1990 K1500 a/c repair

[Steven Petersen]

This all depends what year your truck is and does it have the "max" air button. 88 - 90 (I think) didn't have that button, they will open the recirc door after 30 seconds when the fan is on 2 or 3 and the temp is set to its coldest setting (with A/C on of course). The OE pressure switch on the back was used to close that door but, there wasn't a high pressure shut off switch in those years so, when hot, and shut off for a few minutes, you restart the engine you will get a high pressure spike. This blew out that HP switch on mine - sounded like a shot gun going off. I wound up taking a HP switch, in a new compressor (since the groove was damaged when the previous one blew out) and wired it in series with the LP switch so now, when pressures go over 430 it will shut off the compressor. I did a write up on this a year or 2 ago. Here's the schematic I drew up for that and a relay giving me full battery voltage to the clutch coil. What I added is labelled in blue.

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The problem I had when I put a new compressor is I couldn’t find the old single wire switch. All the replacements offered have a 2 wire connector. I tried to use the old switch from the old compressor, but the plastic disintegrated when I tried to remove it. What I ended up doing was grounding the single wire to the thermostat housing. The recirculate comes on after 30 seconds with the thermostat all the way to cold. To switch it to fresh air, just bump the temperature one notch towards heat.

 

[studigggs]

To switch it to fresh air, just bump the temperature one notch towards heat.

With this wiring mod, you can also direct some air to the floor or select "low" fan speed and the door will move to 'fresh'. This way you avoid introducing any heater core air at all

 

[Steven Petersen]

With this wiring mod, you can also direct some air to the floor or select "low" fan speed and the door will move to 'fresh'. This way you avoid introducing any heater core air at all

Thanks, I didn’t realize that. One notch towards floor would be better than 1 notch towards heat.

 

[PlayingWithTBI]

The problem I had when I put a new compressor is I couldn’t find the old single wire switch. All the replacements offered have a 2 wire connector. I tried to use the old switch from the old compressor, but the plastic disintegrated when I tried to remove it. What I ended up doing was grounding the single wire to the thermostat housing. The recirculate comes on after 30 seconds with the thermostat all the way to cold. To switch it to fresh air, just bump the temperature one notch towards heat.

Yes, the 1 wire switch is obsolete. I neglected to say I grounded that wire too. The main issue with these years is the lack of a high pressure shut off. If you don't have one you're playing with a time bomb, IMHO.

 

[Steven Petersen]

You got plenty of time to plan this out. It’s damned near impossible to charge a system when it’s cold. I had to wait until June last year to do mine.

 

[OLD BLUE 88]

Hello Fellow Old Chv Trk Guys,

I'm new to this forum. Did a complete rebuild of AC system all the way through w exception of original control headset in dash. Pulled it apart, cleaned contacts and tested bulbs. Made the ground off second wire coming off back of R4 compressor. (should have converted to scroll compressor, maybe next time). New actuators, air boxes, etc. All is working as it should for the first time in decades.

Can someone confirm the proper pressure readings coming off low and high sides? Had a tech charge the system and I'm not sure he got them right.

Also, getting the knock knock sound from the recirc end of the box, although it's working as it should (I think).

Thanks for sharing your knowledge, it helped immensly.

Cheers!

 

[Schurkey]

"Proper" pressures are dependent on temperature, humidity, and air flow through the condenser and evaporator. Too many variables to give a single answer. If the radiator fan clutch is out-of-spec, air flow won't be optimum, and the high-side pressure will be different (probably higher.) If the evaporator box is packed full of leaves or pine-cones, you'll have crappy air flow to the dash vents. Pressure may be too low on the low side.


Thousands of years ago, before anyone knew about Ozone Holes, folks charged the A/C by adding just a bit less than their "best guess" or "sticker rating" of system capacity, then added a little at a time until it cooled the best according to a thermometer stuck in a dash vent. Then they added 1/4 pound additional. This took a long time, because the system has to stabilize after each addition of a bit of refrigerant. Nobody does this any more because the labor cost would be astronomical. So now they look at the sticker that tells them how much the system capacity is, and they blow in that much, and drive the vehicle out the door.