A/C pressure at 400 psi
 

What's happening here?
This is on my 92 D250. I've been trying to resurrect the A/C system and can't control the high side pressure. It's running up to 400 psi at idle. I dropped in a new condenser to fix a leak and did a retro to R134.
The low side is hovering around 25-30 psi. I don't dare put any more refrigerant in because the high side is out of control.
We got it to hold at 250 psi by running a large fan against the grille.
I know the truck had to have reasonable pressures at idle when built. My cooling fan is in good shape, the fan clutch was replaced today, the shroud is intact, and there are no obstructions to air flow.

What am I missing?

did you draw a vacuum on the system before you charged it? when you put the shop fan in front and it dropped back down to 250, this is normal, isn't it? When I charge a system I put a fan in front since the engine fan doesn't pull quite enough.

a/c retro
 

if you change to r134 a, you need to install the correct pressure lowering h block expansion valve, although without replacement, your pressure should not exceed 300, still too high, under stickies ac there are dodge correct change part numbers, take those to your local aftermarket supplier to identify part, the compressor will not handle the higher pressure very long. any questions, email me, joe.

I converted everything necessary for 134 a couple years ago.

The pressure drop with the fan is normal but these vehicles were made to idle with the A/C on without exploding. This is true of all stock vehicles.
What I'm asking is not extrordinary. I simply want it to operate as it was designed.

You know you only put in 80 percent of original charge when converting to 134a right? Sounds like it is just over charged.

We were charging it using pressure instead of volume.
The system had stabilized at 250 psi but the low side was down to 20. We added 0.1 lbs and the pressure went through the roof.
That small of a charge should not make the pressure skyrocket like that.
As it sits, the low side is still a wee bit low.

Where are you at? Whats the outside temp/ humidity today? That will all affect the pressures too. On an H- valve system the low side always seems to be lower with r134a to me. And the high side a little higher. 400 is too high obviously, but I would be o.k. with 250 at idle - maybe even 300 ( rather it be lower though ) as long as it was cooling well. Was the system vacuumed out? Air will make the pressured go high too. Will make the low side freeze too.

I work at a Mercedes/Prosche/Maserati/Bentley/Rolls dealer and had the A/C guys up there charge the system. It was properly evacuated (15 minutes) before charging.
The temp was between 75 and 80 in the shop at the time. Humidity has been stupid high in Dallas with all the rain we've had (exceeded yearly precip last month) but the shop A/C cuts that down quite a bit.
The system blows cold but I don't want to blow it up.

I would ask them how much they put in. Then consider letting a little out. I still think with the system blowing cold, and the low side pressure looking o.k., it has too much freon in it. JMO

Okat, From what i read here,,, There must be a restriction in the evap coil. The coil which is under the dash,,, As the refridgerant enters the coil as a liquid, It goes through a metering device which appears to be slightly clogged with particulate matter, therefore creating a HIGH high side, No amount of vacuuming can correct this,,, Mabey you can take compressed nitrogen and purge the system in the reverse direaction and cleae the blockage,,, In the low with 200 psi, and out the high side........
Or you have contaminated refridgerant, But i am guessing that there is an obstruction.
Peter.

You could be right, but the odds of having a restriction in the evap is pretty low. Considering the system works, and the only issue is a high, high side pressure, I would still have to bet on the system being overfilled. If the expansion valve was clogged the high side would be low as well. Here i s a diagnositic chart that might help. It somewhat contradicts what I say, but never the less! .........

Not sure how they are charging by using the gauges pressures.. there's too many variables.. You need to be charging by volume.. with the pressures you listed.. I'd say it over charged... If there's a plugged passage in the Evaporator, you'd have lower pressures.. (simialar results to the chart posted for a TXV (expansation valve.) stuck closed.) A quick check on the TXV to see that its not stuck... run the system, with your gauges hooked up.. and get some air duster .. (or refigeratent) and with the can upside down, spray the TXV.. this will close the valve.. your low pressure will go to a vaccum and you high side pressure will drop..
Or you have a plugged passage on the high side.. (with the new condensor, there's probably not a plugged passage..)
I'd also try running the vaccum pump a little longer.. Especially with the humidity we have been having..

Make sure your air flow is good.. (sounds like you have covered the majority of the parts.. ) just make sure it was the correct fan clutch...

Good Luck,
Bryan