I have sent this up the chain through support, and Bill suggested I post here since they can't find anything in the software:
Setup:
2005 Rainier, 5.3L LH6, stock engine other than DoD removed.
Cooling system mods- Mercedes PWM cooling fan (selected for fit), it has an internal controller, so big wires power and ground then a small wire for PWM input. Tested with a generic PWM generator and it needs reversed duty cycles, i.e. 10% PWM= 90% duty and 90% PWM= 10% duty. It is wired via the old electro-mechanical fan connector- I re-routed the proper wire from the VCM to the new fan, and then spliced it to the Mercedes pigtail.
I have the tuning credit for this VCM and have used it to adapt the fan. Using the inverted settings, the fan works great based upon coolant temp- usually need only about 30% duty cycle on a hot day at idle.
Problem:
The fan does not come on at all based upon AC pressure.
When in VCM scanner, I found I can see the AC pressure and volts. I can also manually activate the fan by setting a duty cycle within range. The sensor is working properly, and its output is making it to the VCM.
Question 1:
I read some helpful posts from blindsquirrel in this thread that dealt with a similar issue on an E38. Is it possible the truck VCM has some difference in internal circuitry vs the Corvette (it doesn't seem like this would be significant, see OS data below where Corvette used all 3 numbers, but maybe it's a hardware issue)?
Question 2:
Would a segment swap be in order here as was done in that thread? If in fact there is an internal difference between the Corvette and Buick VCM, that may be a way to overcome the problem.
Not a solution:
I did try switching from PWM-Electric (Corvette) to PWM-EV (Buick) and it did not turn on the fan. It also did not allow me to manually turn it on via scanner.
Additional data:
I looked up the AC pressure sensors on a parts site, and they are different part numbers, but both are 5V sensors, so the output is the same. I also checked some OS data:
My VCM is using OS 12598983.
I opened a stock 2005 Rainier 5.3 tune from the repository and it had 12596660.
I opened one from a 2005 Corvette and it used 12605740.
From what I've read on another forum, all three of these were used on the 2005 Corvette. The member there reports the 8983 and 5740 use the same calibration but different OS, while the 6660 uses a different calibration.
I have a GM Service Information DVD and looked up the AC sensor data on both vehicles. Here?s what I found:
Both sensors are 3-wire units. All pins have the same functions and go to the same PCM terminals (the 5.3L LH6 Buick; the inline engine is different but that is not a factor for me):
1.Ground
2.5V supply
3.Signal
In the system description, here is what it says about the pressure sensors for both. It looks like both use the same 5V reference setup. I have marked some differences in red, and my comments are in red italics:
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2005 Buick Rainier AC Pressure Switch Operation:
A/C Refrigerant Pressure Sensor
The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts. The powertrain control module (PCM) converts the voltage signal to a pressure value (does this happen in C6? No information.).
The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high pressure condition exists. The PCM disables the compressor clutch if the A/C pressure is more than 2413 kPa (350 psi) (2850 for C6). The clutch will be enabled after the pressure decreases to less than 1578 kPa (229 psi) (2100 for C6). This sensor does not have a low-pressure protection setting as the C6 one does.
There is no mention of the PCM sending an invalid message via class 2 serial as there is in the C6.
2005 Corvette AC Pressure Switch Operation:
A/C Refrigerant Pressure Sensor
The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts.
The A/C refrigerant pressure sensor prevents the A/C system from operating when an excessively high or low pressure condition exists. The ECM disables the compressor clutch under the following conditions:
- A/C pressure is more than 2850 kPa (413 psi). The clutch will be enabled after the pressure decreases to less than 2100 kPa (254 psi).
- A/C pressure is less than 204 kPa (30 psi). The clutch will be enabled after the pressure increases to more than 220 kPa (32 psi).
If the ECM detects a failure in the A/C refrigerant pressure sensor or circuit, the class 2 message sent to the HVAC control module will be invalid. The HVAC control module will display A/C OFF on the module as long as the condition is present.
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I'll attach my most recent hpt file for reference.
Rainier_122120_C6 fan mod raised ECT and AC press numbers.hpt