Blower motor

[Happytexan]

Hello all. This is my first post on the forum. Truck data first: 1974 C10 Cheyenne super, factory 454, factory a/c still working (with an issue), th400, 2wd, factory 3:73 12 bolt out back.

the a/c blows ice cold but only blows on high. That was the only issue with the a/c until I remounted the recirc door actuator someone had left dangling so they could install kick panels with speakers now the recirc door operates as it should, but it still only blows on high and it stops blowing after 10~20 minutes, then will start blowing again if I turn the a/c off and back on. Sounds like a circuit breaker to me, but I can’t find one. I have replaced the resistor, blower relay, and blower motor. Not firing the parts cannon, just freshening things up while I’m in there. Side note, key on engine off, the blower blows on low speed like it should. As soon as I slide the mode switch from off to a/c it stops blowing, if it’s acting up. Is there a circuit breaker for the blower motor I’m not seeing? Do I need to pull the hvac control panel and dive into that? I greatly appreciate your time and help.

[bd]

No circuit breakers from the factory.  Verify that the blower motor case is suitably grounded by a dedicated ground wire firmly attached to the motor flange.  Check the purple wire for a clean and firm connection to the blower motor.  Check the inline fuse that connects to the firewall junction block (15-amp spec'd, but 20-amp is common) - the plastic holders have a propensity for melting because of connection issues that generate excessive heat exacerbated by engine compartment heat.  If the holder is melted, convert it to an Mini-ANL 20-amp fuse and holder.  Note that the inline fuse is only for supplying high blower speed via the blower relay, so make sure the relay connections are not compromised - inspect connectors for melting.  Intermediate blower speeds are powered through the blower switch.  Use a test light to probe for power at the resistor in low and medium switch positions.  The blower switch receives power from the mode selector switch - if the resistor is dead then remove the dash control head and check the selector switch and blower switch connections and functions.  The selector switch is plastic and subject to connection issues and resultant overheating that burns the switch.  As you follow power from one switch to the next, replace switches and connectors as needed.

[Happytexan]

Thank you! I used a rollock disc to take the paint off the mounting hole that the blower ground wire is on. It may take me a few days to check everything and post back with family visiting for Independence Day. Stay safe and have a good weekend.

[Happytexan]

I followed your t/s instructions and found the fuse had been replaced with a breaker by the PO. All connections good, ground is good. My first truck I had almost 30 years ago was a 78 big 10 camper special with a 400 small block. Being a 78, that truck had different a/c modes. The 74 I now have only has one a/c option. No max option. So when you put the mode switch on a/c, it switches the blower to high speed, the same way my old 78 did on max a/c. If I switch it to bilevel, then all 4 blower speeds work as they should. This now has me wondering if the later hvac controller with multi a/c settings will work in my 74. Thanks again for the help and tech info.

[VileZambonie]

74 is a different system that uses a thermostatic switch which you can see in the engine bay side of the evaporator case and a high side pressure switch in the liquid line. Later models just used a low pressure switch at the accumulator. Low 2 Mediums & High are your only options for fan speed with technically no fan off option. Key on it should come on and run at whichever speed the selector is at. The ground wire should jump off the blower housing and to the firewall. When the blower stops working you need to be ready to pinpoint where it is dropping out and why. It sounds like you could have compounded issues, so I would address them one at a time.

[Happytexan]

Wow! Thank you! The blower does run all of the time with key on. Have been told before that it’s to keep fresh air in the cab to combat CO from accumulating. Thank you for explaining the thermostatic switch. I was wondering what that is. The trucks I’ve had experience with have all been 77-79 models. It’s interesting to see the progression made in this body style. The other issue I’m trying to figure out is why the idle mixture screws on the q-jet do nothing. Found the carb mount gasket bad, good size vacuum leak. Replaced it with a new fel-pro gasket. Have capped off all of the vacuum ports on the carb accept the booster and vacuum advance. Put new hose on those. Need to cap off the intake manifold vacuum outlets for the trans and hvac to eliminate those possible sources as well. With the mixture screws in, I can turn the idle down 550 rpm and it purrs just fine. I backed the idle screw a half turn past the point of the engine dying to see if the blades were past the proper transfer slot alignment. Turned the screws out 2 full turns each and it made 0 difference. No start/no idle.