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700r4/np208 issues

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Smokeyk5

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Howdy all,

I just got another K5 and am having some issue with the drive line. It's a 75 that has a 700r4 and np208 swapped in, originally was a np203 and th350, hubs are still slugs and not selectable. Front d44 and rear 12 bolt with LSD, 3.73's on 33's. They guy I got it from said he had not tried it in 4wd as it just sat in his garage (it hasn't been driven 4000 miles in almost 15 years). Currently the issue is when I put it in 4wd low or high I can drive a couple feet but it feels like to drive line is binding and the tires just chirp, doesn't feel normal or right. My guess is it's an issue with the tcase but I'm not sure where to start. I just hooked up the mounting plate on the shifter yesterday (the one that keeps the stick in place has that funky L shape). Any help would be appreciated.
 
Actually that is normal for a part time system. The front and rear axles are never going to turn at exactly the same speed except by accident.
There will always be some slight differences in tire size, whatever.

So, if the front and rear ends are locked together, something it going to have to give. In this case, its the tires slipping.

Full time transfer cases have a differential in the transfer case to let the front and rear driveshafts turn at different speeds until you put it in lock.
 
You could have different gears between the front and rear axles. If it was originally a TH350/NP203 combo truck, it should be geared WAY higher than that. 2.73 or 3.08 probably. I'd say he swapped rear axles with something to get the 3.73s in it and never changed the front. Will it eventually bog down and stop moving?

Also, my Dad worked at a Chevy dealer in the late 70's and they checked in a brand new off the truck K10 with the 350/TH350/NP203 setup that came with the gearing miss-matched. 3.08's in the front and 2.92's in the rear maybe? Possibly more split than that, I don't remember right off. Anyways, he said it would tear the steering wheel out of your hand if you punched it in AWD and would crawl a few feet and stall out in the LOCK positions. They wound up re-gearing a brand new truck to correct the problem.

It's possible your truck may be kin to that one and have too much miss-match in the gearing to run with a part time case and if the original owner only used LOCK in the snow, it's possible he never knew about it.
 
Thanks for the replies, ya it will go a few feet then bog down and stop. I guess I will start by checking to make sure I have the same gears all around. Any other things I should check, maybe disconnecting one of the the drive shafts and driving in FWD then RWD to make sure the tcase is not shot? I know there should be a little bit of chirp maybe at lower speeds turning, but this feels like the truck is binding up way to much just driving straight at a couple MPH's, if I try to go faster the whole things just start wobbiling. Again new to the 208 but is it nessecary to put it in reverse to get out of 4wd? It wont allow me to go from 4wd to 2wd without going into reverse to unlock it. Thanks
 
My 205 sometimes requires me to be in reverse or move it around a bit before it will unlock. Dont remember that being a problem in my 208s though. Drain the fluid, maybe even all of fluids in the truck, and see what type of fluid comes out and in what shape.

I would think a huge difference ie 3.73 vs 2.73/3.08 would literally rip the case in half. I drove a Jeep with 3.54s in the rear and 3.23s and it acted like that. And that is nowhere near even the 3.73/3.08s.

Best thing to do is pull both axle covers, and check gear ratio. My bet is they didnt swap out the front to match the rear doing the ol ASSUMING method it was correct. 10$ on the rear he may have switched to 3.73, but the front is still a 3.08 axle.
 
To answer your one question. There is nothing wrong with pulling a driveshaft and driving around with the other. You just might feel the gear ratio difference when doing that. Also, change all fluids.
 
Its not unusual for a truck to bind up like that. Mine does it everytime and everything is matched on it.
But it takes a few dozen feet before I start feeling it. Some of the older 4wds had the front end a fraction of a number faster in the front than the rear.

For instance, 4.11 in the rear, 4.10 in the front. The old military Jeeps were that way. They were designed to spend most of their time in 4 wheel, and it was thought it would drive smoother that way.
I think I read where some Toyotas still did it. Maybe other trucks.
 
I was reading a little more and found some info saying that unless the t case has a internal diff that 4wd cannot be used on pavement or hard surfaces and it's only for offroad. Is that true to the 208 as well? I wasn't able to find a direct answer, some say it cannot others don't say anything. Seems kinda of goofy not being able to use 4HI on the street in bad conditions, but I maybe the goofy one. Either way I still think it's mistached ratios, but was just wondering about using 4wd on the street, I know 4LO is a no-no, but wasn't sure about 4HI for street use.
 
4wd on the street makes it bind up and steer funny. It shouldnt buck or bind though to the point where it actually stops the vehicle going straight. Now in a turn, if you left off the gas it will bind to a stop, but thats because its in 4wd. I would check ratio before anything just to be sure.
 
OK, if its a part time case, like a 205, you are not supposed to use it in 4WD on hard surfaced roads due to the binding problems we are discussing.

And I am fairly vigilant about not doing so with my truck. However most of the guys here have modded and overbuilt their trucks to the point where the weakest point is the tire grip on the pavement.
After all, rock climbing is by definition operating it on hard surfaces.......

But, since I do all my 4WD in mud and sand, I have not upgraded to that point. When I am going to bad places, I lock in my hubs, and drop it into and out of 4WD as needed.
If I wait too long to take it out after I get out of the slick stuff, it binds my transfer case up to the point I cannot get it out without backing up to ease off the tension.

No matter how strong you have built your system, it is a heavy strain to run in 4WD with good traction.
Its up to you if you want to do that.
 
Good to know thanks, currently I'm going to be building this soley as a classic driver. May see some trails but more than likely will just be used for simple mountain trips and cruising. I just wanted to make sure I had a direction before I start taking things apart that don't need to be.
 
Many years ago I bought a '71 K5 cheap,because someone had swapped a 3:73 rear axle into it,and the original front axle was 3:08 gears...you could go about 5 feet forward with the hubs locked,then it would bind up,if you forced it to go any further you'd feel the frame torque up,then one of the hubs would release with a loug BANG,like a gunshot...needless to say,I didn't try driving it in 4wd after the first time I tried it..(truck had a NP205 and a SM465 tranny)...I checked the rear axle ring gear for the ratio numbers and soon learned someone stuffed any old axle they found under it just to make it roll and sellable...(I paid 300 for it,so I wasn't that upset)..was a decent truck otherwise..

I found another 3:73 front axle for it cheap,but never got around to installing it...sold the truck not long after that,and I gave the guy the axle,he never used it either--instead he put larger tires up front and smaller rear ones to help "match" the ratios...it worked well enough to use 4wd on snow or sand,but not on asphalt...

I've read even GM doesn't have "exactly" matching ratios from front to rear,a truck with 4:11's will actually have a 4:09 ratio up front,due to the different sized ring and pinions...they claim thats "close enough"....I have driven a few of my trucks with NP 205's on roads with the hubs locked that were snow covered in some spots and bare tar on others,never felt any binding or jerking at all,only a bit when taking sharp corners on dry asphalt...so long as all 4 tires are the same size,tread pattern,and inflated equally,you shouldn't notice much lurching or jerking with the hubs locked if the axle ratios are matched...

My bet is you have two that dont match...or maybe two tires are not the same diameter up front compared to the rears ..they would have to be at least 2" different to be noticeable I'd guess though...dont go by the sizes printed on them,measure the circumfrence with a tape measure...for example,one brand's 235/75-15 can be a lot different diameter than anothers,I learned that when selling used tires at the salvage yard...
 
Thanks for the input, currently it won't pop or make grinding noises but just doesn't feel "normal" to me, then again I've only dealt with the 203 on my other K5 and current 4x4's I had read that there can be a slight difference in the ratios can't remember the exact % but it was fairly low something like <5%. I know all the tires are the same brand definently not same diameter they are all 33 Goodyear MTR's but the front has a lot less tread than the rear atleast 1/2 inch if not more. I was going to swap my new 33's from the other rig which are pratically brand new so I can see what happens there, but still will check everything out.
 
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