Originally Posted by guangui

What AC you have? Manual control with 4 settings? Or, automatic, where you set the temperature you want and it controls itself?

Originally Posted by jrmdir

After further thought and looking at the wiring diagram, I'm more inclined to believe it is a switch or wiring problem.

The resistor pack has three resistors wired in series. On position 4, voltage is applied directly to the fan motor (no resistors in the circuit) On position 3, one of the resistors is in the circuit. At position 2, there are 2 resistors in line in the circuit and at position 1, all three resistors are in line and in the circuit.

So when you say that in "failure mode", positions 1 and 2 still work, then current has to be flowing through all of the resistors - thus none are blown out.

The fan switch not only selects which resistor gets voltage, it also activates the heater blower relay so it's possible that your switch is losing contact with the relay on positions 3 and 4.

PM me and I'll try to send you the circuit diagram. WIth a 12V test lamp you should be able to trouble shoot the problem.

Ron

Originally Posted by jrmdir

After further thought and looking at the wiring diagram, I'm more inclined to believe it is a switch or wiring problem.

The resistor pack has three resistors wired in series. On position 4, voltage is applied directly to the fan motor (no resistors in the circuit) On position 3, one of the resistors is in the circuit. At position 2, there are 2 resistors in line in the circuit and at position 1, all three resistors are in line and in the circuit.

So when you say that in "failure mode", positions 1 and 2 still work, then current has to be flowing through all of the resistors - thus none are blown out.

The fan switch not only selects which resistor gets voltage, it also activates the heater blower relay so it's possible that your switch is losing contact with the relay on positions 3 and 4.

PM me and I'll try to send you the circuit diagram. WIth a 12V test lamp you should be able to trouble shoot the problem.

Ron

Originally Posted by jrmdir

Thanks Will:

Good info. The higher the setting, the greater the current flow. If the switch contacts or wiper arm are pitted and not making good contact, then since more current = more heat, at some point something loses contact and stays that way until it cools down. For this to happen sooner on 3 than on 4(and not at all on 1 and 2) would seem to make sense.

Here's a place you can call to check pricing on a switch (or maybe the whole panel if cheap enough) Used Mitsubishi parts, Used Saturn parts, Used Subaru Parts, Used Toyota parts and Dodge Import used parts

Let me know if you want a copy of the wiring diagram. It would be fairly easy to confirm that it's the switch using a test light or continuity tester.

Ron

Originally Posted by deusued

Ron,

Thanks for the responses.

I've been out of town since I posted this and will start my search for a switch shortly - looks like a dealership might be my only recourse.

An additional note: if the truck is started with the blower on setting 3, it will work for about 5 minutes; if the truck is started with the blower on setting 4, it will work for about 3 minutes. once "failed", the switch needs to be set off for about 5-10 minutes, then can be restarted. settings 1 and 2 never have this failure, as mentioned above. after the truck has been running 1 or 2 for a while and we switch it to 3 or 4, it fails --again as mentioned above.

I add this note b/c I spent some time driving around right before I left to try to qualify all of the means that the failure condition occurs.

Does the above change your recommendation at all?

- Will