Idle Not Increasing With Ac On

[Scott7512]

Why does my idle drop instead of increasing when I turn the ac on? I have an electric fan and a new 120 amp alternator Also have a new iac. I was told the car has a x303 cam. New bbk mas air and the throttle body is spotless. Even put on the larger wire from alternator. The ac blows cold. I have done the idle reset procedure many times. The alternator is putting out over 14 volts but spikes down when the ac and fan kick back on after tripping the wot switch. If I set the base idle below 1000 the car will stall out after the wot switch is tripped or if I press the clutch to stop at a light. The car has underdrive pulleys on it but I have installed an overdrive pulley on the alternator to get it back to the correct speed. The only things I can think of now is the compressor is going bad and putting a huge load on the motor or maybe there is something wrong with the battery. I just keep throwing money at this pleas help before my wife finds out how much I have spent!!!

 

[Scott7512]

Just checked battery voltage after sitting for 8 hours and it is 12.6.

 

[jrichker]

The IAC isn't working correctly.

You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix.

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 159,000 hits, which indicates it does help fix idle problems quickly and inexpensively.

 

[Scott7512]

Thanks for the link but I have been through it before. The car idles smooth and doesn't surge at idle. It drops 100 rpm when the ac is on. Also with it on, when the ac kicks back on after a big rev the iac can't catch it and it dies. The only way I can get it to work is having the idle set at 1100 and then the iac catches it. I guess I will have to live with 1100 rpm with the X303 cam.

 

[Phillip123]

If I got this right, when you switch on the AC that is a comand to the PCM, it than turns the compressor, so it is not you who turns it on. Than, the PCM also monitors the RPM and will compensate for the load if the compressor drops RPMs. Now, if your AC is broken or smth, the PMC will know it and the idle will not change.

 

[Scott7512]

The air blows cold and the compressor is not kicking on and off a bunch so I believe it has enough refrigerant.

 

[jrichker]

What happens when you do the base idle set procedure as described in the Surging Idle Checklist ?

 

[Scott7512]

Same thing. The only way to get it to idle with ac reliably is to set it at 1100. When the ac kicks on it drops to about 950 and after revving the engine the iac catches it. The tps is at .945 volts also.

 

[jrichker]

Just for the sake of a little review, lets take a look at the procedure to make sure we are both on the same page...


Setting the base idle speed:
First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical adjustment. The 600 RPM base idle speed is what you have after the mechanical adjustment. The IAC increases that speed by supplying more air under computer control to raise the RPM’s to 650-725 RPM’s. This figure will increase if you have a wild cam, and may end up between 800-950 RPM

Remember that changing the mechanical idle speed adjustment changes the TPS setting too.

This isn't the method Ford uses, but it does work. Do not attempt to set the idle speed until you have fixed all the codes and are sure that there are no vacuum leaks.

Disconnect the battery negative terminal and turn the headlights on. Leave the battery negative terminal disconnected for 5 minutes or so. Then turn the headlights off and reconnect the battery. This erases the computer settings that may affect idle performance.

Warm the engine up to operating temperature, place the transmission in neutral, and set the parking brake. Turn off lights, A/C, all unnecessary electrical loads. Disconnect the IAC electrical connector. Remove the SPOUT plug. This will lock the ignition timing so that the computer won't change the spark advance, which changes the idle speed. Note the engine RPM: use the mechanical adjustment screw under the throttle body to raise or lower the RPM until you get the 600 RPM mark +/- 25 RPM. A wild cam may make it necessary to increase the 600 RPM figure to 700 RPM or possibly a little more to get a stable idle speed.


Is this the procedure you were using to set the base idle speed?

 

[Scott7512]

Yes. But with the cam 650 rpm just won't work. The ac will kill the motor right away. Either the computer and iac can't keep up or the ac compressor is putting too much load on the motor. I have given up and am just going to leave the idle at 1100. I can live with it. Thanks for trying to help.