99 gt ac problem!

99mustanggt2018

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May 16, 2018

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Jun 14, 2018

#1

• Jun 14, 2018

• #1

Ac not working I have tested all connections everything is getting power
Also when compressor is plugged up car starts “loping” up and down about 900rpm to about 1400 back and forth when compressor is unplugged car idles fine.
When wiring straight from battery to compressor it kicks on and cools the car just fine.

Any help would be appreciated thanks!

 

wmburns

SN Certified Technician

Aug 14, 2009

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Jun 14, 2018

#2

• Jun 14, 2018

• #2

99mustanggt2018 said:

when compressor is plugged up car starts “loping” up and down about 900rpm to about 1400 back and forth

Click to expand...

^^this is a sign that the system is short cycling. What are possible causes of short cycling? Low on Freon.

The PCM maintains two IAC idle trim values. AC on and AC off. When the AC is low on Freon the compressor does not stay running long enough for the PCM to re-learn new IAC trim values for the AC on.

How to fix? Add Freon. Another option is to temporally jump the low pressure switch. This is force the AC to stay running. Allow about 5 minutes with the motor at full operating temperature with the AC on.

Note, after a battery disconnect the PCM will "forget" the IAC idle trim values. So EXPECT the motor to idle poorly after a battery disconnect.

 

99mustanggt2018

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May 16, 2018

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#3

• Jun 14, 2018

• #3

Thank you! I’ve tried to add Freon but it will not take anymore?

 

wmburns

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#4

• Jun 14, 2018

• #4

Then try the "jump" low pressure switch method. The idea here is that the compressor has to stay running long enough to allow the PCM to learn.

Have you double checked that the cooling fans work on BOTH speeds?

Is the outside coil and radiator clean?

 

Last edited: Jun 15, 2018

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99mustanggt2018

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• Jun 16, 2018

• #5

Okay jumping the low pressure switch make the car idle perfect and ac cools great when connecting it back it kills the power to the compressor and starts loping again

 

wmburns

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#6

• Jun 16, 2018

• #6

When the idle is loping is the AC clutch cycling on/and off? Please bear in mind that with the low pressure switch disconnected the AC clutch should never kick in.

If the AC clutch is not engaging while the idle is loping, the I suspect the unstable idle is for other reasons.

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

What is the low and high side pressure while this is happening?

What is the temperature of the inside AC discharge air?

Have you CONFIRMED that the cooling fans work? Are the cooling fans running continually during this test?

Note, it's also possible that the low pressure switch is bad. The part is cheap enough to replace for diagnostic reasons. Be advised that rarely will this "fix" the problem (mostly just wishful thinking).

 

99mustanggt2018

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#7

• Jun 16, 2018

• #7

Just went and bought a low pressure switch from advance and fixed my issue! Thanks for the help!