2008 Trailblazer P0411 (and other fun)

[BustedTrail]

Hello! I've been a long time user of the "historical data" over at TrailVoy and all of the information provided by this community has been a great help to me over the last couple of years.

But now I'm running up against something that forum searches don't seem to help, so here I am posting for the first time. Hopefully someone has some idea of what's going on and can help me get to the bottom of it.

Here's my story:

A couple of months ago I picked up a bottle of this Restore-brand "engine restorer and lubricant". Basically an oil additive off the shelf from Wally World. I added it to the oil, checked that my oil wasn't overfilled, and started the engine. Immediately I had very rough idling and shaking and foul-smelling exhaust. The OBD threw a code about a bad camshaft position sensor and I figured it would just be best to drive it over to the station and get the oil changed.

On the way the rough idle cleared up and the engine started running smoothly again. I cleared the error code and it's never come back. This may or may not have anything to do with my current issues but the timing if it is significant.



A day later I got a new code. P0411 - Incorrect secondary air flow. After a lot of research on TrailVoy and other sites, I took a guess that the check valve had gone bad and replaced it. No luck. I left the hose off the check valve on a cold start and determined there was no air coming out of it, so last night I replaced the air pump and relay as well.

This morning, on another cold start, I still didn't hear the air pump come on (it used to sound like a turboprop and made the engine sound like it was a V10 for the first 20 seconds). At this point I think I've replaced all of the mechanical parts which means there must be an electrical problem, and I don't have the tools or experience to start diagnosing that on my own.

Another item which may or may not be important; sometimes when sitting at red light I hear what sounds like some kind of grinding or crackling from behind/under the instrument panel. Like cheap plastic gears on a kid's toy moving around. When I try to flip from 2WD to 4WD (which also doesn't work), the sound behind the panel gets louder or has a more regular, rhythmic noise added to it for a few seconds. I believe this is something trying to send a signal to the encoder motor on the transfer case, which itself definitely makes no noise when I try to switch 4x4 modes.

I have a new encoder motor on hand and was going to replace it this weekend, but I'm wondering now if it's even worth trying.

Long story short, I could use some assistance.

Thanks, Nation!

 

[The_Roadie]

Welcome! Glad you found our previous hangout to be a good reference library. As you noticed, there's probably no community left back there after the new corporate owners sucked the spirit (and profits) out of the place by shoving ads in our faces.

1) The engine restore goop products have a history of damaging our engine's performance. You're lucky it didn't require 3-4 oil flushes to get the crap out. It's our variable valve timing solenoid (CPAS) that is the first to get affected. You might have had to replace that along with the oil flushing.

2) The SAIS (secondary air injection system) does indeed have some electrical requirements to run the turbine pump, and open the exhaust manifold solenoid valve, and on later models like yours, sense the increased pressure when the pump's turned on. A dealer or competent independent mechanic with a GM Tech II or equivalent high end scan tool should be able to command the system to turn on and read the sensors and see where the problem is.

3) Plastic gear noises behind the panel are almost always coming from one of the five HVAC system actuators. Have you had any problems controlling the temperature of each side, or where the airflow goes to? Much less likely are the stepper motors that run the instrument panel gauges. Ever see the gauges point in a wrong direction?

4) The 4WD system really doesn't have any mechanical motor-driven parts behind the panel. That system is my specialty though, so let's work on that one too. How many miles do you have, and have you changed the transfer case fluid religiously every 50K? When was the last time you checked the differential fluid levels or changed them?

Describe the behavior of the lights on the mode control switch exactly - do they all flash on when you turn the ignition on? That's the lamp test. When you have the switch in 2HI, and just turn one position to A4WD, what do the lights do? If you've read my previous posts, you'll know there's a front axle actuator that's supposed to make a higher pitched noise ONLY in the 2HI->A4WD transition and vice versa. Do you hear any whining noises from the passenger side of the engine at the inboard side of the CV shaft at the splined disconnect assembly?

I wouldn't be shotgunning parts without knowing the commands are even being sent to the encoder motor. Do you have a buddy who wants something you can provide (beer, babes, mechanical help, ??) who knows electrical troubleshooting?

 

[BustedTrail]

Hey Roadie, thanks for the reply! To be sure some of your past posts have saved my bacon, so I'm glad to have your help on this.

I'll have to take it in to the shop and have them check out the electrics, I guess. Sad news for me, even most of the independents around here are shop manual book rate crooks.

If those grinding noises behind the panel are normal, that's a relief. I've never had issues with the HVAC or instrument cluster.



As for the 4WD, here we go:

Ignition on, engine off, the 4WD makes zero noise when I flip modes aside from that faint rhythmic gurgling/whining behind the instrument panel. No encoder motor, no front actuator. My understanding was that if the encoder motor isn't turning, anything further down the chain (ie, front actuator, spline disconnect, etc) won't even try to activate.


The lamp indicator behaviors are as follows:

Ignition on, engine start: All lamps come on as expected to test. Then A4WD (where the selector is set) blinks a few times before going solid.

If I switch from A4WD to 2WD, the 2WD lamp blinks for a while but eventually the 4HI lamp comes on solid instead.
If I then switch back to A4WD, the lamp will switch from 4HI to A4WD after a couple of blinks.
Shifting to neutral and trying to go to 4LO, the lamp will just blink and blink and blink.
I'm similarly not able to put the TC into neutral (no lamp activity at all, light just blinks at 4LO)

 

[The_Roadie]

Thanks for the kind words.......but I have to remind you you ignored some of my questions, especially about maintenance history. In general, everything I ask has a diagnostic purpose.

The encoder motor function is totally separate from the front axle actuator. Each one can work or not work independently of the other.

The behavior when the 4HI lamp goes on and you didn't turn the switch there points to a bad switch before anything else. GM redesigned them in 2006 to be more reliable (and cheaper, even at the dealer!) so I would pry that out and swap it before anything else.

On the HVAC actuator question - I didn't say it's normal. Just that those kind of noises can ONLY come from the actuators. They're actually signs of a failing one, unless they're really faint and they function normally. That's why I asked if you were able to get proper control over temp and airflow modes.

 

[BustedTrail]

Ah, sorry! Didn't mean to skip those questions, I just got all excited to talk about the lamps. The Trailblazer only has 43,000 miles so while it's coming up on its first transfer case fluid change, it's not a case of neglect. I haven't checked the differential fluids either, again due to the low mileage.

On the matter of the switch, how much of a process is that to replace? I'll do a search for a how-to in a moment too.

 

[The_Roadie]

Cool. If you were at 129K miles and never touched the transfer case this conversation was about to end.

 

[BustedTrail]

I've gotten the replacement 4WD selector switch ordered off Amazon (GM Part #25965895), which is quickly turning into my favorite parts supplier thanks to my Prime subscription. I should have it swapped out in the next couple of days and I'll update here with the results.

 

[BustedTrail]

Well I had the Trailblazer up on stands today to replace the outer tie rods. I didn't have a large enough socket to get them off, but figured since I had it up I'd take a look at the encoder motor and do some more troubleshooting.

I pulled the old motor off (had to bang it with a hammer a few times to loosen it up) and had a chance to listen more carefully while I changed modes - heard some clicking and whining coming from the encoder motor; definitely seized up. I hooked up my replacement motor to the power and confirmed it moved around when switching modes, but then it stopped responding and the 4WD service light came up on the dash.

The current position of the new encoder motor's shaft and the spindle coming out of the transfer case makes it impossible to install - I just can't get them to line up. Even to get the old motor back on I had to get it on the spindle at an angle and then rotate everything to get the bolt holes lined up. The new motor's even further off and I can't use that trick.

Any ideas why it was moving at first but then stopped responding? Any ideas how I can get it to line up with the spindle coming out of the transfer case so it can be installed (assuming I didn't fry it somehow).

While I had the whole front end lifted, I also spun the driver's side tire and observed the passenger side wheel rotating in the opposite direction. I think this indicates that it's stuck in A4WD? Removing fuse #8 clearly disengages the front actuator and both wheels spin freely and independently.


As a side note, I also decided to double check my work on the secondary air pump and relay and found I'd bent one of the relay blades. I straightened that out and reconnected it; we'll see if my pump kicks on in the morning.

 

[budwich]

IF you run the motor when it isn't connected to the shaft, it runs past its normal "scope" of movement and the system basically shuts it off to prevent a problem. You can "bench" it with powering the motor leads along with the "brake connection" to cause the motor to continue to turn until it is in position to be reseated with the shaft. That's how I did it this past spring after doing a similar "check" to see if the replacement motor worked... :-(


Note: the pins on the motor are close together so be careful with your connection... use electrical tape or likewise to ensure nothing sparks...

 

[BustedTrail]

Well that's bad news!
Would I need a 12V battery to "bench" it like you describe? Would stripping some wires off a cigarette lighter adapter do the job?

Which contacts would I want to energize to get the motor and the brake? Any diagrams out there?

 

[The_Roadie]

4WD system schematics have been posted here and on trailvoy dozens of times. Recommend a site-restricted search like "TCCM schematics site:gmtnation.com" and when the list comes up, hit the "IMAGES" button.

 

[BustedTrail]

4WD system schematics have been posted here and on trailvoy dozens of times. Recommend a site-restricted search like "TCCM schematics site:gmtnation.com" and when the list comes up, hit the "IMAGES" button.

Thanks Roadie. I was specifically looking for a diagram of the 7-pin connector on the encoder motor with clear "put positive here, put negative here" instructions, but I don't think anyone's put those together yet.

Using a wiring diagram of the encoder motor and related systems that I found at trailvoy, I was eventually able to decode the color references and see that they matched the wire colors leading out of the connector on my replacement encoder motor. From there I jerry-rigged some wires off my battery terminals (Actually jumper cables hooked to the terminals) and was able to energize the motor. It took me over an hour fiddling with everything with the battery installed in the Trailblazer and the motor sitting on the fuse box cover, but would take someone with a proper bench no time at all to do this.

In case someone comes to this thread down the road and needs this information, the 7-pin connector basically looks like this:

_______-_-_______
| 1 2 3 4 |
| 5 6 7 |
-------------------------

Pins 6 and 7 are the negative (6) and positive (7) terminals for the solenoid brake, respectively.
Pins 4 and 5 are the negative (4) and positive (5) terminals for the motor, respectively.

I was surprised to find that the motor will turn even with the brake engaged, but I figure this can't be healthy and should be avoided while working to get the shaft back to 12 o'clock (or wherever you need it). It's important to note that the motor will spin in either direction depending on polarity, so the easiest thing to do is wind the same negative wire around pins 5 and 6, then put positive to 7 (to disengage the brake) and lastly put positive to 4 (to turn the motor). All this using a 12V battery, obviously.

The motor will 'coast' a little once you remove power, so it might take a few rotations to get it where you want. Alternately, you could remove power from the motor and the brake at the same time and it should stop immediately.

 

[BustedTrail]

SOLVED!

Well, all of my issues are now a thing of the past. Last night I jacked the Trailblazer up and a swapped out the encoder motors. Sure enough, she's shifting modes again without any trouble (well, there's an alarming metallic bang when going from 4LO to 4HI but I think that's supposed to happen?).

My P0411 code was being caused by either a failed relay or failed pump - considering how loud the old pump was back when it was running, I have a feeling all that noise was caused by it being on its last legs. Replacing the pump and the relay at first didn't work, but that turned out to be because I bent one of the relay terminal blades when connecting it to the truck's wiring. Straightening this blade and reconnecting it seems to have gotten the pump working - I still don't hear anything when I start the engine, but it's been 3 days with no P0411 code (or any other code).

The unresponsive 4WD was caused by a seized encoder motor. Replacing it had some obstacles (see the above posts) but was pretty straight forward. Removing the #8 (15 Amp) fuse from the front fuse box with the engine off, ignition on is a good diagnostic test. You should hear the front actuator disengaging if the transfer case is stuck in Auto 4wd. I don't know if pulling the fuse will do anything if stuck in 4HI, but it might still be a way to get into 2HI until the encoder motor can be replaced. A seized encoder won't be audible inside the vehicle, but you'll be able to hear it click and whine if you can get underneath while someone else tries to switch 4x4 modes. While the encoder is seized, the front actuator won't move even if you switch from 2HI to Auto 4wd - you'll have to pull that #8 fuse to check if the front actuator and splined connectors are intact.

Hopefully this post will help someone else in the future. I'm sad that I've only just now discovered this community since I'll be selling my Trailblazer soon. I'll be sure to let the buyer know about this forum!