Piston Slap: The Wheels on the TrailBlazer Go "Thwup-Thwup-Thwup"

Sajeev Mehta
by Sajeev Mehta

TTAC Commentator Episode19 writes:

I have a 2002 Chevrolet Trailblazer LT 4×4 with less than 134,000 miles. It has been a great vehicle – a dozen trips around the country, suburban family driving, and daily commuting – with the only real mechanical issue being a replaced fuel pump at about 70,000 miles. Now I have two conditions that I wonder if the best and brightest can help diagnose.

First, the HVAC control has a dial with five speeds for the fan. The first three settings do not work – you can turn from off to 1, 2, or 3 and nothing happens. When you turn to 4 the fan engages at the fourth highest speed; and when you turn to 5 it maxes out at the highest speed. What gives? In the guide I can’t identify any specific fuse that would control the lower speeds but not the higher speeds.

Second, and more importantly, I have noticed a rough ride from both the front wheels for the past month or so. There is a slight vibration/roughness in the steering wheel and a noisier ride from the front end at speeds greater than 30 mph. There is also a slight “thwup-thwup-thwup-thwup…” while turning the wheel towards the right at around the same speeds – whether accelerating, decelerating, and coasting. The “thwup” sounds as though it is coming from the front driver side. No “thwup” while turning left, though the rough ride feeling is almost continuous through the entire turning radius of the vehicle. Any insights into these two issues would be greatly appreciated.

Sajeev answers:

The first question is simple, as most blower motors have relays controlling the fan speed. Google tells me your TrailBlazer is no different, and I suspect the TrailBlazer forums make this fix a cakewalk: sometimes the relay goes bad, other times the fan switch itself bites the dust. Either way, HVAC systems are designed to “fail” in only the lower speeds, because higher speeds working (no matter what) relates to safety when windows fog up.

Because you have a 4×4, the second problem sounds like a bad CV joint on at least one side of the front end. CV joints are common wear items on any axle that does double duty: steering and acceleration. I’d inspect the CV boots and joints, and price those parts respective to a new axle shaft. Normally the shaft is a better deal because of the labor savings. Now, I think your poor ride quality is a separate issue: your shocks are almost as FUBAR as a certain 1996 Ford Explorer that really woke up after receiving a fresh set of Bilsteins.

[Send your queries to mehta@ttac.com]

Sajeev Mehta
Sajeev Mehta

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  • Alex L. Dykes Alex L. Dykes on Sep 25, 2009

    It's the blower resistor, as has been said it is a common failure. GM created a revised version that seems to be more reliable. It is an easy swap, I've done it myself.

  • MattMan MattMan on Sep 26, 2009

    Is the title of this post inspired by a line from the film "Mystery, Alaska"?

  • Carguy949 You point out that Rivian and Tesla lack hybrids to “bring home the bacon”, but I would clarify that Tesla currently makes a profit while Rivian doesn’t.
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  • Jrhurren I had this happen numerous times with my former Accord. It usually occurred when on a slow right curve in the road. Somehow the system would get confused and think the opposite lane (oncoming traffic) was an impending head-on collision.
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