charged a/c and it cycles off and on but stays on constant while rad fans on whats up with that. darn thing starts cyling off and on again when temp drops and fans off!


i read about a relay or something that should keep the fan on when a/c is on, if so where is it on the 89 2.8 mpfi

 

AC compressor clutch cycles on and off as system pressure requires it. Once system pressure reaches about 250 psi, rad fans come on to pull air across the condensor and bring pressures back down. AC clutch cycling has nothing to do with how often or how long the fans stay on. If you're cruising down the highway, there's normally enough air flow across the condensor that AC pressure won't build high enough to require the fans to come on. If the AC clutch stays on constantly for any length of time and there's no way to drop the system pressure down, the AC system would ice up and stop working. When it cycles, a little bit of heat is introduced into the system to melt any ice thats started to form. If the rad fans never turn on preventing air flow across the condensor and bring the pressures down, pressure would build up so high that something would burst.

The AC and rad fans will also come on and off when the defroster is turned on.

 

does it not seem unusual for the compressor to stay on when fan is on and cycle rather often when not your saying that this is normal ....

any one else care to tell me how thiers is operating

thanks

 

I don't have the V6, but on my '89 w/5.7, fans stay on all the time the A/C is on, at leats at idle, regardless of whether Compressor is cycling or not.

Suppose to turn off at some MPH I think, based on VSS input, but never had the oppurtunity to look on my engine bay at speed to confirm it

What Stephen is saying, is pretty much right on.

Could be that's the way a V6 is designed, or, you don't have enough charge to maintain pressure to activate the highpressure switch in the A/C system?

 

Thanks for the input .If the pressure gets to high then the fan should be on so as to keep head pressure down .I know there is a relay at fire wall drivers side and is there another next to evaporater I think but don'tknow for sure. could any of these be bad or how do I adjust pressure switch if thatt would even help?

NEED GM SERVICE MANUAL so much for haynes and chiltons

The a/c system has a new orfice and the system was vacumed down before new freon was introduced my friend has many years of service in a/c mostly refrigeration and he seemes to also think the fan/cooling system should stay on while a/c is on at idle at the least ; since you do not have air moving across condenser
when charging we also had the low pressure switch bypassed and the compressor would still cycle with out rad fan on and i mean quit frequently. Then it stay on constant when fan was on.
any more thoughts or tales from other owners about what your system is doing? THANKS

 

I see you have an '89...just so happens I have an '89 Helms, I'll look into it later tonight for you. I should have picked up on this earlier.
But, for grins, look just downstream of the AC cannister, see if you have a 2-wire connect plugged in...if so, then that goes to the High pressure switch in your AC system and is what drives the fan relay. My Helms may or may not explain "how it works" but who knows. Give me a few hours, I have to do some "Honey-Do's" right now.

 

yea there is a second plug-in down stream. And if you say it goes to the high pressure switch then maybe my high pressure switch is weak or going bad. Thanks for the input.

 

hi pressure switch is 70 somthing dollars and none in stock right now guess i'll be whating till this weekend or better information

any replies needed as to what your ride is doing

 

The secondary cooling fan is controlled by a temp switch, on the passerger side head, and a pressure switch, mounted on the back of the A/C compressor itself. The switches in the lines control compressor cycling. The fan will come on at some predetermined pressure, regardless of what speed the car is going. It is purely mechanical in nature, the computer has NO control whatsoever. The primary fan, however, IS triggered by the computer, but the computer pays attention the the secondary fan, and when the computer sees that the secondary is on, it will activate the primary as well. (not sure about at speed though....)

If your A/C system shows the correct pressures while operating, and the fan actually does come on, the system is operating as it should, and you should sit back and enjoy the nice cold air.

 

Quick look through the schematics show the A/C pressure switch triggered through the ECM...my guess would be that based of VSS input for speed, that "trigger" can dissappear...otherwise i see no reason to have it tied into the ECM.

Both V8 and V6 schematics are similar. I'll look through the operation and troubleshooting area and see if any more info is there.

Well,

Nothing I scanned through mentions VSS, so I'm probably wrong...only mention of the highpressure switch to the ECM os for idle issues.

The high pressure switch does have fan relay control though. You V6 might be operating normally if you have only one fan. Based on the circuit test and escriptions, it mentions the pressure at which the High Pressure switch is closed...so could be you don't generate enough pressure to keep it closed unless the compressor is on, or as i mentioned before, might be normal for a V6...

Really helpful, huh?

Maybe a post on the V6 forum will get you a quick answer....shouldn't take someone more than a few minutes to verify it for you.

Good luck.

 

On the v6 the ac relay should turn the fan relay on anytime the ac is turned. The fan then runs all of the time the ac is turned on regardless of whether the clutch is cycled on or not. The system should never blow up under any circumstances. It has a high pressure cutout that would disengage the clutch if the pressure goes too high, which it will if the fan is not running while parked at idle. On the v6 only, the ecm will cut out the clutch whenever the engine is under load. G.M. knew the v6 was underpowered and didn't want to slow it down so much when climbing a hill.