So the blower operation in my 2008 Trailblazer starting acting up. It has the automatic climate control. There are three main assemblies involved; the control head, the blower module and the blower motor.
Symptoms were a bit random but primarily the blower running at low speed regardless of the setting. If I cycled power with it set to a higher speed, it sometimes would start at the requested setting. If I attempted to change it, it would increase or decrease but once at a lower setting, it would not go back up.
I back probed the control head output with an oscilloscope and witnessed the base PWM frequency at around 30 Hz nominal and the duty cycle changing within the 20-80% range depending on blower speed setting. This seemed pretty consistent although the signal looked a bit noisier than I would expect. I wasn’t sure the control head was outputting a valid signal due to this but it seemed to be working.
The output of the blower module would change with setting but when the system was acting up, it seemed to be stuck at a consistent output. I ruled out the blower motor at this point.
Before I could dig further- I went on a short errand and during my goofing with the settings, the blower went to maximum and would not shut off even if the engine off and key out of ignition. Had to go home as leaving it that way would drain the battery pretty quick. It died on the way home and the blower never worked after that.
Removing the blower module was straightforward although it reignited my dislike for the connector retention mechanisms used inside the vehicle. Complete overkill from a reliability standpoint once the vehicle is assembled and in service. Even with the latch disabled, the removal force is like 20lbs of pull force with wiggling
I disassembled the blower module and spotted the problem immediately. There is a high power FET on the output that feeds the blower motor. It had overheated and failed. The tell tale marks of die overheating inside the package were obvious-
The backside of Q1 is discolored.
Looking at the construction of the module, it has a significant heatsink for this part that sits up into the duct when installed. However, the method they used to interface with the heatsink is where the design falls short.
These parts typically specify the force needed to be maintained between the heatsink and the part to ensure proper heat transfer. This FET is no exception:
20N clip force, right in the table of recommended operating parameters.
So what did they do?
They had a dollup of thermal RTV between the parts and relied on the enclosure halves to establish the distance between them. What’s worse, it’s actually designed to have a gap!
Why? The only reason has to be that they did not want the heatsink to be live. The back metal of that part is connected to the drain and so may have the operating voltage on the heatsink if the thermal compound is squeezed out and they make electrical contact. So they purposely designed in a gap. The end result was predictable- premature failure.
Later, looking online, I find all kinds of references to this weakness in the design. They attempted to correct it with a modified design at one point but that was just a spacer to push the board closer to the heatsink. Still a gap and no clamping force.
The module can be repaired for less than a dollar and I’ll have a spare.
When they released the updated module, it went from a separate 3 pin connector to the control head and a 2 pin flying lead connector to the fan motor, to a single 5 pin connector. Exactly the same pin styles used. The upgrade kit instructions have you cut the existing control head wires and splice them into the new harness from the blower module.
I figured why do that? I’ll simply push the pins out of the old connector and install the wires into the new connector. Simple, not. The pins are identical except in one way- the retention mechanism. The old pin is a spring fish hook and the new connector has a ramp in the housing making them incompatible. However, this is the one time I actually liked the secondary retention mechanism- the TPA or Terminal Position Assurance (yes, they have a name and acronym for it) when installed keeps the pins from backing out regardless of the terminal retention mechanism…
Sorry for the lengthy post, maybe you even got this far…