Background: I did away with the huge mechanical fan on my 2005 Rainier 5.3 LH6 (E40 PCM), rewired for a PWM fan and added a large one from a Mercedes that uses similar settings (except for PWM polarity) as the C6. I registered the PCM in HPT so I could make the needed mods. Since the PWM polarity is reversed, so is the duty cycle- i.e. 90% is 10% and 15% is 100%. Using anything outside of this (such as 5%, 95% or 100%) shuts off the fan. I tried to mimic the C6 PWM fan settings when I did this.

The operator of the truck complained about a humming sound that I traced to the PWM duty cycle being set too low (90% commanded or 10% actual). I modded the activation temps so it will not kick on until 203 degrees (up from 192), and it then runs at 80% commanded, 20% actual. That appears to have fixed the humming complaint. I also had a P0128 code, coolant below thermostat temp. One of the causes for this was shown as the fan running all the time. AFAIK, the truck has an OEM thermostat in it, 187 degrees or so.

The fan cools well, in today's 65 degree ambient it never had to go over the 20% to keep the coolant below 203.

The issue now is no matter how high the AC pressure gets, the fan will not activate. I must have missed a setting somewhere.

I am attaching the latest tune file as well as a scan file I made this morning. Thanks for any advice.

Rainier_122120_C6 fan mod raised ECT and AC press numbers.hpt

Rainier122120.hpl