code 79...Even when in off position

90mustangfan

New Member
Oct 18, 2005
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Ok every time I do a KOEO test I alway first get code 79(A/C or defrost on during self test.) Even when the switch for the a/c is in the off position....I recently had all the a/c components replaced(comporessor, condesor, hoses, freon.. whole a/c system) And when the switch is in off position, When I check the a/c compressor the a/c clutch is kicking on. cycling on and off, like I have the a/c switch in on position, when I have it the off position. and when I unplugg the wire to the a/c clutch it stops cycling...also when I have the Key in the ignition turned all the way on where it primes the fuel pup and all the gauge lights and CEL is light up, but not starting the car...IF I turn my a/c switch onto max or vent or and position, It interupts the starting and redoes the fuel pump prime, and will make it flash the CEL lights in the dash every time I a/c switch to a different position...I know this is a electrical problem, any one know where to start looking? Could it be a bad ground?
 
Code 79 - A/C is on, or computer pin 10 is shorted to power.

See the diagram below and you will find the very first wire starting from the bottom of the computer connector is a pink/lt
blue wire that connects to pin 10. Disconnect all the items plugged into the pink/lt blue wire like the pressure cutout switch and the
A/C defrost switch. Look for the 12 volts to go away when the pressure cutout switch and the A/C defrost are disconnected. If they
don't, the WOT relay wiring may be bad. The WOT relay is located on the inner wheel well under the Mass Air meter.

Diagram courtesy of Tmoss & Stang&2birds
88-91_5.0_EEC_Wiring_Diagram.gif


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif
 
Thanks alot, I test all the wiring and figured out I have a bad WOT relay...I guess the people before me put a wire to make the a/c clutch come on all the time, and then I tested the relay and found it was bad and thats why they had it wired incorrectly...Now all I need is a wot relay....Im not sure wheres the best place to get one...autozone has one for 15 bux, should I check else where ?
 
How was the WOT relay rewired? In stock form, power to the clutch travels through the common and Normally closed terminals. WHen the puter tells the puter to shut off the AC, it sends a ground pulse to energize the WOT relay, thus opening the circuit.

I initially thought someone might have wired your compressor clutch to VPWR at the relay, but if it cycles it doesnt sound like this was done.

Good luck.
 
on the relay, when you open it, they put a wire that connects the power or something so it gets constant voltage, and always makes the compressor clutch to stay on, I tested the relay and its bad.