02 trailblazer 4wd issues

[stohlimann]

Hi GMTnation! I have been doing research on the 4wd system of the trailblazer/envoy etc for about two weeks now trying to narrow down my problem. Originally, the A4wd and 4hi were acting erratically on my 02 trailblazer, sometimes working sometimes causing dash service 4wd light to come on. Then over a period of about 5 days the system failed and the lights on the selector switch wouldn't illuminate after the 2 second initial start up. For the last 7 days I have been doing research on this site trying to narrow down what my problem could be. Yesterday morning I got in the trailblazer started it up and no service 4wd light on dash and the 2wd light on the switch illuminated. Switched into A4wd and then into 4hi, they worked. Tried switching out of 4hi to 2wd and the system failed again. Replaced with known TCCM, same problem. Checked front axle actuator, it was covered in grease and sand, pulled it and connection pins looked great. Nice and clean. Then I inspected the wiring near the axle actuator and noticed the brown wire was cut pretty bad. I was so happy that I found this problem as I thought it was my problem. Not quite. Soldered wire and replaced actuator, same problem. I thought it would be fun to check the encoder, so I pulled it and inspected wiring. Looked fine. Put it back together, not fun at all as I couldn't get the shaft splines aligned easily. Also, the shaft coming out of the transfer case for the encoder would not spin more than 10-15 degrees.(should I be able to rotate this?) I then pulled the cover off of the encoder motor and rotated the internal gears until I could get the female shaft splines close to where I thought they would go on and then I reassembled the motor and installed, after aligning splines better it slid right on, imagine that! Plugged it in and checked for 4wd operation. Now no service light and the switch flashes continually as if the its trying to engage into A4wd or 4hi, then goes black, if i try to switch modes after it goes black it flashes is that mode for a while then stops, black again, try another mode same thing. The red N light is on constantly now but it still rolls drives in 2wd. It will not and I don't hear the encoder trying to turn, or turning. But I couldn't hear it before I pulled it either.

Is there a way to bench test the encoder motor?
Any other ideas?

This is my first post, this community seems awesome!! Thanks in advance!!

Transfer case fluid was changed recently and condition looked acceptable for being 55,000 old.

 

[6716]

Welcome aboard!

You are way deeper into the 4WD than I have ever gotten, though.

Wish I had something to offer on your question.

 

[IllogicTC]

Bench-testing the encoder motor:

This is much easier with an assistant. The ignition should be in RUN, but the car doesn't have to be started.

Pull the encoder motor and position so that you can observe its motion. Then, have your assistant turn the dial, while you watch the motion. I'm not sure on the specific details of how it should turn, etc. - usually The Roadie will show up and can explain this in more detail. However, this can be a preliminary test to ensure that the encoder motor does operate. If the system acts fine, doesn't flash anything to your assistant, the problem may lay inside the transfer case but I can't guarantee that.

Have your assistant turn the dial back to the appropriate position (you said it was stuck in 4HI, right?) then reinstall the encoder motor. I am not sure if significantly changing the rotor position in the encoder motor has affected performance.

 

[stohlimann]

I'm 99% sure its in 2hi. When I cycle through the choices on the switch it flashes for about 15 seconds and then just goes black. Except the neutral light remains red.

 

[Matt]

Just pissing in the wind, but have you replaced the selector switch? These, on the 02's especially, were badly designed and an updated design was used for later years. It may be as simple as replacing the switch for a new, redesigned one...about $30 at any auto parts store.

 

[The_Roadie]

The shaft coming out of the transfer case should rotate a lot more than 10-15 degrees. Assume you found the NP226 theory of operation writeup on offroadtb.com?

The red N should never be illuminated unless you selected it the way explained in the Owner's Manual. But GM, in their infinite wisdom, made sure that the N mode is what the TCCM wants to put you in if it detects a high resistance on the rotating switch. So if the switch is bad, or the wiring back to the TCCM has an open or high resistance connection on the sense feedback wire, that would explain some of your symptoms.

I think you have at least two problems going on, like layers in an onion.

 

[stohlimann]

Thanks for the replies. When I said rotates 10-15 degrees I meant thats how much play was in the shaft. I couldn't turn it any more than that by hand. Should it turn more freely?

Thanks for the suggestion to check out the offroadtb, I hadn't read that.

Tomorrow morning I am going to remove it, the encoder, again, leave it plugged in and try to cycle it through the different modes. Does anyone know which wire provides the power for the encoder motor so I can check it with a meter?

 

[stohlimann]

Oh yeah one more question. Is it possible that I changed the resistance of that brown wire on the actuator by soldering it? Would that be causing any problems?

 

[linneje]

Oh yeah one more question. Is it possible that I changed the resistance of that brown wire on the actuator by soldering it? Would that be causing any problems?

Yes, that will change the resistance of the wire. The question, though, is does it matter? I don't know. Some components are very sensitive to changes in resistance (I have read that wheel speed sensors do not do well if you solder the wiring because of the resistance change), but in this case I don't know. Try popping the actuator off (2 8mm bolts) and leave it connected to the electrical harness. Have some one turn the truck on and try the switch from 2WD to 4WD and see if the actuator plunger moves out ...

If that works, try switching back to 2Wd and see if it moves in, and check the response of the selector lights inside.

 

[The_Roadie]

Oh yeah one more question. Is it possible that I changed the resistance of that brown wire on the actuator by soldering it? Would that be causing any problems?

Yes, and no. Soldering only reduces the resistance of a wire or connection and makes it conduct better. It can't cause a fault.

The folklore about wheel speed sensors is not in the resistance, since a wire should be extremely low resistance anyway. It's that the signal coming out of a wheel speed sensor is tiny, and easily disturbed by outside interference. The two main ways to shield small signals from external interference are twisting the wires, and using coax/shielded cable. If you have a damaged wiring harness coming from a wheel speed sensor on our vehicle, the repair is relatively easy and soldering is no problem if you do it properly and use insulation (tape or heat shrink) and don't short out the two wires. Repairing a coax cable takes more skill and experience and it's easy to damage it by soldering it as if it's a speaker wire. So maybe that's where the warning came from.

The movement of the actuator is easy to see and hear when it's in mid-air.