1970 c20 transmission

[1970c20owner]

I have a 1970 c20 camper special with a 350 with a four barrel holley carb. My automatic transmission developed a problem and I am looking into maybe upgrading it. I was told that I can install a 700r4 instead of the th400 that it currently has. I would like to be able to drive on the freeway without having my engine pulling high rpms, but I also have s 30' travel trailer that I want to be able to still tow. Anyone out there has any suggestions on how I should proceed?
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[Wrenchbender Ret]

There is another thread about this somewhere that has been active. The 700R4 is a great 0-drive trans but according to the guys it won't stand up to that kind of use.

[WIDESIDE72]

Originally Posted by Wrenchbender Ret

There is another thread about this somewhere that has been active. The 700R4 is a great 0-drive trans but according to the guys it won't stand up to that kind of use.

I have to disagree with them not holding up to towing. When properly built they are a great all around trans. Why did GM put them in half ton trucks for nearly 20 yrs?
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[Vintage Windmills]

Originally Posted by WIDESIDE72

I have to disagree with them not holding up to towing. When properly built they are a great all around trans. Why did GM put them in half ton trucks for nearly 20 yrs?
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Stock they weren't all that great but they can be built for 450hp/450ft lbs quite reasonably. I wouldn't run a stock one unless not towing. My dad's '93 K1500 had its redone by '94 and he didn't tow much. I'm looking at a bowtie overdrives built one for my 72 K20

[motorcritter]

The 700R4 and earlier versions of the 4L60 had a single element sprague clutch, and when in OD, it ran power through that not-so-robust sprague clutch. It wasn't until '94, IIRC, that they went to a double-element sprague, and solbed the problem. Best way to run one (700R4 or 4L60) is to leave it in 3rd until you get up to cruising speed, then engage the OD. That is if you are hauling something on the lighter side. Full-capacity loads, leave it in 3rd- direct drive doesn't go through the sprague, and avoids the nasty mess that can occur. I worked in a transmission shop, and we used to see a lot of these units get beat to death. The upgrade was probably the best thing for those transmissions. If you're going to haul heavy stuff, it would be best to get a heavy-duty unit built for you- a good transmission shop can do it, and they're available on the 'net. No matter what you do, run the biggest auxiliary cooler you can hang on the front, and use synthetic fluid- heat kills automatic transmissions, synthetic holds up under extremes of temperature. Those two items are a lot cheaper than a transmission.
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[SS Tim]

Welcome to the board. It is good to have more from the CA desert. Tell us some more about your truck and remeber we LOVE pictures.

If it were me with a C20/350 I would look for a used Gear Vendors O/D. It gives you a total of 6 speeds with a TH400 and in between gears when you are pulling a trailer up and down the grades like the Cajon Pass. What does you 30' trailer weigh?

Also what problem is your trans. having? TH400s are fairly tough to break.

[1970c20owner]

Thanks everyone for the input.
TIM: As far as my trailers weight I believe is around 7500lbs. but not sure I would have to verify that. The problem that developed with my th400 was that I began to hear a rattling noise one morning leaving home. When I stopped at an intersection and tried to take off, truck did not go. I shifted manually into low and it went, I had to shift it that way all the way til drive, and you could hear metal noise in the transmission. I drained ATF at home and found large amounts of metal showing at the bottom of the pan.
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[SS Tim]

Sounds like something serious broke and was chewed up. Thats not good, but is possibly fixable. Still think a TH400/GV O/D might just be the best choice. Do you know much of the trucks history?

[MARKDTN]

700R4s get a bad reputation. My '83 K20 went nearly 100k miles pulling trailers and other heavy use before the pump went out. There is nothing wrong with a properly built 700R4 IMHO. You need at least 2 things to have one last.
1-Use an '89 up core. The later ones have better guts and a different pump. I got a GM rebuilt one and it has lasted. It is a '91 core.
2-The TV cable MUST be properly adjusted from day 1. Do not drive it even a few feet without the TV cable adjusted. It will burn it up fast.

You might consider a 4L80E which is basically a TH400 with overdrive. There are places out there that make stand-alone controllers for them. Or might be a good time to convert to TBI fuel injection like a mid '90s 350. Then the ECM would control the engine and trans. You could use either a 4L60E or 4L80E.

[steelhorse]

The 4l65 is stronger than both the 700r and the 4l60 and easy to find. You would need a stand alone controller. The 4l80 is the strongest if you do a lot of towing.

[motorcritter]

That's a lot of weight to pull. The 4L80 should do nicely- I had forgotten about that one- that's what I get for posting late at night. Converting to fuel injection and running the electrically controlled transmission will allow you to increase reliability and economy.
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[SS Tim]

Any progress on the trans.? Found a couple of CL links that might be helpful in the long run. They are for GV O/Ds, one is TH400 specific and comes with trans. that has been modded (core??). Another needs a TH400 adapter/speedo change. The third is a whole square CC Dually that you could pull the trans. and O/D from and part the rest or even in a moment of maddness install the 454 in you truck as well.

http://lasvegas.craigslist.org/cto/3325690902.html

http://lasvegas.craigslist.org/pts/3354991529.html

http://inlandempire.craigslist.org/pts/3347461220.html

[bollybib]

I just looked and all the postings have been "deleted by author".

[SS Tim]

Originally Posted by bollybib

I just looked and all the postings have been "deleted by author".

The ads were good when I posted them, as I had rechecked the links during pasteup. Point is there are deals out there and a GV's O/D giving 6 speeds can be affordable. They were all about $1200-1400 including the truck! There was even an inline version that can be used to replace the carrier bearing up in SF Bay on CL. It went even before I could tell my other half how good of a deal it was.

This was relisted, the most expensive for a TH400 application of course.

http://inlandempire.craigslist.org/pts/3363964742.html

[jocko]

JMO, but I think a 2004R can be built as strong and as economically as a 700R4. However, with an approx 7500 lb trailer, I'd probably not put anything but another TH400 in there. GV od's are practically indestructible, so no matter what they are placed behind, the weakest link will be the trans itself. So, I agree with Tim, in your particular applicaiton, a rebuilt/replacement TH400 would be where I'd start, and then add the GV od while you're swapping, or add it later when money allows.

Having said that, if you want to dump the money in up front (and it would probably be less overall than a new or rebuilt TH400 + a GV od unit), the 4L80 is a good bet. That would be the only o/d trans I'd be comfortable pulling a 7500 lb trailer with behind a 3/4 ton truck.

I am considering upgrading my TH350 (don't laugh Tim...) to a 2004R in the future sometime - it should be sufficient for my plans for my C20. If it ever arrives...
But I may want to re-think my overhead camper with a 2004R or better beef it up pretty good. (Art Carr, the "other" Art Carr, in socal, makes some very nice and strong 2004Rs and prices aren't half bad. Having said that, IF I buy one from a mfgr like that, I'll probably work with them on a specific recipe for a truck to haul a camper since they primarily focus on street/race transmissions). But I figure if it works with a GNX, then it should suffice for my beater...

[GMCJOE]

There are many opinions on what overdrive transmissions to run for whatever your application/use is going to be. I have been thru a few 700r4's myself, but they was stock and my foot wasn't so nice and I was over loading the trucks way past the limit for many of salvage yard runs to make a few extra bucks as a teen in school. I shelled out a 4l60e also, virtually a 700r4, doing the same.

But, I do have to agree with if you build them up right they will handle the abuse. A 4L80E should take the abuse you want to put it thru running a standalone for it, that would be my option for the swap pulling a 30ft trailer. They push the up armor Humvee's around quite well with all the weight they carry and the abuse we soldiers put them thru.

I am purchasing a 700r4 from www.transdepot.com in the near future with there stage 1 build. But I won't be towing anything, just a heavy foot. They have many options of transmissions. IMO gearvendors overdrive is a great system and holds up to some abuse, but my pockets aren't as deep as what they ask for there systems.

Best thing to do is research a few different applications and see what fits your needs and budget. Then make your decision.

[mrein3]

Originally Posted by motorcritter

The 700R4 and earlier versions of the 4L60 had a single element sprague clutch, and when in OD, it ran power through that not-so-robust sprague clutch. It wasn't until '94, IIRC, that they went to a double-element sprague, and solbed the problem. Best way to run one (700R4 or 4L60) is to leave it in 3rd until you get up to cruising speed, then engage the OD. That is if you are hauling something on the lighter side. Full-capacity loads, leave it in 3rd- direct drive doesn't go through the sprague, and avoids the nasty mess that can occur. I worked in a transmission shop, and we used to see a lot of these units get beat to death. The upgrade was probably the best thing for those transmissions. If you're going to haul heavy stuff, it would be best to get a heavy-duty unit built for you- a good transmission shop can do it, and they're available on the 'net. No matter what you do, run the biggest auxiliary cooler you can hang on the front, and use synthetic fluid- heat kills automatic transmissions, synthetic holds up under extremes of temperature. Those two items are a lot cheaper than a transmission.
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I agree with motorcritter. Don't tow big loads in OD. 3rd gear in a 700r4 is EXACTLY the same deal as 3rd in a th350 or th400. It will last forever in that gear pulling loads.

If I was you I'd get a good 700r4. Set it up properly. Only use OD pulling that 7500 pound trailer on the flattest of slabs. I wouldn't use it at all with that large of a trailer.

A 700r4 in 3rd gear will turn slightly less rpms rolling down the highway because it has a lock up converter where your th400 does not.

[White70C20]

My '70 C20 has the TH350. I don't like driving it on the highway because the RPMs are kinda high. This results in horrible mpgs and impossible conversations (I have old mufflers now and plan to run straight pipes). I don't tow anything, and the truck is basically a toy that I would like to drive to work once in a while. Would a "CL find" 700R4 work for my uses?
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[motorcritter]

Originally Posted by White70C20

My '70 C20 has the TH350. I don't like driving it on the highway because the RPMs are kinda high. This results in horrible mpgs and impossible conversations (I have old mufflers now and plan to run straight pipes). I don't tow anything, and the truck is basically a toy that I would like to drive to work once in a while. Would a "CL find" 700R4 work for my uses?
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You would probably do well with a 700R4, or a 4L60. Having an overdrive is worth whatever the transmission upgrade costs- fuel savings and less wear 'n' tear on the engine are the obvious benefits. Just replace the seals, filter & fluid- drain the torque converter and refill it prior to installation. The only additive to use is LubeGard. It is the only transmission additive that OEMs recommend for useage. It's a friction modifier, actually- synthesized sperm whale oil, exclusive to their products- and it can cure a few ills, and helps it run cooler. I'm not a stockholder, it's just that I've seen it work well in a number of vehicles. It was the only 'magic juice' we used at the shop in our rebuilds. It won't eliminate severe slippage, or correct extremely poor shift quality- if you've got serious problems, nothing you can pour into it is going to fix it- except money! Automatic transmissions, for as much beating they take, can use everything to help ease their load.
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[White70C20]

Thanks Motorcritter!

Does the 700R4 require its own actuator?
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[motorcritter]

The 700R4 uses a throttle valve cable to operate the shifting operations, but the torque converter lock-up is electically controlled, by computer. There are kits that allow vacuum operation of the TC lock-up. Earlier models (pre '88) had to have TC lock-up or the fluid flow was reduced, cooking the transmission. Later ones used a different design, and didn't require lock-up, but there would be slight amount of slippage, ie, efficiency loss. Controller adapter kits are available from most performance outlets (JEG's, Summit) or a good local transmission shop- one that does race/heavy-duty/off-road work- should be able to set you right. Just remember that the used transmission may have hidden, and unknown, problems that can end up more trouble than the 'savings'.was worth. Rebuilt units are pretty cheap these days, getting one can eliminate lots of headaches, and should have a warranty. Most problems with reman units show up pretty quick, so the shop can fix what needs fixing. With a used unit, from a private seller, it's a gamble. A used unit from a wrecking yard, with a warranty, is better. It comes down to what you want- and what you require, performance-wise.

All that is one reason I run manual transmissions, whenever possible, on my own vehicles.
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[White70C20]

Ok, cool! Thanks.

Another idea I had was the Gear Vendors OD. Will those fit on the th350? If so, where can I find one? My th350 shifts fine, and I had the thought that maybe keeping the truck as original as possible would be better? Do those GV ODs work pretty good?
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[motorcritter]

GearVendors overdrive- pricey, but nice. I believe they are sold direct from them- there are some postings regarding them elsewhere on the board. I'm not directly familiar with them.
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[SS Tim]

1970c20owner, Are you still around?
As this post has been hijacked from the original poster's question and to bring it back I want to give my reasons for recommending the Gear Vendor's O/D vs a 4-speed O/D trans. swap.

First up the "High Desert" where we live, is a series of large bowls about 2500-3000 ft high. Going anywhere is pretty much a climb going away from or to return to in most cases. The truck is a little down on power from the altitude to start, so best case he is starting out at 95% rated power. Then this number depends on what has been done to the truck and how worn the engine is. The trailer weight is over the original recommended limit for the truck. In fact his truck may not even have deep enough gears depending on the ratio and tire selection. The chart below is for a '72 which like the 71 IIRC are rated about 1000lbs higher than earlier years. This I believe is due to the addition of disc brakes. Figuring the weight of his truck and trailer his GCW (now GCVWR) is 1500 to 3500lbs over recommendations for a '72. Hopefully he has already upgraded to front dics but it is very unlikely his truck has 4.57s. So here we have a truck, occasionally heavily loaded and possibly underpowered trying to climb or descend fairly steep grades. That can be a lot to ask of a 42 year old truck!
By being able to split the gears, he will have maximum power available to him which should help pulling ability greatly. Plus on the flats or running with the wind and especially empty the truck will benefit from an O/D gear allowing lower RPM's when the extra power isn't needed.
Now please don't get me wrong the 4-speed O/D autos. are great for many applications. The general light truck user and especially the type of use most board owners want from their trucks is well suited to this type of trans.. Though for some powertrain/load combinations the 1-2 gear jump of the 700R4 is excessive. For many a 200R4 is a better choice. However neither of these transmissions should be considered straight from the junkyard as bolt-in replacements. There are specialists and parts to adapt them to many specific uses. However in the end a lot can be spent to "build" a trans. that might be better spent on a Gear Vendors unit and thus have more flexibility.

Of course if you can afford a 6L80E I'd probably start there!

[White70C20]

Sorry, I didn't mean to hijack this thread. I saw that this guy had a similar problem (high rpms) and thought others could learn from our problems. Plus, I was under the impression that you're not supposed to post a new thread if you can find one with a similar topic. I apologize.
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