sowise

I feel like I have searched all over this site and the Internet and can't quite find anything specific to match the problem I am seeing.
First this is on a 2007 Ford F150 4.6L STX

Problem: Initially the fan blower would only work on High and so I followed simple suggestions and replaced the blower motor resistor. Everything was fine for a day or two and then I noticed it was again only working on High. I bought another register and installed it and checked the operation, everything was working fine until I touched the resistor and it was very hot and starting to cause the black plastic coating to melt and drip. I quickly disconnected it and have run it on High with no issues since. I have recently replaced the blower motor since I couldn't test the amperage draw and well there is no change the resistor still gets burning hot. Both the old and new fans work fine and there is no obstruction inside the fan housing since this problem happens when I run the fan outside of the housing. As a last ditch effort I also tried a used dash switch panel did a continuity check of the harness with no apparent shorts to ground other than the ground wire. I swapped the relay with the one for the headlight bright switch relay and still the resistor is burning hot. I have been through at least 4 resistors, one of them actually caused smoke to come out of the ducting and burning plastic smell. I think I have covered pretty much all of the components in this circuit but can't seem to figure out what is causing this. Any ideas or help would be appreciated.

homer

I had to recently replace my blower motor resistor as well. It will generate heat but if you take the resistor board out of the mounting spot, run the blower motor on high, how much air flow do you feel through that hole? The board will generate heat, it's unfortunately dependent on the ducting to cool it back down. If I take mine out of the mount and hold it in my hand on setting 2 or 3, it'll get hot... unfortunately that's the design. However, in your case it does sound like it's getting air flow across it as you're smelling it through the ducting. Can you check though that you feel air flow with it removed in the mounting hole? Otherwise you have far too much load for some reason, but the new blower motor drawing that much doesn't make sense either...

sowise

Thanks for the reply and the suggestion Homer. I have been a bit concerned about reinstalling the resistor since the smoke incident. I did think it would be normal to get warm since it isn't much of a heat sink at all for a resistor but I have been operating it on blower speed 1 mostly for check out. Air pressure has been great through the ducting I can feel it at all speeds when I have everything connected. The thing is I do think the A/C would help with cooling it somewhat but what about when the heat is on and blower is turned low? Am I being overly cautious after replacing everything? I am starting to wonder if I am so I will go run the truck with the resistor installed for a good bit to make sure.

PerryB

Sounds like your fan motor is drawing excessive amperage. Maybe the bearings are dry or the grease has congealed and its dragging ?

Great Ridin

sowise,
Did you solve the issue with the resistors burning up? If so, what was the solution?

I just experienced the same problem today... The fan was only working on high so I put a new resistor in and it burned up within 5 minutes of driving around. I'll be checking out the motor next, to make sure it is spinning freely, and check the motor for excessive current draw.

homer

I only had the resistor go once on mine, but as PerryB wrote if your resistor is generating that much heat there's likely too much draw. I would measure the draw on your blower motor.

sowise

I did get this to finally stop burning out but it is difficult to say what specifically fixed the problem. What I did in the end was replaced the resiator multiple times with at least two burning out I think it was the third or fourth one that I was using to check the temperature of it by hand while the blower was on different speeds. It always got hot except when bypassed at full blower. I did replace the blower and checked it was still getting hot I didnt want to burn it out so I did a continuity check on the harness and no shorts or issues found. I then replaced the manual heater switches (non electronic) and it still was hot when operating but I just installed it anyway and so far it hasnt burned out. I may have been overly cautious but it really is odd to get too hot to touch under normal operation.

woody2010

This is a really old thread, but I'm curious if anyone else has had to replace the blower motor resistor multiple times. I replaced mine on my 2010 for the first time about 2 months ago, and today I'm doing it again.

SCORGE

New parts can fail prematurely. I'd say if it happens a third time... you may have a wire issue somewhere

woody2010

Thanks, at least the part is cheap.