A/C clutch not activating

[chessy56]

Not sure this is the proper place for this post, but....
2001 Silverado 2500-HD. LED on A/C pushbutton illuminates, but I have no 12v to the plug at the compressor clutch. The pressure sensor switch on the freon line (before the compressor) is shorted (I'll assume I have sufficient refrigerant pressure unless otherwise informed). I've checked the fuses- nothing blown from what I've seen. Is there a TBC involved? Any ideas on where to start?

Additional find: I found I can disconnect the harness from the accumulator switch, short it, and the clutch kicks in. Plug it back onto the accumulator switch and I get no clutch operation. Is this a bad switch or a pressure problem in the lines?

[DmaxMaverick]

Sounds like low freon. No surprise with a 12 Y/O truck. The pressure switches rarely fail, but freon leaks out over time on most of them. You can try an A/C service/repair kit (local parts store, Walmart, etc.). Get the one with a gage so you can see right away, before you try to put some in. The kit should have a hose, gage and a bottle of R134a (with or without dye or sealer, or "conditioner", your choice, but you shouldn't need oil unless you've leaked it out). If it's low, just add until the pressure comes up and the compressor stays on (gage needle in the ideal zone). Once the hose is hooked up, start the engine, select A/C with blower on full, add a little at a time and watch the pressure. Stop as soon as the compressor engages. Keep adding a little at a time until the compressor stays on (low side pressure at the accumulator drops as the compressor runs). It may come on and go off until the pressure is enough to satisfy the low side pressure while running. Once it continues to run, add until the gage stays in the ideal range. When it's cold, like now, don't try to get the gage up to the top of the range. Fill it only enough to keep it running. Do this again on a hot day, or you may have too much when that time comes. If this is the first time you've noticed it not working, and it worked before, 1 can will be more than enough freon.

If it doesn't need freon, then the switch is bad. The switch connects to a schrader valve (like a tire valve stem), so you won't loose any freon while replacing it.

[Robyn]

X2

If the low side switch is not allowing the compressor to run, then its usually because the refrigerant pressure is LOW.

In some cases at low temps the system may not activate, but that would be very low temps.

As maverick mentioned, check the low side pressure and if its below about 40 PSI with the sytem off, then there has been a leak and the pressure switch is protecting the system.

The low side switch does two things, it protects the compressor if the refrigerant level is lowand also cycles the compressor off and on during normal operation.

These systems are refered to as a "CCOT" system.
"Clutch cycling orifice tube"
Many older GM systems used a far more complex system with a whole host of other controls.

These systems had a thermostatic expansion valve, a suction throttling valve and in some cases a thermostatic switch iin the evaporator core that shut the compressor off in the event the core got too cold.

These systems were used in the older GM vehicles such as the Caddy's, large Olds, Buicks, Pontiacs and large Chevy passenger cars.

The CCOT system was introduced to cut down on the needed system components and of course cut costs.

The downfall of the CCOT system is the extra wear and tear on the compressor clutch as it cycles on and off all the time the AC is used.

There is also a high side switch that opens in the event of the high side pressure going beyond safety limits.

This can be in the plumbing from the compressor or on the compressor itself.

Some vehicles used a trinary switch that controlled the compressor on and off as well and protected the system from too high or low of a pressure and controlled an auxilliary fan in the grill or raised the engine idle speed to help cool things off.

Look over your system, especially hose connections and crimped areas (hose to metal tubes) and the clutch area. Look for signs of an oily residue in these areas.

Any time refrigerant is leaking there exists the real possibility that some oil will seep out too.

Adding a small can of tracer oil (has a zyglow additive stuff) that once it has leaked out, the use a of a Black light will show it really well.

The other option is a 134A sniffer unit that will squeal like a little Piggy when its sniff probe is passed over a leak.

These things are very sensative and will sniff out even the smallest leak.

The shaft seal/clutch area of the compressor the first place to look. If the clutch shows signs of oily residue "Bingo"
The areas where the hoses crimp into the aluminum piping are the other biggy.

Any threaded connector or quick connectors (have O rings) are always suspect.

Again, the oily residue is a dead giveaway.

As Maverick mentioned, 12 years and even the best system can need a little juice added.

My Old 1989 K5 gets a small shot in the Summer and its good for the season.

Its an R12 system, which is obsolete now due to enviro laws.
The 134 is not nearly as efficient as 12 was and is a poor choice for a retro on the older rigs.

The 134 though is readily available in handy little cans without the need for an HVAC license.

134 is some nasty stuff, not to mention that it like all refrigerants is very cold and will cause serious injuries if it gets on you (eyes are a biggy)

Be safe and wear glasses or a face shield when uncoupling any connections and doing a recharge

Keep us posted.

Missy

[chessy56]

Initially, the gauge read ~35 psi w/o the compressor running. I added a shot of R134a and got the compressor to continuously cycle on/off. When the compressor cycles on, the pressure drops to around 5 psi- at which time the cluch drops out. The pressure climbs to around 45-48 psi and the compressor kicks back in. In reading further on the web, it appears that this will continue cycling until enough freon is added to the system. Should the pressure on the low side be ~35 psi with the compressor running? Fully charged, what would I expect to see the low side pressure at with the compressor idle (assuming I have ~35 psi with it running)?

I forgot to mention that I sustained front end damage thanks to a deer running in front of me on the highway a couple of years ago (hardly see 'em in the woods wouldn't you know). Anyway, a whole bunch of stuff got replaced up front, condenser coil included. My guess is I've got a slow leak due to that. If this recharge doesn't work I was going to try a R134 with the sealer (some cans come with a conditioner, sealer and red dye all in one- found at NAPA).
Anyway, I'm more interested in the low pressure side's reading both with the compressor on and off. Comments?
Thanks folks.

[DmaxMaverick]

That's normal. It should be 40+ PSI while running on a cool day, and will be significantly more on a hot day. If it's still cycling when it shouldn't, it's still short on freon. Add until it stays on when it should. If it takes more pressure than is "safe" on the gage to keep it running, the compressor is probably shot, or a line is damaged/blocked.

[chessy56]

Thanks for the tips- all better now that I'm back to ~35 psi with the extra freon. I'm sure I'll be revisiting this, so next time I'll add a dye so I can find the leak. So far, all is "cool"....