Tbi ac

[tsr2185]

Should a properly charged 91 TBI ac compressor cycle on and off ever in above 75deg weather? Referring to below 20psi low side pressure? Can anyone give me the best reference chart for pressures of 134 in our converted systems?

I charged mine last year when temps were around 95 and now they are around 80 and the ac is cycling on and off. 25 ish psi low, 230 high side. Getting 55deg vent temps.

 

[Schurkey]

If that were R12, I'd say it was a little under-charged. I was taught that with R12, high-side pressure would be somewhere around 3x ambient temp. 80 degrees F x 3 = 240 psi on the high-side.

But I'm WAY behind the curve on R134a; which is too bad. By the time I figure-out 134a, it'll be as illegal as R12 and R22.

You're sure your air doors are working properly? Not getting blended air from the heater core, are you? Failing actuators, or air doors that just don't seal?

 

[tsr2185]

If that were R12, I'd say it was a little under-charged. I was taught that with R12, high-side pressure would be somewhere around 3x ambient temp. 80 degrees F x 3 = 240 psi on the high-side.

But I'm WAY behind the curve on R134a; which is too bad. By the time I figure-out 134a, it'll be as illegal as R12 and R22.

You're sure your air doors are working properly? Not getting blended air from the heater core, are you? Failing actuators, or air doors that just don't seal?

My blend and mode doors seem to be working fine. My rec door was working but motor crapped out so I unplugged it nowbits constantly open. But I would think outside humid air would make the ac pressures higher anyways...

Ac cools off cab no problem and I'm fine with 55deg vents temps. I'm just concerned about the constant cycling of the compressor will make ot crap our on my 4 hour work trips and leave me stranded. It cycles on and off about every 5-10 secs, at hwy speeds.

 

[Schurkey]

Research R134a pressures. Seems under-charged to me.

55 degree vent air would piss me off. I remember the days of GM A6 compressors, POA-STV valves, and ice crystals flying out of the vents. CCOT could never duplicate that, of course.

 

[L31MaxExpress]

Research R134a pressures. Seems under-charged to me.

55 degree vent air would piss me off. I remember the days of GM A6 compressors, POA-STV valves, and ice crystals flying out of the vents. CCOT could never duplicate that, of course.

55F air would not cut it here either. Even my old 83 G20 with dual air and the stock R12 system converted to R134a would pull down into the 38-40F range going down the highway.

I do concour with it sounds to be undercharged to me.

Even properly charged dual evaporator systems will cycle on the highway in 105F.

 

[Erik the Awful]

High side pressure on R-134a should be even higher. It's under-charged. Your A/C should cycle, even in 100 degree weather, but it shouldn't cycle often.

 

[PlayingWithTBI]

Even my old 83 G20 with dual air and the stock R12 system converted to R134a would pull down into the 38-40F range going down the highway.

I do concour with it sounds to be undercharged to me.

Agreed. Yesterday I was driving all over the valley in my 88. In ~90° ambient my R4 compressor and 134A was blowing ~46° air and, I know mine is a little low on refrigerant. I may be about 1/2 can or more low. At idle it was cycling about every minute or so.

 

[L31MaxExpress]

Agreed. Yesterday I was driving all over the valley in my 88. In ~90° ambient my R4 compressor and 134A was blowing ~46° air and, I know mine is a little low on refrigerant. I may be about 1/2 can or more low. At idle it was cycling about every minute or so.

Have you put a R134a specific cycling switch?

 

[PlayingWithTBI]

Have you put a R134a specific cycling switch?

Nah, it's still the R12 one but, I've had that for over 10 years since I converted to 134A. 4 years ago I redid my system with a parallel flow condenser, an orifice tube from a later model 134A truck, new compressor, and new receiver/accumulator, after a thorough flush, of course. I do have a small leak on the high side Schrader valve adapter where I had to put double O-rings. Every summer, when it gets nice and hot, I add a can and I'm good for another year

 

[L31MaxExpress]

Nah, it's still the R12 one but, I've had that for over 10 years since I converted to 134A. 4 years ago I redid my system with a parallel flow condenser, an orifice tube from a later model 134A truck, new compressor, and new receiver/accumulator, after a thorough flush, of course. I do have a small leak on the high side Schrader valve adapter where I had to put double O-rings. Every summer, when it gets nice and hot, I add a can and I'm good for another year

4 Seasons makes a switch for those early systems like yours that have the R12 service fitting, but it is calibrated for the R134a cut-off pressure. Will bring it down to where it is cycling at 19-20 psi rather than 28ish. Will get your discharge air closer to the 38-40*F range before cycling off. 46F would have me sweating here. With an adjustable switch I have my van down to about 34F when it cuts the compressor.