NP8 transfer case w/4-button dash control.

When I select Auto 4WD, 4HI or 4LO, the correct light illuminates on the dash switch but it just keeps flashing...nothing happens @ the TC. Interestingly, if I depress & hold the 2HI & 4LO buttons simultaneously for 10 seconds, the TC will drop into Neutral...just like its supposed to do.

I replaced the Encoder with a new GM unit since that seemed like the most probable cause, but nothing changed...same problem.

(Now, I either have two bad, or two good encoders and it doesn't seem to be possible to test the damn things off the vehicle. The only information I've found says to NOT apply power to them unless they're installed.)

The Service 4WD indicator lamp is not on and all mechanicals/wiring/connectors are in good shape. I just bought this truck so I don't know it's full history, but it's really clean & solid...no rust either.

The FSM talks about being able to deactivate/reactivate both the Encoder & corresponding lock-out with an appropriate scan tool. Is it possible to turn the Encoder off & forget to turn it back on?

Sure would appreciate any advice. Kinda stuck at the moment. Thanks!

 

So you're saying that it's stuck in Neutral, or will it still go into 2WD when that is selected? Can you click through all the positions using a wrench on the shaft when the encoder motor is off?

Have you seen this? https://blazerforum.com/forum/tech-a...rmation-41198/

 

Hi Les. Thanks for replying. No, the TC isn't stuck in Neutral...it will return to 2WD when the 2WD button is depressed. Haven't tried rotating the selector shaft so I'll do that.

Have discovered that the FSM diagnostic procedures cover two different scenarios of the button lights flashing:
1. Flash for 15 seconds & stop.
2. Flash continuously.
I have #2 and the FSM points to the TCCM as the likely culprit, so I guess I'll remove the RH kick-panel & see what's up in there. Thanks again.

Dene

 

It may just need the terminals cleaned. Look for corrosion, especially on the big red wire at the TCCM.

As you are finding out, it's difficult to go through the diagnostic charts in the FSM without a Tech 2 and you need to improvise for diagnostics. Once you completely understand how the system works/controls, then you can develop your own little tests that don't require a Tech 2. Or you can shotgun parts using the S.W.A.G system if your time is more valuable than a TCCM or if you simply don't really understand how it works.

Sometimes shorted encoder motors can take out TCCMs (did in mine). My TCCM was convinced it was in-between positions because the board got hot during encoder motor short and apparently melted the two adjacent positions on the board (encoder motor position inputs) together. All I can say is that you could pull down the reference voltage on both circuits through either of them, and that the two reference voltages always were the same during operation. My new encoder motor was going to extreme position one way trying to find where only one circuit was pulled down. No codes were present. Not saying this is your problem at all, just an example of alternative testing. Tech2 allows you to see what position encoder motor is in at any time, and I'm sure that doing so was part of a diagnostic chart. I had to do it with a volt meter. It was a real education on the 4 button 4WD system.

Captain Hook (moderator and resident Blazer Guru) has said that you need to clear codes after replacing TCCM or Encoder motor in order for the system to work correctly. I figured out a way to communicate with the TCCM and send commands to read and clear codes - by sending hexadecimal codes through an OBD2 terminal program. You might want to consider this (or having someone clear them for you), at least down the road if you still have problems. I never tried operation without clearing the codes in mine (after parts replacement), so I can't say what happens if you don't. Others have reported no problems.

 

Appreciate those insights, Les. Yeah, GM obviously built those procedures for their technicians who have ready-access to a Tech 2. The rest of us just do the best we can.

I should be able to clear any DTC's by just disconnecting the battery, yes/no?

Also, since the "Service 4WD" lamp is not illuminated, I would assume that no DTC's have been recorded. Is that not a safe assumption?

Dene

 

No, 4 button doesn't work that way.

No, old DTC can be in memory - but it is a good sign.

 

Hi Les,

Reactivating this thread...had to park it for a while and attend to other stuff. I decided to just bite the bullet & have my local GM dealer perform a Tech 2 diagnostic. The good news is that there are no stored DTC's & all transfer case functions are normal.

The problem lies within the front differential:

1. When I select any 4WD mode, vacuum energizes the actuator & draws the cable.
2. I've verified that the cable is attached at both ends.
3. I removed the 4WD Indicator Switch, confirmed that it's plunger is moving freely & (using an ohmmeter) verified proper open & closed circuit function.

That being said, I'm still getting a flashing dash light in any selected 4WD mode & the front axles are not engaging. It appears that interference within the differential is preventing the shift fork from being drawn fully into position. My next step is to remove the inner axle assembly (Ugh!) to reveal the problem. Any other suggestions?

Thanks!

 

I would suggest you do the diagnostics for the front axle engagement that is found in this tech article. https://blazerforum.com/forum/tech-a...4hi-4lo-41226/

That will definitively tell you if you have a problem there and how to fix it.

 

Thank you for that information, Les. I suppose it's possible that the interference I'm envisioning as being inside the differential is actually a binding cable...I will test for that.

One question:

I've searched hi & low for the procedure to disconnect the cable at the differential. I removed the bolts holding the cable housing but am unable to withdraw the assembly. I assume this is because the shift fork is engaged with it's clutch sleeve. It appears that the only way to remove the cable assembly is to also remove the inner drive shaft assembly. I don't see any other way to gain access to the end of the cable & unhook it. Is this correct?

Dene