Progress Thread 96 GT Convertible when defrost turned on wants to kill it ..

[mike7880]

Have a 96 GT Convertible 4.6. Every time I turn the defrost on it gets lugged down and tries to die out. It's only when defrost is turned on. Both panel and floor heat is fine.

 

[Mustang5L5]

Defrost will engage the AC compressor, putting a load on the engine. Does it also do it when you turn the AC on?

With engine running, unplug the IAC. If the engine stalls, this is your issue. Find the throttle stop screw on the TB and open it up a bit. Restart the engine and the idle should be higher. Unplug the IAC again and hopefully the engine remains running. Now, back the throttle stop screw down to lower the idle down to a stable idle, a little on the lower side. Shut off engine and plug IAC back in. Restart and it should relearn idle and bring it down to a normal idle within a min or so. If not, you can unplug the battery to reset the ECU and then try again.


Reason it bogs and stalls would be that the throttle blade is closed too much meaning the IAC is 100% controlling idle. When the AC compressor engages, it can't compensate fast enough and you get a bog stall. IDeally you want the TB blade to provide 95% or so of the air needed for the idle and the IAC just tips in a little as needed to smooth things out or compensate for the load of the AC compressor

 

[mike7880]

Defrost will engage the AC compressor, putting a load on the engine. Does it also do it when you turn the AC on?

With engine running, unplug the IAC. If the engine stalls, this is your issue. Find the throttle stop screw on the TB and open it up a bit. Restart the engine and the idle should be higher. Unplug the IAC again and hopefully the engine remains running. Now, back the throttle stop screw down to lower the idle down to a stable idle, a little on the lower side. Shut off engine and plug IAC back in. Restart and it should relearn idle and bring it down to a normal idle within a min or so. If not, you can unplug the battery to reset the ECU and then try again.


Reason it bogs and stalls would be that the throttle blade is closed too much meaning the IAC is 100% controlling idle. When the AC compressor engages, it can't compensate fast enough and you get a bog stall. IDeally you want the TB blade to provide 95% or so of the air needed for the idle and the IAC just tips in a little as needed to smooth things out or compensate for the load of the AC compressor

Awesome. Thank you tons. I'm going to go out and do that in just a minute. Didn't even think about the fact that the defrost uses AC. And this motor was just out on the car a week ago. And has low miles but eas sitting for a while when I got it. . so thanks for taking time to read and give me some pointers. I'll let ya know what it does . Thanks again .