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Old 12-21-2010, 06:01 PM   #1 (permalink)
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78 Chevy t-case issues

78 Chevy 1/2 ton 4x4 turbo 350
As far as I can tell this has the "original np 203". I am going off of the picture in chiltons and the two rods from the shifter to the t-case. From searching around I found that a np 203 was full time 4 wheel drive, and has a differential in it. Then when you shifted into lock it locked the differential and both front and rear drivelines locked together. When not locked if one driveline is removed the t-case acts like an open diff. and no power to the other driveline.

Correct me if I am wrong, which from some searching many people will do.

I had the front driveline out for a couple weeks while I changed the u-joints. I drove it during that time. So either someone did some work to the t-case or it was stuck in lock, but I don't think so because now I cant get lock to work in either high or lo very well. My truck also has selectable lock in hubs instead of the full-time hubs like are on my brothers Suburban.

When I lock the hubs in and try it out in the snow or mud I have gotten the fronts to spin a couple of times but it feels like the teeth are not meshing on the gears very well. When I do get it to lock in it pops out whenever I let off the gas or coast. I have been on the creeper under the truck and have watched as my brother ran the shifter and figured I could adjust the linkage for the lock. I tried and have not had any luck to get it to go in lock and stay there. This is my daily driver so if anyone has any suggestions please throw them out there for me.
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Old 12-22-2010, 02:43 PM   #2 (permalink)
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Well maybe was too long of a post. I just thought more info was better.

I am just wanting to know if from what I posted if my t-case has been modified or if something is internally wrong with it.
Also trying to figure out how to get it to stay locked in.
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Old 12-22-2010, 07:03 PM   #3 (permalink)
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There were aftermarket kits back when those trucks were new(er) to make the 203 a part time case. Your's may have one, esp if it has lockout hubs.

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Old 12-22-2010, 08:08 PM   #4 (permalink)
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Yep, since you have locking hubs, it probably has a part time 4WD kit installed ^^^.

Maybe the chain has stretched, that's a common problem with 203s.

If for some reason, it was not converted, and it was driven without a driveshaft, it can/will burn up the diff inside the 203.

The only thing a 203 is good for is doubler parts.
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Old 12-27-2010, 10:48 PM   #5 (permalink)
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The only thing a 203 is good for is doubler parts.
OK I now know I have a "junk" t-case on my truck but am going to have to drive it till I can figure out what to do to about it. Fix the shifting into Lock, change t-case or scrap truck are all options at this point.

When we were trying to trouble shoot it we noticed it was low on fluid and put some 90 weight in it. I now know that was the wrong fluid and have since drained it or at least as much as possible by taking the back cover off. I probably only added half a quart and drove only 15 miles.
Would I be alright to just replace the back cover, fill with 10-30 and drive? Or I was reading right the front half is also full of oil and by now the 90 weight would have mixed in the front and it would also need to be drained? Or would it have because the t-case wasn't shifted into lock and the chain didn't spin.
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Old 12-27-2010, 11:07 PM   #6 (permalink)
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You shud be ok with the 90 weight in your 203. But really the best thing you can do is unbolt that case from the trans, take it apart and save the range box part, and chunk the chain drive end in tha river with a $20 bill stuffed inside so you can say you threw sumthing away
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Old 12-28-2010, 08:00 AM   #7 (permalink)
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You shud be ok with the 90 weight in your 203. But really the best thing you can do is unbolt that case from the trans, take it apart and save the range box part, and chunk the chain drive end in tha river with a $20 bill stuffed inside so you can say you threw sumthing away
I realize that would be the "hardcore" answer to everything "Junk the 203". That is not my near future plans, I still want to drive it like it is and be able to get it into 4 lock in case it snows and I have to get to work.

Guess I will just put it together and fill it up and drive.
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Old 12-29-2010, 07:29 PM   #8 (permalink)
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since this setup is something that your wanting to work properly when you need it then it might be worth the $ for a 203 gasket kit and tear into it and find the problem.

my personal opinion is to trade that 203 out for a 205, should be a mostly bolt in exchange. plus 205's are really bulletproof. save the 203 range box and adapter for someday when you might build a doubler or sell it to help pay for a 205. you may even find someone to trade you that 203 and adapter for a 205 with adapter.
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Old 12-30-2010, 04:34 PM   #9 (permalink)
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While swaping in a 205 would be nice, it's not really budget friendly for alot of people. 205's to fit th350 trans are not that common, then you have to add the price of driveshaft mods.

From your description of the front not working so well, I'd say your chain is stretched/worn. There is some adjustment on the shifter rods, but I don't know if that would do anything. 203's are pretty easy to find around here for $100 or less. Other than being heavy, they are pretty easy to swap. I've got a whole truck with one, shipping might be a little much though.
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Old 12-30-2010, 09:41 PM   #10 (permalink)
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since this setup is something that your wanting to work properly when you need it then it might be worth the $ for a 203 gasket kit and tear into it and find the problem.

my personal opinion is to trade that 203 out for a 205, should be a mostly bolt in exchange. plus 205's are really bulletproof. save the 203 range box and adapter for someday when you might build a doubler or sell it to help pay for a 205. you may even find someone to trade you that 203 and adapter for a 205 with adapter.
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While swaping in a 205 would be nice, it's not really budget friendly for alot of people. 205's to fit th350 trans are not that common, then you have to add the price of driveshaft mods.

From your description of the front not working so well, I'd say your chain is stretched/worn. There is some adjustment on the shifter rods, but I don't know if that would do anything. 203's are pretty easy to find around here for $100 or less. Other than being heavy, they are pretty easy to swap. I've got a whole truck with one, shipping might be a little much though.
Still trying to figure out how to "Multi-Quote"

Which is it? 205 is bolt in mostly or a 205 for a turbo 350 are hard to find and not very budget friendly. I have been reading on posts all over the site and conflicting/confusing answers. Like I believe I read there are four different input shafts for the 203 and I would have to find the right 205. So how would I tell what I have to find parts before I tear my 203 off the trans?

I have already tried adjusting the linkage how I thought it should be, that didn't work so I tried how it said in Chiltons and I can get it to sometimes lock in while in Low, but then when I stop or change directions the damn thing pops out of lock.

I am not against replacing the whole damn thing, I am on good terms with the driveline shop. I just need to know what to look for before tearing this one apart in the middle of my garage.
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Old 12-30-2010, 10:57 PM   #11 (permalink)
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Im pretty sure you can find an np205 with adapter for a th350 in the for sale section if you look hard enough.

It is a complete bolt on swap except for the driveshafts need to be modded. You need a 27 spline male input in the 205 along with the adapter and the coupler sleeve to bolt it up to you existing trans.

If you want to fix the 203 you could just get a rebuild kit and a new chain but it will probably cost just as much as switching to 205.

Does it make any funny popping noises from the front end when it "pops out" of lock?
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Old 12-30-2010, 11:24 PM   #12 (permalink)
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Have you checked to see if front and rear gear ratios are still the same? The 203 would accommodate different ratios open if the diff hadn't been locked with a part-time kit, but it could not tolerate different ratios in lock whether it was modded or not. Before you worked on the driveline did you use lock without any problems? Running the 203 with either driveline out won't hurt its differential at all because it must be locked to do so. If you don't need to put it in lock to run with one line out, then the diff had to be locked with a kit. The part-time hubs are a very good indication that it was.

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Old 12-31-2010, 08:13 AM   #13 (permalink)
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Does it make any funny popping noises from the front end when it "pops out" of lock?
No, it makes clunking sounds, like a u-joint, when I change directions. But they are coming from inside the transfer case. When I do get it to lock when it comes unlocked I don't hear anything, just lose traction. But also only locks predictably in LOW so engine rpms are up. In HI I can't seem to get it to lock predictably more like random. I do the same thing and sometimes it locks but usually it doesn't.
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Old 12-31-2010, 08:24 AM   #14 (permalink)
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Have you checked to see if front and rear gear ratios are still the same? The 203 would accommodate different ratios open if the diff hadn't been locked with a part-time kit, but it could not tolerate different ratios in lock whether it was modded or not. Before you worked on the driveline did you use lock without any problems? Running the 203 with either driveline out won't hurt its differential at all because it must be locked to do so. If you don't need to put it in lock to run with one line out, then the diff had to be locked with a kit. The part-time hubs are a very good indication that it was.
No haven't checked ratios
No never had any luck with lock before I pulled the driveline. That was what led to pulling the front driveline was I tried to use 4X4 and it didn't work. I crawled under it and looked and the front driveline had bad u-joints. After I replaced them it started to get snowy and I started trying to lock in the front end. Not knowing at that time I had a full time case that had been modified to part time.

Thanks for all the help, I guess the search is on for a 205.
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Old 12-31-2010, 10:22 AM   #15 (permalink)
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not sure what rock mafia is talking about and maybe back east th350/205's may be hard to find, but in the last couple of months at the local u-pull it yard here in Ca. i've seen at least 3 or 4. all to be had for less than $100 (i think they go for $75). if you grab the front driveshaft at the same time, as it will be a bolt in fit. if your lucky the rear driveshaft will be there also and useable, if it's from the same wheelbase truck as yours. or if you have to it's usually not that expensive to have a driveshaft re-tubed to make it longer. its still stronger and more reliable in the long run to have a 205.
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Old 12-31-2010, 10:31 AM   #16 (permalink)
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your problem sounds more like a shifter linkage issue to me. My stock hurst style crap shifter would pop out of gear all the time. It would only sometimes engauge and mostly wouldnt. I ended up twin sticking mine so i have a lever for lock and unlock and hi n lo. Solved all the issues plus it looks cool.

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Old 12-31-2010, 10:48 AM   #17 (permalink)
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not sure what rock mafia is talking about and maybe back east th350/205's may be hard to find, but in the last couple of months at the local u-pull it yard here in Ca. i've seen at least 3 or 4. all to be had for less than $100 (i think they go for $75). .
I havn't seen that many in local junk yards here in the last 10 years, maybe because all our trucks have rusted away

If a $75-150 205 is an option, I'd be going that way.
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Old 03-15-2011, 10:19 AM   #18 (permalink)
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Old topic but new questions

OK I guess I didn't update this thread but the 203 leaked oil out of all the seals. It quit leaking like 5 days after fill-up. It has a terrible clunking in it when going from forward to reverse. I quit driving it and have been looking for a replacement 205. I have not found one but did come across a 203 hooked to a Dodge 727. Will it work? I know the typical answer is junk the 203 but this is not a hardcore rock crawler. This is to get me to work and go pull my brother out of trouble when he gets stuck. It will see some 4X4 time but not anything too extreme.
So will the Dodge 203 work?
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Old 03-15-2011, 10:59 AM   #19 (permalink)
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Old 03-15-2011, 07:11 PM   #20 (permalink)
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just keep searching for "205 transfer" on craigslist or on here. they come up all the time.
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Old 03-15-2011, 11:34 PM   #21 (permalink)
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Old topic but new questions

OK I guess I didn't update this thread but the 203 leaked oil out of all the seals. It quit leaking like 5 days after fill-up. It has a terrible clunking in it when going from forward to reverse. I quit driving it and have been looking for a replacement 205. I have not found one but did come across a 203 hooked to a Dodge 727. Will it work? I know the typical answer is junk the 203 but this is not a hardcore rock crawler. This is to get me to work and go pull my brother out of trouble when he gets stuck. It will see some 4X4 time but not anything too extreme.
So will the Dodge 203 work?
The input shaft is different on the dodge, and I doubt the Chevy addapter will bolt up to the dodge case.

Both are pass side drop.
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Old 03-16-2011, 09:07 AM   #22 (permalink)
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the adapter will work afaik, but your are right about the input shaft, and it is the absolute last thing i would ever want to do.

you will have to do a full tear down of a 203 to install the input shaft and you are talking about dumping, finding and reinstalling nearly 200 loose roller bearings. and in sure reconcceting the locking lever internal linkage would be a blast.

you can find hundreds of 203 on here and craigs list.
OK will keep searching

The Dodge t-case was just an idea to get this truck back on the road. I have not found one local yet. Closest one so far was 300 miles and $200
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Old 03-16-2011, 10:09 AM   #23 (permalink)
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Sound like linkage isuues. Just because it has locking hubs doesn't mean it was converted. Some people put the hubs in free and put it in hi or lo lock for 2wd. But the gears will spin still in the front. To tell If yours was converted, lock the hubs, and Jack the front up with it in park. In hi or low(not lock) it will roll while in park. You can get hurt jacking the front or rear up with fulltime.

Fulltime 4wd is bashed all the time, but most of that comes from people who never owned one. It's point and shoot, it works very well on twisty wet roads. Smooth and predictible and excelent wet or dry even with bald tires.

If cost is a factor, get another 203 and run it full time (hubs locked.)
Or get a divorced 205


I've heard that converting 203's to part time starves them of oil. That may be the problem too.
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Old 03-17-2011, 08:45 PM   #24 (permalink)
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your problem sounds more like a shifter linkage issue to me. My stock hurst style crap shifter would pop out of gear all the time. It would only sometimes engauge and mostly wouldnt.
This.^^^ Installing the PT kit is pretty easy but the linkage must be adjusted just right. Many people don't take the time to do it properly, the thing starts popping out of gear, strips the engagement teeth (I'm guessing this is what has happened to yours), and then they declare the kit and/or the T-case "junk." It's not a 205, but I really wouldn't call it junk. They are pretty strong in stock configuration, and I've beat on 'em with a properly installed PT kit without ever having a real problem.

I'll bet your engagement teeth are partially stripped and the PT kit parts that replace the spider gears in the diff are worn and rotating a bit when you change gears, causing the "clunk" you're hearing. Mine started to do this a bit after a few years of hard on and off road use. Yours might have been pounded pretty hard.

You can check all this pretty easily. Pull the rear driveshaft, diff housing on the T-case, and the output and diff and check the teeth on the front side of the diff carrier. Check the output to diff connection for slop. It should be pretty tight (if PT kit is installed). If this is out of a '78, you'll probably have the cheap POS stamped steel diff carrier which is in fact junk and could be the cause of some of your trouble (the engagement teeth are tiny and it doesn't support the PT parts well). If this is the problem and the slider teeth (in the T-case that mate with the diff carrier teeth) are still good, you can replace the whole carrier with a good cast version out of any older 203 and install a new PT kit if yours is real sloppy ($40-$50). Then shim it properly (as per kit instructions), carefully adjust the shifter, and I bet you'll be back in business.

EDIT: All that said, if you can find a 205 that will work and can afford the driveline alterations, you'd be better off in the long run. I imagine fixing the 203 might be pretty cheap and easy if you can find a good cast iron diff carrier, but the 205 would definitely be an upgrade.
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Old 03-20-2011, 02:16 PM   #25 (permalink)
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Is it really that hard to find a 27 spline np 205 ?? Trying to keep the 203 is really just a bad idea all around. And are you sure you would need to change your drivelines just for the 205 swap? I think your over complicating things man.

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