Chevy Astro and GMC Safari Forum banner

Easy manual shift NP-231C T-case swap How-to

17304 Views 51 Replies 14 Participants Last post by  markmitch
OK, I've been meaning to do this, so here goes....

I found out about this swap on the S-10 forums, and once I heard about it, I couldn't get it out of my mind.... So finally I just did it. Since I already had a completed 233 swap that had been working for years, a logical question is "Why?"

Well, it was precipitated by my electronic shift setup failing once, which honestly wasn't that big a deal, just a loose connection, but somehow that got my mind to chewing on it and one thing led to another..... Once I read about this swap, I just had to find out if it was doable....

Well, it is.....

And it's pretty slick, all the way around. The downside is that the parts are a bit harder to source than the 233 t-case - not by any means unavailable, just less common.

The basis for this swap is the transfer case from 95-97 S-10s and Blazers with manual shifted 4x4. Didn't know they made those? Me either, but it turns out they did, and quite a few of them. It was an option for those buyers who were skeptical of the newfangled electronic shifted system GM was moving into. I found quite a few of them listed on and eBay, some with pretty low miles as they were removed from vehicles a while back and are just sitting on shelves waiting for new homes. The listings will most often say "without option F-46, floor shift"... I'm not sure why they specify without F-46 (AWD) but they usually do. The floor shift part is the key. That will get you an NP231 case instead of an NP233. Be careful to double check, though, because there was also an option for a floor shifted auto trans, and I saw at least one listing for the floor shift t-case that was actually out of an electronic shift 4x4 that had the floor shifted tranny....

So on to the case. It's a 231-C that has a splined front output instead of the yoke we usually associate with the 231. It doesn't have a separate designation because New Process just considered it a running change to the same transfer case rather than a new configuration. And it has the same 32 spline rear output as Astros.




Inside, it's a beefy case with the 1.25 inch chain and a 4-gear planetary.


Outside, it's virtually identical to the NP-233, except just over by where the shift lever is. Here's a picture of them together, the 233 is on top:


To go with it, I got the shifter and cable from a 96 Blazer at Pick'n'Pull. This is what the donor looks like:


And here are the parts removed. Basically, there is just a shifter assembly, a cable, and a bracket.



I don't see any reason why you couldn't use this case with the shifter from any NP231 equipped vehicle, if you can't find the S-10 shifter setup, though it might take a little fiddling. Because this case is meant to be manually shifted, the tension on the shift mechanism is lighter than the 233 and it has neutral position, which all the factory shifters are set up for.

The GM shifter is nice because it attaches directly and works without any mods, plus it has a built in range switch that gives you your 4LO ground signal for the ECU. If your donor is complete, you can also use this slick range indicator thing that is in the front part of the console:


Once you have the parts, you can install this directly, with no more complex fabrication than just using a hole saw.
See less See more
1 - 20 of 52 Posts
Let me be the first to say, congratulations!

The thought of using the manual t case had crossed my mind when doing mine years ago, but dismissed it as I couldn't find one and had access to a cheap 233.

Is the shifter bolted up yet? interest in seeing how you mounted it to the floor
So now on to the install.

First I got everything ready for the shifter. I took out my console, and mocked up the shifter location, then used a piece of wire the length of the cable to play with routings to get everything to work. Considerations are the length and limited flexibility of the cable, positioning of the shifter so it doesn't interfere with doghouse removal, yet still is convenient to tug on from the driver's seat. Then I could find a good spot for the cable to feed through the floor so that both ends wind up where I could use them. It turned out that it works if the cable runs down through the middle of a reinforcing channel under the floor behind the doghouse. I used a hole saw (I think it was 1.25 inches, but could have been 1.5, sorry about the vagueness) to drill through the floor, then held the drill at an angle and went on through the reinforcing channel underneath. Looks like this when done:


The I straightened the bracket that clamps the cable grommet to the floor (it's curved as installed in the Blazer)


And screwed that to the floor. The grommet can be slid a little back and forth on the casing of the cable, so there is some slack for getting it all right.


Then I made a bracket/box thing to mount the shifter to. In my installation, I was trying to keep as much of my original floor console as I could, plus meet the other considerations above. Getting the angle right took a bit, but here is what it looks like. It's a mixture of wood and metal, and is plenty strong for this purpose.


I cut off a bunch of the extra bracketing the shifter originally had attached to it (it's mostly for holding together the Blazer console and whatnot) and then bolted all that together.



Swapping in the case is just like swapping in a 233 - basically you take out your 136 AWD case and put this one in. Use the studs from the Astro case because the Blazer case was installed with bolts, and the studs make it easier to line up.

The bracket for the shift cable attaches to the two studs (or bolts) on the upper LH side of the case, and due to a minor difference in the t-case adapter I had to notch it a wee bit for it to fit correctly.


The you just attach the cable and you're done underneath. Well, OK, remember to put your driveshafts back in. And add tranny fluid. And, well, you get the idea. Here is the install underneath:



And close up the fit of the shifter cable and bracket with the Astro tranny shift cable threading down through them - it all fit together perfectly.


This is the basic install, and at this point the 4WD was working. It actually took me a moment to wrap my head around it, I kept thinking it should require more fiddling, but no, I just tugged on the shifter and it went into 4HI. Tugged again and into neutral. Once more and 4LO. :rockon:

So from here it's all just about accessorizing.

I've used this a bit now, had it in for a couple months. The shifting action is smooth and precise. It requires enough grunt that you can't do it by accident, but it doesn't balk or complain or give you a workout. I'm very happy with it. Next post I'll show how I wired up the range indicator, and put my floor console back together and whatnot.
See less See more
Great work!

I don't see a picture of the indicator ;)
What every astro should have had as an option...nice clean where is (as Paul Harvey would say) the rest of the story....
Thank you for the props, guys, I appreciate it..... The rest of the story is coming.... Things sometimes move slowly in my world..... Or maybe it's me that sometimes moves slowly...
Wow. I dig. Great work.

Sent from my iPad using Tapatalk
Wait until you see the finished product!
OK, finally to finish up..... I've had really poor internet for a while, so talking advantage of a good signal to get this done.

In our last installment, the 4wd was working, but we were wondering how is this gonna get tidied up?

I finished up the shifter by making this frame and sewing up this vinyl boot:



BTW, I'll just mention this, since it took me a while to figure out.... GM floor shifters use a hidden U clip to hold on the knob, it's on the invisible side of the knob, and isn't easy to feel with your fingers, but once you find it, you just slip in a screwdriver and slide the clip out:


Then I had to remake my console to make room for the shifter. Here's what it looked like before:


So I moved the main box back, put the cupholders inline, and reshaped the whole thing so it looks like this:


If you don't have a console or other things in the way, you could do something much simpler, but unfortunately that isn't how I roll. S10s with bench seats a have a short console that you could probably easily incorporate into the space between the Astro seats. Sorry for the crappy photo, it was all I could find in a hurry:


So back to our story. I then hooked up the 4wd range indicator which I removed from the Blazer console. I had to extend the wiring harness, since it's made to be installed in the little console immediately in front of the shifter, as opposed to all the way up in the dash where I have it. The connections on one plug just go straight to the shifter, while the other plug has the connections to the vehicle:

grey - dash lighting
black - ground
pink - hot in run
grey/black - low range ground signal to PCM
white/black - 12v from axle engage switch


Wait a minute, what's that last one? On S10s there is a signal sent by the front axle engaging mechanism that tells the computer that the axle has engaged fully. On vehicles with the electronic shift 4x4 it turns on an indicator light that says, creatively enough, 4WD. On vehicles with the manual shift 4x4, it skips the computer and turns on a light that illuminates the front axle illustrated on the indicator. Since the Astro has a full-time front axle, I decided to connect this using a relay wired so that it is one whenever the shifter is NOT in 2WD. I connected the relay such that it would be active when the shifter is in 2WD, and then the wire for the light is connected to the 87A pin, which means it has power when the relay is not active. I have not verified that it's working right because the light bulb is burned out and I haven't had a chance to take the dash apart again to fix it.

To fit the indicator into the dash, I hacked a whole bunch of it off:


And then the assembly fits under/behind the A/C bezel which I have cut up rather a bit, too:


Once back together, it looks like this:


And here is a shot of the whole dash with everything back together:


So that's how I did it. Most of the work went into redoing the console, honestly, without that complication it would have been done in a day.
See less See more
Can you chat about the ECM connection?
I can see some pros and cons of this job.. doing it this way. Building your shifter mounting assembly looked like a fair amount of work, and would not be easy for some. Then you additionally had to design a boot and way to mount it too. Of course, ultimately you had to redesign your center console to accommodate as well. Congrats on a nice creative job.

But there are many different options for shifters that can still work with your nice TC case too.

The TC case and the rest looks pretty easy however. It was a great discovery you've found with this year specific model that will interchange so easily.. if you can find one.

The illuminated dash mounted display is an awesome addition too! Very good!

Great job, pictures, and excellent write-up.
In the end, you have a nice reliable custom job (done your way) you'll enjoy for years to come. :D
Thanks for the props, guys, it does make me smile....

MM, I agree that doing it the way I did it is a lot of detail, but if you don't have a whOle custom console to deal with (and aren't as fussy as I am), you can make it a lot easier on yourself. The shorty console I showed in that one photo would be an easy way if you can find one of those.

The real gold here, though, is the case itself which should work with the shifter out of any NP231 equipped vehicle.
gavio said:
Thanks for the props, guys, it does make me smile....
Can you go into any detail on the console sewing , attachment or anything about methods you might have used in hindsight ?
Some here are not so handy w/ a sewing machine or upholstery.

So, can you talk about the grounding part?

Time for a skid plate now.
So, can you talk about the grounding part?
The 4LO ground signal's purpose is to tell the computer to upshift at lower vehicle speeds, so you can get all 4 gears in 4LO. With it unconnected, you only get 1st and 2nd. On the S10, it goes to the PCM and this pinout for the 2001 Blazer PCM connector C2 shows where:


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.


Using this S10 setup with the floor shift, the range switch on the shifter has a yellow wire running up to the indicator, which both turns on the appropriate light and connects to the signal output, which is a grey/black wire the vehicle side of the wiring harness. That grey/black wire just gets extended out to the PCM and connected to that pin #16. Note: Earlier Astros (before 2000????) have different plugs to the PCM, and I don't know what pin you use in that application.

If you have the shifter, but not the indicator, you can just use the yellow wire directly to the PCM, then run the black wire to ground.


If you don't have the shifter with the range switch, then you'll need to improvise. IIRC, Music Man shows this in his manual shift 233 t-case thread.
See less See more
Can you go into any detail on the console sewing , attachment or anything about methods you might have used in hindsight ?
Some here are not so handy w/ a sewing machine or upholstery.
Hmmm, I didn't take any pictures of that process, so I'm not sure what would be helpful. Basically, I just made triangular templates of all four sides, with the shifter at each end of its travel, then sewed them together. Which is a very unhelpful description, I know.

If I just had the shifter itself (which doesn't have terribly long throw and isn't very big), I think I would just go through Pick'n'Pull looking at shift boots 'til you found one that would work. That's how I started out, and I found a boot in an older Toyota 4x4 with manual transmission that was plenty big and would have worked for that. I ended up having to make one because I need to fit such an unusual shape with my console.
gavio said:
... I think I would just go through Pick'n'Pull looking at shift boots 'til you found one that would work. That's how I started out, and I found a boot in an older Toyota 4x4 with manual transmission that was plenty big and would have worked for that.
Thanks very much - I was thinking just that - JY and make mods to an existing one.
Hate to have to try and reinvent an already made idea.

Nice job again - thanks for sharing and the photos.

Thank you. The pcm piece is key I think. I guess if one didn't have the wiring, they could just rig up a switch.
You're welcome..... I think the pcm connection is a refinement, really..... Even without it, the t-case is functional, and LO range will still pull you out of a stuck.... Back when I first swapped in the 233 case I used to have, it was months before I got around to hooking that up, and I did use LO range at least once.... You just can't drive much because it won't shift past second..... Its nice to have it all working right, though....
1 - 20 of 52 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.