New member. Didn't see this one in a search.
Less than 2k miles on a new 5.7 . most all components are new. Been running strong then started a chugging when engaged ac fan on high. Ses code 43. Changed out knock sensor . New Fuel pump/filter. New ac comp.
new delco alternator ( had to add excite wire to this one as voltage output wasn't charging) also have a isolator in the circuit for aux batt. Getting up to 14volts out.
Anyway. When engine warms up. Idles fine. Turn ac fan on to high engine shakes. Even on vent. No new codes.
Anyone think my voltages are a factor?

 

What's the battery voltage at that time? Just with blower on high?

 

About 12.2. Turn fan off an it'll jump to 13.+

 

Two things, I can't see this being a problem without blowing the fuse. Second, I'm thinking on high with your vintage truck it might be straight battery voltage, and not through a resistor block. If this only happens under what you post, I'd replace the blower motor.

 

Finally got a code.
let engine cool down. Restarted road tested and of course ran fine, even with ac and fan on high.
ran around for awhile ac and fan on and off.
When temps came up of course it started up hesitating. And now high fan makes it backfire.
Code i got was a 44. O2 sensor. Not sure how fan high position figures in. Doesn't do it on low or med. Voltages vary 12.2 to 14.1.( i have a digital meter plugged into lighter socket.)
gonna get it on a real machine tomorrow.

 

Does the AC compressor have to be engaged for this to occur or will it happen if only the fan is on high?

 

Nope. Does it with ac or vent or heat. High position only.

 

So, with fan on high it drops the voltage by 2 volts? Again, I'd think it would blow a fuse (you might make sure the circuit is fused in high speed), but I guess at this point look at replacing the blower motor.

 

Could there be a blower resistor that could be changed??

 

Due to the position of the blower motor & resistor, the ECM, and related wiring, It’s possible the high speed circuit is shorted to one of the ECM circuits or that it’s inducing electromagnetic interference.

On low or medium, voltage is fed to the resistor; but when high speed is selected, the circuit bypasses the resistor, providing full battery/charging voltage to the motor.

Try unplugging the blower motor and installing a test light in its place. If the truck runs smoothly with the test light, the blower motor is faulty and causing interference - if it’s still running rough, see what happens when you unplug the test light. Note that it may require shutting down and restarting the engine to find out if leaving the circuit open makes a difference.