I had my system converted to R134a a number of years back, probably like 5-6 yrs ago. New compressor too.

I was noticing that system's not really blowing cold anymore. I had also checked the condenser's outlet side not too long ago and it wasn't hot. I can still hear the A/C clutch kick in and the compressor run, it's not really noisy. Hope that's a good thing. I don't recall seeing any oil leaks but the whole thing's pretty messy. I have oil spilled from clumsy filling. I went over it with a UV LED light and didn't see any glowing leakage.

So, I know not to take A/C lightly. I already own a vac pump and can get A/C gauges, I consider myself a smart guy and wouldn't mind learning this skill. I read up on some pages that mainly addressed doing the initial conversion, and they said to fill until either there's no bubbles in the sight window or the high side reaches a max allowable pressure- "High side pressure in PSI should be about 2.2 to 2.5 times outside air temperature in degrees F. Example if it's 90 degrees F, high side should be no higher than 225PSI."

Well, the problem cannot be conclusively nailed down as being low on R134a yet. What's really the best diagnostic procedure here?

 

If your AC hasn't had any service in "probably like 5-6 yrs" it could very well be simply low refrigerant level. Double checking the spec for high pressure & attaching a reliable AC guage manifold is "the best diagnostic procedure". Refrigerant commonly leaks w/o leaving traces of the oil which contains the UV indicator. You need to pressurize the system & check for leaks w/R134 (Freon) detector, if pressure is below spec.

You'd do yourself a favor by adding what model/year/system you're dealing with. Those w/rear HAC have a reputation for rusted lines running the length of the truck. More than likely there's room for a can or 2 of R134 & that's all that's needed. However I wouldn't look at the condensor's outlet-I'd touch the evaporator's outlet to judge if its cold. Low refrigerant will barely liquify & just get circulated, making components warm. If you had a big leak pressure would be so low the limit switch should've switched off the AC clutch.

Also confirm normal airflow. Evaporators can get covered w/debris dramatically reducing the flow of air thru them.

 

It's an '87 E150, no rear air.
If I'm adding R134a, is that stuff about how much to add accurate? No bubbles or that pressure calculation I quoted?

One thing that bothers me is I don't see any oil leak. This would tend to answer my lingering question- if I'm adding R134a, shouldn't I add oil too, but how would I know how much?? If I haven't lost oil then I wouldn't worry.

Will the gauges be able to tell me if it's low, or am I blindly adding it on a theory? Aren't fluorescent oil leaks supposed to show up when the system leaks, suggesting this isn't a leak?

I have cleaned that evaporator core once before. And yeah it's dirty again- really what's the best way to clean it? It's incredibly difficult to access from inside. Is there a way to open up that evaporator box under the hood and get enough access to clean it?

 

Sorry, I'm less familiar w/'87 systems, its been a long time. I don't have R134 specs memorized & never heard that "pressure calculation" before. However "225PSI" sounds low compared to my memory of the last recharge for my '92. While "bubbles" are a good indication that a system is low "no bubbles" is not a good way to tell if a system is full.

Gauges will tell you the low & high side pressures. If pressure is not high enough cooling will be poor. However a leak isn't the only possibility. The compressor or evaporator tube may be at fault. I also use an AC thermometer to show active cooling.

As I said: "Refrigerant commonly leaks w/o leaving traces of the oil which contains the UV indicator. You need to pressurize the system & check for leaks w/R134 (Freon) detector, if pressure is below spec." Particularly over winter layups, refrig oil pools at low points & Freon can leak above w/o blowing any oil. If you can't detect indicator oil having leaked there's little reason to think it needs more oil. If unsure its easy enough use a can of the oil/charge mix. A bit more oil won't hurt, it just reduces the capacity of refrigerant slightly.

Evaporators are a PITA to clean. I found the 'Purple' cleaner, w/copious rinsing, worked best, after trying everything in the house & shop. Minimal access is gained just thru the opening left by removing the blower. On my '92 there was just enough hose to allow the evaporator to be flipped out & up onto the radiator support, after the housing was dismantled from under the hood. This gave total access to the evaporator. I covered the front end w/plastic & went at it. Sorry, but I don't recall how an '87 comes apart.

 

Well, I set it up and it was showing pretty low pressure when I hooked up the gauges.

Now I've added a can and started on the second one, but I only have 45 psi on both the high and low side. The center vent is only 74F and the air inside and out is 85F. There's condensation on the return line and the high pressure side is hot but not really burning hot. The compressor clutch is engaging and the compressor is turning, I can hear the "rrrrrr" when I turn the AC fan on low and switch from Vent to AC (the cabin fan's on either way so I can tell that's not just the fan). It is not especially loud as Ford AC compressors go.

When I shut off the R134a can and turn off the vehicle the pressure jumps to about 85psi on both the high and low side.

Well, seeing as we don't seem to be able to build a pressure differential and adding coolant didn't do squat, what's my next step? I mean if we can't build pressure either the orifice is somehow stuck open or the compressor's doing nothing. AFAIK it's impossible for the orifice to be stuck open, I thought they only got clogged. In fact any clog in the system anywhere would have shown up as an excessive high side pressure, right?

AFAIK the whirring sound when turning on the AC conclusively rules out the AC clutch or the low pressure or freeze switch or anything, right?

So... basically my compressor's blown then and I'm stuck with flushing, getting a new compressor, new dryer, and evac/refill?

 

The high side gets to 135F, the condensor output is 110F, the return line is 48F. Actually now I'm seeing 68F or so in the center vent, maybe I just pushed the probe in further.

 

This is fascinating!

Your "next step" might be to get an expert 2nd, hands on, opinion.