NEED HELP 4x4 Already in 4Low at Startup?

[AC75TB07]

Couple times in the past month when I start my truck it's already in 4Low...

I drive for a bit, stop, restart and it's back in regular RWD. No switch adjustment at all. The light indicator changes but that's all. The traction control dash light is on but as I understand that's normal when in 4Low... That light also goes out again when the system reverts to RWD, too. No other dash lights. No Service 4x4. Nothing.

I did replace the transfer case motor earlier this year with a used one. I tried aftermarket new but it was junk out of the box. Everyone said to find an OEM replacement. Also replaced the TC Control Module thing behind the dash below the steering wheel with a known good one.

Any thoughts? I exercise the 4x4 system a few times a month and have had all the fluids changed in the last year.

 

[mrrsm]

Visit THIS Link and Drill Down there for some Diagnostic Information on your 2007 Chevrolet Trailblazer on various sub-systems and subjects:

https://charm.li/Chevy%20Truck/2007/TrailBlazer%204WD%20L6-4.2L/Repair%20and%20Diagnosis/

NOTE: The Source of THIS Suggestion comes from @TJBaker57 as an excellent resource.

 

[AC75TB07]

Already read through that actually but was more looking for others opinions based on the symptom. I do not own a scan tool that can read 4x4 codes.

 

[budwich]

I drive for a bit, stop, restart and it's back in regular RWD

Does this mean you are actually driving in 4wLO or just the light is "wrong".
My guess, you have a switch issue.

 

[AC75TB07]

No, it's in 4Low. The first time it happened I thought it was a stuck caliber, making the truck feel like it was pulling against the brakes. Thankfully I only drove it a few kilometers before seeing the switch was illuminated on 4Low. I've used the regular 4HI a few times since and everything seems OK.

 

[budwich]

OK... but I think it is still a switch issue. The system is reading the position of the switch. The position ISN"T digital, it is based on a resistance / voltage reading. Even if you didn't physically move the switch, at startup, the readings could be wrong and the system "thought" you had selected 4wLO. Since the truck wasn't in gear (either park or neutral is required), the system responded and put things in the "selected state". On stop / restart, the reading changed and the truck again responded.

The issue could be the switch itself which can be checked with a meter or the wiring going to the switch. Again, just my read. I don't believe that a 2007 has TCCM issues of soem of the earlier years. Someone here will likely have more say.

 

[AC75TB07]

If it's the switch, would trying to change the position do anything, or is it like an internal thing that won't respond to the switch being moved?

 

[mrrsm]

This TSB may provide some additional insights on reported problems with the TCCM on certain GM-GMC Model Vehicles:

 

[Mooseman]

Could be a dirty switch. With the ignition OFF, operate the switch through all modes several times to clean the contacts. Put it back in 2wd and see if it happens again.

 

[AC75TB07]

OK, nothing today. Normal all the way. I did turn the switch a billion times. We'll see if it happens again over the coming days... It wasn't an everyday thing anyway.

Question:

If it was/is the switch is there any reason why it would go to 4LOW compared to any of the other settings? Why not Auto or 4HI? Is it just totally random?

 

[Mooseman]

The switch works on resistances so if there's a bad contact, it can change that resistance and may mimic the one for 4lo. If still happening, I'd try to find another one in a pick-n-pull yard and replace it. If it still happens, I would then suspect either the TCCM or the encoder motor is not reporting the proper position/resistance.

 

[mrrsm]

Highly Detailed Information on how to Read TCCM Codes on Inexpensive Code Readers:

How To Guide: Reading TCCM Codes

Last year I spent a lot of time trying to pull TCCM codes from an 02 trailblazer. At the end I was able to access them manually via the command line. Below is the writeup on it. Hopefully this helps some folks diagnose their own 4WD issues :) There is a lot of info below, if you are looking...

gmtnation.com

...and from "The Roadie"...

offroadTB.com • View topic - Diagnosing a TCCM or wiring problem

 

[AC75TB07]

The switch works on resistances so if there's a bad contact, it can change that resistance and may mimic the one for 4lo. If still happening, I'd try to find another one in a pick-n-pull yard and replace it. If it still happens, I would then suspect either the TCCM or the encoder motor is not reporting the proper position/resistance.

When you say "mimic" do you mean that as if it's just the wrong light that's illuminated, and the 4x4 system hasn't actually changed settings?

Because my truck was definitely IN 4Low, not just that light on the switch...

 

[budwich]

The light is not set by the switch directly, The system lights the light as determined by the "reading" that the TCCM has done (along with some checks).... in your case, at startup. The "mimic" is meaning, the resistance is the same as being associated with the 4LO.

 

[TJBaker57]

When you say "mimic" do you mean that as if it's just the wrong light that's illuminated, and the 4x4 system hasn't actually changed settings

I think I can shed some light here.

The indicator lights are actually not controlled by the switch at all. Those indicator lights are wired to the TCCM. The TCCM reads a signal from a sensor inside the transfer case shift encoder/motor that reports the position of the shift motor as a variable voltage.

So the shift motor reports its position to the TCCM and the TCCM turns on the correct indicator for that shift motor position.

In a similar fashion the switch reports its position as a variable voltage to the TCCM. The TCCM reacts to this variable voltage by moving the shift motor to the position indicated by the variable voltage returned by the 4wd mode switch.

A picture may be useful here. This is a 4wd switch taken apart for cleaning.



Inside that top part, see the little metal bit with 3 raised dots on it? That rotates around as you turn the switch.

See the shiny metal pads on the bottom piece? Thats what the other piece with the 3 dots rests against when assembled. The piece with the 3 dots connects different pads together as it is moved to different 4wd selections.

Those metal pads are connected through a pack of varying resistances such that a different resistance is placed into a circuit in each 4wd mode selection.

Now then as things age and lubricating grease ages those pads may not make reliable connection through the moveable 3 dot metal piece. This can result in an incorrect resistance getting in the circuit.

This can make the TCCM think the switch is somewhere it is not.

 

[TJBaker57]

As an addendum to my post just above:

It's not only the switch that can cause a bad switch position signal. The wire terminals at the switch harness connector and the wire terminals at the TCCM connector can become loose.

I don't mean the tightness of the entire harness connector but instead I am speaking of the individual pins and receivers within the harness connector.

If either or both of these get loose the same result can happen where a bad switch position signal can get to the TCCM.

I have experienced this exact issue of a loose sensor terminal pin/receiver.

 

[AC75TB07]

I've been turning the 4x4 switch everyday since my last post and haven't had the original issue since. However today when driving, the Service 4x4 light came on. I drive for 10 minutes, stop for 20 minutes, light is gone out on restart. Is that still consistent with a faulty switch?

 

[budwich]

Potentially... without reading the codes, you will never know. :-( Based on the problem history, the system is likely not able to get a reading and set the warning light.... just my guess without codes being read / posted.

 

[AC75TB07]

The switch is about the only 4x4 item that hasn't been replaced. The TC motor was used but came from a low mileage Envoy, so I don't WANT to believe it's failed already. A switch is cheap enough that it will still be less costly than the nearest place that can read 4x4 codes and the petrol to get there. If this keeps up, I may just take the chance and throw a new switch at it.

 

[AC75TB07]

Quick update: 4x4 was behaving normally until yesterday. The Service 4x4 light and the 4low light came on. I tried changing to other settings and the light just kept blinking at each switch setting. Finally it worked and I was able to switch through ALL settings and could hear the TC motor beneath the truck switch through each 4x4 setting.

I have ordered a new switch anyway, but do these latest symptoms still point to that as a possible culprit?

 

[Mooseman]

I'd be leaning towards a failing encoder motor. Sounds like the position sensor inside the motor isn't working right or possibly the wiring to it. Maybe getting it working like that cleaned the contacts inside of the motor.

I know that @TJBaker57 has a lot of info on that.

 

[TJBaker57]

No amount of opinions, experiences, guesses and so on will ever replace reading the codes.

 

[AC75TB07]

Thanks everyone for the replies. Today I started up as usual without issue. I drive for 20+ minutes, park and shutdown. About 30 minutes later I startup again and boom the 4x4 service light is on again and NO lights are illuminated on the 4x4 switch at all. I shut the truck off and flick the switch back and forth a few times, restart and every setting works as it should and the Service 4x4 has dissapeared. The encoder TC motor goes through all settings and it makes its normal noise under the truck...

The encoder TC motor is a used replacement, OEM, bought tested only last Fall... Obviously I know it can be faulty but it had less than 100,000miles on it.

 

[budwich]

The story will continue... until you get codes read OR start replacing the cheapest thing (the switch) and see if there is an impact... before moving on to other components... again working on price and easy of replacement.

NOTE: that your "flicking the switch back and forth" while the truck is off does NOTHING to the transfer motor. Thus, if that "flicking" solves your problem even temporarily, that would likely indicate the switch (or wiring therein around) is the problem.

 

[AC75TB07]

I have a switch ordered. Getting to a place that can read 4x4 codes properly requires a day off and a tank of gas. That's why I like to hear others thoughts and experiences.

 

[Mooseman]

You could read them yourself with a cheap ELM327 Bluetooth OBD adapter and an app on an Android device. The Veepak from Amazon is a favourite and some apps that can read the codes. Car Gauge Pro and Chevrosys should be able to read them or @TJBaker57 can provide instructions on using a command line app to get the codes.

 

[AC75TB07]

An 07 Trailblazer LT 4x4 system is an NP8?

 

[Mooseman]

NP226

 

[TJBaker57]

An 07 Trailblazer LT 4x4 system is an NP8?

NP8 is a General Motors RPO (regular production option) code for a New Process/New Venture 226 transfer case.

You can read about the history of New Process Gear and New Venture Gear in Wikipedia...

New Venture Gear - Wikipedia

en.m.wikipedia.org

 

[AC75TB07]

For the purpose of buying a replacement TC motor, most sites offer several options... NP8 with 7 pin plug is what I need with a 2007 TB LT...? I think the other options are for the trucks with AWD?... I know the Rainer shows different results than the Trailblazer and Envoy AFAIK...

I'm just checking prices of what's out there should I need another.

 

[TJBaker57]

NP8 with 7 pin plug is what I need with a 2007 TB LT...?

That is what you have, yes.

3 pins are for the encoder (range position sensor), 2 pins are for the internal brake, and 2 pins are for the actual motor.

And while your checking prices, a Veepeak OBD2 adapter that can allow you to read your own codes can be had for under $15 (US).

 

[AC75TB07]

Thank you. Gonna install the new switch before buying anything, give it a couple days and see what happens... I hope that's the end of it...