Three common possibilities. One is a locked-up AC compressor. The stalling will only occur when you switch to "defrost" mode, not floor heat. The AC system runs in defrost mode to condense the humidity out of the air before it is blown onto the cold windshield where it would cause fogging. If the compressor is seized, it can stall the engine when its clutch engages.
A second, even more common problem is the charging system. If one of the six diodes inside the generator fails, the most current you will be able to get is exactly one-third of the generator's rated maximum current. That may not be enough to run the entire electrical system under all conditions. The clue you need to look for is lights dim noticeably when you turn on the heater, and/or the engine will stay running if it is at a higher speed when the heater is switched on. Also, there will be less tendency to stall if the heater fan is on a lower speed.
On many cars of this era, the heater fan is powered through the same part of the ignition switch as that which turns on the Engine Computer. When the heater fan is used on the highest speeds a lot, that stresses the contacts in the ignition switch and can overheat them. With that type of circuit, a total switch failure will cause the engine to not run at all. When the bad contacts are not quite so severe yet, they can limit the amount of current that can get through. You might be able to run the engine or the heater fan, but not both. A potential clue you can look for is to observe how fast the heater fan runs with the engine off, then whether the fan slows down when turn on another system that is on the same circuit. That could include the wipers, the radio, or even the power windows.
Monday, April 3rd, 2017 AT 7:07 PM