Automotive Air Conditioning Information Forum

This is my fiancé's Dad's truck that he bought used quite a few years ago. It has about 230,000 miles on it with what he believes to be the original AC components. It has the 6.0 diesel in it. It has been having intermittent AC issues where it would cool great for a while and then not.

He took it to a shop because he was going to have them replace the compressor because of the mileage and they told him the clutch gap was too wide and was causing the issue. They adjusted the clutch and replaced some shims or something and recharged the system. He said it still wasn't cooling correctly and that it was cooling better at idle compared to going down the road.

He brought it to the house so I could measure the vent temps and at idle with the blower on high, the vent temps were about 62 degrees on a hot and humid 92 degree day. I lowered the blower to low and the temps dropped to about 57 degrees. I also revved the engine up to 2,000 and the first time I did it, the vent temps went up to about 68-70 degrees at the center vent. One of the other times I revved it up that high, the temps went down to about 57 degrees.

This truck has a cycling switch with an orifice tube. I hooked up my scanner to it to look at the clutch circuit and cycling switch circuit while it was idling and at the higher rpms. At idle, both of these circuits stayed on. As soon as I revved it to 2K rpms or above, the cycling switch began cycling off at a rapid rate. I was graphing these PIDS and it looked like a square wave signal on both based on how often it was going on and off. As far as he knows, the cycling switch is also original.

My question is this, is this a normal cycling pattern at higher rpms for the system on this truck. It seems like it shouldn't be cycling that quickly or that much but I don't know how this system is supposed to work. Why would the vent temps sometimes go up at higher rpms with the cycling going on and off like I described and sometimes the vent temps would go down while cycling the same way?

This is an automatic heat/air system and the control module has been replaced with a new Motorcraft part not that long ago.

What else should we be looking at in the system? Thanks for any ideas or information.

Sounds like it's just low on charge. Best idea is to take it to a pro and have them recharge to the weight shown on the sticker.

A few possible scenarios come to mind, but it sounds to me like a low-charge issue as well.

Do you happen to know how much refrigerant the shop put in it? The specifications that I have for this truck show 45 ounces (2.8 pounds) of R-134a unless the under-hood decal says otherwise. That's a lot of refrigerant, so I wonder whether the shop might have undercharged it.

Thanks for the replies. No, I don't know how much they put in. Her dad just said they recharged it and assumed it was the correct amount. I will have to ask him if he can get more info. I think he plans on calling tomorrow to tell the shop that he doesn't think it working correctly.

70monte wrote: ↑Fri Jul 15, 2022 12:02 am Thanks for the replies. No, I don't know how much they put in. Her dad just said they recharged it and assumed it was the correct amount. I will have to ask him if he can get more info. I think he plans on calling tomorrow to tell the shop that he doesn't think it working correctly.

While he's discussing this, see whether the shop will provide low-side and high-side pressures at about 1,800 RPM engine speed and the ambient temperature at which they measured the pressures.

He is going to bring it over and I'm going to put my gauges on it and see what the pressures are at.

Ok, here are the results. Ambiant temperature today is 95 degrees and humid. Static pressure when I hooked the gauges up before starting the truck was around 110 on both gauges.

With the truck at idle with the blower on high and max AC, low side pressures were 47-50. High side was around 200-205.

With truck at about 1,800 rpms, low side went down to about 22 and high side from 205-225.

When bringing the rpms up for the cycling switch to kick in and shut the clutch off, low side went to 55 and high side came down to about 150. About 2,000 rpms and above is where the clutch starts cycling on and off consistently.

I didn't see any signs of leaks or oily residue on the lines or the face of the condenser. I sprayed both charging ports with soapy water and there were no leaks there.

I found out from her dad that the shop just added two cans of refrigerant to the system so they didn't start from scratch. I couldn't find any stickers under the hood or otherwise saying what the AC capacity is supposed to be for this truck.

Never add ANY sealant or refrigerant that contains sealant. UV dye is fine, and recommended, as likely a leak.

Cusser wrote: ↑Fri Jul 15, 2022 11:57 am Never add ANY sealant or refrigerant that contains sealant. UV dye is fine, and recommended, as likely a leak.

I know not to do that but I don't know what the shop did. They said that they didn't see any signs of leaking anywhere. The owner has had this truck for quite a few years and has never had anything done to the AC system and that includes recharging it before now.

He will call the shop back Monday and discuss the situation. They closed early today so he can't give them the pressure readings we got.

70monte wrote: ↑Fri Jul 15, 2022 12:02 pm I know not to do that but I don't know what the shop did. They said that they didn't see any signs of leaking anywhere. The owner has had this truck for quite a few years and has never had anything done to the AC system and that includes recharging it before now.

He will call the shop back Monday and discuss the situation. They closed early today so he can't give them the pressure readings we got.

Well, 17 years and no AC service - that's very good. Yes, it's likely the issue is just low refrigerant due to small engineered-in seepage over those 17 years, to lubricate the compressor seal.