Fuse pops when heater is turned on

[DonSTP]

My heater used to work great but recently I turned it on and blew the Heater fuse.  So far the only thing I can tell is the fuse ONLY blows when the heater switch it moved to the ON position - in other words there doesn't seem to be another circuit conflicting with the heater, thereby causing the fuse to blow.

I have the schematics, and upon initial inspection everything seems to lead to the RELAY assembly, RESISTOR assembly or BLOWER MOTOR. 

Anyone have any experience with this?  Any suggestions?

Thanks,
Don
68-RS

[Bjv69ss]

I'd reach under the fender and unplug the orange lead to the blower motor.  Use a test light to see if there is power when the swith is switched on, if you get a light and not another blown fuse your blower motor is stuck.  Either the motor is siezed or there is something stuck in the fan keeping it from turning.  If the fuse blows you have a dead short.  Look for the orange wire to be chafed through and touching ground. 

Not a nice job to change a blower motor if it is one. 

Good luck!
Joe

[rich69rs]

Don,

This is a 30A fuse, so if it is blowing, there is a very significant current draw.

I would concur that the most likely causes are possibly that the blower motor is tight and drawing too much current or there is a direct short to ground.  If it happens immediately, I'd be more suspicious of a direct short to ground. 

As Joe suggested, check the orange wire (blower motor lead) carefully to be sure that it is ok.  Blower motor grounds through the casing, so if you unplug the orange wire from the blower motor to test the circuit, your test light will need to be connected between the orange wire and ground in order to complete the circuit.

Good luck,

Richard

[DonSTP]

Interesting.  I had a 25A fuse in this slot. 

I'll double check the docs and get the right fuse and then run the test you guys suggested.

Thanks for the tip.  I'll respond back with what I find.

Don

[DonSTP]

Ok - finally got around to trying this experiment. 

First, I wasn't aware that the blower motor was in the front fender well.  Not knowing where to look, I actually snooped around just above the tire under the fender - both the driver and passenger sides.  No orange wire or blower motor. 

Then I opened the hood and low and behold there was a bright orange wire leading from the fire wall to the passenger side front fender.  The blower motor is actually just under the fender in the engine bay, up by the windshield.  Again, if you follow the orange wire you can't miss it.

Next, I disconnected the orange wire and hooked up a volt meter with positive lead to the orange wire and negative lead to the negative on the battery terminal.  I inserted a 25Amp fuse (incidentially this is the correct fuse rating, not 30Amp as was previously suggested), turned the ignition to BATTery position and moved the heater switch to HOT.  I checked the volt meter and there was no power to the orange lead.  This told me I had a direct short to ground somewhere...not fun.

Fortunately I found the ground short pretty quick.  I had installed an original AC Delco AM radio last summer and although it worked great the ground lead to the radio was shorting out the heater circuit.  Since I didn't need the heater in the summer (I live in New England) I didn't find this issue until late this Fall. 

So - I pulled the ground wire from the radio and retested the power to the orange wire and WALA...12volts!

And the heater still works great after reconnecting the orange wire.

Thanks for the tips on this one.  If Rich and Joe hadn't pointed me to the orange wire I would probably still be scratching my head since my schematics DO NOT show an orange wire going to the blower motor. 

So all is well and once again the CRG saved me!   

Thanks Rich and Joe!

Regards,

Don StP
68-RS

[DonSTP]

   Correction! 

I just looked at my Chilton's manual and it actually shows an orange wire going to the blower motor.  The schematics I purchased (Jim Osborn Reproductions, Inc.) don't show an orange wire. 

Anyway, problem solved.

End of thread.

[Bjv69ss]

Glad to have been able to help!
Joe

[Marty]

Your factory AM radio should only have three wires from the back. Two are for the speaker and one is power. The radio is grounded by the mounting screw to the support bracket to the car body. Do you have the mounting bracket, p/n 3886996. It's all in the Assy Manual, U63, A3.