Chevy Astro and GMC Safari Forum banner

Wiring the NP-233 T-Case with 3-button control

13547 Views 15 Replies 9 Participants Last post by  J.Stice
I thought I'd put down in one place what I did to hook up the electronic controls for the np233 transfer case installed in a later model Astrofari…. I see the question come up from time to time, and I certainly had questions when I went to do it, and I couldn't find the answers any one place. So maybe this will be helpful for someone.

I started with a thread on the other forum that detailed the installation of the 233 with 3 button controls in a gen1 Astro… That was helpful, but there are some differences in the later models….

Let me just say that I don't know for sure whether this will work for you, I just know that the following worked for me. My van is a 2004 Safari AWD, into which I swapped the t-case out of a 2000 Blazer. The physical install was bolt-in, and required no mods.


So now, to make it work. I chose the electronic controls because I don't have welding and metal fab skills to make a manual shift setup, but I do know how to connect wiring together. I extracted the 3-button switch, encoder motor, TCCM and as much wiring as I could get from a 97 Blazer. I took the dash apart in the donor car and removed the full harness - this saves building it. For reasons I will discuss later, I would suggest using the setup from a 98 and later Blazer instead.

I found that the full Blazer harness gave me enough wire to install the switch to the right of the speedo, the TCCM under the driver's seat and run the encoder wiring out through the grommet in the floor (by the B pillar) that the original AWD wiring ran through. I like having the TCCM there because if it breaks, it takes 1 minute (if I'm slow) to swap in a spare - this put my mind at ease about possible unreliability issues. Once all the wiring harness was threaded into place, it just remains to connect it.


SO this pinout chart is the basis of the hookup.


Most of the wiring is internal to the system, which is why the entire harness is a blessing - it just leaves a handful of external connections to be made. The external connections are:

C1 - 4Lo signal - more on that later
C6 - battery power
C8 - Ignition only power
C10 - ground
D2 - PNP switch output - positive
D3 - TCCM diagnostics
D8 - VSS signal
D10,12,13 - grounds
D14,15 - battery feed
D16 - either PNP signal ground, OR crank feed.

I did a lot of head scratching to figure out D16 - how could it be both positive and negative? Also, just where do you get the PNP signal from, since the Astro has a different switch configuration? Then I found this diagram for the 1999 Blazer and all became clear.


Later Blazers use a PNP switch much like the later Astros. The PNP signal actually runs up to the stoplight switch, and that's where to find it. The earlier electronics, like I had, use a second (ground) signal from the older style Neutral Safety Switch that is not present on later models (of Astros or Blazers)…. Later Blazers use the crank feed in this location.

So, with the wiring harness threaded into place, it's time for some connections. The way I routed the harness, all of my loose wires from the TCCM ended up along the driver's kick panel area, so I looked for as many of the hookups to be on the convenience center (CC) as possible. Here goes:

C6 is connected to the courtesy light circuit - I used the wire running up to the visor vanity lights at the upper left of the CC.

C8 is connected to a pink wire found at the steering column running from the ignition switch.

C10 & D10,12,13 are connected to the old AWD system's ground in a plug on the CC.


D2 connects to the PNP wire found at the stop lamp switch - it's a light green wire.


D3 is left unconnected - I found out after much head scratching that if the diagnostic signal is hooked up, it disables the TCCM. Leave it disconnected and then, to run the TCCM's diagnostics, hook it to ground and the codes are read from the flashing of the lights in the switch….

D8 is the VSS signal - If you have the overhead console, with DIC (trip computer), there is a signal running up it that can be found on the DIC plug in the convenience center- look for a dark green wire with white stripe. This signal may also run to your radio if you have the speed control volume feature. If you have neither of those, then you probably need to pull your gauges out and find it in there.


D14 & D15 I hooked to the power feed for the AWD at the plug in the CC.

D16 - since I had the earlier Blazer TCCM, I connected this to the e-brake switch down in the kick panel - to shift into 4LO, I would apply the e-brake, which grounds that wire….

Later, I switched to the TCCM from a 2001 Blazer, which required that I change this. I found a crank feed in the back of the fuse block - a purple wire that's by itself (position E5 below). Now, to engage 4LO, I just put it in neutral at a near stop and it drops right in.


What about C1? It goes to the PCM and this pinout for the 2001 Blazer PCM connector C2 shows what is needed:


The Astro uses the same basic PCM connector configurations with just some minor differences in what wires are hooked up - one of those being position 16 on C2 - that's where that 4LO signal goes - on Astros, it's empty.


I haven't hooked it up yet on mine. It's purpose is to allow the tranny to upshift at lower vehicle speeds, so you can get all 4 gears in 4LO. With it unconnected, you only get 1st and 2nd. Next, the switch.

I was unhappy with the choices for mounting the Blazer switch, which is very strange, in the Astro dash, so I found a 3-button switch used in Tahoe/Yukons from around 95-98 or so that I thought I could fit into the space below the right A/C vent. Well, there's not really enough space there, but I made it work out...

Here's the Tahoe switch:


Basically, the left part of the switch doesn't contain any circuit board, so I cut it off.


Then I modified the A/C outlet by removing the lower vent:


Then I added a piece to the instrument panel face:


And then I rigged up some fairly cheesy hocus-pocus behind it all to hold the switch in, cleaned up the evidence, and from the front, it looks about right


That particular switch uses the same inputs and outputs as the Blazer switch, so integrating them was easy. Only difference is that the Tahoe/Yukon switch adds backlighting, easily tapped into the dash lighting circuit.

The part with the switch actually took longer than the whole rest of the conversion, but I'm happy with the final product:


And the super-good news is that it works!

Except for the backlighting bulbs, which are, of course burned out.
See less See more
1 - 16 of 16 Posts
I know some of these pictures are sideways and what-not, but it's ben a long day, so I'll fix it tomorrow....
I think its an excellent write up! Nice work.
Thanks, I've been meaning to do this for some time now.....
The next part of the write up needs to be the proper use of the transfer case. I've got the same set up, but without the front differential being swapped/converted with S-10 parts, disengagement/engagement and use isn't the same as the vehicle the transfer case came from. On dry pavement with 4wd engaged, it's bound up on my van when trying to disengage with a solid clunk. I destroyed the first transfer case from trying to use 4wd on a snowy day, but when there were large patches of dry pavement. 4lo is great for popping up on car ramps, pulling a boat out of a lake, etc. Unless I've got a different gear ratio from the front to rear axles in my van, this isn't a system for dry pavement.
Here's this section from the 1998 Blazer Owner's Manual:

If your vehicle has four-wheel drive, you can send your engine's driving power to all four wheels for extra traction. To shift out of two-wheel drive and into four-wheel drive, push the 4HI or 4LO button on the transfer case switch. You should use 2HI for most normal driving conditions. Use these switches to shift into and out of four-wheel drive. You can choose among three driving settings:

2HI: This setting is for driving in most street and highway situations. Your front axle is not engaged in two-wheel drive. When this lamp is lit, it is about one half as bright as the others.

Driving in the 4HI or 4LO positions for a long time on dry or wet pavement could shorten the life of your vehicle's drivetrain.

4HI: This setting engages your front axle to help drive your vehicle. Use 4HI when you need extra traction, such as on snowy or icy roads, or in most off-road situations.

4LO: This setting also engages your front axle to give you extra traction. You may never need 4 LO. It sends the maximum power to all four wheels. You might choose 4LO if you were driving off-road in sand, mud or deep snow and climbing or descending steep hills.

Indicator lights in the switches show you which setting you are in. The indicator lights will come on briefly when you turn on the ignition and one will stay on. If the lights do not come on, you should take your vehicle in for service. An indicator light will flash while shifting. It will remain illuminated when the shift is completed.

Shifting from 2HI to 4HI: Press and release the 4HI switch. This can be done at any speed, and the front axle will lock automatically.

Shifting from 4HI to 2HI: Press and release the 2HI switch. This can be done at any speed, and the front axle will unlock automatically.

Shifting from 2HI or 4HI to 4LO: To shift from 2HI or 4HI to 4L0, the vehicle must be stopped or moving less than 3 mph (4.8 kmh) with the transmission in NEUTRAL (N) in vehicles equipped with an automatic transmission or the clutch pedal engaged in vehicles equipped with a manual transmission. The preferred method for shifting into 4LO is to have your vehicle moving 1 to 2 mph (1.6 to 3.2 km/h). Press and release the 4LO switch. You must wait for the 4LO indicator light to stop flashing and remain illuminated before shifting your transmission into gear or releasing the clutch pedal. If the 4LO switch is pressed when your vehicle is in gear and/or moving, the 4LO indicator light will flash for 30 seconds and not omplete the shift unless your
vehicle is moving slower than3 mph (4.8 kmh) and the transmission is in NEUTRAL (N) or the clutch pedal engaged. On automatic transmission equipped vehicles, if your transfer case does not shift into 4L0, your transmission indicator switch may require adjustment. With your transmission in NEUTRAL (N), press and release the 4LO switch. While the 4LO indicator light is flashing, shift your transmission into PARK (P). Wait until the 4LO indicator light remains illuminated before shifting your transmission into gear. This will get you into 4L0, but you should take your vehicle in for service to restore normal operation.
See less See more
Shifting from 2HI to 4HI: Press and release the 4HI switch. This can be done at any speed, and the front axle will lock automatically.

Shifting from 4HI to 2HI: Press and release the 2HI switch. This can be done at any speed, and the front axle will unlock automatically.

and that is the inherent issue with just swapping out the transfer case. You lose the shift on the fly function as the front axle cannot disengage or freewheel to take off any binding for a proper engagement in the transfer case.

In my case, it wouldn't fully disengage and would eventually jump out of engagement in the transfer case. Enough times and it would bind and not fully disengage even at a full stop. It would require backing up to undo any preload.
You lose the shift on the fly function as the front axle cannot disengage or freewheel to take off any binding for a proper engagement in the transfer case.
I see what you're getting at - yes that's true.... I didn't think about it in my own situation as I only have the 4x4 for off-road situations, since I don't really like snow very much.... It would be possible to add in the Blazer axle and use the vacuum switching setup to do the job - the switch is built into the t-case.... I know it's sort of considered trouble prone, but perhaps better than nothing...
Gavio, thanks so much for posting this up for the forum. This is a huge resource for me and others when we decide to swap our TCs over. Having met Gavio and seen the rig in person, he's a great guy and and I'm a big fan of his van. Glad to see how all the pieces came together. That switch install is :rockon: and looks factory.

Well done :clap: :clap: :clap:
Thanks for the props, mr Z.... I do hope this gives somebody a boost at some point...
Thanks for posting this.... That's the switch that comes from 95-97 Blazers.... I was going to do that until I thought of something more complicated :D
I have a couple questions if someone us around and can help. I was curious if one were to want to put in a manual linkage and have to bypass the electronics all together so it would be more reliable what would they have to do to maintain full function of the transfer case?

Thanks for any help
A 231 case is a manual shift. I suppose with a little machine work you could away with the electronic actuator and make a lever for the stub shaft to make it manual. Good luck.
I believe HotWire went the mechanical route with his. The details are in his build.
I may be dredging up an old thread, but I wanted to thank Gavio for the excellent write-up.

My project started a while back when I ordered a 236 to replace the 136 AWD case and instead was shipped a 233, which bolted right in and solidified my decision to lose the clutch packs forever, but necessitated re-wiring. That was a few years ago.
Finally getting around to the control module, this thread has made it possible. Thanks a million!

The s-10 I got my wiring from was a newer model and has the purple d16 wire for the crank feed, which would be nice as the old 136 AWD TCCM had a purple crank feed from the Convenience Center connector. However someone had apparently replaced the S-10's TCCM with an older model and I did the d-16 to e-brake contact and it works just fine for now.

My AWD connector in the CC did not have BATT power, I pulled it from another single-pin point in the CC panel. The AWD connector did have the RUN power (Brown). The fused 12v supply for the lights circuit (orange), the Panel Lights (Grayish Tan), and LED Dimmer (purple/white) wires were tapped from the headlight switch. I just fed the wires out at the instrument cluster and taped the weird shaped newer S-10 selector switch to the steering column for now, it works ok there.

On my 4.3L I have the four-connector computer. Connector 4, a 24-pin connector Black or Smokey Gray shield inside, will be where you put the 4-Lo signal pin. You will have to add a barrel-socket connector and wire to #23 in this connector. I used an old female hard drive power wire connector, but I recommend pulling the right barrel socket from a junk vehicle computer connector if you have a junkyard nearby. I am happy to report that this worked perfectly and allowed 4-LO shifts through all four gears very nicely. This is worth doing as the 4L60E is a true four-speed transmission.

I also ran the s-10 Encoder harness all the way up to just under the e-brake. The connector hides just fine under the little corner trim panel there. The TCCM is now located in the same place the old AWD TCCM was located.
See less See more
1 - 16 of 16 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.