Automotive Air Conditioning Information Forum

Hi y'all,

New to the forum, but read many posts; enough to buy a Sanden to replace my burned out HT6.

This is for a 1999 GMC Suburban K1500, 5.7l with dual heat and A/C.

Also replaced condenser, receiver/dryer, orifice and TXV. Rented a/c solvent canister, bought a/c solvent, opened system, removed old parts and flushed it very thoroughly. Installed new parts, added correct oil and quantity, pulled down about 550 microns. Charged it, worked well, except for rear a/c.

The small tube didn't fit into the TXV, no auto stores had another, truck was ripped apart, so holding valve upside down, we carefully drilled it out to accept short tube. Cleaned it well and installed it. Since the front worked and the rear didn't, we thought debris go into it in spite of our efforts to prevent it, so we replaced it. New one fit, no problem. Removed, cleaned and reinstalled the orifice too, just as a precaution.

Pulled the system down to about 550 microns again, and started charging it. System takes 56oz, got 14oz into it, and it began cycling off on high head pressure.

I purged enough 134a to stop the cycling, interior dropped about 5* and rear EVAP was condensing, as evidenced by condensate dripping from drain tube.

Started to add a little refrigerant back in and it began cycling on high head pressure again. I'm wondering if there's a restriction somewhere, but using an infrared thermometer, I haven't found it yet or I don't know what I'm looking at.

Any guidance is truly appreciated.

Thank you

Can you please provide hi and lo readings at whatever RPM you are using (s/b 1500). Also provide vent temp with a proper vent thermometer, an IR gun does not work well for this either
I have come across new condensers with aluminum stamping shards blocking the ports upon close inspection. Now I check them all very carefully and blow dry shop air thru to test patency. We also paint cars here so we have a pro air drying system.

As you know if there is a restriction the piping will be cold right after, so keep looking. Use your hand. IR guns do not work so well on aluminum tubing.
Is the suction line to the comp at least cool? Is the high side from the comp at least warm?

Listing pressures would be helpful, idle and 1500 rpm.

Another thing to consider is that the charge goes in much easier with the interior fan on low speed. Yes, the clutch will cycle but running the fan will increase the load and slow down the charge time.

Agree with above recommendation as well.

Why do you believe it was cycling on "High head pressure" with only 14 ounces of refrigerant in a 56 ounce system?

FYI, the best way to charge a large system like that is with a bulk cylinder, with the engine stopped. Feed liquid directly in to the High side of the system, and be sure to close all of the manifold valves before you start the engine.

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Thank you for the quick replies.

We knew it was shutting off on high head pressure because we watched the gauge set as it cycled - was cutting out at about 425lbs.

My digital air temp thermometer is broken. I have the infrared and a probe type with analogue gauge, which is in my front dash vent.

Engine off, hooked up gauge set to high and low ports, only reading about 77 pounds on both sides. Don't know if this is relevant, but it's what I found. Dash vent temp was 90* to start with.

Fired up the engine, a/c on, fans at medium. Began cycling quickly: low side at shut off was 25lbs, high side at shut off was 300lbs, RPM at idle was around 633. Let it warm up for about ten minutes, cycling slowed down, as low side took longer to drop to 25lbs (is that the set low side pressure shut off, does anyone know?). Vent temp dropped to 80*, orifice was covered in condensate, rear evap had condensate dripping from drain hole.

Tried to up RPM to 1500, sort of held somewhat steady at about 1900 (was using a 2x4 from electric seat sliders to accelerator pedal, best I could think of).

Cycling then increased rapidly and continued to cycle rapidly after. Low side fluxuated from ~70lbs down to ~32lbs; high side reached greater than 450lbs (cut out about 425lbs) and then dropped to around 100lbs. Vent temp stayed around 80*, though I didn't watch as closely with it cycling so quickly now.

As for a new condenser having debris in it, I did consider that...after I installed it. Sigh. It was well sealed when received and in my prior life as an A/C&R tech, I never received any components with debris inside, so this is a painful lesson, well learned; if that is the cause here.

You have blockage in the system somewhere. Use compressed air to find the section that is blocked.

Fudge. Thank you for the reply.

This is a dual system, any suggestions on how to isolate most probable location?

Before ripping the entire system apart, mist the condenser with water while running and see if head pressure drops.

Backing up a bit, are you sure you thoroughly removed all the liquid flushing chemical from the system?

Also, presuming you started with a completely "dry" system, how much oil did you put in before you evacuated it, and where did you put it?