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A test for A/C operation

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Since I have caught a NASTY summer cold....
nothing GOOD on TV... can't sleep....

I'M POSTIN'

So it sez in the Holy GM book ( paraphrased)

You hit the A/C on....

A few moments later the MAIN fan will come on.

REGUARDLESS OF ENGINE TEMP.....

So mine didn't.

But the main would come on @175deg.
(Hypertech chip)

and the AUX would come on@200
(Mid America fan sender)


So here was the rub.......

In the late afternoon...leaving work
(engine cold/outside HOT ) I would fire up the car... turn on the AC.. and in 25 or so seconds the compressor would start cycling.



BUT the minute either of the fans came on the A/C would be fine... and NOT cycle.

And when the fans were running the AC charge was OK.

So here's the punchline.

The COOLANT FAN SWITCH on the High side A/C line is a normaly closed switch UNTIL the high side pressure reached 240(ish)psi

This switch needs to be OPEN before the main fan will run.

But here's the clinker...

The HIGH SIDE COMPRESSOR SWITCH would OPEN @ 230 (ish)psi disconnecting the A/C compressor clutch.

So the HIGH side compressor switch is outa spec.

The cheapo fix...... anytime you call for A/C.. and you UNPLUG the coolant fan switch.. the fans will come on in 30 or so seconds.


Did you also know that the ECM shuts down the MAIN fan once you get past 40 mph... betcha ya didn't!

Science ROCKS!

Vig!
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Oh yeah

The COOLANT FAN switch is the SMALLER of the 2 connectors... the Bigger one is the compressor HIGH side cut out.

The BOOK shows the FAN switch closer to the firewall....mines reversed!


FYI


Vig!
 

fixvet

Well-known member
Joined
Apr 4, 2002
Messages
139
Location
Clever, MO
Corvette
1998 Two Tone Torch Red& Black Coupe
Vig,
I think the Haynes manual states the ecm will cut off the fans once you hit 55mph........unless you have an excessive overheating situation.
Later,
Fix
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
From the holy scriptures 1988 Electrical supplement

6E3-C12-4

The ECM will remove the ground to crkt 335
(fan control relay coil) if the vehicle speed is over 40mph/ Unless the engine is overheating.



Ohmmmmmmmmmm

Vig!
 
L

LarryBible

Guest
Are you talking about your '88 or your seventies car?

I have an '88 and have been into the a/c lately. Your description doesn't sound like the '88.

My '88 compressor is cycled by the pressure cycling(PC) switch on the LOW side. It could be that you are seeing the 240 cut out the clutch thinking that it is the high side switch that is doing this, when in reality, the PC switch is cutting it out.

I am assuming that you have gauges attaced to have learned this. 240 sounds low on the high side, but it is, of course, dependent upon ambient temp.

If it is short cycling until the aux fan kicks in, it sounds low on refrigerant. When the air across the condensor increases, it will bring the high side down which will bring the high side up, keeping the PC switch from reaching the lower limit which disengages the clutch.

With gauges attached, watch the low side and see that the clutch disengages when it gets down to about 24.5 PSI. If it is short cycling I feel quite sure you are low on refrigerant.

Is this still an R12 system, if not and has been changed to r134a, you need to adjust the PC switch via the screw between the terminals, for a low side pressure of about 21 PSI. After doing this, then charge for good cooling with the least amoutn of high side pressure. The PC switch is at the top connection on the evaporator.

I hope this helps,
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Sorry Larry

This is for my 88....

and when the A/C is running and I'm working on it ( usually mornings when it's cool ).... the suction side is about 36 pounds for a 34 degree discharge... the system IS charged correctly.


I posted this to describe a trickey malfunction.....

And BTW the AUX fan 86-89 is a hard switch...no ECM control.


Vig!
 
L

LarryBible

Guest
I guess I have the fans mixed up. I haven't messed with that in a few years. I remember that one is basically turned on, I think at 228 degrees or something directly by a switch in the head through a relay. The other is determined by the ECM based on a few programmatical things.

Yes, if the low side does not get below 34, it should be properly charged, but then what is cutting off the compressor clutch.

Maybe I need to study your problem description more closely.

Good luck,
 
L

LarryBible

Guest
I owe some apologies here. I had perused the post and jumped in too quickly.

I understand now. The high side compressor switch which is supposed to cut out at over 400PSI was cutting it out at 240PSI. When the fans came on, they brought down the high side pressure to a point that would allow the a/c to work fine.

I know that the PC switch goes over a shrader valve and can be changed without blowing the charge. I would assume that the high side switch also can be changed.

Sorry for jumping to conclusions, when you said cycling, I thought you meant short cycling, indicating a different kind of problem.

In the Army, we had a saying "RTFQ" it stood for "Read the......Question."

Have a great day,
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
No problem

I have learned that there are 3 switches.

1) Low side lack of charge cut off for compressor.

2) High side Over pressure cut off for compressor

3) High side high pressure ( slightly above normal ) pressure switch...( really sender to the ECM ) for fan OVERRIDE on A/C.



It was the interaction between these 3 switches that was my confusion.. and now I understand what is "Normal"operation for the system.



Vig!
 

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