Help! A/C techs!
 

1989 K3500 7.4L Silverado.
I replaced the compressor, clutch, orifice tube, dryer/accumulator and all o rings on my truck in preparation for a switch to R134. Took it to my local a/c shop for draw down and charge. After draw down he said that there is no voltage to the clutch. I jumpered the accumulator cycling switch wires and the clutch engaged. The switch showed open so I replaced it. No joy. He is adamant that I should have a high pressure switch on the compressor but the old one and new one donÂ’t have it. Nor can I find a plug that would connect to one. I have seen wiring diagrams the show 1 accumulator cycling switch and others that show the cycling switch and an a/c air inlet valve motor pressure switch. IÂ’m horse trading with him or IÂ’d try a second opinion, which is why I am here lol

Any help would be appreciated,
Thanks, Dan

Re: Help! A/C techs!
 

Is your AC clutch relay working?

You can download the 1989 OEM GM CK light trucks wiring diagrams for your 1989 T400 chassis CK truck from the service manual project link below.
HVAC Theory of operation is on Page 68
HVAC troubleshooting tree starts on page 69 with wiring diagrams following.

Re: Help! A/C techs!
 

This will work. I am going to use one soon too.
There are 2 different kits depending on the threads.

https://coldhose.com/collections/swi...35532366151840

Re: Help! A/C techs!
 

Some trucks have a second switch and some don't. It's a high side low pressure cutoff switch. This is the one on my 93. The one on the accumulator is the low side low pressure cutoff switch.

Re: Help! A/C techs!
 

You can get binary HPCO/LPCO switches to replace the LPCO switch on the accumulator that are direct plug and play replacements. I'd use one of those if you're installing a binary switch. The fact that your AC guy didn't immediately suggest this makes him more than slightly suspect IMHO.

Folks are frightened of AC work. It's not oogie boogie. If you have any mechanical aptitude and some patience you can do your own AC work on an already empty system.

Download the 1973 GM CK light truck service manual and read the AC section. It's a fairly extensive overview theory of operation service methods etc. 73 used a TXV system rather than an orifice valve but the broad strokes are the same.
The GM service training group AC textbook is scanned and downloadable amongst the 73-91 manuals as well.

You can usually rent a vacuum pump and R134a manifold gauges from The Borg Collective stores. O'Riley, AutoZone, etc. Refrigerant and oil is still reasonably available as long as you aren't trying to by R12.

If you're retrofitting the refrigerant there are several things that need to be on your list so your system hangs together.
The condenser and evaporator and hardlines need to have all the old R12 mineral oil and bugs and crud flushed out. R12 mineral oil is not compatible with R134a so it won't flow around the system with your refrigerand and it'll impede heat transfer in the condenser and evaporator giving you higher vent temps. Bugs and crud will block the orifice valve filter.

I usually dump the new compressor completely out into a graduated cylinder and re-fill with the same amount of fresh PAG or Ester oil. This is only the compressor part of the oil charge. Subtract the compressor oil charge you just poured in from the system oil charge found in the service manual. The remainder is the amount you load in your AC Charge Oil Injector tool and dump in the system after pulling a vacuum for at least 30 minutes & better yet an hour and making sure the vacuum holds for a full hour.

The charge weight you'll have when installing a non-factory refrigerant be it R134a or HFC152 or ... will not have any correlation to the original R12 refrigerant charge weight. There are several methods to charge the AC system with a non-original refrigerant. I learned from an old man that forgot more than I've ever learned about AC systems. This is a fair approximation of his method. https://67-72chevytrucks.com/vboard/...48&postcount=6

Re: Help! A/C techs!
 

Thanks all for the input.

Hatzie, since there is only one switch in my trucks system, wouldn’t that make it a binary switch? Is it possible that there isn’t enough of a refrigerant charge in the system to close the low pressure side of the switch? The switch is normally closed with the proper system charge but right now it is open. I ask because he said he put a partial charge in and the compressor didn’t cycle, tossed it back in my court said there was no voltage to the clutch. I have checked all the wiring for the system, no issues. I have power at the A/C Heat relay and controller in all the right places and the relay operates as it should. If I jumper the accumulator cycling switch connector (close the switch) the clutch engages.
I am not a great fan of this guy but we are trading firewood for his machine and services so I am kind of stuck with him. Our idiot Governor made it illegal to sell refrigerants of any kind over the counter at parts stores. You have to be a licensed A/C tech to purchase or service the system.
I back flushed all the lines, condenser and evaporator, serviced the compressor with fresh 134a compatible oil.
What do you think?
Dan

Re: Help! A/C techs!
 

The LPCO switch isn't what they call a binary switch. And Yes. Technically ON-OFF is binary LOL One of many ways the language gets abused.

If the clutch engages with the LPCO bypassed then the switch is suspect. It's setup for R12 pressures if it's the original part so should be replaced anyway.
Replace it with one of the combination HPCO/LPCO "Binary" switches.

Back in the late 1990's you could get an EPA 608 & 609 cert online.

Re: Help! A/C techs!
 

Thank you, I will order an HPCO/LPCO switch and get back with results.
Dan

Re: Help! A/C techs!
 

is the high pressure cut off and low pressure cut off all in 1 switch?
is it connected to the accumulator? if so why is it necessary to replace the low pressure switch?

high pressure cut off switch goes on the high pressure side of the a/c not the low side.

check wiring diagrams and if power goes from a/c panel to low pressure switch (if ok) it goes to compressor clutch. see if you have power going to low pressure switch.

clint

Re: Help! A/C techs!
 

So this is all that's needed then?

https://www.aircomponents.com/produc...tch-sw-10001c/

Re: Help! A/C techs!
 

Clint,
The original factory part on a truck originally equipped for R12 is just an LPCO set for R12 refrigerant low pressure cutout.

The aftermarket replacements can be purchased with both HPCO & LPCO all in one. They're setup for R134a safety pressures so you kill 2 birds with one stone.


That's the animal you're looking for assuming your LPCO has the same Packard 56 connection type. Somewhere around this year GM started using a Metripak 150 shrouded plug on the LPCO. You can buy either type you just need to verify what you have. It'll take care of both low and high pressure safety cutout without changing the wiring at all.

Re: Help! A/C techs!
 

the r12 and r134a LPCO might not be different at all, the LPCO is there for 2 reasons, turn the power off to the compressor if there is lack of refrigerant - that would be a low pressure, and switch opens. the 2nd is simply to keep evaporator from freezing. the pressure nearly corresponds to evaporator temperature...
there might be some differences in the guts of the switch, but if the evaporate is frozen due to a improper LPCO switch, your going to cause high side pressures to shoot up and blow out of the relief valve in the compressor....

its not 2 birds one stone with that switch. Its a switch that can be installed either on high or low side and opens or closes based on internal guts - open or close... if you had 384 psi, in this sensors case, in your low side, i'm sure your evaporator would pop and fail as its normal operating pressures are around 120ºf just stting out in the sun baking (ambient temp) and lows of about 28ºf

https://www.aircomponents.com/produc...witch-f-r134a/

specs for that sensor
Female R134a 1/4 FF binary high-low pressure switch
low opens at 28 PSIG closes at 29 PSIG
high opens at 384 PSIG closes at 298 PSIG

high side switch should defiantly be installed on the high side line to prevent the head pressure from getting too much and letting the relief valve in the compressor let the refrigerant out.

clint