Don Terrill’s Speed-Talk

I have a 69 chevy PU,I installed aftermarket AC. System blows cold for about 45mins.then slowly diminishes to warm.System is holding pressure,and when system is evacuated,it shows same amount as when charged.This is a underdash setup.Ive had 2 AC shops look at it and check,they come up with nothing.Any Ideas.Ther are no leaks in system.

How long do you have to wait until it blows cool again? Does it drip under the truck for a good long while you shut it off, after the first 45 minutes? Could be evaporator icing up, if they have it running a bit too cold. Stick a thermometer in the middle outlet vent and turn the fan on hi, then start it up and see how cold the air gets. Too much under 40F and that's my guess.

ijames wrote:How long do you have to wait until it blows cool again? Does it drip under the truck for a good long while you shut it off, after the first 45 minutes? Could be evaporator icing up, if they have it running a bit too cold. Stick a thermometer in the middle outlet vent and turn the fan on hi, then start it up and see how cold the air gets. Too much under 40F and that's my guess.

Thanks, I will check that,heres something else,Im not 100% sure but if I turn the temp down when its staring to warm up,it seems as it gets alittle cooler. I know very little about AC systems so please forgive my ignorance. The last time it happened I parked the truck overnite, started in AM and blew cold?

So plenty of time to thaw out anything frozen. Do the thermometer test, it's easiest.

I will do the temp test tonite,thanks. Ill report back.

I used an infrared temp gun,92outside temp,35degrees out of vent in cab.

If you are getting 35F air temp out of the vent, the evaporator has to be below 32F and it is condensing water out of the air and freezing into a block of ice that builds up and blocks the airflow, which is when things start to warm up. You need to take it to a shop that is familiar with that brand/model of aftermarket unit (or is competent enough to look at it and know what the parts are ). If the system uses an orifice tube then it needs a bit more refrigerant (if R134A the suction side is probably 26-28 psi now, bump it to 30-31 psi and check), if it uses some kind of thermostatic expansion valve it needs adjusting. There may also be a low side pressure switch that needs adjusting, too. You are not very far off now, shoot for 38-40F air coming out of the vent and you should have a 2-3F margin of safety.

Thank you,and thanks for the explanation,I understand now.

put some gauges on it.38 to 48 temp was the rule of thumb for years.48 is a bit high though.i do a lot of a/c.and modern cars will go to 40 easy.

What was the outcome?
Where are you located. Often if there's somebody close by, they'll lend a hand.

Sadly, we will never know.
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