Rear AC Switch Keeps Melting

[teafreak35]

Thanks in advance for any help you all can offer.

2002 Chevrolet Astro with rear AC. About a year ago, REAR ac switch stopped working on high. Medium would only work if you moved switch between the medium and high marks. I figured it was the switch so I ordered a new one. When I pulled off old switch, I noticed that the connector and switch were burnt. Figured the switch had gone bad so I popped the new switch in and everything worked fine for a couple of months. After a couple of months I noticed a burning smell in the car AND a wisp of smoke coming from the switch. Pulled the switch out and it was beginning to melt a little on back side and connector was toasty too. About this same time, I decided to have AC compressor replaced because of a leak. Once the compressor was replaced, I had cold air again and the rear switch would work on medium & low so I drove it that way for a couple weeks after repairs thinking I'd look into rear AC switch issue later. Towards end of the second week, the rear air got to where it would blow, but it wasn't cold. Felt lines going to rear of van and neither was cold so I assumed no coolant was being circulated. I thought that perhaps trash had gotten in line when they replaced compressor or an expansion valve had gone bad or something.

Took it back to repair shop and they said it was the rear evaporator core. Ordered a new EC and they put it in, but still no cold air in rear. Next, they said it was the resistor pack. Ordered a new resistor pack and they put it in and the cold air was back. However, driving home from shop, the REAR AC switch started smoking AGAIN. Pulled it out and it is toast, along with the connector...both melted beyond use.

Here is what I'm thinking after looking at wiring diagram in my Hayne's manual. The rear AC switch connects to AC Resistor Pack and then to rear blower. I've been thinking the culprit was the AC switch, however, now I'm thinking that the problem is the rear blower motor. Theoretically, if it is pulling too much current, the weakest part in the chain would suffer the most...in this case the front AC switch that is made of plastic. The next thing to go would be the resistor pack, I would think. When they pulled the resistor pack out, the coils were burnt on it, so I'm thinking that the extra current eventually got the resistor pack. Here are a couple of questions for you guys:

1 - Even though the blower motor works on all speeds, could a worn out blower motor make toast of the resistor pack and the switch?

2 - Does the resistor pack have anything to do with opening a valve or something in the Evaporator Core? In other words, is the coolant ALWAYS circulating through that EC if the compressor is running OR does it circulate only when the rear AC is activated at the switch? Another way of asking this is, could a faulty resistor pack really cause the rear air to blow, but not be cold?

I think I've had a chain of problems that probably began with a worn out blower motor. However, I'm tired of just throwing parts at this van so I'd like to have a better understanding of how the system works before I purchase any more parts.

Thanks again for any guidance you all can provide!

[Mmusicman]

Yes, as a motor gets older, it can most certainly start to slow a little, drawing more current.

But it's a guess to probably say the motor is now exceeding the switch's capacity, whereas before it was ok.

Keep in mind that the slightest resistance at a connector will generate heat. As the heat increases, so does the resistance. It's a chain reaction, until the connection, the connector, or the switch, burns up and fails.

The resistor pack falls into the same problem area. Usually where the parts are burned, that is where the problem was.

Sidenote: I had a powerful electric radiator fan that kept burning up connections on a Bosche relay. Eventually I soldered the connection, and problem never happened again.

[Leeann_93]

Absolutely.

The blower motors on these vans are brush motors - you actually can pull the blower motor, remove the rear cap, clean the brushes and dress them with a file, blow out the dust, then put it back together and have it work like new. Unless, that is, the motor itself has gone bad.

I bought a new blower motor for the white '02 because the original wouldn't blow at all when turned on unless you gave it a pretty hard whack, then my husband showed me how to take apart, clean, dress and test the old one - and it was just like new. Once it was cleaned out and the brushes dressed, it spun out of my hand when connected to a 9v battery. So I have that one on the shelf as a spare.

So, replace the switch, connector, resistor AND blower motor. Double-check your ground connections while you have things apart, just in case.

[teafreak35]

Thanks everyone! You've confirmed what I thought the issue was. I'm going to go ahead and try to find a new wiring harness for that switch while I'm at it since the other is burnt pretty bad.

[teafreak35]

I still have a question about the operation of the evaporator core. Does refrigerant circulate through the EC whenever the compressor is running OR only when the rear AC switch is engaged on the dash? I'm trying to diagnose whether or not the problem I had with the rear AC blowing hot air could have been related to the resistor or switch burning up.