool air. I have a video I can send you if you message me your email or phone number in a private message. Also keep in mind I have a brand new compressor, condenser, new clutch switch and pressure sensor.

 

I know just enough about AC to get myself into trouble.

In the inclosed link, there is reference to static pressure.

Understanding Normal AC Pressure Gauge Readings

Learn why normal AC pressure gauge readings are essential for diagnosing car AC problems. Don't rely on just the low-pressure gauge

ricksfreeautorepairadvice.com

ROUGHLY, it should be the same as room temp.
BUT , underline BUT , correct static pressure does not indicate correct, too much, too little amount of Freon.

That said, looking the other way, if the static pressure is wrong, then your charge is wrong.

The only way to be doing this is a manifold, reading high and low , putting in freon by weight.
Also correct amount of oil
Then looking at high vs low running pressures

 

The A/C clutch kicks out when the low pressure side gets too low (not enough R134a). As you add more and more, it will stay on longer.

Make sure the fan is on high and A/C is set to max/coldest temperature and the windows open.

Keep adding R134a and the low side will come up while it's running. Ignore the low side gauge when the compressor is not turning.

The low side gauge will start high 80F-100F or more. When the compressor kicks in, the low side will drop, as it gets below about 35F the clutch will kick out - this is the low pressure switch doing it's job preventing the evaporator from freezing up. As the low side pressure comes back up, it will kick in again - This is normal operation. You want to target 35F-40F on the low side while the compressor is engaged. You should have water dripping off the low side line back at the compressor (or as close as you can get to it) and the compressor should stay engaged 100% of the time with the engine at idle, fan on high, windows open, max A/C, etc.

 

Your A/C is not going to work without the low pressure switch that goes on the receiver dryer.

 

Ok the repeated cycle is ok. If You don’t want it to repeat cycle at idle. Service with Freon a little at a time slowly while at idle until the compressor stops cycling then stop the Freon flow.

 

I usually set it at about 2,000 rpm’s