[HO455]

Originally Posted by toolboxchev

Test it on the high setting, you will find that those things draw as many as 40 amps or more depending on what is feeding them.

Higher output alternator is not helping also.

Since completely redoing the underdash wiring harness I do not run the fan on high. Have tried with a 10,15 amp fuse and I keep blowing them so..............

All the fan motors are coming from 2 sources now, and they are rated for both AC and non AC applications. Problem is the non AC truck wiring is only good for a 10 amp motor. Do not know about the aftermarket wiring so it sounds like you have to update that to accept the higher amp motors.

The alternator is not an issue here as the engine is not running or hasn't been running long enough to start charging by the time the key is released from the start position. Actually wen the engine is running the newer alternator should provide better voltage control when amp demand is high thus reducing amperage draw across the board. It's that Ohms law thing, when voltage drops, amperage increases.

Heater fan amp draw doesn't vary much from low speed to high speed as the fan speed resistors add to the amperage drawn when they are energized.

Running the AC fan motor with up sized conductors will only lead to heater switch failure. (A lesson I learned the hard way many years ago on my GTO) That's why GM spent so much money designing and installing a A/C fan control relay in A/C equipped vehicles. Instead of up sizing the conductors to run the A/C motor is is much better to install an A/C fan control harness. Something that I am actually considering doing to my Burban. A Suburban has the same heater as a pickup but 4 times the interior volume to heat. I get plenty of heat but not much circulation. Which leads to whining from the back seats. An A/C blower fan may help me with that, but I digress.

I believe I may end to look into getting a different amp meter to troubleshoot this further. Flyback diodes may be a good addition to the circuit but I would like to have more accurate information on the circuit amperage before investing the time in adding them.

Yet another rabbit hole opens .