Jooosshua

The ac on my truck works perfectly fine until you stop and sit still for a couple of minutes the compressor starts cycling on and off rapidly but when you start moving it works fine. It also cycles on and off when you crank it after it has warmed up. Any help or advice is appreciated

scope103

Will it stop cycling if you ever-so-slightly raise the rpm? There may be a control labeled "rpm" on the A/C "amplifier" located on top of the Cooling Unit. I'm not 100% clear on its use (it is not well documented). But I think it adjusts a minimum engine rpm, below which the A/C won't run. You could try that.

Jooosshua

I will try to adjust that and see if I get any change

yota tay

I'm pretty sure thats normal operation. How do you expect the condenser to dissipate heat without airflow? Far as I can tell there's only the clutch fan for pulling air across the coils at a stop, and that isn't big or strong enough in stock form for cooling duties at a stop. Add a condenser fan motor and you will be happy.

Jooosshua

I have a fan in front of the condenser for that purpose but I had unplugged it because it was cutting the compressor on and off sitting still, after I unplugged it it would run normally, I believe it has something to do with my idle

Paul22RE

Does anybody have a link on how to correctly adjust this amplifier? I turned mine accidentally without making note of where it was set at.

Edit: played with the amplifier, basically it changes the idle when AC is on. Adjuster can increase/decrease about 250rpms. I put mine at lowest setting, idle now sits at 1000rpm when AC is on. Fully counter clockwise puts idle at about 1250rpm. Have to recycle AC switch to see result of adjustment, just turning knob while AC is on does nothing until main switch is recycled.

Jooosshua

Tried adjusting the amplifier and it’s at its lowest setting but when the vehicle stops and idles the compressor cycles on and off rapidly still. I have no other ideas

Paul22RE

Low pressure switch shuts off compressor. Your pressure is good?

old87yota

What type of refrigerant is in the system?

Has the refrigerant ever been changed to a different type than what was originally in the Truck?

Is the A/C system properly charged?

Jooosshua

The system has been converted to 134a was r12 originally, but it is charged correctly and the pressures are reading right. I vacuumed the whole system and it held vacuum.

Jooosshua

Here is a video of the idling

Jooosshua

I can’t seem to get the video to upload, I can email or send the video however if anyone wants to see how it idles

Paul22RE

Here is the FSM for 89-95s. Since it sounds like the compressor is losing it's voltage signal I would check the operation of the thermisitor, pressure switch, amplifier & relay. All you need is a multimeter. FSM requires a manifold gauge set to test the pressure switch, but it can be easily bypassed to see if that is the issue. Bridge the 2 contacts in the plug for continuity. If compressor runs fine with pressure switch out of the equation, you have low pressure or a bad switch. I have a few pics from the '87 FSM I can post to test these components:

https://ether3al.com/93fsm-mobile/airconditioning.html

Jooosshua

Do I need to unplug the switch or just probe into the connector? Those seem like some high pressures.

Paul22RE

Unplug the electrical connector to the switch, bridge the two contacts in the plug (on the wire side, not the switch side). Turn AC on and see if compressor stays on. What year is your truck? 22RE I assume? Pre-89 it is called a low pressure switch, 89-on it is called a dual pressure switch. Or better yet if you can probe it while connected, you could see if it's cycling on and off with the AC running. Any chance your pressure might be too high?

Jooosshua

Thats what it is. Put gauges on it and the pressure is getting to high, but I know it’s charged the right amount I put in 1.4 pounds with an ac machine. Does that sound like the expansion valve or the compressor? I doubt it’s the compressor because it does blow cold when you are driving but when it’s at an idle the pressure gets to high

Paul22RE

High pressure could be a few things:

Overcharged (too much refrigerant)
Too much oil in the system
Wrong oil in the system
Debris in the system
Clogged dryer
Crimped line
Bad expansion valve
Air contamination in system
Water contamination in system
Poor airflow over condenser
Dirty condenser coils

Jooosshua

Do you have any idea on how to get the evaporator box apart without taking the whole dash apart? or do you have a link to somewhere I can look that might can explain how to do it?

Paul22RE

FSM is here, click on "Cooling Unit" in this link: https://ether3al.com/93fsm-mobile/airconditioning.html

When I changed the heater core in my 92 I also wanted to clean up the evaporator without disconnecting the lines. The box seemed tricky to get off so just cleaned each side of the evaporator by removing the blower motor assembly on one side and had heater core out on other side. I did not see an easy way to get that box off of a connected evaporator. Why do you want to take the box apart?

Maybe someone else here can chime in on how/if they got the box apart without disconnecting the lines?

old87yota

I have seen here in the past when these R12 systems get charged with R134A, you can get similar symptoms as you are getting.

It may be possible that there just isn't enough air flow going through the condenser at an idle to make the A/C system happy with R134A, but that is just a guess.

I don't have A/C in my truck, so I am just passing along what I have seen on this forum.