Air Conditioning Clutch Voltage is 6.2 Volts.

I am reinstalling my air conditioning system in my 1983 308 QV. I removed it last timing belt change because the clutch had gotten so hot it melted the air conditioning v-belt.

Despite extensive inspecting and troubleshooting of the mechanical system, I have never been able to determine the cause of the overheated clutch.

Today though, when I tried to activate the newly installed system to load it with refrigerant, I discovered that the clutch was not closing at all. With the engine off, I unplugged the positive trigger wire at the clutch and confirmed the clutch closes just fine with an external 12-volt battery. I also confirmed the ground wire is fine. I checked the fuse. I then started the engine, turned on the a/c system and measured the voltage going to the car's positive clutch wire. I was very surprised to get only 6.2 volts. I assume the clutch normally gets 12 v and the weak voltage maybe why the clutch was slipping and overheating.

I am kind of baffled as to how I am only getting half voltage to that trigger wire. I need suggestions as to how to troubleshoot such a large surprise voltage drop.

 

I am getting full system voltage at fuse #5 (Heater Fan Motors - Air Conditioning System). There does not appear to be a relay for A/C clutch trigger. Relay D (Relay for radiator right fan with operating air conditioning) is the only thing mentioning a/c. I swapped Relay D with Relay E (External Lights) which had no effect on the trigger wire voltage.

 

.... it always pays to ask for help. I think I found the voltage drop.

I pulled out the wiring diagram (and a large magnifying glass). The clutch trigger wire DOES run through a relay, but it is Relay C (Relay for radiator right fan).

On the wiring diagram, I traced the wiring back from the clutch (through a connector) to the next inline components which were the pressure switches at the front of the car. Indeed, the voltage drop is across the pressure switch that screws into part # 106646 (Union for Pressure Switch).

Now, I must admit my lack of knowledge ... is this the low pressure switch or high pressure switch and what is it's part number?

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So, ... it's a low pressure switch and it appears to be more-or-less working.

I pressurized the a/c system with nitrogen to 50 psi and suddenly I was getting battery voltage to the clutch trigger wire. I bled the pressure down and the switch cut the voltage to about 4.6 volts (with the engine off) at around 25 psi. To charge the system, I need to jumper the switch until system pressure rises above 25 psi.

The question now is why doesn't the voltage drop to zero when the low pressure switch opens up? And, I guess, is the switch performing satisfactorily since, presumably, the system never goes near 25 psi when the system is charged with refrigerant? Now is time to change it, before I pressurize the system with refrigerant.

 

The switch shows a 17M ohm resistance below the threshold pressure.

I am going to buy a new pressure switch today.

If anyone knows of an appropriate brand/model, I'd appreciate getting a recommendation.

 

Excellent. Thank you. It looks like the 328 uses a female thread while my 308 needs a male thread.

It turns out these a/c switches are extremely generic and readily available. I purchased one on Amazon for $9.42. Mine has a low pressure switch at 2 kg/cm2 (28 psi) and high pressure at 32 kg/cm2 (455 psi). The higher high pressure cut-off is a little concerning but seems to be the standard.

Some cross-reference model numbers are:
‎23203; 954086; SP4086; 5811374; 50008; MT1481; SW569.
I believe you can search on any one of those and find it.

I will also post this cross-reference in the cross-reference thread.

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The new switch does indeed read open circuit at 0 psi.

As you know, I burned out my clutch and could not find any smoking gun (just a smoking v-belt). I can't help but wonder now if the bad switch was only sending 6 volts to the clutch, lightly clamping it and causing it to slip and burn.

Tomorrow, I put it all back together again.

The a/c system on these cars is miserable in so many ways.

... you know, I take that back: only the stupid compressor installation is miserable. The rest is actually nice to work on. Everything (else) is readily accessible. Now my 1987 BMW: that's miserable. Everything is buried deep in the dash.