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73-87 Chevy _ GMC Trucks => Electrical => Topic started by: 78Copperhead on March 01, 2016, 10:34:52 am

Title: 78 Scottsdale Blower Motor and Fuse
Post by: 78Copperhead on March 01, 2016, 10:34:52 am

Hey all!  I've used this forum for some good info on various issues with my 78 Chevy Scottsdale but even after reading posts that are related to the problem I have, I'm still coming up sort trying to fix my issue.  My blower motor does not work and for several months before it went out all together it was only working about half the time.  I started simple and replaced the resistor.  That didn't work so I replaced the relay.  That didn't work either so I replaced the motor and still no dice.  I've taken the in-dash temp control switch our and cleaned it as well as bypassed the radio interference thing from the relay to the motor and still nothing.  Still the blower motor does not run on any fan speed but when I switch it to the high position, the fuse blows. 

Thanks for any help you can give me!

Title: Re: 78 Scottsdale Blower Motor and Fuse
Post by: bd on March 03, 2016, 10:06:54 pm

Welcome to the site!

1978 Wiring Manual (http://www.73-87chevytrucks.com/techinfo/7387CKMans//Wiring/ST_352_78_1978_Chevrolet_Light_Truck_Wiring_CK_10_30_Only.pdf)

With or without A/C? 

If it stops blowing the fuse on high when you unplug the blower relay, then the blower relay needs to have two of its wires reversed in the relay connector.  Service blower relays contain a clamping diode that is inappropriately forward biased when installed in the GM trucks that effectively creates a short circuit on high speed.  It is a problem that results from trying to make one part fit too many applications.  When properly wired, the diode helps suppress electrical arcing across the blower switch points when high blower is deselected for a slower fan speed.  The symptom of blowing the fuse probably began with the installation of the new relay.  The wiring to the relay is easy to correct.  More on that later.

It probably has a faulty blower switch or burned wire connections to the switch.  But, rather than throwing more parts at it, what happens if you apply battery voltage directly to the blower motor terminal using a jumper wire from battery positive?