1978 F150 output temperature variation

[57chevyeddie]

Good evening. My name is Eddie. I’m working on my 1978 F150 AC. I installed a new Classic Auto Air unit in my truck about a year ago. The temperature at my center vent varies 15 to 20 degrees as I’m driving on these hot days. My temperature goes down to 37 and up to 57 over and over. I just fixed a leaking condenser line and recharged it a week ago. I vacuumed the unit down and put 24 ounces of R134 into it. The guages read 35-40 low side and 275 high side with ambient temperature around 90 degrees. The system cools good but climbs to 55-60 degrees while the compressor is off. I’ve checked my water valve. Bypassed the heater hoses. I lifted the thermostat tube about an inch from inside the case. Monitored my engine water temperature. I’ve contacted the tech support at CAA for advice.

This is a OEM truck with no bell or whistles. The only thing that’s not stock is the CAA air conditioner unit. It has performed this way since day one of use.
I will gladly consider any ideas that would clear this up for me. Thanks in advance for the advice.

[Cusser]

Disclaimer: I'm not an AC professional.

If my truck, I'd test the fan clutch first.

Second, when the cooling stops, I'd pull over immediately, leave engine and AC switched on, and visually check if the AC compressor drive plate (the center disc) is engaged, turning at the same speed as the outer AC clutch pulley and belt. You might need Mr. Flashlight to see this. If the center drive plate isn't turning at that speed, USE CAUTION (beware belts and radiator fan) and tap on the drive plate with a broom handle, see if the AC clutch suddenly engages. An alternative to this would be to have a 12 volt test light or multimeter immediately available and see if the AC clutch wire is getting positive voltage while the drive plate is not spinning properly.

I experienced the second one almost a decade ago on my 1998 Frontier, the AC clutch "gap" had increased/worn over the years. The fix was to remove one AC clutch shim to decrease the gap to within specifications, no issue since. Took me 15 minutes and I didn't even need to remove the AC drive belt. See post #73 on this link to read and see what fixed mine. https://www.clubfrontier.org/threads/19 ... lQ#replies

[JohnHere]

In addition to what Cusser suggested, does CAA specify exactly 24 ounces of refrigerant by weight? And how much oil, in fluid ounces?

The pressures seem elevated. The low side should be ~30 PSI, and the high side should be ~225 PSI at 90°F with good condensing.

Is the fan shroud in place, and do you have an operational fan clutch? Any auxiliary electric fans?

After verifying those amounts, the fan arrangement, and the clutch engagement, I would wait for CAA to respond to your inquiry.

[Cusser]

The guages read 35-40 low side and 275 high side with ambient temperature around 90 degrees.

Were those pressures measured at about 2000 rpm? Pressures measured at idle speed are not as useful.

[57chevyeddie]

I have been working on factory connections that started leaking Freon. I’ve replaced the faulty lines for the forth time. I’ve vacuumed and recharged the system with 24 oz. Of 134a. The pressures are 23 on the low side and 175 on the high side. The ambient temperature is around 75 degrees. My output temperature at the center vent while driving averages from 37 to 45 over and over. The outside temperature is around 70 degrees. When I drive later in the day the outside temperature is above 90 degrees. The compressor runs until the temperature is 31 at the center vent when the compressor finally cycles off and the temperature climbs quickly to around 60 degrees. The compressor cycles on and slowly drops the temperature to the low 30’s which seems to freeze my evaporator. It goes from working great early in the day to freezing up when the temperatures outside have climbed to the 90’s. My fan has a shroud and the water temperature is staying in the normal range of under 150-175 driving and up to 200 at a fast idle. Any ideas on what would cause this Classic Auto Air to perform this way. I have checked everything my tech support from CAA has recommended but still have trouble in hot weather. Thanks in advance for any suggestions. Here’s my email address if you can help me. Eddie.

[JohnHere]

First of all, I presume that your truck did not have factory-installed A/C. If that presumption is correct, then it seems that you installed one of two Classic Auto Air kits:

1. 1973-79 Ford Truck Non Factory Air - Model Specific Fit Elite Series, SKU 20-2095
2. 1973-79 Ford Truck Non Factory Air - Smart Series, SKU 20-3095

Which one?

I'll then read the appropriate Installation Manual to see what if anything jumps out at me.