Low idle when AC turned on
Thread Starter
Pro
Joined: Nov 2006
Posts: 727
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From: Ocean City Maryland
Low idle when AC turned on
I have an 88 Corvette and it runs ok except when I turn on the AC. The car idles between 700 - 800 RPM with the air off, but when the air is turned on it drops between 500 - 600 RPM but when it hits 400 it stalls. I suspect an intake leak but is there anything I can do to compensate and keep the ECM happy until I can get the intake leak fixed?
Thanks in advance
Thanks in advance
Instructor
Joined: Mar 2010
Posts: 216
Likes: 4
From: Shrub Oak NY
Sounds like the ECM doesn't know you turned on the AC. Before ECM's there was a vacuum diaphragm on the carburator which raised raised the idle when the AC was on. Now I think there's an input to the ECM to tell it the AC is on so the ECM can raise the idle.
Team Owner
Joined: Jan 2003
Posts: 20,306
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From: Summerland B.C. Canada
ECM does get an AC signal.
Does your cooling fan come on when AC is on ?
The idle air control valve should be open more to allow for the extra load. Is the IAC connected and working ?
Le Mans Master
Joined: Dec 2000
Posts: 7,839
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From: San Diego, Ca
HVAC controls simply sends a voltage to the ECM when a/c is requested. You verify/diagnose with a scanner which will show a "Yes" in the a/c field. You will also want to look at Targeted RPM vs Actual RPM, IAC counts and Load Factor.
Thread Starter
Pro
Joined: Nov 2006
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From: Ocean City Maryland
I will have to check and see if the fan comes on and get back to you. The IAC is connected but not sure whether its working or not. I was going to do the minimum idle adjustment and the tps adjust to make sure both are correct
Thread Starter
Pro
Joined: Nov 2006
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From: Ocean City Maryland
Scanner
Wish I had a scanner but not, so Im going to try some adjustments
Thread Starter
Pro
Joined: Nov 2006
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From: Ocean City Maryland
ac idle
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Team Owner
Joined: Jan 2003
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From: Summerland B.C. Canada
Yes, it should come on when AC head pressure reaches 233 psi.
AC input to ECM comes from HVAC control head. ECM just sees voltage from engaging AC and should adjust IAC to take up the extra load.
Here is some info from my 86 FSM
http://members.shaw.ca/dankai/HVAC%20Electrical.pdf
http://members.shaw.ca/dankai/HVAV'S.pdf
AC input to ECM comes from HVAC control head. ECM just sees voltage from engaging AC and should adjust IAC to take up the extra load.
Here is some info from my 86 FSM
http://members.shaw.ca/dankai/HVAC%20Electrical.pdf
http://members.shaw.ca/dankai/HVAV'S.pdf
Thread Starter
Pro
Joined: Nov 2006
Posts: 727
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From: Ocean City Maryland
minimum idle
Yes, it should come on when AC head pressure reaches 233 psi.
AC input to ECM comes from HVAC control head. ECM just sees voltage from engaging AC and should adjust IAC to take up the extra load.
Here is some info from my 86 FSM
http://members.shaw.ca/dankai/HVAC%20Electrical.pdf
http://members.shaw.ca/dankai/HVAV'S.pdf
AC input to ECM comes from HVAC control head. ECM just sees voltage from engaging AC and should adjust IAC to take up the extra load.
Here is some info from my 86 FSM
http://members.shaw.ca/dankai/HVAC%20Electrical.pdf
http://members.shaw.ca/dankai/HVAV'S.pdf
Thanks for that information. I started doing the minimum adjustment procedure but I dont think I have the tps adjustment right and Im going to relook it tomorrow. I did however start it up and the idle was about 700 rpm and when I turned on the AC the fan did come on and the idle changed to abot 800. Only thing is that I have service engine light which means it threw a code but I ran out of daylight. I did test drive it and it ran the best it ever has, but when I returned the idle jumped to 1200. I'll get back to it tomorrow.
Melting Slicks
Joined: May 2009
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From: Decatur IL
2025 C4 of the Year Finalist - Unmodified
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I'm having similar issues with my 90. I'm leaning on the iac valve. That valve is supposed to keep the engine from stalling under load changes.
Thread Starter
Pro
Joined: Nov 2006
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From: Ocean City Maryland
I realized over night that prior to my test drive I forgot to plug the MAF back in. I guess it could explain the change in idle. Going to plug it in this morning and dee what happens
Thread Starter
Pro
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From: Ocean City Maryland
Thanks
Le Mans Master
Joined: Dec 2000
Posts: 7,839
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From: San Diego, Ca
What else can we tell you? A/c request to the ECM so that it can handle the load. See if you can download something to your cell phone to scan it (you can for OBD 2). My guess is that your ECM is toast, but only because that's fairly common when this happens - you could also try an a/c shop, though most only have OBD deuce stuff these days (which is kinda mandatory for modern a/c).
Thread Starter
Pro
Joined: Nov 2006
Posts: 727
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From: Ocean City Maryland
What else can we tell you? A/c request to the ECM so that it can handle the load. See if you can download something to your cell phone to scan it (you can for OBD 2). My guess is that your ECM is toast, but only because that's fairly common when this happens - you could also try an a/c shop, though most only have OBD deuce stuff these days (which is kinda mandatory for modern a/c).
Le Mans Master
Joined: Sep 2006
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From: Phoenix AZ
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When my 86 was doing the idle dance,
I unplugged my ignition control modul over by the heater, and forced it to go into limp home mode. I shut it off, then plugged it back in, started it back up, and the hunting was gone.
Sort of like you gotta get them ol electrons moving again..............
Can't explain it, but I was trying anything I could think of, and that worked.
I unplugged my ignition control modul over by the heater, and forced it to go into limp home mode. I shut it off, then plugged it back in, started it back up, and the hunting was gone.
Sort of like you gotta get them ol electrons moving again..............
Can't explain it, but I was trying anything I could think of, and that worked.
Thread Starter
Pro
Joined: Nov 2006
Posts: 727
Likes: 0
From: Ocean City Maryland
When my 86 was doing the idle dance,
I unplugged my ignition control modul over by the heater, and forced it to go into limp home mode. I shut it off, then plugged it back in, started it back up, and the hunting was gone.
Sort of like you gotta get them ol electrons moving again..............
Can't explain it, but I was trying anything I could think of, and that worked.
I unplugged my ignition control modul over by the heater, and forced it to go into limp home mode. I shut it off, then plugged it back in, started it back up, and the hunting was gone.
Sort of like you gotta get them ol electrons moving again..............
Can't explain it, but I was trying anything I could think of, and that worked.