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Air Conditioner Compressor-"On Signal"

O

omega1940

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I have an 88 with a 383 stroker. I had some of the "O" rings replaced on the air conditioner as it was leaking Freon. When driving home, I had plenty of cold air, (which we need in Phoenix!) but I noticed that when the compressor kicked on, the idle dropped pretty dramatically. I don't remember actually feeling the RPM's drop before when the compressor clutch engaged.

I checked my Haynes manual, and it talks about an Air Conditioning On Signal for 1985 and up. When the A/C selector switch is turned to on, this is supposed to send a signal to the ECM to adjust the idle speed. I'm wondering if somehow this signal is no longer being sent to the ECM, so when the clutch engages, the idle drops too much.

Anyone know where this signal comes from? Is there a sensor that does this? The drop in RPM's is significant, and the clutch seems to cycle on and off every 10 seconds or so and it is really irritating.

Thanks for any help you can provide.

Mike
 

G Winter

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Messages
2,177
Location
NW Iowa
Corvette
1990 red convert 6 sp
The compressor kicking in and out is usually a sign of low freon or a faulty pressure switch. I thing the ECM get its signal from a pressure switch.


Glenn
:w
 
O

omega1940

Guest
Glenn, do you know where the pressure switch is?
 

G Winter

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2,177
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1990 red convert 6 sp
There may be a couple of them and where they are located in the line varies from yr to yr. On my 90 which has the compressor in a diff location than yours, there is one in the low pressure line near the condensor. there are also two in the high pressure line not far from the condensor. Larger line is low, smaller line is high. I don't see one on mine but I know some compressors had one in the back of them. You need a manual to find out what each does.

Glenn
:w
 

Robertwav1

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Joined
Jan 7, 2003
Messages
822
Location
Ocala, Florida
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1993 Blue
Just for the record. I've never replaced a low pressure sensor yet because it was bad, at least not on a vehicle anyway. Your problem is probably low freon....I would address that issue first since it's the most common problem. This will also buy you some time in locating the low pressure switches. Good Luck!
 

dkfitness

Member
Joined
Mar 31, 2010
Messages
10
Location
Desoto, TX
Corvette
1988 Blue/White
A/c

I have replaced everything on the C4 A/C unit and the clutch does not engage. i jumped the clutch and it came on to take 2 cans of Freon. It's full now but the clutch willnot engage on its on. Whats the problem?:w
 

boomdriver

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Dec 18, 2008
Messages
1,888
Location
texas
Corvette
87 z-51
I have replaced everything on the C4 A/C unit and the clutch does not engage. i jumped the clutch and it came on to take 2 cans of Freon. It's full now but the clutch willnot engage on its on. Whats the problem?:w

Its better to jump the low pressure switch so you don't damage the clutch coil with hi amperage.
Test your coil. Check your cycling switches.

If all checks out ok, its likely the Power module in opt 68 or the compressor relay on the manual a/c system.
There should be power to the clutch plug at all times. The PM regulates by the ground path.
 

boomdriver

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Joined
Dec 18, 2008
Messages
1,888
Location
texas
Corvette
87 z-51
I have an 88 with a 383 stroker. I had some of the "O" rings replaced on the air conditioner as it was leaking Freon. When driving home, I had plenty of cold air, (which we need in Phoenix!) but I noticed that when the compressor kicked on, the idle dropped pretty dramatically. I don't remember actually feeling the RPM's drop before when the compressor clutch engaged.

I checked my Haynes manual, and it talks about an Air Conditioning On Signal for 1985 and up. When the A/C selector switch is turned to on, this is supposed to send a signal to the ECM to adjust the idle speed. I'm wondering if somehow this signal is no longer being sent to the ECM, so when the clutch engages, the idle drops too much.

Anyone know where this signal comes from? Is there a sensor that does this? The drop in RPM's is significant, and the clutch seems to cycle on and off every 10 seconds or so and it is really irritating.

Thanks for any help you can provide.

Mike

Thats possibly a sign that the IAC is dead. A/C systems cycle on/off with hi-pressure, or low pressure. Its normal.They even have an inline switch to cause them to cycle occasionally. They do that to avoid over-heating or freezing up in different areas. It's managed by the pressures.
The a/c system sends a signal to the ecm thru a dedicated circuit where the ecm sends a signal to the IAC to idle up whenever the compressor circuit is complete. That is relayed thru the power module and cycling switches..
 

Schrade

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Not in CF; BANNED!!!
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'90 LT 5
The a/c system sends a signal to the ecm thru a dedicated circuit where the ecm sends a signal to the IAC to idle up whenever the compressor circuit is complete. That is relayed thru the power module and cycling switches..

OP - you need FSM ECM pinout schematic, to determine which circuit carries this signal. This is the first thing to check in my opinion, like you mentioned in the first post.

But with a swapped motor, you might be in the dark on diagnostics, even WITH FSM yikes.

Hayne's is worthless, except to hang in the 'library', where the seat has a hole in the middle. Read a page, tear off, wipe, flush, repeat if necessary. 'Krinkle up' between 'tear off', and 'wipe', if you're tender in the :booty uh HUh.
 

boomdriver

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Messages
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Location
texas
Corvette
87 z-51
OP - you need FSM ECM pinout schematic, to determine which circuit carries this signal. This is the first thing to check in my opinion, like you mentioned in the first post.

But with a swapped motor, you might be in the dark on diagnostics, even WITH FSM yikes.

Hayne's is worthless, except to hang in the 'library', where the seat has a hole in the middle. Read a page, tear off, wipe, flush, repeat if necessary. 'Krinkle up' between 'tear off', and 'wipe', if you're tender in the :booty uh HUh.

I respectively disagree...

The haynes manual pages are too much like wax paper to be useful to the throne.. The covers make great pressed paper gaskets though..

Mine serves me well under a table leg. ....:thumb
 

Schrade

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May 2, 2009
Messages
1,315
Location
Not in CF; BANNED!!!
Corvette
'90 LT 5
I respectively disagree...

The haynes manual pages are too much like wax paper to be useful to the throne.. The covers make great pressed paper gaskets though..

Mine serves me well under a table leg. ....:thumb

Just takes a little more 'krinklin' if yer tender :D uh huh yup

Besides what's the alternative?

From a dorm stall wall:
Here I sit in awesome vapor,
some SOB has stole the paper

No longer here shall I linger,
beware hole, here comes the finger

Hmmm ... waxy paper, or the 'alternative'??? :eek:hnoes

I'll 'krinkle'.
 

boomdriver

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Joined
Dec 18, 2008
Messages
1,888
Location
texas
Corvette
87 z-51
Just takes a little more 'krinklin' if yer tender :D uh huh yup

Besides what's the alternative?

From a dorm stall wall:


Hmmm ... waxy paper, or the 'alternative'??? :eek:hnoes

I'll 'krinkle'.


when ya put it like that.....:ugh

Me too......
 

Schrade

Well-known member
Joined
May 2, 2009
Messages
1,315
Location
Not in CF; BANNED!!!
Corvette
'90 LT 5
Its better to jump the low pressure switch so you don't damage the clutch coil with hi amperage.
Test your coil. Check your cycling switches.

If all checks out ok, its likely the Power module in opt 68 or the compressor relay on the manual a/c system.
There should be power to the clutch plug at all times. The PM regulates by the ground path.

OP - you know how to jump this switch? I had a pic somewhere of this test; I can't find it now. Post back...

If your lines are charged, and the jump test passes, you could have a clogged orifice filter. Not a big deal, but you'll have to have the lines evacuated to replace it.

You can check RELATIVE flow with a Mickey Mouse gauge from Advance / Zone. If the low side port is near 0, and the high side is normal (or high), you've got blockage.
 

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