Nvg233/gm rpo np1
 

Hello

I’m having an issue with my 1998 Chevrolet Blazer LT. It’s got the NP1 RPO/NVG233 transfer case. It has difficulty getting back into 2HI from 4HI or 4LO. The 4WD itself is working as designed. T/case and front diff locking and unlocking once it’s back in 2HI. When 2Hi is selected on the 3 botton switch, the encoder motor can be heard trying to shift the t/case back to 2HI but it’s unsuccessful. With the trans in neutral and in 4LO, when I attempt to switch it to 2HI, switch indicator blinks and goes to 4HI. After that, I have to hit the 2HI button a random number of times and eventually will shift to 2HI, front axle and front driveshaft will disengage.
I have scanned the system for codes by jumping term.13 of ALDL to ground and have no DTC’s. Reset system several times, checked connection at TCCM and encoder motor and all good. Vacuum circuit of the system is working properly. This Blazer has 327k miles on it, no engine, ABS, SIR codes stored. Interior is leather with few cracks, no tears. However, body can use some attention. Overall great vehicle other than this 4x4 issue.
I’m guessing encoder motor but I would like to get some more input from others on the forum,

thanks,
Dedaike

I can't help with the encoder motor but will ask about the TCCM. Have you heard it clicking many times when it is prompted to change the drive configuration? The relays in the TCCM can go bad. One of the symptoms is being able to hear the relays clicking a bit before the new setting is found, if at all.

I can hear the TCCM clicking when I’m selecting the different ranges. The 2HI light flashes and goes into 4HI.

[QUOTE=Dedaike;744073]I can hear the TCCM clicking when I’m selecting the different ranges. The 2HI light flashes and goes into 4HI. Does the TCCM click only once for each range?/QUOTE]

I'm sorry I should have been more clear. If you can hear the relays of the TCCM repeatedly clicking before engaging the selected new mode or not engaging at all, then it is possible the TCCM is going bad.

When mine was going bad, I could hear the relays clicking many times, as it the TCCM was trying repeatedly to have the system make the switch from one mode to the other. And over time, this clicking would go on longer and longer before the new mode would engage. Off the top of my head I'm pretty sure you should only hear a click or two from those relays when switching from one drive mode to another.

Does this sound like what is occurring with your TCCM?

Hey Chritine

Thanks for replying. I don’t hear multiple clicking, one click going into 4H or 4L, and all engages properly. I’ll have to verify what I hear going into 2H. I’ll let you know this afternoon. I’ve never had a TCCM apart, are there multiple relays in it?

Yes, if I remember correctly, there are several relays attached to the printed circuit board. I do not believe they are serviceable. I bet a google search could find you some pictures of the inside of a TCCM.

Alright. I looked into the TCCM. When I hit 2H, I hear the TCCM/relay click and the encoder trying to engage right away. This goes on for 2-3 cycles at which time the 2H light flashes and goes back 4H. After cycling from 4H to 4L several times and cycling the 2H button, the system goes to 2H.

When you say "cycles" is it as if the TCCM is repeatedly trying to get the mode to change or do you mean going through the switch from one mode to another 2-3 times?

If the former, then that doesn't sound right.

I had to replace my TCCM. I was fortunate to find a NOS one but this was 10 years ago.

I press the 2H button and I hear the TCCM click 2-3 times as well as the encoder motor trying to shift the t/case as well.