I have a '95, F150, 4.9. The whole AC has been flushed, new compressor, new orifice tube, properly evacuated and then the problem began. After
24 oz. of 134 was put in, the system pressure went very high. High side and low side. When the high side hit 430 lbs. it vented at the back of the compressor. The system is cooling, but not like it should and the high pressure has me worried. Any thoughts or ideas are greatly appreciated.

 

By "evacuated", do you mean you pulled a vacuum on it with a vacuum pump? If so, what did the vacuum reading go down to? How long did you hold vacuum on it?

What are the low and high pressures now?

 

Sounds like a blockage somewhere, in a way..

Also, a 1 hr vacuum is the minimal you wanna pull.

 

Agreed Wes. Although with both sides high and warm vent temps, I'm thinking air or other non-condensables in the system.

 

Could be moisture if he's in the south...

 

if a 1 hour vac has been pulled one trick that I know of to get the high side normalized is to shock the system, just take a garden hose and cool water and spray the condencer down for a bit and that could help, I did check with a shop that does AC work on this and they said its a trick that does work and help at times,, but I agree with the others you might possibly have a block or other non condencable in the system still...

 

Like I said, I did a complete flush of the system, new compressor, new orifice, and pulled a vacuum on the system for over one hour. I did not try the shock this time, but I am going to add a fan next.
And, Bob, I pulled vacuum for over an hour and then system pressures vary from
30/280 to 90/430 when it vents.

 

I cannot disagree with a possible block, but when I blew out the flush solution, it flowed as though there was no obstruction.
Everything I have done is exactly the same I did with my '96 F250 and it works just like it should.

 

If the low side pressure is high then it is not a blockage.
Did you replace the accumulator/filter drier? Replace it if you didnt.
How long did you pull a vacuum for?
How low did the gauge read vacuum? it should read 30" (even though it is truly not 30"). Once you get to 30" on the gauge continue pulling a vacuum for at least an hour.
How long did you hold the vacuum? After pulling 30" for 1 hour isolate the gauges and go eat lunch. After you eat lunch(at least 30 minutes) you should not have any noticeable drop in vacuum?
Did you test the fan clutch on the radiator fan? even though it is spinning it may not be spinning fast enough to pull enough air to properly cool the condenser.
Answer these questions and then I/we can tell you what your next step will be

Matt

 

Bad pressures switch? The switch should cut out the compressor at about 40 low, 300 high. At 430 high something is going to blow. That is R410a pressures.

 

Low pressure switch is 25 psi not 40, optimum low pressure for R134a is 35 psi. I don't think our trucks have high pressure switches.

Matt

 

I would guess (just a guess) that your condenser has collapsed inside or is partially plugged. Couple reasons, one is that they are prone to do that in certain years, often the directions you get with the new compressor will mention this.

Other reason is the flush, not a big fan of that stuff. Have had it clog up several condensers when we first started using it. At first just figured they were bad, but it got to be pretty regular. Cut one of the condensers apart and it seems the flush turns the stuff inside to a weird crystal like sticky goo, which of course doesn't flush out very well at all and seems to like to clog up the condenser more than anything.

Since you can get the system to work sort of, you can feel the condenser while its running, or feel close to it if its hot, if you find that its temp is not the same all over it is almost certainly restricted. Also, If you run a garden hose on it like mentioned earlier it will help it work better if its clogged, but probably won't fix it other than temporarily.


We quit using the flush and if we don't just go ahead and replace the condenser with the compressor anyway -which we always recommend, but customers don't always want to do- we just leave it alone and have had way fewer problems.

I know some compressor manufacturers want you to flush the system in order for them to warranty the compressor. Its pretty rare we have one fail, and when we do the parts guys know us well enough they just replace it without any questions. But if you are concerned about that I'd buy the flush at the same time you buy the compressor and give it to somebody you don't like.

Low side pressure seems fine, we usually fill them to the point where you can run the engine at 1500 to 2000 rpm and the compressor doesn't kick out. The 90 psi on the low side you saw is a bit high, but not unusual if you were filling it, especially in hot weather, always best to fill them slowly.

 

That's the switch, compressor cycling switch. It should turn the compressor on at about 25-30 psi, and OFF at about 45-50 psi.
Depending on ambient temp and airflow across the evaporator, the system might not reach the 50 psi cut out pressure (over 90 outside and fan running on high,) so compressor might run continuous, but not to 400+ lbs high side and 90psi low side. At switch FAILURE the back-up safety measure is the blow off valve, as the OP mentioned was happening on his system.

 

First, what was wrong with the system before you flushed it? Did it work at all?

Next, I am wondering if you added oil?
for a Ford, your year and model, (I don't know if I can add links) it should be:
R134a. 2lb 1oz, OR 33oz. PAG 46 oil, 7oz (TOTAL Not including what is already in the parts that were not replaced.) is required to lube the system (Per AllData Specs). Just to make sure the levels are correct.

Next, if your truck is like mine, the accumulator and dryer are all in one. It is my understanding that is the area most likely to get plugged. Just wondering what was wrong with the system before you started to work on it.