Blower motor

[Smokin Diesel]

Never thought about it until I was reading through here. Why did GM use a high speed relay on the AC equipped trucks, but none on heat only trucks? I still have to factory cover over the blower motor/resistor, so I can't take any voltage readings to see how many amps it's pulling, or if a relay would help.

 

[Jgonick]

My guess why- It is generally harder to blow cold air than hot air because cold air is denser, meaning it exerts more pressure and is therefore more resistant to being moved compared to less dense hot air; essentially, it takes more effort to push a mass of cold air than a mass of hot air.

 

[Smokin Diesel]

My guess why- It is generally harder to blow cold air than hot air because cold air is denser, meaning it exerts more pressure and is therefore more resistant to being moved compared to less dense hot air; essentially, it takes more effort to push a mass of cold air than a mass of hot air.

Though the blower is before the air is conditioned either way, so especially during the winter, 10 degrees per se, it should take more to push the air than on a 110 degree day. I also know that at least my experience with axial fans on grain bins(i've never dealt with centrifugal fans on them) that the higher the static pressure, like the grain is deeper so it has more to push though, amperage goes down due to it physically moving less air. I do know from my research, a centrifugal fan can develop a higher static pressure though for the same horsepower, but don't know how that translates to hvac systems.

 

[Jgonick]

Also it could need more air flow dealing with heat transfer aspect of the evaporator. More air flow increases heat transfer & cooling.-

-don’t know- just another guess.

 

[Ricko1966]

Never thought about it until I was reading through here. Why did GM use a high speed relay on the AC equipped trucks, but none on heat only trucks? I still have to factory cover over the blower motor/resistor, so I can't take any voltage readings to see how many amps it's pulling, or if a relay would help.

Maybe it's a perceived usage over time scenario. The heater equipped truck will see occasional use 3months out of the year,but the AC equipped truck will see the AC running those same months and another 6 months of 24/7 AC on full blast. I can see this being harder on the switch and wiring and relay is cheap insurance.

 

[75gmck25]

I don’t have a good explanation or rationale. However, the HVAC fan in many houses also runs on a higher speed for A/C than heat, and when you manually turn on the blower it goes to the higher speed.

 

[Jgonick]

Another- Maybe
took a quick look at the wiring diagrams this morning-

Possibly the A/C setup had too much of a voltage drop on the circuit to use high setting.
With A/C the circuit also powers pressure control switch/compressor and the fast idle solenoid. Low, Med1, & Med2 all go through a resistor setup. Hi of course switches relay.
On the Non A/C it appears only the Low & Med settings go through resistors where HI does not.