My 89 F-150 with a 4.9l engine with a 4 speed trans when I turn on the ac or defroster the idle stars surging sometimes to the point of cutting off. When I turn the a.c. or defroster off it idles fine. Replaced the iac and no better. Any ideas what this could be?

 

Did you get this figured out... I have the exact same problem and haven't been able to figure it out. Any help would be great!

 

clean the ISC-BPA really really good and be done with it.

 

been there...hell still there. cleaned in really really good......still aint done with it

 

ok I reread the opening, 'surges to the point of cutting off' - these are 2 opposites. is it surging (racing ahead) and then falling back, or is it normal then almost stalls when the compressor clutch engages?

if the latter, find the wiring diagram and wiggle test line 348 in the harness (ac demand)

 

Sounds like your IAC(Idle air control valve) is bad. I had the same problem with my 92 F150. I replaced the IAC, and now the AC/defroster works fine.