GM 700R4 trans - can it be upgraded for HD use?

I just lost 3rd and 4th gear in my Chevy Blazer diesel mini-motor home. I drove 120 miles home in 2nd gear which was not easy. Had to stop about every 15 miles to let the engine cool down. Tne normally 2 hour trip took 6 hours, but we made it.

I was always nervous about having the OEM 700R4 in there anyway, but now I'm trying to decide what to do.

I'm well aware of all the changes and improvements to the 700R4 or 4L60E over the years. I'm also aware of the many ads that sell "extreme built" transmissions and torque converters for over $2000 that hold up "to anything."

My problem now is . . . how much is hype? I don't personally know anybody that has actually used one of these upgraded transmissions, used it heavy, and had it last for a long time. Most claims I see are related to racing, not long-term, over-the-road heavy use.

My 1986 Blazer weighs 7000 lbs. Has 4WD, 3.08 axles, and a turbocharged 6.2 diesel that probably puts out around 200 horse and and 300 lbs. of torque.
Most of our driving is up and down mountain roads, i.e. very little flat-easy driving. I baby it and do not use 4th/OD when climbing hills. The trans in it was supposedly rebuilt 30,000 miles ago, but only lasted 1000 miles on this rig (when it became a small motorhome).

I have several TH-400 transmissions around, all setup for 6.2 diesels. I'm confident the TH-400 is fine for this rig, but . . . it lacks a creeper low gear, lacks overdrive, and lacks any converter lock-up. So, I'd rather have the 700R4 if I thought it was able to hold-up.

By the way, I also know about the the "converted" TH-400 that led to the 4L80E with lock-up and OD. I just don't want to deal with the extra changes, expense, computer controls, etc. That is out of the question.

I figure my most reasonable options are, upgrade the 700R4, or use a TH-400, or . . . install a manual trans. On the latter, I have several New Process four-speed manuals with overdrive already setup for the 6.2 diesel. But they are light-duty setups and not any stronger the the NV3500 five-speeds that GM later used. They are rated for a GVW of under 5000 lbs. and engine torque of 300 lbs. max.

I also have several good working and newer 700R4s in mini-school bus 1991 4WD diesel Suburbans. But again, newer yes - but good for HD use - I doubt it.

My preference is to stay with automatic since there is limited room in front for a gear-shift coming out of the floor. That because we have a bench seat where three passengers ride. 6 year-old kid rides in the middle and his legs just barely allow gear shifing now (in my Dodge diesel).

From GM, the heaviest rated 4L60 is listed for a max engine torque of 360 lbs. The 4L65 is rated a max engine torque of 400 lbs. Max GVW of 8600 lbs. but . . . can't say I've seen a GM truck that heavy with that trans.

Do some research on Allison automatics that GM builds. Did GM ever put an Allison behind a 6.2 in a medium-duty, such as a school bus? If so, that's what you need to seek out.

Ever notice the new pickups with the Duramax diesel and automatic? I believe EVERY one of 'em I ever saw said "Allison" on the fender tag. So will the bellhousing pattern from the Duramax mate to your 6.2? If so, there's your answer. Fabricating the electrical/electronic hookups may not be easy...but if you were looking for easy, you wouldn't have made an '86 Blazer into a mini motor home, IMHO.

But I think it's time for you to give up on the idea of putting a PASSENGER-CAR automatic behind a diesel, and look for a TRUCK transmission. Just my opinion, and worth every cent of what you paid for it.

GM used the TH400 in school buses with 6.2s.

The Allison is out of question simply because of the complexity and expense. 4L80E is the HD option for use with the 6.2/6.5 series of engines, but it is also expensive. It is based on the former TH400 with some added gears and lock-up.

Now, don't take this the wrong way...but it sounds like you want to have your cake and eat it, too. You want something stout enough to stand up to the demands of a diesel, AND be essentially under a load condition ALL the time...yet you want something cheap. Sounds like my buddy back in the '70's who wanted a car with gobs of horsepower and torque, that would smoke the tires in all the gears, but also wanted to get 30-35 MPG.

I've seen the ads in the street rod publications for GearStar transmissions. They advertise a Level 4 700R4 which they say is good for 600 HP and 650 lb/ft of torque...for around $3500,PLUS packaging, PLUS ATF, and PLUS shipping. So you're looking at somewhere in the area of $4 grand for a 700R4 to do what you seek. Are you trying to say you can't get into an Allison for $4 grand? [I don't know,I'm only asking.]
GearStar 700R4 packages

In '93, GM used a stand-alone controller to operate a 4L80E behind a mech-injected 6.5 tubodiesel.

I wonder how hard it would be to find one of those controllers and a good 4L80E?

GM PP has a stand-alone controller, GM12497316.

<img src = "http://www.gmpartsdirect.com/part_images/12497316.jpg">

They can be had for $950, I've seen used ones for sale for half that.

That would allow you to put one %^## of a tough setup in yout truck.

I'm sure you know a bad trans when you see it. However a friend had a 700R4 do the same thing and it was some kind of sensor . It seems that the A/C or the speedo quit while the trans wouldn't work. replaced sensor and everything wored great.

Gm made many changes to the 700R4 over the years. Several of them were directed to third and fourth gear, and most of those changes post-date the 1986 model year. A performance rebuild could/should have incorporated those changes but would have cost more vs. a minimum cost generic rebuild.

If you have access to a 1991 700R4 designed for a Diesel app it should bolt right in to your Blazer. You'll need an aftermarket computer or a simple toggle switch to control torque converter lock-up but otherwise the trans will shift itself mechanically/hydraulically

I have to believe those Suburban based mini-school buses weigh in around 7000 pounds, so if the 700R4 trans holds up OK for that it should work for your mini-motor home.

Alternatively, you could rebuild your current trans with all the latest mods.

Whatever you do, Add an aftermarket trans cooler and a trans temp guage.

I've added a pointer to an article discussing the 700R4 below.
evolution of 700r4

I've got a 1991 700R4 in a diesel Sububan that I can use. It does NOT use any computer for the trans as far as I can tell. Still uses the TV cable just like the 80s versions. Probably because it's diesel and not gas. If I decide to rebuild a 700R4, I'll probably use the 1991 version and not the 86.

Check out Monster transmissions site, lots of information and pricing. About $950 for the kit to to diy if you can rebuild one. New sprags ,planetary's etc.

Yes, I've been thinking about it. Just dreading the cost of the controller, mods, plus the cost of buying a complete 4L80E transmission. 4L80E cores out of diesel 4WD trucks are priced $750 - $1000. If I found a core with converter and controller in that price range, I'd rebuild the 4L80E and use it.

No, I don't want to "have my cake and eat it too." I'm just trying to get the most for my labor and money. I learned a long time ago not to fall for everything I read. My engine being diesel has very little to do with anything. Non-turbo diesels are inherently LESS powerful then equal sized gas engines, not more. Since mine does have a low- boost turbo, it's equal in power to a good gas 350 engine. The main issue is the weight I'm pulling all the time.

I already have several 700R4s and several TH400s. If I was confident one would do the job with proper upgrades, and it also was the most cost-effective - why wouldn't I go this way?

I know the TH400 is plenty heavy enough, but I'd like to get a converter lockup and OD. Thus the question about beefing up a 700R4. I don't mind spending a $1000 if the thing is going to hold up. I'm not willing to so just based on hyped up ads about 700R4s used with 600 horse in racing.
I tend to wonder why GM came out with the 4L80E if the 700R4/4L60 could be made just as strong??
My gut-feeling is . . . it can't be made that strong regardless of the claims by places like Raptor.

I friend has a 4X4 4L85E core he'll let go for $200, but shipping from ND would be a KILLER.

My son and I have been toying with getting it and rebuilding it for a project but it's just one of those things that will never happen.

I have a 1994 6.5 TD pickup with over 225,000 on it's 4L80E.

I've got no doubt the 4L80 is a good trans. It's based on the TH400 which is also excellent as a rugged non-lockup and non OD trans). My 87 diesel 3/4 ton Suburban had over 500K when the 6.2 engine blew to pieces and the TH400 was still fine. Even wore out the rear-axles splines twice, but the trans kept on going - and still is. My son took the Suburban to Colorado, put in a used 6.2 along with a turbo, and still drives it on hunting trips to Wyoming with 33" tires stuck on it.

I've also got an 83 Blazer diesel plow truck with a TH400 - along with a mechanical-modulator. Also been great.

The thing is with this mini-motorhome is . . . I baby it. No towing or beating on it . . . but there's no getting around the fact that it weighs 7000 lbs. and I climb a lot of hills at 60 MPH. If I felt confident that I could build a 700R4 to stand up to that use for 100K I'd do it. I'm already sure the 4L80 and TH400(3L80) can, . . . but it's a lot more money, including the controller, throttle-position sensor I'd have to rig up, etc. So, I'm researching a bit. If I had to tow anything, I'd never consider the 700R4.

Also, if there was a torque converter available for the TH400 with less slippage loss at crusing speed, I might try that also. As I recall, there is a 15% loss all the time, which I'd like to dimenish or eliminate.

Your best bet is to use the '91 4L60. It was the strongest of the non-electrics.

Their ONLY enemy is heat. Get a FACTORY add-on cooler (the square one) add it in series DOWNSTREAM from the stock cooler; also a tranny temp gauge.

Keep that oil below 270 degrees and it'll probably outlast you and I summed together. :>)

Allan

Diesel engine firing torsionals are MUCH higher than any gas engine. The torsionals are absorbed by the torque converter if it is an open converter but if it is a lockup then the input spring damper must absorb the torsionals before they get to the transmission hard parts or they will tear up the internals.

Yep, I've got the 4L60 in my 3/4. Bought new and put 2 qts. of Lucus Tranny oil in it and towed 200,000 miles back and forth across the Rockies, now another 215,000 heavy hauling and some towing, still going strong, have never changed the fluid. Actually have changed the filter once, replaced 2 qts. Dex 6, and 1 qt. Lucus Tranny Oil at 230k.

Bulletproof.

I hear you can get a new Jasper one for $2,500 installed.

http://www.summitracing.com/parts/HUP-K74-10XD/

http://www.summitracing.com/parts/TCI-371403/

http://www.summitracing.com/parts/TCI-371003/

I've got a good working (but rusted out) 1991 diesel 4WD Suburban with a 700R4. Probably close to same as the 4L60. It's a ex-school transport rig with 130K and drives great. That's the trans I was thinking of pulling apart, inpsecting, maybe put a kit in, an use in this 86 Blazer. Just trying to make sure it's worth the effort.

As to heat, all my automatic trucks have big coolers. In fact, my Ford F250 has a huge cooler with a thermostatic bypass so the cooler doesn't work in extreme cold weather.

This Blazer didn't get hot when it failed. Just all of a sudden, when 2nd tried to shift to 3rd - it acted like it was fighting itself (like in two gears at once). Then it broke loose but slipped like crazy in 3rd - and would not shift into 4th at all. So, I had to drive home in 2nd. Whatever the failure was, it was not gradual. Worked perfect - and then - it worked lousy.

I don"t suppose a Gear Vendors under/overdrive unit behind a TH400 would solve your problem? Solid setup, though a bit pricey.

Good luck with your project.

Anthony
Gear Vendors home page

4x4 info page

I've had the Gear Vendors OD in some other rigs. Also the Ranger OD unit which I like ever better. The Ranger goes in front of the trans so it works in 2WD or 4WD. The Gear Vendors only works in 2WD since it goes in after the trans. Problem is, an OD unit doesn't solve the problem of torque-converter slippage-loss. No matter how much OD you stick in, you still get around 15% loss without a lock-up clutch inside the converter.

If you rebuild it yourself, put a shift kit in it. Sounds like you have some parts sticking in the valve body.

Just curious,

Did you loose reverse too?

Allan

Nope. Reverse, 1st and 2nd work fine.

Okay, you nailed it.

She for darned sure pulled thru those upper clutches.

Like I say, GM finally got that transmission straightened out in '91 and then went electric in '93.

Oh, and just for what it is worth on the cooler thing; those 'aftermarket' things are a total waste of time (kinda like clutch hubs at the fan). :>)

I'd sure plumb in the factory aux cooler and use 5/16" steel brake line to do it. There's only 7 lbs of pressure at the cooler circut, but hoses can come off in the dead of winter when the oil is thick on a cold start.

Allan

Okay, you nailed it.

She for darned sure pulled thru those upper clutches.

Like I say, GM finally got that transmission straightened out in '91 and then went electric in '93.

Oh, and just for what it is worth on the cooler thing; those 'aftermarket' things are a total waste of time (kinda like clutch hubs at the fan). :>)

I'd sure plumb in the factory aux cooler and use 5/16" steel brake line to do it. There's only 7 lbs of pressure at the cooler circut, but hoses can come off in the dead of winter when the oil is thick on a cold start.

Allan

While you're at it, switch out the axles for a set of 3.73's.

You burned up the transmission lugging it through those 3.08 gears with all that weight up top.

I don't lug it. My low-torque 6.2 engine wouldn't last very long if I drove like that.

I've had 3.73s, 4.10s, and 3.42s. The 3.08s work best overall for what I use the rig for. I don't use OD when driving up in the mountains and only drive in 3rd. When driving 65-80 MPH on the highway, I use 4th OD.

In 3rd and driving at 55 MPH, my engine is at its peak torque-curve at 2000 RPM. That's all I use in the mountains.

On the highway, in 4th at 70 MPH, the engine is still in the peak curve at 1800 RPM.

I might stick in the 3.44 axles sometime, but I don't want the 3.73s. Too much revs. If I was doing any towing, then I'd definitely want the 3.73s or 4.10s.

I built this thing as sort of fuel-mileage experiment, thus my concern for revs. I've been fooling around for years with motorhomes and campers, trying to come up with a high-fuel-mileage rig. So far, this 4WD Blazer has done the best at 17 MPG.

My Dodge with Cummins 5.9 and truck camper on the back did almost as well at 16 MPG. It has 3.50 axles and I wish it had 3.07s instead. I suspect it would be the best mileage rig if I had those axles, but they are scarce for Dodge diesel trucks.