AC cycles constantly after 2 mins

[Milktruck]

Started working on this last year and gave up as weather got cooler. My AC compressor (clutch) switches on and off and drags after a couple of minutes of use. Blows cold til then.

New compressor (York)
New clutch
New pressure switch
New hoses
New dryer
New expansion valve
R134



Thoughts?

[REDONE]

Sounds like a low charge, needs more refrigerant.

Basically, there's not enough refrigerant to build up a supply in liquid form in the condenser. It tries for a start-up period, but after that, if it doesn't see the pressure difference it want's to, it cycles looking for a change.

[440sixpack]

Or too much charge. that will do it as well.

[mx71]

I second low charge. I don't know if there's a high pressure cutoff, but there would definitely be a low pressure cutoff. Check for leaks. The pressure switch is cutting the power to protect the compressor.

[Milktruck]

Thanks for the replies. I’m gonna go look for my notes on how much we put on pressure-wise. As I remember there is a conversion for the R12 to R134. Anyone have a R134 number that they’ve used that is standard?

[Milktruck]

I’m seeing 2 1/4 lbs R12 recommended for the system (Using the 1979 Field Manual since it is a York and not Sanden compressor). If I’m using a standard 80% conversion to R134 that would make my target: 1.8 lbs R134. Does that sound about right?

[Milktruck]

Went out and found the tech’s scribbled note and he put 1.68 lbs in. That may explain it. I guess its relatively cheap to bump up. Still will wait to hear others feelings about this.

[440sixpack]

If the conversion was done half way correct you will have a binary switch. that will turn it off in an overpressure situation.

You can't go by how much was put in you have to use gauges. any competent A/C mechanic can test it and get it right.

[Milktruck]

There is a low pressure cut off switch, as in the stock install. I am assuming that is NOT what you’re speaking of when you mention a binary switch? Where would a binary switch be added to the system?

[sonoraed]

Back in the day when we were doing a lot of R12 to 134A conversations GM recommended minus 10% minus 2 oz, so if 2 1/4 pounds = 40 oz of R12- 134A charge would be 34 oz, binary switch is a low pressure/high pressure cut off switch usually on the drier, power comes from t-stat on evap thru sw and to comp clutch, you need a set of guages to see whats going on if overcharged high side will hit 300 lbs depending what the spec's are for the binary and sw will kill power to clutch, if low on refrigerant low side will be well low (less than 20 lbs ???) also if the engine fan is not pulling enough air thru the condenser at idle or low speeds will cause the high side to go nuts, but need pictures or details to be sure, Cheers

[Milktruck]

Thank you sonoraed.

Quick question about how you arrived at the 40oz number you’re using for R12. Is that to compensate for losses in the charging equipment? I was using 16oz/lb which would give a starting point of 36oz for R12. Then using your conversion would yield (36*.9)-2 = 30.4oz. Then to get pounds: 30.4/16 = 1.9 pounds. Just wondering if I’m starting with the wrong assumptions regarding weight. I also read this and wondered about adding oz to compensate for equipment losses:
https://www.factoryairtemp.com/Upload/F ... 20CANS.PDF

Regarding the binary switch, the stock drier doesn’t have a port for connection. I see the value of having one, but wonder if it is necessary since I’d most likely need to change drier. Or would it be possible to put in place of low pressure switch if it was dual purpose?

Thanks for the comments!!

[440sixpack]

Normally they're on the drier but it could be on a fitting somewhere. yes the low pressure switch can be replaced with a binary switch. are you sure your switch isn't a binary switch anyway ?

Don't worry about how much refrigerant you have. in a morgified conversion you're just making a guess to what it would need. get it checked and get it right.