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AC Causing Drop In RPM At Idle.

 
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  • 2 Post By techno2
  • 1 Post By ptcruisersteve

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  #1 (permalink)  
Old 14 Jun 2015, 12:59 am
Fresh Cruiser
 
Join Date: Apr 2015
Posts: 12
Smile AC Causing Drop In RPM At Idle.

On A 2007 PT Cruiser 2.4 L gas

When ever the Air Conditioner compressor would start up, the engine RPM
would drop below 500 and the car would want to stall. With each cycle of the AC clutch on the Compressor, the engine would stumble again. Drove me crazy. I thought it was a problem with the AC ... Maybe a partially seized up compressor ... or clutch assembly. After a long cold winter here in Montreal, Quebec, Canada. the cars sure take a beating from Mother Nature here.
Then I happen to come upon this Great forum and after reading many posts, I stumbled upon this piece of Golden Information about the IAC on PT Cruisers:

IDLE AIR CONTROL MOTOR (IAC)

DESCRIPTION

The Idle Air Control (IAC) motor is mounted on the
throttle body. The PCM operates the idle air control
motor

OPERATION
The PCM adjusts engine idle speed through the
idle air control motor to compensate for engine load,
coolant temperature or barometric pressure changes.
The throttle body has an air bypass passage that
provides air for the engine during closed throttle idle.
The idle air control motor pindtle protrudes into the
air bypass passage and regulates air flow through it.
The PCM adjusts engine idle speed by moving the
IAC motor pintle in and out of the bypass passage.
The adjustments are based on inputs the PCM
receives. The inputs are from the throttle position
sensor, crankshaft position sensor, coolant tempera-
ture sensor, MAP sensor, vehicle speed sensor and
various switch operations (brake, park/neutral, air
conditioning).

When engine rpm is above idle speed, the IAC is
used for the following functions:

Off-idle dashpot.
Deceleration air flow control.
*** A/C compressor load control (also opens the pas-
*** sage slightly before the compressor is engaged so
*** that the engine rpm does not dip down when the
*** compressor engages)

Target Idle

Target idle is determined by the following inputs:
Gear position
ECT Sensor
Battery voltage
Ambient/Battery Temperature Sensor
VSS
TPS
MAP Sensor

So, I ordered a new IDLE AIR CONTROL MOTOR (IAC) and will
replace mine as soon as I get the time.
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Old 14 Jun 2015, 02:05 am
Obsessed Cruiser
 
Join Date: Aug 2014
Location: Thousand Oaks, CA
Posts: 7,234
Default Re: AC Causing Drop In RPM At Idle.

Is your engine light on? If so what are your engine trouble codes?
Is both speeds on your radiator fan working?
With the engine cold, when you first start the engine, turn on the A/C does the radiator come on automatically? If not you need to find out why the radiator fan is not coming on.
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Old 14 Jun 2015, 11:21 am
Ray_Dockrey's Avatar
Senior Cruiser
 
Join Date: May 2012
Location: Mustang, OK
Posts: 1,213
Default Re: AC Causing Drop In RPM At Idle.

I am with Steve. What you are describing is a classic symptom of the high speed fan being out which is very common. They seem to fail at the 70,000 to 80,000 mile mark.
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Old 14 Jun 2015, 12:00 pm
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Join Date: Apr 2015
Posts: 12
Default Re: AC Causing Drop In RPM At Idle.

Is your engine light on? No... But, it was on before I did a few replacements, such as AC low pressure switch ...cam position sensor ...O2 sensor (lower).

If so what are your engine trouble codes? None

Is both speeds on your radiator fan working? No. Aha! Fan appears to not work!

With the engine cold, when you first start the engine, turn on the A/C does the radiator come on automatically? If not you need to find out why the radiator fan is not coming on. Fan not working .....
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