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70 F250 R-134a retrofit with freezing evaporator

fireguywtc on Tue July 03, 2012 3:26 PM User is offline

Year: 1970
Make: Ford
Model: F-250
Engine Size: 390
Refrigerant Type: R134a
Ambient Temp: 95
Pressure Low: 25
Pressure High: 275
Country of Origin: United States

Hello all!

I have been fighting with my retrofit in my 70 Ford F250 for two summers now (this is the second one). The pickup has the factory under dash A/C but most of the equipment needed replaced so I decided to convert to R-134a at that time. Everything in the system is new except the evaporator and expansion valve. It has a brand new York compressor and I replaced the factory condenser with a 37"x13" fin and tube type, much larger than the old one. Since the addition of a new condenser, I am not sure how much refrigerant it should have by weight. I did not charge it. I did all of the work until it was time to install all the new refrigerant lines and charge it, so I had a pro do it.

I live in Central Texas, so this spring I checked everything out before it got too hot and found my truck to be low on refrigerant. When I checked it, readings were about 20/175 at idle and would drop to below 10psi on the low side when I increased the RPM. Ambient air temp was about 95. My A/C isn’t leaking, that is what the service placed charged it to. The compressor is the kind that stays on all the time with a thermostatic switch, mine doesn’t seem to be working as it doesn’t cut off. After determining it was low, I added 12 ounces of refrigerant, a little at a time but ended up adding the whole can because the pressures topped out at 25/275 at idle. I varied the RPM’s up to 1500 and got 20/250 and at 2500 rpm 15/225. 2500 rpm is important because that is my typical cruising speed of 60 mph. When I am driving down the road, the evaporator seems to be freezing with lots of frost building up on the tubes of the evaporator. Cab does cool down after a while and vent temps average mid to upper 50’s with the fan on high. I forgot to mention the fan motor is brand new as well. Lastly, I have setup a redneck A/C recirculation in my truck by blocking off the fresh air intake with a piece of cardboard. Trust me the A/C unit itself is getting plenty of air flow though as there is a six inch gap now at the A/C box intake. I can provide pics of needed.
My question is what advice do you all offer me on what to do to rectify this. I am leaning towards needing to replace the TXV since it is the original one. I am afraid to add more refrigerant since I already have a pretty high “high side” reading, but maybe it is still low and needs more? If the expansion valve needs replaced, I need help finding a replacement because the original style doesn’t appear to be available any more. TXV part number: C8TZ-19849-A
Thank you all in advance, Bill


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1970 Ford F250 4x4
2007 Ford F350

mk378 on Tue July 03, 2012 4:24 PM User is offline

Is this a POA system? POA could be stuck open. Properly working, the POA won't let low side go below the high 20's, which prevents overcooling of the evaporator and freeze-up.

A TXV does not try to control the evaporator to a specific absolute temperature. It tries to keep it as cold as possible given the pressure in the low side.

fireguywtc on Tue July 03, 2012 6:29 PM User is offline

No, this is not a PAO system. From what I have been able to find out the only regulation the factory system had was a thermal switch that would cut off the compressor when the evaporator gets too cold. But I think that is not working. However, it woudn't help me much either way if I can't get my low side above 25psi at it's highest idle pressure. I would think that my system would operate more efficiently with the low side in the 30's for pressure.

I should have mentioned before that I am a shadetree mechanic and I have been learning A/C stuff fairly recently. I used fordification for my reference and here is a section that references the thermstatic switch: http://www.fordification.com/tech/images/acservice12.jpg

Thanks, Bill

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1970 Ford F250 4x4
2007 Ford F350

Dougflas on Wed July 04, 2012 6:22 AM User is offline

Where are you connecting the low side hose? If directly on the compressor, the reading can be slightly lower than 30psi at 1500rpm. I think that possibly your air over the condenser may be the problem. You stated that the high side was lower with increased rpm's, right? So typically, if the high side is high, the low side will be higher also. This would indicate that a low charge is present being over shadowed by a higher high side, hense a freeze up.

Mist the condenser with a water hose. If the high side drops like a rock, this is the area to check. If the high side drops, the low side should drop lower than you are at. Radiator cooling fns (clutches) do get weak. If you place a 20 inch box window fan directly at the grill, this may help your airflow for testing.

This model vehicle had a york/tehumseh compressor that was a cycling clutch system controlled by a switch sensing the evap.

fireguywtc on Thu July 05, 2012 2:57 PM User is offline

Sorry double post. Was replying from iPhone since my internet is down.

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1970 Ford F250 4x4
2007 Ford F350

Edited: Thu July 05, 2012 at 3:16 PM by fireguywtc

fireguywtc on Thu July 05, 2012 3:15 PM User is offline

Yes the readings are taken at the top of the compressor at the valves. The new 134 fittings are screwed into the valves.

Eaerlier today I did what you advised and retook Gauge readings. Ambient air temps were about 90 and ac fan on high. At idle high side was 225, low 25. When I increase rpm the high jumps a little to 250 initially then quickly returns down to 225 and stay around that pressure. Low side drops from 25 down to 15 as rpm's increase but goes no lower. Misting water and blowing a box fan on the condenser causes no significant change.

I should also mention that the motor has been rebuilt about 1500 miles ago. Has a new radiator and the engine fan is direct drive (no clutch) with an improved flex fan and a shroud I am getting air flow though I know it's not perfect parked. Should I add some more refrigerant to see if I can get my low side up? Thanks Bill

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1970 Ford F250 4x4
2007 Ford F350

Dougflas on Thu July 05, 2012 8:14 PM User is offline

you do not want the evap to freeze up. I would add more R134. 2 oz at a time. Give it 15 minutes each time to normalize. Id the TXV bulb mounted tightly to a clean evap pipe and insulated?

fireguywtc on Mon July 09, 2012 8:51 AM User is offline

Ambient Temp: 77
Pressure Low: 20
Pressure High: 225

This morning I finally got around to adding more refrigerant to see if that would remedy the problem. Long story short I ended up adding another 12 oz and pretty much only the highside got higher. When I started the process, the low side was about 20 at idle and 175 on the high side. After I added the 12 oz the high side I was up to 225 and low side stayed around 20 at idle. fyi, all of the readings were taken with the a/c fan on high and both windows open. Granted the abmient temp was 77 but the evaporator and the return line to the compressor will frost over at any rpm over idle (650 rpm). I am curious what the readings will be when the temps are in the 90s.

Quote
Originally posted by: Dougflas
you do not want the evap to freeze up. I would add more R134. 2 oz at a time. Give it 15 minutes each time to normalize. Id the TXV bulb mounted tightly to a clean evap pipe and insulated?Yes the TXV bulb is stuck directly into the evaporator, that is the way it was from the factory. I also insulated the TXV with that sticky tar tape.

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1970 Ford F250 4x4
2007 Ford F350

mk378 on Mon July 09, 2012 9:38 AM User is offline

Like Dougflas said, the non-POA systems have a temperature switch on the evaporator to stop the compressor before it gets cold enough to freeze-up. Apparently yours is not working.

Edited: Mon July 09, 2012 at 9:40 AM by mk378

fireguywtc on Thu August 09, 2012 5:53 PM User is offline

An update: I was given some advice to try tapping the TXV with a wrench or something similar to see if it was sticking. I did that and immediately my engine started running smoother and I can hear a difference in the system. I hooked up the gauges my readings changed to 32/350 at 100*ambient temp. It is no longer freezing while going down the road. I am guessing that I will need to replace the TXV here pretty soon.

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1970 Ford F250 4x4
2007 Ford F350

Dougflas on Fri August 10, 2012 12:47 PM User is offline

350 is too high. You may be overcharged as you earlier stated you added 12oz of refrigerant. I also noticed you stated that the TXV bulb is in the evaporator. It should not be. I think you may be mistaking the switch sense bulb which should be in the evap. The TXV bulb should be on the evap line.

fireguywtc on Fri August 10, 2012 3:42 PM User is offline

Quote
Originally posted by: Dougflas
I also noticed you stated that the TXV bulb is in the evaporator. It should not be. I think you may be mistaking the switch sense bulb which should be in the evap. The TXV bulb should be on the evap line.Yes, you are correct, thank you for pointing it out. I realized my mistake earlier after looking at it but never posted it. Its all plumbed correcty, my ignorance led to my mistake.
Its working okay at the moment but I will likely recover some the charge here pretty soon to get everything where it should be.

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1970 Ford F250 4x4
2007 Ford F350

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