Well, you've confused me with this "It blows COLD 34-36 degrees at idle in 70 degree outside temps when the main fan comes on.". I thought you said the main fan doesn't come on. Which is it? Going back to what SunCr said, what you have to understand is that the A/C switch is not a main fan switch! There is a switch in the A/C high pressure side. When this switch opens, it signals the ECM to turn on the main fan. What SunCr was referring to was at cooler temps, the high side pressure will not go high enough to open this switch, so the fan won't run. Also if the refrigerant charge is low, it won't go high enough to open the switch. In most instances, with outside temps above 70°F, the switch should open pretty much as soon as you switch on the A/C.

You claim the A/C is fully charged, it cools fine and outside temps are 70°F or greater, but the main fan doesn't come on. You've disconnected the high side pressure switch and the fan does come on, so the wiring to the ECM and the ECM's A/C input are good. That leaves the High Side Pressure Switch. Easy Peasy!

 

Sorry, The main fan comes on thermostaticly only, and cycles with the engine temps. I think it's like on at 225 off around 200 or 195. The A/C works well on the freeway or at cruising speeds when there is good forced air flow and when the fan comes on (just in testing in garage) it blows 34-36 so I would expect about 50 out of it when it's 90 or so out. The main fan does not come n when I turn on the A/C swithch though as I understand it should.

OK I guess "ll change the high side pressure switch and see if that does it. There are two guages on the A/C guage. I'm not in front of it right now, but I recall thatthe high side was cycling between 150(fan on) and 250(fan off). My friend described this as temps vs. pressures. I don't recall the values on the low side guage but he assured me that the charge was good. Thanks for hangin with me on this :cheers: I'm sure that I could find the answers somewhere else but I'm trying too learn this for myself and not abuse my friendship, this guy is busy so I hang out and try not to bother him more than is neccesary. Besides, the more I do the this the faster I learn how to type :lolg:

 

I'd get the a/c checked out or double check those vent temps with another thermometer. 34 to 36 degrees at idle on a 70 degree day with no fan would be an engineering miracle and even with the fan on, the evaporator would probably ice over in a couple of seconds with a corresponding rise in vent temps. A fully charged system, with the engine at 2000 rpm & the fan blowing might see 36 degrees, but at idle, I think you might have icing. Speaking of which, is there any ice on the ac lines? Check the low pressure or larger line from the evaporator to the accumulator at the radiator. Or do the vent temps rise dramatically after 3 or 4 minutes at idle? I agree that the fan switch may be a problem, but it could also be that your charge is low and that the low pressure switch is faulty or adjusted too low causing the compressor not to cycle as it should which could also be why the main fan isn't coming on. Having someone hookup a manifold gauge set would help you pinpoint the problem. Pressures for a 70 degree day, main fan on, with the engine @ 2000 rpm, should be about 170 on the high side & no less than 22 on the low to produce a 36 degree vent temp with no icing. This simulates driving; idle vent temps & pressures are going to be higher.

 

I think that I have misunderstood the fan operation. I thought that when you turn your A/C on that the fan (now I know, main fan, originally I thought aux fan)was supposed to come on full time. I guess that's not right. The fan just comes on with the regular engine temp cycles which I guess coincides with the cooling A/C cooling system pressures high and low side swithches or seem to from what I observed.

I've got a good charge and misunderstood what my friend said about the guages. It is pressures, not temps. I'm cylcling 150 -250 on the high side (right,red guage). I don't recall what the low side pressures were, but I'll find out tomorrow. I'll have to look at the guages and see. I was using an infared digital temp guage at the center vent. Just as the fan would switch off with the high side pressure at 150 (I think, this was now several days ago) I was getting temps at the center vent wth max A/C and int. fan on high at 34-36. It varied a degree or two. I observed several cycles of the compresser over about 20 min. or so. I did not note any abnormal icing and he didn't point any out. He would have if he'd seen it.

I have an A/C license that I got about 5 years ago, but I haven't done any A/C work at all since 6 mos after that, so I've really forgotten almoswt everything I learned and never did enough to really know it well. This guy does A/C work during the summer as part of his general automotive repair business. He called a highly awarded GM tech in Hawaii yesterday and the guy instantly said that the main fan is cylcing exactly exactly as it should. My freind then proceeded to tell me that in about 84 or so that the full time fan operation was discontinued due to the A/C activation with defroster operation and the fan operating in cooler temps being uneccesary or couterproductive I guess.

I was originally trying to make sure the system was operating properly because I was hping to get it to blow colder during in town and stop and go driving. I think that what I'll do is put a manual switch on the aux fan so I can flip it on under those conditions and see if that will improve the performance.

How's that plan sound? If the charge were low I think that my compresser would cycle abnormally frequently. What would be the purpose of a manifold guge be, if you still have any patience with me? Thanks for all the input and suggestions :flag

 

Your vent temps at idle, will be higher than at speed. If they are higher at speed, that's another indication that the charge is low and that the evaporator may be icing over. At idle, air flow over the condensor is less (even with the fan on) and the compressor doesn't reach maximum efficiency until about 1500 rpm. That's why your vent temps seem unrealistic & unless you've got a freezer under the hood, your 2000 rpm temps based on idle temps, would be below freezing. If you have R12 in the system, the compressor should cycle off when the low side is drawn down to 25 - 28 psi. There is an adjustable switch on the evaporator. Remove the connector and you will see a small screw between the terminals. Turning it counterclockwise reduces the pressure setting the switch will open at to shut off the compressor. Turning it clockwise raises the pressure for cutoff. If you are testing on a 70 degree or better day, the system may not draw down low enough to cycle the compressor. To determine the setting of the cycling switch, leave the a/c on high & disconnect the blower. The lack of air over the evaporator should allow the pressure to drop and the compressor to cycle. Since R12 boils at a temperature of 30 degrees @ 28psi, it's important for the low side to maintain a pressure range of 25 to 28 psi. Any lower, and icing will occur. Back to the fan switch: It opens at 233 psi, signaling the ECM to turn on the fan. It closes at 190 psi, turning it off. Running both fans for an extended idle taxes the Vette's charging system. With the a/c on high, & both fans going, the alternator can't keep up. Voltage drops, fans slow down, engine temp goes up, a/c is less efficient - in fact GM recommends that you turn the a/c off if temps get to high. So, if you want it to work this way, you may have upgrade the charging system. In any event, hook up your gauge set being sure that you purge any air from the lines, and measure your pressures at 2000 rpm. Disconnect the fan switch so that it remains running during the testing and note the air temp at the condensor and of course the vent temps. With that info, you should know if it's working right.

 

PS: you may want to try a digital stick thermometer as opposed to what your using as I think they're better at measuring air temp. If the surface temp is as cold as you registering, another thought is the evaporator is leaking. R12 at sea level pressure is 21 below, though if you have an evaporator leak, you usually get frost on the windshield, especially if the humidity is high.