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Argh! 700R4 Headache

T

Tesla

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So, my '78 Blazer is more of a "project" than anything. The truck needs a lot of work, and while it's fun for a weekend cruise - I wouldn't trust it for a long roadtrip or to get me to work everyday.

With that in mind, I went out looking for a "daily driver," found a nice looking one, and bought it yesterday. Its a Suburban with a 6.2L diesel, 700R4 transmission, and 4WD. I drove it the 3hrs (~170mi) home through the pouring rain. A few miles from my house, I noticed that the transmission wasn't shifting up into overdrive anymore. And when I got to my driveway, I was disappointed to find out that the truck refused to shift into reverse at all.

Today after work I dropped the transmission pan to take a look at it. The ATF was burnt black, smelled awful, and the pan was completely full of chunks of metal ranging from tiny slivers to huge quarter-sized pieces. (see the pic)

So, what I need is some help on the best way to get this rig back on the road. (It sucks to have my new toy roll over dead 3hrs after I bought it!) I don't really have the time or the tools to do a transmission swap myself (much less a full trans rebuild), so if you guys have any recommendations on how to find good mechanics and good places to get rebuilt transmissions, I'm all ears.

Also, I'd like opinions on any modifications that are needed or recommended while I've got the transmission out. I don't plan on putting large tires on the truck, pulling any large trailers, or doing hard-core off-roading. This is just going to be a daily driver work truck, and so I'm kind of thinking that if the original transmission lasted 100k, I'd be ok with a rebuild to the same specs if it'll last another 100k. But, again, I'm open to suggestions.

dead.jpg
 
You could buy a transmission from www.Bowtieoverdrives.com and then either install it yourself (not too hard) or have a buddy help you install it OR take it to your local trans shop and have them install it.
 
The debris in the pan is from an overheated and fragged rear planetary. Usual cause for this is lack of lube from either a restricted cooler path and/or non functioning lockup in the converter. The trans will be costly to rebuild due to extensive internal damage and the coolant system should be replaced if possible to keep the metal out of the new trans. There are lots of solutions for a replacement trans and converter, buy the best one you can afford and you will probably get long service life from it. You will need a dedicated diesel trans and converter as the gas model will not have the same shift points or stall speed. Don't overlook the basics like tv cable adjustment and functioning lockup,(brake switch adjustment and function causes a lot of problems on this vehicle). I have decades of experience with this trans, PM me if I can lend support.
 
Greg Ducato said:
The debris in the pan is from an overheated and fragged rear planetary. Usual cause for this is lack of lube from either a restricted cooler path and/or non functioning lockup in the converter. The trans will be costly to rebuild due to extensive internal damage and the coolant system should be replaced if possible to keep the metal out of the new trans. There are lots of solutions for a replacement trans and converter, buy the best one you can afford and you will probably get long service life from it. You will need a dedicated diesel trans and converter as the gas model will not have the same shift points or stall speed. Don't overlook the basics like tv cable adjustment and functioning lockup,(brake switch adjustment and function causes a lot of problems on this vehicle). I have decades of experience with this trans, PM me if I can lend support.
Click to expand...

A dedicated diesel transmission? All of the sites I've found list 700R4's in various "builds" or "stages," but I haven't seen anyone offer a diesel specific tranny. Can you point me to someone that has it?

I called my local GM dealer and they said that a manufactured transmission for my truck is no longer available (its been discontinued).

Also, the truck is an '89 GMC.
 
The difference is in the governor and valve body which are calibrated to shift in the diesels operating rpm range. The torque converter uses 6 mounting lugs instead of the gas converters 3 lugs and has a much lower stall speed than the gas converter. If you can't find one in your area PM me and I will get a quote for you to consider.
 
It wouldn't hurt to buy a shift kit for the trans if you are going the route or rebuild.
 
dieselndixie said:
what exactly does the shift kit do?
Click to expand...

As 4x4 High says, it will produce a quicker and more positive shift. It really depends upon the contents of the kit how this is accomplished. Some kits are very simple, drilling a few holes in the seprator plate, using a different accumulator or pressure regulator spring to increase available pressure for clutch and band aplication. Other kits include more springs for individual valves to control shift timing and lockup application as well as other components to control line and lube pressure. The basic kits of course are less expensive and easier for a beginner to install. Stay away from modifications that give a super hard shift as these shifts can shorten the life of some internal parrts as well as u-joints and other driveline parts. What you want is a clean, positive shift with minimal overlap between gears and good holding pressure of servos and apply pistons.
 
Sorry to hear of the bad tranny.

OOOOOOOOOOK to start off with the 700R has gone through many udates and changes over the many years that it has been in existance.

If you go searching for another gearbox (with the broken and chewed parts that is a good idea)

"K" case is the buzz word here. The "K" Case tranny has a large K cast into the RH side of the bellhousing.

These are the best boxes to use.

Now inside we go.

The 700R even in the best trim that Ma General ever let loose were not stellar performers.

From the front back.

The "front pump" has had many updates and a 13 vane pump is the one to use.

The next item is the reverse input clutch, good seals and factory clutches will do fine.

Next comes the input drum with all the rest of the clutches in it.

The drum is aluminum and is prone to failure in the spline area where the shaft is pressed in.

There are custom input drums available with an added steel support ring pressed over the aluminum hub to strengthen the area where the input shaft presses in.

The clutches in these trannies are small, much smaller than say a TH400.

The clutches need to be replaced with a set that has more steels and more frictions.

There are several kits that have all the good stuff (Colene steels and red clutches)

Next is the sun shell.

This stamped metal device with a gear on one end is a very failure prone part.

Replace it with THE BEAST sun shell or similar part that has the area around the gear in the back reinforced.

The front sprag needs to be replaced with a HD unit.
The rear roller lock also needs to be replaced with a HD unit.

All bushings and thrust washers should be looked over carefully.

At the least, the front pump bushing (where the converter hub rides, needs to be replaced.

The 2-4 band must be replaced with a performace one.

Check the planet carriers to be sure all is well. There are 4 pinion carriers and 5 pinion carriers.

The little piece you are holding in one pix is a thrust washer from a planet carrer pinion.

The 700R does require a few special tools to install some of the inner seals (Input drum area)

A press to be able to get the clutches apart is needed too.

A Transgo 2-3 shift kit is a must. **** follow the instructions and set all the choices of springs and Mods to FIRMEST SHIFTS

Replace the 2-4 servo with the corvette unit or equivalent

There are some choices of other valves and the install sheet in the kit will point this out.

When I build a 700R4 for a 4x4 I set everything on KILL and they live.

Get a top notch HD converter with a "KEVLAR" lockup clutch
6 lug is recommended.

Some builders use a 3 lug converter housing on all the 700 stuff. 6 is better.

A 6.2 is not going to tear up a 3 lug BUTTTTTTT.

The governor must be for a diesel for the tranny to work well with the 6.2

Now, another area that can be addressed and should be is the lockup controls.

Transgo make a kit called 700LU and this kit will convert the tranny to full hydraulic controls on the lockup.

This eliminates the need to any wires to the tranny whatsoever.

This kit is fully adjustable for lockup speed and gear range.

I prefer to set the 700R to lockup in 3 or 4 at about 45 MPH minimum.

The kit also allows the tranny to unlock with whats called "Part throttle"

So if your cruising along at 45 or so in lockup and press the throttle down quite a bit, the lockup will drop out and then relock once speed is increased and the throttle is eased off some.

This is a great Plus to the 700R The factory settings of low lock speeds sucks.

The 700R is what I call a "Busy transmission" its always doing something.

The transgo 700/2-3 shift kit makes a lot of recommendations in the literature as to what may/can/should be done if the tranny is out on the bench.

Follow these words of wisdom.

The other very important part of a 700R is getting the throttle pressure cable set correctly.

Follow the instructions in the 2-3 kit precisely.

If the throttle pressure cable (Only cable on the box) is not in perfect shape, GET A NEW ONE

This cable is the lifeline of these trannies, its not a kickdown cable as was the case on the old TH350 but instead what controls "Throttle pressure"

Throttle pressure thats wrong (too low) is a death sentence to these boxes, and quick too.

The 700R is in no way shape or form a cheap tranny to build. But, if you want a good dependable box, then the above description of what needs to be done to get there.


Yesss you can cut corners, but I dont recommend it.

The 700R, back in its day, was one of the most common trannies to be seen on the bench in the shops.

The 4L60E thats used in later rigs is a modified form of the same box with electronic controls.

If you search for a "K" Case box, look for one thats in the 87 model range as these have all the needed hydraulic controls to do the 700LU trick.

Good luck.

Snowy
 
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