heater fuse blows when truck is running

[coldgin]

just replaced the heater fan motor on my 69 gmc....non a/c....it quit working Saturday night....so turned switch on and the fan wasn't working...checked fuse and saw it was blown....replaced and fan works....but when I start the truck in about 5 seconds the fuse blows....popped 2 like this?what could be my problem?

[LockDoc]

Originally Posted by coldgin

just replaced the heater fan motor on my 69 gmc....non a/c....it quit working Saturday night....so turned switch on and the fan wasn't working...checked fuse and saw it was blown....replaced and fan works....but when I start the truck in about 5 seconds the fuse blows....popped 2 like this?what could be my problem?

Does the motor seem like it starts up and runs OK? Does it sound like it is dragging?

I guess the first thing I would check is the resistor. It is in the top of the heater box on the inside of the truck. (under the glove box) There are two 5/16 hex head screws that secure it and it has Blue/Yellow/Orange wires that run from the heater switch to it....

Here is a picture of it. I think this picture was originally posted by member VetteVet.

The switch could possibly be bad also. I'm not sure how starting the truck would affect it though.

LockDoc

[HO455]

I suspect you are experiencing low voltage to the heater blower motor. When voltage drops amps go up. So if your motor draws ten amps at 12 volts it will draw 20 amps at 6 volts. If the amp draw is close to rating of the fuse before starting it will rise when the voltage drops as you start the truck and exceed the fuse rating. I would check voltage at the motor (with it on high) then compare it with battery voltage (battery voltage should be between 12 and 14 volts). There should be less than a volt difference. If it is more than one volt difference trace the circuit back checking voltage as you go until you find the voltage drop. If it is less then check the resistance from the motor power connection to ground. It should be a small amount. Then run a jumper wire from the motor case to a good ground and check again. They should be the same or very close. If they are different you have a bad ground to the motor. This should get to the problem. If not let me know and I can look into it further. As LockDoc pointed out the resistor may be the problem. The resistor gets very hot when the motor is running in slow speed. Over time this causes resistance to build up in the connector until the fuse blows. It seems like half the time someone then jams a larger fuse in and everything works until the wiring melts or the resistor burns in two. If your fuse box looks like mine did (see photo) the voltage drop (or part of it. It's quite likely there maybe more than one bad spot) could be in the fuse block.

[El Dorado Jim]

try cleaning up the fuse holders at the fuse block....can cause the problem you are having

[VetteVet]

The relationship between voltage and amperage is directly proportional i.e. as the voltage goes up the amperage goes up provided the resistance stays the same.
If your alternator is overcharging then the amps through the fuse will increase and blow the fuse. Check the voltage at the alternator with the truck running and place the fan on high to bypass the heater resistor. This will give you the maximum current flow through the fuse.

The relationship between resistance and amperage is indirectly proportional i.e. as the resistance goes down the amperage goes up and vice-versa, provided the voltage stays the same.

The heater fuse is rated at 10 amps so you shouldn't be drawing near that much normally.

[coldgin]

FIXED.....bad ground on the motor....removed the screw at the motor with the ground wire and scratched the paint off the motor and put screw back in with ground wire....started the truck and the fan motor worked...glad it was that simple...

[coldgin]

thanks for the input HO455

[HO455]

You are welcome. Bad time of year not to have heat.

[coldgin]

Well I spoke to soon....drove the truck today....after about5 minutes the fuse blew....gotta dig a little deeper in it this weekend....

[dmjlambert]

Are you certain you have a heater blower motor, and not an A/C blower motor, which draws more power?

[VetteVet]

Originally Posted by dmjlambert

Are you certain you have a heater blower motor, and not an A/C blower motor, which draws more power?

Very good point, the AC fuse is rated at 25 amps.

[coldgin]

Got the fan at oreillys....and it is a pm102...my buddy at work said it is the replacement fan for non a/c and a/c trucks.....so I was wandering about a bigger fuse....I know the new fan motor is definitely moving more air

[dmjlambert]

Well that is interesting. I would say if it is a replacement for non a/c and a/c trucks, that means it is an a/c fan. Perhaps the true heater fan is not available easily. The a/c fan draws more power and is more powerful.

The thing is, if a truck has a/c and runs the more powerful fan, they not only have a bigger fuse, but the wiring includes a relay to power the fan at the highest speed so all the current is not going through the fan selector switch. The "four season system" section of the wiring diagram shows the wiring for that, and I don't know if there is an online version of that diagram, but here's one attached. I don't know if Chevrolet is different from GMC but I'm thinking probably not. This is for 1969 Chevrolet. I'm thinking it may be a good idea to use a relay for the high speed, in addition to increasing the fuse rating.

[coldgin]

Thanks for the diagram

[HO455]

Originally Posted by dmjlambert

Well that is interesting. I would say if it is a replacement for non a/c and a/c trucks, that means it is an a/c fan. Perhaps the true heater fan is not available easily. The a/c fan draws more power and is more powerful.

The thing is, if a truck has a/c and runs the more powerful fan, they not only have a bigger fuse, but the wiring includes a relay to power the fan at the highest speed so all the current is not going through the fan selector switch. The "four season system" section of the wiring diagram shows the wiring for that, and I don't know if there is an online version of that diagram, but here's one attached. I don't know if Chevrolet is different from GMC but I'm thinking probably not. This is for 1969 Chevrolet. I'm thinking it may be a good idea to use a relay for the high speed, in addition to increasing the fuse rating.

Don't forget the resistor and the switches are built to handle the larger amperage of the A/C blower motor. Found that out the hard way on my GTO. Yes, the A/C blower made my defroster work ten times better, but when the switch and wiring melted a month later it wasn't so good. And back then switches were a junk yard only part. So it was a cold couple of weeks.

[coldgin]

Talking to some buddies at work they both have 65 c10....have replaced their blower motors....one of them had same problem and just replaced 10a fuse with a 20a....no more problems....it is a non a.c. truck....still haven't got into the truck yet....hopefully this week

[HO455]

If you can get a DC amp-meter measure the amp load as close to the fuse as possible. If is about 15 amps you should be ok. The higher the amp draw is the more likely you will have issues with the switch down the road.