Help! ac problems 93 vette

[GTSDart340]

I am trying to fix the ac on my dad's 93. It had blown out the condenser, so it lost all of it's r12. We decided to replace itwith r134. I replaced the condenser, drier, orifice tube, and flushed out all the rest of the system. I went to recharge it, and got a can into it. the compressor started cycling, but only 3 or 4 times before it shut off and has not come back on. it does have the automatic temperature control system. We tested the high and low pressure switches, and determined we had a bad high pressure switch. (It has code 66 stored in he computer. We replaced the switch, but it stillhascode66stored. IfWeeraseit, itimmediately comes back on. The compressor comes on if We jump power to it. Any ideas? I'm completelylost this time! Thanks!!

 

[tmkassin]

Did you run a vacuum pump on the system before trying to recharge it?

 

[boomdriver]

Wish you had read the FSM before getting this far into it......

Lets assume that you;re standing there with a clean, empty system.

Vac system to max (29hg) and hold that for an hr pump running. D/C pump and observe the vac....make sure it stays where it is. Assuming the new orifice is correct, (blue tubes are most popular or smart tubes) then add OIL charge w/134a FIRST to lubricate the dry system, engine NOT running. Actually, you should have poured an oz in the compressor before assembly and rotated it by hand....
Ok, got 6-8 oz of oil in and now time to start the engine and connect the 1st can of gas. NOW, with the gas connected, jump the LOW pressure cut-out switch so the compressor will run and take the charge. IF you fail to do this the gas will not circulate, the system will not release the circuit and nothing will happen.
The system for opt 68 elect a/c is powered all the time and like everything else on a Corvette, its managed by completing ground paths inside the BCM or control head, whichever the case may be.

Low press switch jumped, gas ON and discharging the compressor should now run to accept the gas. 134a charge will be about 90% of what the R-12 was, so get the FSM out and calculate how much you will need, approx 2 lbs give or take a few oz.
BUT< always use the gauges to know when its full and when its time to stop adding gas charge. Even IF you are 1/2 lb short of the calculated gas charge, STOP when the pressures reach a specific point. 134a is VERY fickle and can work well or not at all with only 2 oz difference. The key is the low pressure. I do not recall exactly what that should be, but its well documented and can be found if you do a google search and do some homework on conversions. You have to stop when the high pressure reaches a certain point, but if the LOW is not high enough its not going to work well...usually this means there is something wrong at the condensor. Add gas slowly and give it time to react before going too far.

The proper amount of oil is important...too much effects the 1 or 2 oz of gas charge.....too little and you risk seizing the compressor.
The system should be sterile before charging. New, clean dryer, tube, flushed lines and compressor, flushed condensor. Even NEW condensors require flushing because they have debris inside that settles in the bottom coils and plugs off rows so you loose heat exchange...and cooling efficiency suffers.

The secret to making a conversion work well is in the condensor and how well you can draw the heat off the thing...mine was all new...everything under the hood. Like a new car, except done by me and not the factory. The heat from my condensor is so great that it drives the radiator temp upward within 3 or 4 minutes.
If my (big all alum radiator) has me running at 187 degrees.....turning on the a/c drives it up beyond 200. As long as you are dumping that condensor heat you are cooling the cabin. If your 134a charge is off and there is too much heating then it cannot cool. If the 134a is too low it cannot collect enough cabin heat to carry to the condensor. So, make certain that your cooling stack is clean,(would be if you installed a new condensor) no leaves or debris trapped in the air-box, and make sure the air-dam is good and in one piece. It ALL matters because these conversions are so fickle. Some will put frost on the windshield while others do good to blow 55 degree air from the vents.
Bottom line on conversions is that you have to follow the pressures more than you do the weights and other specs.

Jump the low pressure switch to make the compressor run but ONLY after you have installed a can of oil charge so there is enough lube in the system to allow it to run for a few minutes and charge. After you get 1 to 1.5 lbs of gas in, take the jumper off and reconnect the low press switch and it should be able to cycle on its own then.

Mine works well enough, BUT, since the street prices of R12 have dropped to around $35 a lb, and 134a is UP to around $15 and climbing, I'd go back. I WILL next time I have to open the system...

One more detail....
you may need t o adjust the low pressure cut out cycling switch since your 134a low side will be a few lbs different than the R12...I think its about 1/4 turn of the screw CC to prevent the switch from cycling the compressor too much.

 

[GTSDart340]

Yes, I did vacuum the system and add oil. That was all implied. I have jumped the low pressure switch.... Nothing happened. That's why We looked into the high pressure switch. It is throwing a dtc 66... Everything I have found shows that being the high pressure switch.

 

[boomdriver]

Yes, I did vacuum the system and add oil. That was all implied. I have jumped the low pressure switch.... Nothing happened. That's why We looked into the high pressure switch. It is throwing a dtc 66... Everything I have found shows that being the high pressure switch.

I no longer believe in coincidence as an acceptable explaination for ANYTHING....so in the case of "it was working until the condensor was damaged" and having a high press switch fail at the same time....I'm not buy'n it. Not easily, anyway.
What I would be inclined to believe is that with work performed and knowing these things as I do, I would look for harness issues. Wire thats been damaged, shorted, broken insulation, something to corrupt the signal and break that circuit. A bad high pressure cut out switch wouldn't cause the system to have an open circuit....and not allow cycling. It might let the system go to high pressure and blow some seals out...but not on the low pressure end of the deal. The simple test would be to jump the HP cut out and see if that made a difference. That circuit should be complete until extreme pressures where the switch would then break the circuit and cause an 'open' that would end the cycling until the temps went down and the pressures were reduced.

Before guessing and making any assumptions about the harness or anything else, I would definately follow the FSM flow charts to their conclusions and go from there. I have yet to have ANY problem that the flow charts did not cover and allow you to diagnose, no matter the cause or how strange.

 

[GTSDart340]

We went through the fsm diagnostic tree, that's how We determined the switch was bad. It isn't the standard two prong switch, this one sends ac pressure information to the computer. The car has the automatic temperature control. That is what is throwing me off.

 

[boomdriver]

We went through the fsm diagnostic tree, that's how We determined the switch was bad. It isn't the standard two prong switch, this one sends ac pressure information to the computer. The car has the automatic temperature control. That is what is throwing me off.

I think that 3rd pin has to do with compressor cycling and ECM managing cut out @ WOT or low pressure to protect the compressor.
Has the control head been checked or replaced?

If the head were faulty it could continue to power the clutch and a dangerous high pressure event could take place. Not sure why it would continue to leave the circuit open now when its empty but thats why they call them faults.......

Without having a scanner, and being there, I know that most ALL mystery a/c elect issues with opt-68 systems can be traced back to the BCM in the evap top. I've seen fans that ran with the key out, clutches that would not engage, or run but slip,. random displays, dead battery...all form of weird stuff...all from the blower control module.
Its an expensive guess, around $125 (even more at GM) but those things can fail in different ways and cause a thousand different symptoms. BCMs are the devil. Been there done that...

Beyond that I have no better ideas..wish I did.