Need advice: TH-400 to 700R4 swap

[dieselcrawler]

I need some advice from they who know more than I. My K-30 work truck, running steady at about #8000-8500 lbs every day with tools, parts and such in a utility bed, has a TH-400 auto trans in it now. First gear is way too high, and with 4.10 gears, anything over 45-50 mph has the ol' 6.2L screamin'. I have a 700R4 out of a 2WD van, an '84-'85 model I think, and a divorce mt transfer case. Is this a reasonable swap, if the trans is rebuilt before it goes in the K-30, has a nice large cooler, and some custom wiring done to help the tork converter work as best it can, as in, switchable by me. I also want you to keep in mind that I tow a trailer from time to time, not often, but #2000 lbs of trailer and 5500 lbs of K-5 Blazer on the trailer makes for a nice load. With the TH400, I have to use low range (!!!) to pull out from a dead stop with that much behind me. I have heard of Brittanic and Arvtech both using the 700R4, one behind a hot turbo'd 6.2, and the other towing RV trailers on a regular basis. Guys, how's it work for you? Tips, ideas? I think I really need the lower first gear, and the overdrive... but I'd hate to waste my time to swap to a weak trans. The key here is that I have the trans, and transfer case, so the parts are here, just need some input to see if you think it's a good idea, and if it's a reasonably good one, some tips to make it last would be great.
Thanks for any thoughts you might have...
Greg

[john8662]

I don't think the swap with a 2wd case will be the best route to take. I think the swap to a 700R4 is a good move though. See what you can find locally for a 700 case, you'll probably want to freshen it up. Here is the ideal situation:

TH700R4 out of 88+ blazer/sub/truck 4x4 with the "K" stamped on the case

An NP208 transission adaptor, and input shaft for the case.

The rest of the mounting hardware, mainly the brackets for the support bars to the engine mounts and the bars themselves.

A TV cable from a 6.2L (this could be 2wd or 4wd) The bracket for the back of the engine to cradle the TV cable, with it's accompanying throttle linkage cable.

You'll also need a TC lockup throttle position sensor to replace the vacuum modulator mounted on the side of the injection pump now.

Now, how do wire it, thats for the rest of those on here that have actually done the conversion, this is from me looking at the parts and having both a TH400 and 700 sitting on the floor in the shop.

[More Power]

You might want to check out Dr. Lee's 700R4 Part I (http://www.thedieselpage.com/members/drlee/lee700r4-1.htm) and Part II (http://www.thedieselpage.com/members/drlee/lee700r4-2.htm) articles. Good stuff!

MP

[arveetek]

Yep, I have a 700R4, and I absolutely love it. The deep first gear is great for getting loads moving. However, when I'm towing my 8500 lb. 5th wheel, I don't use overdrive, because it lugs the engine too much, and that's with 4.10 gears. If your truck is really that heavy, you may find OD to be too tall until you're out on the freeway. The 700 has a really steep overdrive.

I'm using a 1990 model 700, supposed to be one of the strongest that left the GM factory. I'm not sure I would want to use an older one like what you have.... I don't know if all the upgrades that Dr. Lee has discussed in his articles would make that older tranny as reliable as the later models or not.

If you do decide to swap, you'll need the TV cable, and a TC wiring kit, like from Bowtie Overdrives. It will allow you manual control of the TC. Forget the TPS that came stock with the 6.2L's, you don't need it. They're more trouble than they're worth.

Casey

[Stray Cat]

I have a 700R4 in both my '79 Blazer (TH350 orig) and my '83 Suburban (orig TH400). Very happy with both! I tow with both. They need additional cooling though! They will tend to run hot with the same cooling as the stock TH350 or TH400. Add a good cooler and put on a temp gauge to monitor. With the additional cooling, mine both run at around 160 degrees normally, up to 180-200 under heavy load...still in a safe range. They can definitely handle the load if properly built and cooled!

[dieselcrawler]

First... I haven't had a chance to read Dr Lee's articles yet. I will do that as soon as I can find/remember/get my access codes to them.
So, if I were to rebuild the trans I have (because I have it, no cost involved), install a large cooler, ('Heat kills trans' seems to be a safe saying), and modified the T C clutch wiring and used it wisely, I would think it could be made to live, if not abused. With proper driving styles, keeping the temps down, what are the chances? I don't want to spend a lot of money on a trans, just a mostly stock rebuild, but if there are a few small things I can do that will make a big diffrence...

Thanks,
Greg

[Stray Cat]

A good rebuild with a good pump & better (corvette) servo, strap and drum is probably a good start. I put a shift improver kit, for low-end and towing, in mine. I really like the results! It is as solid as a rock.I did the shift improver kit with the trans in. If you incorporate it with the rebuild, it will be easier. I am not a trans expert, but I know that the accumulators were bad on my trans from PATC and caused a lot of problems; you may want to ensure they are in good condition!

[dieselcrawler]

From what I can tell, there are internal parts that can be upgraded, but there are also some newer trans cases that are better... does anyone know the diffrances, from the one I have, an '84-'85 2wd 700R4 case, vs the newer ones. What goes wrong if you use an older case, or 2 wd vs 4 wd? What ways can I change my driving style to get the most out of what I have? If I only have the money to upgrade (beyond stock rebuild) one or 2 things, what should they be? What things are diesel specific? Tork Converter will be I'm sure, but what else? (This trans came from behind a 305 or 350 gas motor)
Another option is to take the 700R4/NP208 combo out of the K-5 Blazer I have, and then put the 400 back in the K-5. Would the stock diesel '83 4wd 700R4 be better suited than the 2wd gasser trans? (With the same rebuild/refreshening before installation) I'm trying to get a game plan together, the truck goes in the shop in a week or two...

Thanks,
Greg

[Stray Cat]

I believe the 4WD case is just a bit stronger, you want it if it is going into a 4WD. I have a 4WD 700R4 with NP205 in my '79 K5 Blazer, behind a POWERFUL (extremely modified) 350...it does great. I have a 4WD 700R4 with NP208 in my '83 K20 Suburban behind a 6.2TD...it does great. Both are built with better parts, both have towing shift kits in them. I think the most important parts for improved performance are the 1-2 servo, 3-4 servo, good band (from 4L60E trans) and clutches, a good pump. The GM Torque Converter for the 6.2-700R4 combination is rated at 1365 RPMs; there are aftermarket TCs rated at 1200 RPMs for the application. They are relatively cheap. I got the GM TC with better bearings and lock-up surface material for $185 through my local trans shop. I installed the shift kit myself after the rebuild, and can attest to the improvement...it was noticeable and nice! Both trans' needed additional cooling as they ran hot compared to the stock TH350 and TH400 that they replaced. I will let the trans experts give the final word as I am just a layman who has been through this several times before. So, there are my 2 cents...hope it helps!

[dieselcrawler]

I think I'm gonna stick with the 2wd case one way or the other, I want to keep the 'married' 700R4/NP208 combo in the K-5, and the K-30 has plenty of room to use the divorce mount 205 transfer case and 2wd trans that I already have. I'll have to make drive shafts, but I'd like the rear one to be shorter anyway. It's a one pc shaft, and longer than it should be to ballance out right. And if I ever find a newer case/better trans, I can swap it out easily.
So, on that note, I will look into finding the upgraded parts. Does anyone have any favorite shops or parts stores, ones you've dealt with in the past as to a good rebuild kit, the better servos and bands, torq convert and maybe the pump, depending on price.
Another part of this deal is a friend of mine who wants to rebuild the trans for me. He's done meny TH-350's and TH-400's, but not a 7000R4. So, he said he's read about them, and he's smart enough... I'm willing to let him try. We'll see how this turns out.
If I were to just walk into the local Advance Auto Parts, and ask for a trans rebuild kit, how diffrent will it be from what else is out there? Would I get better, upgraded parts if I specify a diffrent year trans when I ordered it? What year?
Stray Cat,
You said you like your shift kit, and that it was installed later. What brand, your recomendations?

Thanks,
Greg