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Discussion Starter · #1 ·
Hi,

On a 2007 9-7X (re-badged Trailblazer), 5.3 V8 (with the 4L70E) I have the following problem:

Check Engine light on, most of the time, but not always.
At times, automatic gear shift is "sticky", meaning at lower speeds, it will want to shift gear, but lag and all of a sudden, after the lag, very suddenly engage the next gear up, at times "violently" so--very weird.
At other times, drives fine.

Have had the following problems which may be related:

- car shuddering or stopping with "Stabilitrak Off". Then after a while the message would go away and everything back to normal.
(I mostly ignored this and went on driving the car.)

- this "Stabilitrak Off" no longer appears, replaced by the problem explained at the beginning of this post, ("sticky".)

I've asked one mechanic I use for oil changes, etc., good guy and he tells me it could be a transmission problem but not sure (he doesn't do transmission.) I told him I've only had the car since about 110k miles (has 140k now) and have no idea if transmission fluid was ever changed as part of maintenance, etc. He tells me that if you don't know, it's often best never to change it as some cars will just refuse to drive if there is something wrong in the transmission, once new fluid is in there.

This sounds like a load of crap to me and the "opinion" of someone whom just doesn't know.....

Anyone here has experienced this "sticky" gear shifts with "Check Engine" light coming on most of the time, etc.?

Looking at the Sticky "Common trans failure symptoms and causes", it sounds closest to #3 or #4. Hard to find a good transmission shop (n my area) which will know how to diagnose and fix, without them just saying: Oh, you need a new transmission, etc.

What I would tend do do is:

- new transmission fluid (flush, etc.)
this should include cleaning pan?

  • change front and rear differential fluid
  • change TPS

But I really don't know.

Any suggestions and advice?
 

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First what's the CEL for? My Trans did funky shit when my MAF went bad. Pull the Trans fluid stick when it's hot check level, smell, color and drop some on a white paper towel and see if it has any debris in it. From there you can make a better assumption of where to go next.

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Discussion Starter · #3 ·
I will try to get the CEL code and determine what it's for; but not sure I'll be able to determine, using an phone app for that connecting to OBD2 port.
Will do the fluid check you suggested; and post back, Thanks.
 

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You sure will, that's what I use. Torque app and Bluetooth adapter. It will also let you monitor every sensor it reads if you see something out of whack.

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Discussion Starter · #5 · (Edited)
OK, so I checked the transmission fluid while hot and it looks very clean, no debris, nice and clear. Looks like it comes up to quite a bit higher level than where it should be on the "cross hatched" area for "hot" on the stick but maybe it's me.... Probably because I didn't hold the dipstick down (as specified in the manual, just read that now.)

The 3 CEL codes are:

1) Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance
P0121

2) System Too Lean Bank 1
P0171

3) System Too Lean Bank 2
P0174

This is the LH6 V8, 5.3 ltr.
 

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Code 1, i think would Def screw with the Trans not knowing where throttle position was as to shifting and such. Look at the pedal assy make sure the sensor is plugged in all the way, even unplug it check the connection reconnect clear your codes and check. If it comes back pedal assy are apps $60. On rock auto or try a scrap yard.

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Discussion Starter · #7 · (Edited)
Code 1, i think would Def screw with the Trans not knowing where throttle position was as to shifting and such. Look at the pedal assy make sure the sensor is plugged in all the way, even unplug it check the connection reconnect clear your codes and check. If it comes back pedal assy are apps $60. On rock auto or try a scrap yard.

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I agree on Code 1; it could screw with the shifting. I'm told that there are 2 TPS, one on the throttle itself (for the LH6 engine) and the other one is part of the pedal assy. It could be either of these 2 misbehaving?

I looked at the switch just left of the throttle pedal and unplugged, replugged the switch there; cleared the codes. Not sure they've come back yet but probably will come back. We were in there a few weeks ago, and took out the throttle pedal to have better access to change one of the A/C vent actuators, and possibly messed with something or didn't plug something back in properly, don't know. I can't see anything odd at the moment but will look again.

The other sensor, I believe, is included with the Electronic Throttle Body: this or that
Has the sensor included (for the LH6.)

Would it make sense to consider replacing that?
 

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Link didn't work but correct there are 2 sensors, pedal and tps. Depending on what app you have you can monitor the sensor and see what is and not working.

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Discussion Starter · #9 ·
Link didn't work but correct there are 2 sensors, pedal and tps. Depending on what app you have you can monitor the sensor and see what is and not working.

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I fixed the link, my bad.
 

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I wouldn't worry about the throttle body just yet, go for that pedal sensor, put some mileage on it see what comes back.

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Discussion Starter · #11 · (Edited)
I wouldn't worry about the throttle body just yet, go for that pedal sensor, put some mileage on it see what comes back.

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OK, you're saying replace that pedal sensor with a new one, correct?
So this baby or that one on rockauto here (sensor is included I believe)?

what about this connector here ?
might as well swap that out too, I suppose.

NOTE: funny, I didn't event know I had power adjustable pedals.....
 

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Def not the hole pedal assy, just the pedal switch (which was your code). But then you said you moved it for other reasons, then I said remove the plug check it, replug, clear codes, put some miles on it to see if the code comes back. If the code don't come back I would not buy anything just keep troubleshooting. The only way to figure it out is 1 step at a time, please don't waste money and time just throwing parts at it.

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Discussion Starter · #14 ·
Def not the hole pedal assy, just the pedal switch (which was your code). But then you said you moved it for other reasons, then I said remove the plug check it, replug, clear codes, put some miles on it to see if the code comes back. If the code don't come back I would not buy anything just keep troubleshooting. The only way to figure it out is 1 step at a time, please don't waste money and time just throwing parts at it.

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OK, I agree. So just the connector here, then. That's what you are referring to as the pedal switch? Correct me if I'm wrong.
(I did unplug it, plugged it back in, cleared the codes but the CEL is back now.)

FYI, the sensor itself is not available on its own (or at least I cannot find it on its own); it's part the the accelerator pedal assy, which is why I had linked to it.
 

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This is for my 08. On the site go to interior then accelerator pedal position sensor.


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Discussion Starter · #16 ·
This is for my 08. On the site go to interior then accelerator pedal position sensor.

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Just not there, unless I am blind.
Here is an exploded diagram of the pedal assy.
I don't see the switch available as a single part.
On rockauto, I did go to "Interior" but no "accelerator pedal position sensor".

Here again, it seems to show the sensor part of the assembly.

If I look specifically for part #19121979 on rockauto, I DO get this Dorman switch, the only one which seems to be available as a single unit, $64.79. I think that's what you are referring to.
I don't have good experience with Dorman, so would be good if I could get another brand but not available on rockauto.
Hard to tell if this is compatible with my car....
 

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Ain't that some shit, no pedal with sensor. The sensor only at rock looks like the right sensor. Depends on weather you want to change just the switch or the pedal assy. I just looked up same yr and motor trailblazer as yours and under interior the switch is there. Again only if the CEL comes back on change it out.

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Discussion Starter · #18 ·
Ain't that some shit, no pedal with sensor. The sensor only at rock looks like the right sensor. Depends on weather you want to change just the switch or the pedal assy. I just looked up same yr and motor trailblazer as yours and under interior the switch is there. Again only if the CEL comes back on change it out.

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I ended up going with this one for about $15 more than the Dorman and I got the connector as well for $5, might as well swap that out too.
Will post here again once it's in -- and hopefully the issue will be solved. If not, will go to next step, possibly the sensor in the Throttle Body itself (but hopefully won't be necessary.)
 

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Sorry but i have to ask a sidebar question here ..........

Since the Toyota fiasco where they killed a few drivers in their drive-by-wire vehicles, GM got smart and used dual potentiometers - one starting from LO-Ω and the other at HI-Ω to keep from the ECM thinking the driver was calling for WOT when there was just some electrical 'noise' on the throttle pedal sensor pot.

So now along comes this post and I see that there's only ONE POT in this pedal assembly? Wowee ... that sounds kinda crazy even for GM --- or (shudder) more naturally Ford or Chrysler (who owns Chrysler at the moment, is anybody's guess).

My '05 TB 4.2 utilizes the dual-pot configuration --- so did the GM bean counters win here and did they do away with the duality/parity safety feature and now are subjecting their customers to die like Toyota did ...... splattered on the highway at 300 MPH?
 

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<snippity, snip> I DO get this Dorman switch, the only one which seems to be available as a single unit, $64.79. I think that's what you are referring to.
The part to which that link goes, actually looks like the throttle position sensor that is mounted on the throttle body (butterfly), not on the throttle (gas pedal) pedal.

The picture is actually somewhat distorted and the female electrical connex appears a little too wide if memory assists me - which at my age, may not.

If so it's a feedback device to tell the computer where the throttle plate is in reference to the commanded position and if it is in parity to the actual position.
 
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