Hello, I've been lurking here a while, trying to learn and solve my problem, but still not quite clear on the next step. Thought I'd ask you guys with more experience before doing something stupid, and possibly expensive.

Truck is '98 3500 4x4 with V10, and 208k miles. AC stopped working while I was overseas and my buddy had the truck.. Had been working well since I bought it 3 years before.

With temp control set to max cold, and max fan speed, the compressor "short cycles" - I think that's what it is called.. Compressor clutch will engage for a second, then disengage for 3-4 seconds, then engage, repeating that.. Does not make cold air.

I measured system pressure with engine running. When compressor is engaged, low side is 20psi and high side is 60psi. When compressor disengaged, low side is 60psi and high side is 60psi. Uhh.. According to low pressure gauge, I have enough freon.. Adding any will put measurements into the "danger zone." But compressor does not seem to be making high pressure..? This truck does not have a condenser fan.. Not sure if it should, or not.. Clutch is for sure engaging.

There is an electric sensor on the tube that comes out of the compressor on the high pressure side. The insulation on those wires was badly eroded, they were basically bare wires.. They were probably touching each other and/or the engine block at some point.. I spliced in new wire, covered solder joints with shrink tubing, and put wires inside some plastic protective wire loom..



Knowing that the wiring was bad, could the sensor also be bad as a result? Any suggestions to confirm this or check something else would be most appreciated. I've got a little experience with AC, but so far the solution is not evident from just reading forum posts. (Thanks to all those who comment & help!)

 

I think you're just low on freon and that's why the comp. is short cycling. When comp. is running, low pressure should be 45-50 depending on ambient air temp.

 

^ what he said.

 

Your low on Freon not high on Freon lol..

When the compressor cycles with the engine idling it means its low on Freon.. when at idle the compressor should literally be running all the time and the pressure should stay at a constant.. if its going up and down like that its low.. 20psi is low.. Fill it up to 40-50psi and watch as your compressor stops cycling and pressure holds.. See your thinking backwords you think its normal for the clutch to engage and disengage like that.. It isn't.. It is normal for it to disengage when your rpms go up to like 1500-2000.. it does that purposely.. but as I said at idle it should turn on and stay on if its full of Freon.. if its low itll cycle like yours is doing.

Since you are basically empty on Freon then your system will take a whole 12oz can of those small cans to fill it back up completely.. When you fill it up, turn the can upside down and hold the trigger to fill it up then twist the can upright for a second and then twist it back down and then up and down.. Don't hold the can down and leave it there.. Your suppose to hold it down then up and repeat that's the proper way to fill it

 

Hey Arde, dhvaughan, & candymancan - thanks for the help! I think I got it solved.

What made me think it was overcharged:


Pic was taken with yellow hose valve open and r134a flowing into system. The low side gauge shows the "danger" pressure level around 60psi.. It actually spiked higher pressure than that as I filled it, all the way up to 80psi on the low side.. When I saw that, I immediately quit - thinking I had overcharged it. I guess I did not wait long enough for system pressure to stabilize.

Now at idle, the compressor is staying engaged, and pressure reads steady 40psi on low side, it is not swinging so wildly.. And it reads 200psi on high side. Road test! :-)

Hopefully using it over the next few days will tell if I have a serious leak..

For other AC newbies reading this - add in the refrigerant like was explained in this thread, and let the system run 2-3 minutes before jumping to conclusions about the pressure measurements. Don't panic like I did.

Thanks again, guys!

 

Download the service manual from the FAQ/DIY section. It should have a chart in it for pressures at various ambient air temps.