If charge was low, compressor would at least short cycle. I suspect one of the temp sensors that feed the ECM. BUT what one. Coolant temp is normal, driving Dr info temp readout is accurate. Shows no codes.

 

If charge is gone, low pressure switch keeps compressor from ever coming on. Best to get some gauges on it, find-out for sure.

 

There are two sensors that gate the power to the compressor. The low pressure/cycling switch on the accumulator and the high pressure switch on the back of the compressor. When ambient temps go up, pressures go up so it’s unlikely that 90F temps are causing a low pressure condition. You need to determine if power is being applied to the compressor clutch and if not then determine if either sensor or the actual pressures are the cause. Let us know and I can walk you through that.

There is an even better chance that high temps are causing electrical problems. As suggested, gauges might be required to diagnose eventually but you can start with power.

What year is is the Truck?

George

 

Thanks for the replies. 99 Bravada. And a big howdy, I used to be a regular on here and have a few posts , both asking and answering. And yes, I've seen just about all the trademark blazer quirks.
As I said, the a.c. blows cold until about 90 degrees ambient. After that the compressor never runs. Until I let vehicle cool down and it's below 90. Yes, the low side pressure is nominal for temp. This is highly repeatable. Not sometimes, every time.
I had thought maybe a bad TPS or wire to it because the ECM SHUTS compressor off on WOT. Nope. No other symptoms of bad tps. Or codes.
maybe I'll look at the high side cutout again.
BTW, I've done all my own AC work on house and cars for last 30 years. Doesn't mean I didn't miss something but I do know the basics.

 

Let us know what you find out.

 

The high and low side switches are not in the power path, they are inputs to VCM which then provides the ground for the compressor relay. In each case proper operation is a shorted switch so you can short them in turn to see if that’s your issue. If not then check for power to the compressor clutch from the relay. If not then swap relays, if not then ground the low side of the relay coil.

Report back and we can help you further.

George

 

Being that it's a Bravada, doesn't it have the auto temp AC system? I know that with those systems, they have an outside temp sensor up front on the radiator support that ties into the AC system for it to operate, maybe that's misreading and/or is faulty and is causing the issue? Could always just unplug it and see if anything changes? The vehicles without the auto temp AC controls do not have this sensor; the plug is there on the radiator support, there's just nothing plugged into it (between it and the plug for the sensor for the outside temp display on the roof console, lots of people mistake them for prewired fog light plugs...)

 

Thats a good point, manual or auto AC?

George

 

My 2001 had intermittent compressor operation when hot outside. Drove me bonkers. Turns out the low side switch would get a bunch of compressor oil in it and then go goofy. I've replaced the switch twice in 6 years and it has fixed the problem both times. Guess someone must have put too much oil in the system. This is a very cheap part - you might want to try a new one (you don't lose any freon). Each time I have replaced mine, I got a lot of oil out of the removed part.

 

Thanks for the replies, yes it is the automatic control. And yes, unlike earlier vintage, switcheS all feed the ECM and it controls compressor
Les, that's a good idea. I'll get one, replaced it about 8 years ago and it may be time. Was pretty sure I tested that already but I am an old fart.
BTW, the input impedance on ECM is relatively high compared to a relay and that's how oil pollution could screw it up.
SIDENOTE for posterity: Another cause of this which I have seen twice in last 5 years is excessive clutchplate clearance.. Test for that is to tap the flex plate with screwdriver handle. If clutch engages, adjust shimming. I checked that early on and that ain't it.