Yet another design failure uncovered

Maintenance/Repairs
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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 :grinning:

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-

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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:

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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… :wink:

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Clearly it is a design flaw… but it did last 15 years! :wink:

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ATC blower module failures keeping the blower running, even with key off, has been an issue with GM going back into the late 80’s.

But design failure? Perhaps, although in your case the module has outlasted the expected lifespan of the car.

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I bought the truck used about 5 years ago. It could be the original part but nothing definitive on the case or board to indicate when it was assembled or placed in service. If it’s original, it had a decent run. :wink: The truck only has 97k miles on it. However, if it had been properly heat sunk, I’m betting it would have gone to the junkyard still working. That part has tons of margin and can handle 147A if properly installed.

On another note, this was the third of three recent failures. Yay, I am good for awhile now since they come in threes :grinning: The power steering cooler sprung a leak and certainly looked to be the original (who do I write to about my disappointment with that “defect” :grinning:) What a bummer though, this truck I can have the entire front off in 15 minutes or less. Should have been a 30 minute job to swap it out. However, that plow frame I installed blocked it from coming out. Seven torched off bolt heads later…

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To me the design failure is that it drains the battery with the key out. The rest I chalk up to mediocre design.

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I replaced the module in my olds with a junk yard part for about $35. The fuse was under the back seat so a little inconvenient to get to.

Never took it apart though but didn’t see the Q1. I got lost somewhere around the heat sink. Yeah I think lots of this electronic stuff could be fixed cheaply is the cause could be determined.

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If you liked that design, wait until you do the ATF change due at 100k. I have changed ATF on many vehicles and it never took more than two hours to drop the pan and replace the filter. On my 2007 Silverado with the 4L60E, it turned into three day nightmare. edit: half days actually but still…

The rear bracket for the shift linkage curls under the pan, right on top of one of the pan bolts. The bracket is supposed to be removed to drop the pan, BUT, it is held in place by two blind and inaccessible bolts, on top of which are 40mm torx with the head pointed upward so they hold all the dirt and grime keeping you from putting a torx socket on them. And again they are blind so you can’t see to clean them out.

If you have 4WD/AWD, they don’t tell you in the instructions but you have to disconnect the front driveshaft from the front differential and let it drop down a bit. This is to gain some room to slip your hand onto the bracket bolts. It also is needed to provide the room for prying the bracket out of the way when you realize you are not going to get those two bolts out. All the Youtube videos ended up doing this.

Couple of other things: remove the dipstick (not in the instructions), remove the heat shield on the passenger side of the transmission (in the instructions but it looks like it is not necessary, it is) and go to the pet section of Walmart and get at least 8’ of 1/4 clear vinyl tubing and siphon the ATF out from the dipstick tube, you will be glad you did or wish you did. I did not have to loosen or remove the exhaust pipe as long as you remove the heat shield and bend that bracket well out of the way.

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This is my third Trailblazer so the quirks are pretty well known to me. Trans pan drop is a bit different than the pickup variety and really wasn’t too bad to do. Those kind of troubles you describe are well known to me for other things or GM vehicles I have owned :grinning:

One of those circus contortionists would be a valuable skill to have when working on some of this stuff- especially without benefit of a lift. It took me a day and a half to recover from kneeling on the ground while simultaneously twisting my back to reach up into the dash area with both hands to release the connector lock while simultaneously pulling it out (or struggling to).

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Yeah, now I’m fixin to replace the motor mounts. I’m starting with a three day soak of the manifold head shield bolts. This is day one, I got two bolts on each side slightly loose so far. I’m using BreakFree on them.

I have the old style mounts that have the box style heat shield on them that block access to the bolts to the engine. Fun, fun.

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You did an excellent diagnostic & repair job there. Good for you! It’s too bad there aren’t a bevy of vendors offering inexpensive (around $50) o-scopes for diy auto hobbyists, as you’ve demo’d, that tool is very useful. o-scopes designed only for car diagnosis can be much simpler than for general purpose, so it should be possible to implement a small inexpensive hand-held battery operated version. I had a similar power FET overheat and fail like that on a tv set one time.

If I had that same problem I’d probably put another heat sink on it, if there’s room for one. If the problem returns, the motor may be failing and drawing too much current.

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What is Rds MAX? 0.008 ohms? Probably 0.016 at 175 degrees.

I^2 * R = P
say 2 watts with no heat sink.
I^2 * 0.016 = 2
I = 11.2 Amps

That’s about what a fan motor requires.

With such a low PWM frequency it is just using the motor as a resistor. The RMS current might actually be higher at lower speed, so the transistor would get hotter at lower fan speed.

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Fifty dollars might be a bit optimistic but there are lots of affordable (IMO) options, here’s one-

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DVM alone can display a relative voltage for the pulse width so you can see that it varies with the control signal. However, I also wanted to see the true amplitude and quality of the signal to rule out the control head absolutely and for that, an o-scope is irreplaceable.

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It’s not that simple.
Rds(on) has a static value but in this case, it is dynamic based on the driving conditions. And we’re driving an inductive load, not a strictly resistive one.

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Plus, your “cooling” air going over your heatsink can also be 50 degC or thereabouts…

The key point is that the semiconductor die is small so even a relatively small amount of internal heat can lead to thermal runaway if it is not effectively dissipated.

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$130 seems within reason, but that one has 40 MHz capability. It would be a good choice for an electronics technician or engineer, but auto-diagnosis doesn’t need nearly that much bandwidth for most diy’er testing. I expect a $50 version with 1 MHz or even 100 KHz bandwidth would be more popular among the diy’er community.

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I’m thinking the market for one of those is pretty small, not big enough to get costs low enough for $50 sales price.

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You’re going to have to give up a whole lot more than sample rate to cut the price by 60%. At some point, it loses most of its usefulness. Frankly, I was surprised they had come down even this much. I might have to get one to see if it can reasonably replace my home scope for this duty that is fairly small already but needs AC power.

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Are there any control units for oscilloscopes that interface to a computer/tablet/smartphone? Maybe that could save the money that @George_San_Jose1 wants. Limited market may be an impediment to sales in this case too.

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Rds (on) is only 1mOHM at 25C? The part wouldn’t even need a heat sink. Did they not use a bypass diode in the circuit so the mosfet is having to absorb the full back EMF from the motor on the falling edge of the PWM cycle? A lot of heat could be generated from that, even though the part is avalanche rated.

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Yes there are, some made by “Pico” as I recall, but that configuration is cumbersome for car diagnosis. It’s hard to say what the diy’er market for a $50 stand-alone battery-operated portable o’scope w/ 100 KHz car-diagnosis functionality would be. The raw ingredients are mostly sand & oil, so production costs per unit could easily meet a $50 unit retail price — much more sophisticated dvd players are sold for less — but the retail price has to produce a big enough per unit profit for the developers to recoup the development costs.

This sort of device could be used for more than car diagnosis. It could be used for diagnosing home appliance & general purpose home electrical system use. The biggest market limitation I expect is that using it & correctly interpreting the results requires quite a bit of training.

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Raw materiel cost has nothing to do with product cost for something like this. And I doubt they’d be selling tens of thousands of these, unlike dvd players. If a full-featured one sells for $150, a stripped version might sell for $120. Not $50.