Technical > Automotive > [Tech] 1988 Buick A/C excessive head pressure?

[Tech] 1988 Buick A/C excessive head pressure?

A good test of the condenser cooling is lightly spray water on the condenser when reading pressures, if it drops like a rock, there is your problem, not enough heat dissipation.

It has been a while since I have done A/C, and I am sure being 1988 it has a direct drive fan for cooling the radiator and condenser. If you feel enough air is passing the condenser to cool it and you are not overcharged on the system, from the top of my head sounds like the Orifice Tube may be clogged unfortunately you must discharge the system and it is located in the low side pipe by connection at the evaporator. Maybe let someone else chime in.

reply to OvrQualified
First, which refrigerant are you running? Is it still an R12 system or is it R134A?

Second, the water hose on the condenser is *NOT* a valid test. ANY condenser running at ambient temperature will drop like a rock when you spray 50-degree water on it. Has nothing at all to do with the performance of the condenser, you have simply dropped your ambient by 50 degrees+, so it had better drop...

Third, that system should have an electric motor-driven cooling fan, (more likely 2) behind the radiator inside the engine compartment. It's a transverse engine, so no direct drive fan on that one.

And....it *may* be a clogged orifice tube, but unlikely since it doesn't drop below 30 on the low side. You should expect a low-to-vacuum if that plugs. And another thing, the orifice tube is located in the *high* side line going to the evaporator from the condenser.

(Sorry to contradict everyone, just trying to keep it factual...)

My first reaction would be to evacuate the system completely, check for leaks, and recharge to the proper fill of refrigerant. Air can make high-side pressures spike, and cause a drop in cooling performance. I would go from there, check your results and see what is next.

-Matt

Thanks Matt, I've put in a call to my shop (with a machine) to do an evac/recharge along with an acid test with a request to leak check (shouldn't be too difficult, the shop that replaced the condenser three years ago inserted UV dye).

The system was converted to R134A nine years ago and received a new compressor/flex hoses/accumulator three years ago. No maintenance has been necessary since that service.

Both the radiator and condenser fans come on, with the condenser ("high speed") fan kicking on at 200psi, per the Buick technical manual.

I'll report back when I know more ... Thanks!
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The enemy of my enemy is my WHAT?!

I think you are going to find that the TXV is icing up, causing a low side restriction. Any time you get a high side pressure like you have, there has to be a restriction.

If the compressor had failed previously, there may have been material left in the system that is clogging the orifice tube. I don't recall that the R-D/A contains the OT, rather it is in the line. It doesn't have to be fully plugged, which may be why you don't have a vacuum on low side.

I have heard that PAG oil and the older 525 oil don't mix well, but three years is admittedly a long time for a problem to show up for that. Maybe it is a combo of things.

Another thing: how well does the radiator work? What is the cooling system temperature? That car has an aluminium radiator which may be clogged by this time.

This is a CCOT system -- no TXV, just a fixed o-tube and a low pressure switch....the o-tube is in the liquid line, just downstream of the high side service port. The desiccant was changed three years ago by virtue of changing the accumulator...the system was pulled down to 250 microns and held when it was serviced three years ago. A waterlogged desiccant isn't high on my "probables" list.

The system was never retrofitted with PAG...the conversion was to POE, and I made sure that the repair three years ago continued use of POE...didn't want to take any chances with PAG/mineral oil interaction.

The cooling system is fine, holding around 190F with the high speed fans on...this car has always been odd in that it runs cooler with the A/C on rather than off

I checked the liquid line with a Raytek, and didn't find any evidence of a restriction upstream from the o-tube, so it's either in the o-tube, or, as Mattmag said, there's air in the system somehow...
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The enemy of my enemy is my WHAT?!

reply to mattmag
Matt,

Your excellent post reminded me to get back to basics. From residential A/Cs, I remembered that sometimes calculating the subcooling and superheat was instructive for diagnosis of some conditions.

So this morning, I pull out my gauges again, along with my trusty Raytek MT4. I measured the liquid line post-condenser and came up with an actual temperature of 140F. The table says that 400psi refrigerant (the head pressure it was holding this morning before the sun came up) should be at 183F. 43 degress?! Holy subcooling! I seem to recall that non-capillary condensers should have around 15-20 degress of subcooling at most. A quick check of my old A/C book concurred: out-of-this-world head pressures + excessive subcooling = non-condensables in the system.

Luckily my shop can fit me her in tomorrow, but I have a feeling that changing the accumulator after checking for moisture/leaks will prove instructive.
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The enemy of my enemy is my WHAT?!

reply to mattmag

Now that's a vent temp!

Nice outlet temp there!

Most often a trip through the basics pays off in most everything automotive, not just A/C systems. Once a person gets diverted by "it's probably this or that" then you're on a tough road to get off of, and it has probably resulted in parts getting thrown at a problem that were not needed. A good understanding of what is happening in an A/C system goes a long way toward properly evaluating what those pressure gauges are telling you.

Your knowledge of refrigeration system operation is excellent, so you could make good use of the diagnostic info you had to work with. I REALLY liked your use of the Raytek. That was always one of my favorite tools to get a true look at what is happening. I haven't heard anyone mention subcooling for about 15 years....

Glad it worked out well!!

-Matt