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Going from a Th700r4 to a TH400 in my diesel

eodcoduto

eodcoduto

We could have been closer.
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So the tranny is slipping all over the place in my newly aquired '85 K20 6.2 Suburban, but I knew that when I bought it off the guy! I have the full drivetrain from an M1008 CUCV that will be giving up its TH400 and NP208, and eventually its axles as well. It looks like quite a few guys have done this and I will miss the overdrive, but the cost of a rebuild th700r4 is ridiculous. This is my "just in case" vehicle, and the th400 will fit the bill. I hope to start making progress on it soon.

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Yea its not the best combo but it is free, so that is the major factor. I will be using it for local camping in the desert and with the 4.10s in it right now it will be bearable. We shall see.
 
I'd just use what you have (the TH400/NP208)--and in the meantime,keep an eye out for a later 700R4 to have rebuilt that has the updated pump spline count,and auxillary valve body ('87 & newer)..

My pickup has that combo (TH400 & NP208)..and probably 3:23 or 3:42 gearing (haven't had the diff cover off to check)---while take offs are a bit on the slow side from a standstill on an incline,I don't really notice it feeling too high geared all that much..
(I also put 265/75-16's on it,2" taller than what it came with ,that helps none too!)...I rarely ever use low range even when plowing,mostly I use low range only if I get stuck and need to rock it out gently..the diesel is more tolerant of high gearing than a gas engine..

If you have 4:10 gears you can put bigger tires on it to slow the rpms down on the highway some,and still have decent pulling power in first gear...and there is always low range if that isn't enough!..

I know the 700R4's overdrive and lower first gear are desireable qualities,but we got by just fine up until 1981 without them, when they decided to start adding overdrive and lower the first gear ratio.

The TH350's were actually a bit on the weak side for a 4x4 truck,especially ones used to tow or plow,the 700R4's were even weaker..at least the TH350's made it past 100K before dying..

If the 700R4's were so great,why are there none in the trucks for military purposes ?...in stock form they probably wouldn't last long under that kind of use,that is why they went with the TH400...
I think the trade off for a few less miles per gallon and a few less seconds on your 0-60 mph time is not as bad as many will say it is..
I would rather have the stronger transmission and a few less mpg and a bit slower take offs...
 
Yep it is what I have so that is what is going in it. I will eventually get the 255/85R16s back on it so that will give me about an inch over the current tires. For the military, when our HMMWVs went from the old th400s without park to the newer ones with the overdrive and park I believe they used a 4l80e. I hate electronics and this is my bug out vehicle so the simpler the better.
 
I hate electronics in vehicles too--older is better as far as I'm concerned..I feel computers belong in a nice dry,temperature controlled environment,where they will perform better as intended..vehicles rarely have that kind of atmosphere--I find it amazing what they do withstand none the less though..

My biggest peeve with my 6.2 lately is getting it to start when its below 20 degrees...my batteries are ancient and likely junk,but even when I had two good ones I often had to crank it a long time before it would finally fire up,and the glow plugs work and are good...well,7 of them are..:blush:...one wont come out..it feels like if it had 40 more RPMS it would fire right up...15W-40 might be too thick for it..?.
 
Mine starts pretty good, but it has a hokey manual glow plug set up. One of the previous owners just ran some 10ga wire to a switch under the dash and used the stock glow plug timer for a power source, that is going to get fixed next weekend. I might also add a few glow plugs to the intake manifold, drill and tap them in and It could act like an old flame heater on a M35 or Perkins engine. Might help or it might not, but I think it will be worth the experiment.
 
A lighter weight oil will make starting easier in winter. We regularly stay below zero for weeks. Block heaters are a wonderful thing. My 6.2's start well to about -40. Then after that nothing starts well. If I forget to plug it in it starts but it takes several cycles of the glow plugs. Hot batterjes, oversize cables and good glow plugs make for a very happy 6.2


On the gearing note: 6.2/400/205 and 37s with 4.56 gearing top out at 62 inside the sweet rpm range. You can over rev and certainly do more. Changing mine over to a nv4500 was a game changer.
 
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My glow plugs are manually controlled,I rigged a push button to ground the relay,and unplugged the controller...I use AC 60G glow plugs,but had to put a 9G in one cylinder a few months ago after one died..I prefer manual control for the glow plugs,and the cold start advance on the injector pump..

I agree lighter oil would help--last year I was using mostly "used" oil from a friends shop,drained from high end cars owned by rich folks who had oil changes done every 3000 miles ,and no doubt it was a mixture of 5W-30 or 10W-30 and some was probably synthetic--my engine was leaking badly and I didn't want to take it apart in the dead of winter when I needed it to plow..and at 5 bucks a qt,15W-40 wasn't in my "budget"..I figured keeping it full of anything,was better than letting it run low --or dry!..

The thing spun over nice with that oil in it...15W-40 turns to grease around 20 degrees,it pours like molasses at that temp when you add some..but when spring came,the oil pressure dropped a lot hot at idle,so I jerry rigged the leaks and got them down to a trickle and went back to 15W-40..

My block heater is a tank type in one of the heater hoses,only 450W--it doesn't seem to do much other than make bubbling noises and give me heat faster--it barely heats the engine...so I rarely use it..its not that often it gets below 20 here anyways..

I'd love to get a 8D battery out of a semi and put it in the bed under my tool box--then I'd have no lack of cold cranking amps!..and I could put something else in place of the original batteries like a storage compartment under the hood..the weight of a huge battery would aid in plowing too..
 
Are you certain it has a 700r4? I have an 85, K20 diesel, 3/4 suburban and it has the turbo 400 in it.... that said, you can build a 700r4 to be every bit as tough as a 400 - and still have overdrive without the $1000 stand alone computer.

If you do have the 700r4, it has the same size output shaft as the turbo 350, the turbo 400 had a larger shaft and the adapter is a 6 bolt instead of the 4 bolt on the 700 and 350
 
Yup...
A TH400 will have a NP208 with 32 spline input..
700R4 have a 27 spline input on the T-case..


An '85 3/4 ton could have came factory with both a TH400 or a 700R4..

My '85 Suburban is a K10 and has a 700R4..the owners manual states a TH400 was optional if desired..

Also,anything that old may have already had one or more tranny swaps done too!..(so you have to crawl under it and count the bolts as suggested--the dash indicator may have the "D" in a circle indicating a 700R4, but someome may have swapped a TH400 in it)..

I "think" a TH400 & a mating NP208 will bolt up in place of the 700R4 setup,the overall length may be a little different,but there should be enough travel in the driveshaft slip yokes to let it work ok..
 
you should be fine as to the length. The 400 is a touch shorter then the 700r4 (less then an inch). Given you have the parts, that should be a pretty inexpensive swap.

Ah, I thought it was a 3/4 ton you couldn't get a 4 speed 3/4 ton with the until 89 or 90 then it was the 4l80e - but that certainly doesn't mean GM didn't do it, I keep telling them to check with me before they do those changes; but they ignore me... :cautious::biggrin:
 
superbuickguy said:
you should be fine as to the length. The 400 is a touch shorter then the 700r4 (less then an inch). Given you have the parts, that should be a pretty inexpensive swap.

Ah, I thought it was a 3/4 ton you couldn't get a 4 speed 3/4 ton with the until 89 or 90 then it was the 4l80e - but that certainly doesn't mean GM didn't do it, I keep telling them to check with me before they do those changes; but they ignore me... :cautious::biggrin:
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I didn't think you could get a 700r4 either from reading and looking around, but mine has it and it is on the RPO codes in the glove box. The previous owner said the injection pump was rebuilt, so if that was true I would guess the TV cable wasn't adjusted correctly when they put it back together. I pulled the TH400 and NP 208 yesterday, depending on work I might get it put in this weekend. I still need to grab vacuum T off of the CUCV and the kickdown switch. Then I will start saving my pennies to rebuild the 700r4.
 
Got the th700r4 and transfer case pulled on Monday, then the th400 and np208 pulled Tuesday. The 400 looks clean and had 60k miles on the odometer, and since it was from a CUCV I will say that's the original mileage. I am waiting on a filter, gasket, and dipstick grommet to come in then I will get it put in. I also have to put the vrv on the injection pump, and wire up the kick-down switch. Hopefully I'll have it running again by Sunday.
 
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Well my flexplate is a different bolt circle than the Th400, and there was fresh antifreeze on the torque converter bolt threads which is odd. The rear main seal needs replaced so I am leaning towards pulling the engine and putting new gaskets and head bolts.
 
I pulled the engine and spent three hours with a steam pressure washer on the engine and engine compartment. This thing leaked more oil than any vehicle I have ever owned. Tore the engine down and the drivers coolant passages were blocked with junk, and there are 5 main web cracks, with one starting to go horizontal. Time to throw this engine in the trash and and start hunting for a newer one.
 
Sorry to hear that. 6.2 engines are fairly common in junk yards and Craigslist. Might be a good opportunity to pick up a mechnical IP 6.5 to drop in there.
 
eodcoduto said:
I pulled the engine and spent three hours with a steam pressure washer on the engine and engine compartment. This thing leaked more oil than any vehicle I have ever owned. Tore the engine down and the drivers coolant passages were blocked with junk, and there are 5 main web cracks, with one starting to go horizontal. Time to throw this engine in the trash and and start hunting for a newer one.
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The one I just tore apart was covered, I mean covered in oil. Transmission as well. I had to pressure wash and gunk the whole thing before starting to take it apart. I wish you were closer, I have a local guy that hoards 6.2's and I have bought several low mileage 6.2's for 250 bucks complete. I would go get you one.
 
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