Hi,

I just converted the AC system on my 1993 F350 7.5L from R12 to 134a.
I replaced the high and low pressure hoses and the drier and all new O rings.
I drained the compressor, but no old oil came out. I put in about 6 oz of oil. I've put in about 30 oz of 134a and it's still blowing hot. The compressor cycles on and off.

I pulled vacuum on it and it held prior to filling.

The compressor makes a grinding sound when it comes on: Should I add more oil?

High side shows 150 psi when compressor not running and 175 psi when running.
Low side shows 45 psi when compressor not running and 25 when on.


Also, I don't know if the AC was working before the conversion.

Any help is greatly appreciated.

Thanks.

 

What was the outside ambient temp when it was running?

Looking at the pressures you were running you are still a little low on refrigerant.

Did you flush out the condenser or evaporator?

If it were me I would have also replaced the orifice tube.

Matt

 

I don't what the orifice tube is, but I'll replace it if necessary. It was about 80 degrees outside. I did not flush any part. I could not find a sticker on the truck telling me how much refrigerant is needed. This info would be helpful.
I just checked the pressures with truck off. Both low and high are showing about 125.
With pressure switch jumped so compressor stays on, low side is about 20 and high side is about 125.
I checked high and low side on my 2002 F150 for comparison. With AC on the low side was about 40 and high was 225. This is about a 100 PSI more than the F350, making me think the compressor is bad.

I also added another 2 oz of oil for a total of 8 oz, the compressor is still making noise.

I'm thinking now the compressor is bad. It has been replaced in the past. It has a Ford remanufacture sticker on it.

Thanks

 

You are low on refrigerant, add more refrigerant until the low side is 35-40 psi. You had to jumper the low pressure switch because it is low on refrigerant. If the compressor were bad you would not see much difference in the high and low pressure.

The orifice tube is the device that converts the refrigerant from a high pressure liquid to a low pressure gas. It has a fine mesh screen to prevent the orifice from getting clogged with dirt.

Add more refrigerant

matt

 

Ok, I'll try more refrigerant. But seems about 30 oz would be enough to at least start to see some cold air?

Thanks

 

A/C systems 101

The compressor takes a low pressure, high temperature gas and turns it into a high pressure, high temp gas. That gas then goes through the condenser where it gives up heat and condenses back into a liquid. The high pressure liquid then goes through the orifice tube variable orifice valve (VOV) or TXV (thermal expansion valve) and is converted into a low pressure liquid. Think jets on a carburetor. The low pressure liquid then goes though the evaporator where it absorbs heat and evaporates into a gas. The low pressure gas then goes back to the compressor and starts over. Now there can also be things like receivers, dryers, accumulators, etc in the system but I've left them out to simplify the system.

Now... This is the cool sciency part. Refrigerants have unique properties in that once they evaporate fully they can continue to absorb heat. This is called super heat. The opposite is super cool where the refrigerant condenses back to a liquid and continues to give up heat. Now, since a liquid can't be compressed you want to make sure that the refrigerant stays a gas till it goes through the compressor. You do that by super heat. So the system is engineered to turn to a gas and continue to accept heat and raise the temperature say another 10°. That's super heat +10. On the flip side we don't want the liquid to evaporate till it's in the evaporator or we're loosing efficiency. So we super cool. This is where the proper amount of refrigerant is needed to develop the pressures to make all the magic work. If you're close it'll kind of work but if you're far off it won't do squat...

Each gas has different temperature/pressure points but R12 and R134a are close enough that the switch works out pretty well. Noisy compressors are typically a sign of a low charge, although they can simply be bad. The clutch cycle switch on the accumulator can be adjusted for 134 to make the system perform better. Basically you want the evaporator just above freezing, about 34°.

Here's a chart that shows you the relationship for different temperatures so you can help tune it in.

 

Should sticky this one, well done

 

I have been doing this a long time, I don't know how accurate the guestimations are on the chart posted, as far as PTR charts go that is not very accurate.
If you have R134a running 300 psi your system is not going to work very well and you need to clean your condenser or check to see how much air is actually being moved by your fan. With a 110 degree ambient temp your high side should be about 215 psi.
The low side temp has more to do with the temp of the air being cooled going into the evap and not the outside(figuring most people run their system on recirc-MAX A/C)

Matt

 

So this is where I'm at now.

I have about 35 + oz of refrigerant and 8 oz of oil.

With the compressor off I'm getting about 80 PSI on both sides.
With compressor on (jumped) I'm getting about 5-9 PSI low and still about 80-85 PSI high side.

Looks like there is no real difference on the high side when compressor running, but big difference on low side.......What's next?

Thank you all.

 

I would lean towards the orifice tube screen is plugged. With the suction pressure running so low the compressor is trying to pump, but there is nothing returning to the compressor for it to pump so it is trying to pull into a vacuum. The discharge is low because there is no refrigerant being pumped. I would recommend replacing the orifice tube, and while you have the system open again run a can of flush through the condenser and the evaporator. Pull a good vacuum for a couple of hours, then recharge

Matt

 

Sounds good. Should I drain the compressor again and refill with oil, or should I just add more oil. Also, how can I check to see if the compressor is good. I'd hate to waste even more money of refrigerant.
Where is the orifice tube on these?
thanks.

 

Since you'll open the system again, drain the compressor. The orifice tube is located inside the lower connector pipe coming from the evaporator (just below the dryer).

 

with the pressures that you have given us the compressor is doing its job and working.

It won't hurt to drain the oil, and put in another fresh oil charge

Matt

 

I'll see if I can get an orifice tube locally. If I can, I'll post back to update progress.

I really appreciate your help.

Thanks

 

Here is some info on the tubes, color, orifice size tolerance and Four Seasons part # from my notes on converting my 86 to R134.

Blue - .0655-.0685 - FS Part # 38621
Red - .0605-.0655 - FS Part # 38635 (standard cab)
Orange - .056-.059 - FS Part # 38639 (Extended/Crew cab)
VOV - for under 105 degree averages - FS Part # 38902
VOV - for over 105 degree averages - FS Part # 38904

Norfolknova, you're correct. The PTR chart is a guess. The best way to do a 134 system is to weigh in the correct amount of refrigerant. But, armed with a PTR chart and thermometer you can get pretty close by guesstimating.