Car stalls when turning on heat/defrost

[STL79Coupe]

My son's 83GT stalls when you turn the heat/defrost switch on. You can restart it in the heat position but not the defrost setting. Is this a electrical or vacuum problem?

[grabbergreen84]

Sounds almost like the compressor is locked up.

[STL79Coupe]

I forgot to mention the A/C has been deleted.

[mudgepondexpress]

But are the wires for the fan clutch terminated correctly? Are they grounding out on the block when he turns on defrost?

[Matt J]

The dashpot on the carb/cfi activates when the AC is turned on, it's only a wild guess on my part, but it's possible that either the unit is leaking, or there's another vacuum line someplace that needs to be plugged. If it actually stalls the car, it would have to be a pretty huge leak, though, and I expect you'd be able to hear it.

[STL79Coupe]

It stalls at idle I haven't tried it while driving yet. We just bought the car and now we are finding all the problems unfortunatly.

[Jerry peachuer]

Don't turn the heat on and make sure the defrost is off when you start it that should work for you ....ha ha just kidding.

The defroster wire is a pretty big gauge wire look at your rear window to find the color if it has not been spliced trace it down and un hook it just to see if that eliminates the possible direct short to your solenoid area where it hooks up
As for a vac leak. A simple gauge test would rule that out quickly
What is your RPM set at?? the initial draw or load on the system could overcome your curb idle just bump it to 900 rpm see if that works or if it is any better some cars have a kicker solenoid to bump idle up to help the strain from the on demand power accessories also check fuse panel and make sure fuses are properly sized and if you find some real artist work look the car overt with a magnify glass to be certain it's safe to drive
Congradulations on your purchase

[JACook]

Let's be clear here - We are talking about the HVAC system, not rear window defrost?

The A/C idle kicker on an '83 GT is electrical, with no vacuum connection. (There were no CFI GTs in '83.) And I
have never known the HVAC vacuum system itself to flow enough to cause a stall if you switch to a mode with a
leaking vacuum motor. And besides, defrost is the one setting that does not operate any vacuum motor.

The compressor would have been a great guess if this car still had one. Apart from that, the compressor clutch
circuit is still highly suspect, unless this doesn't happen when the control is in either of the A/C positions. Likewise
if the throttle kicker is involved in this somehow.

[STL79Coupe]

Let's be clear here - We are talking about the HVAC system, not rear window defrost?

The compressor would have been a great guess if this car still had one. Apart from that, the compressor clutch
circuit is still highly suspect, unless this doesn't happen when the control is in either of the A/C positions. Likewise
if the throttle kicker is involved in this somehow.

I just tested it in the A/C setting and it dies. It will die in the Defrost setting and the A/C settings, but is OK in the heat and vent settings. How and what do I look for to fix this? I also noticed that the flow of air only comes out of the defrost vents. I can post a picture of the engine compartment if that helps.

[mudgepondexpress]

I would turn in the ac and try to start it with a spare plug inserted in the coil wire. I'll bet you have a really weak or no spark. If the heater box hasn't been changed then you should have a pressure switch on the dryer (big silver can). Disconnect this and try again. It really sounds Luke a dead short to me and you need to elinate the power that went to the ac pump.

[JACook]

If the A/C system is empty, the Low Pressure CutOut switch is as good as unplugged already. But that doesn't
eliminate the kicker solenoid from possibly being involved in this somehow. So that would be a good thing to
try disconnecting.

But if any of these things is enough to electrically cause the engine to die, you really need to examine your
ignition switch, sooner rather than later.

[STL79Coupe]

If the A/C system is empty, the Low Pressure CutOut switch is as good as unplugged already. But that doesn't
eliminate the kicker solenoid from possibly being involved in this somehow. So that would be a good thing to
try disconnecting.

But if any of these things is enough to electrically cause the engine to die, you really need to examine your
ignition switch, sooner rather than later.

I finally replaced the ignition switch and no change at all.

[KevinK]

Since the car had AC, it would be easy to rule out the HVAC control panel by blocking off the vacuum source under the hood. It's a simple test. The HVAC controls fall apart when the bracket breaks on the back side.

[STL79Coupe]

Since the car had AC, it would be easy to rule out the HVAC control panel by blocking off the vacuum source under the hood. It's a simple test. The HVAC controls fall apart when the bracket breaks on the back side.

I'll be checking the vacuum line, but the fan isn't working as well.

[STL79Coupe]

Finally got this all figured out. Most of the problem was a ton of bad wiring and one vacum line was unhooked. The switch panel was also broken.

[mustangxtreme]

Congrats on getting to the bottom of this one.