Heater fan plug melting

[BB72CHEVKT]

I had a local shop install a new heater control, blower motor and the heater core back in November. The truck has a new painless wiring harness that I thought the shop (different shop) replaced all the wiring but it appears from my investigation under the dash that all wires may not have been replaced. I have been smelling the plastic from the fan switch melting every time I turn on the heater but I was thinking it was some by the heater core like plastic debris. The hot wire that make the fan come in is getting hot for some reason. Here is a pic of the plug coming from the blower motor that plugs into the back of the fan controller. There is also usually has a black wire plugged in as well but it melted. The yellow wire is also a little burnt were it goes into the connection. Any ideas why it is getting so hot?

[Mike C]

That plug goes to the resistor assembly inside the ventilation plenum. The resistor is what gives you low and medium on the fan speed. The high speed should run straight from the fan relay. Looks like someone has incorrectly wired something.

[BB72CHEVKT]

Originally Posted by Mike C

That plug goes to the resistor assembly inside the ventilation plenum. The resistor is what gives you low and medium on the fan speed. The high speed should run straight from the fan relay. Looks like someone has incorrectly wired something.

It seemed like there was only a high setting. The fan would not come until the lever was slid completely over.

[BB72CHEVKT]

Talked to the shop and theyclaim it was just plug n play on their end. Any ideas why the connection is getting so hot? It appears I only had one speed which was high if that has anything to do with it.

[BLT gmc]

The fan will have three speeds. I suspect the wiring was either made or installed incorrect. This is what most likely caused the wiring to get too hot. As Mike C stated the low medium wiring is run through a resistor that is screwed into the plenum, behind the glove box on the bottom left. Wiring diagram will be needed. Unfortunately, the wires will need to be tracked one by one. I am assuming this is a non-AC truck. The wiring should be fairly simple and straight forward. Harness is available from vendors here on the board or manufactures like M&H. Also need to verify the proper function of the switch. Best Wishes, Bruce

[slikside]

Sub'd. Good info here. My fan only works on med, and hi.

Good luck on the melting plug BB72CHEVKT.

[72MARIO]

I wonder if the blower motor resistor ( what that plugs connects to in the heater box) is bad ? Or the blower motor is drawing to many amps

[BB72CHEVKT]

This is a heat only truck. I just checked and it is new wiring from the switch to the resistor. I am not sure if the painless kit had this part premade or if the shop had to wire it and got something backwards? Here are some pics of both ends.

[BB72CHEVKT]

This is a heat only truck. I just checked and it is new wiring from the switch to the resistor. I am not sure if the painless kit had this part premade or if the shop had to wire it and got something backwards? Here are some pics of both ends. The end that goes to the switch which is melted has blue wire on top, yellow with white line in the middle, yellow bottom left and there was a black power wire on bottom right. The second pic goes to the resistor and has blue wire on top, lower left is yellow and white, lower right is yellow wire and orange wire going to blower.

[mrein3]

Years ago I converted my heat only truck to a factory AC system.
While collecting parts in yards and from used suppliers, I found that ALL the used plugs on the back of ALL the used fan switches were in various states of meltedness (new word - meltedness).

In the end I decided to simply order a new switch (still available from GM) and a new plastic plug for the back of the switch. I then bought correct gauge and color wire and rewired the whole harness. My thinking was the melting came from 40 or so year old wiring getting furry and losing conductivity.

Fast forward to this past spring. My new switch started acting wonky. I took a look at the plug and it was melted again.

I ordered another new switch and plug and put it back together with the intention of jumping up a size in the wire.

Long story short, don't go blaming anybody about your wiring. I'm starting to wonder if those resistors in the resistor block get too hot then transfer that heat back down the wire. I'm tempted to jump up a size in wire but that might not be the correct solution.

What happens in that resistor block is that the resistor drops the voltage down to 3 and 6 or 6 and 9 volts (I can't remember the exact numbers but I measured them the first time I rebuilt it). The lower voltage simply spins the fan motor slower. The rest of your 12+ volts gets turned in to heat which ends up wrecking your plug and switch.

In a perfect world a guy would design a circuit that does the same thing without those big resistors under there generating heat. Sounds like a future project.

[BLT gmc]

BB72CHEVKT-Over what peroid of time did your wiring melt? And how old is the wiring? I just want to make sure I have read post #1 accurately. Bruce

[davepl]

After 40 years the (bigger) connector on my AC truck was a little melty. Bothered me.

To be clear, I think only the AC trucks use the relay to trigger the fan. I doubt GM did that as a comfort/cooling-performance thing, so likely the larger fan required it.

If it was wiring, or a short in the resistor or motor, it should blow the fuse. Make sure it is actually plugged into the correct fused circuit. Maybe a PO got tired of it blowing fuses and plugged it into the IGN UNFUSED which has heavy enough gauge to melt the plastic. Wild guess.

If I had to make a more educated guess, I'd look at the blower motor resistor.

I could never find out what the ohm ratings are supposed to be, I did measure mine, wish I'd written them down, but what was for an AC truck anyway. Certainly not zero or infinity, but either of those would be obvious. So I tried to think of a way the resistance would be low enough to melt the plastic but not trip the fuse:

Mine was a little corroded. I wonder if you get a fine accumulation of gunk, debris, and oxidization that it might cause the resistance to drop. That would give you the "similar speed on all settings" feature and cause the resistor to get really hot when on any setting other than High. Remember these are right in the air intake path from the outside air. They've seen a lot over their lifetime.

So that's my guess (this last part).

[BB72CHEVKT]

[QUOTE=BLT gmc;7068558]BB72CHEVKT-Over what peroid of time did your wiring melt? And how old is the wiring? I just want to make sure I have read post #1 accurately. Bruce[/QUOT

All new wiring, connectors/plugs, and new heater/ fan control. It was almost exactly 90 days. It was apparently melting a little each day. I thought the smell was from the new heater core but it was apparently the power wire going to the fan controller. I have ordered a new wire and resistor.

[Ugly 61]

The blower motor resistor on factory air trucks is mounted inside the case on the inside of the cab in the airstream from the fan so that the fan airstream will keep it from melting, on the factory air trucks the blower fan relay is energized when the fan switch is moved to the high speed position, this takes power directly from the fuse block and sends power directly to the fan motor completly bypassing the blower resistor. I rebuilt the factory air unit on my 72 GMC and for some time I only had low and med speed on the blower it turned out the power wire to the blower relay had come off at the fuse block the connecting plug on the back of the switch was fine.

[special-K]

Is the heater itself getting that hot to touch? Is air coming out full force? Could be it's plugged up in there from mouse nesting or whatever. I've seen melted flappers inside from that