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Discussion Starter · #1 · (Edited)
I have a 700r4 in a 1987 full size Blazer with factory 5.7l. I have owned this truck since 2000 and am on my third 700r4. The current 700r4 was installed about 20000 miles ago. It was purchased from Advance Auto Parts, built by Accurate Transmissions as their Bulletproof model with upgraded pump and clutchpacks and more, remanufactured for heavy duty 4x4 applications or Rv use. This is a 4x4 K case 700r4 transmission.
Everything was fine until one day driving on the highway about 65 mph in overdrive, I came to a slight incline and as the downshift occurred, it felt and sounded as if I had hit a parking block in the middle of the highway. The rest of the way home all seemed normal. I think this was a shift with a quick high rev then slamming into drive? Not sure?
I let it sit for a while and researched the problem, but haven't found anything that matches the problem. Now on a cold startup and placing the shifter in Overdrive, the truck does nothing as if it is in neutral. I cycled through all gears, Park to 1st and back several times with no change. But the truck pulls strong in forward and reverse gears. Driving, the 1/2 shift seems more harsh than usual. It had shifted hard since installed, but seemed normal due to product description. I jacked up the rear wheels and ran it through the gears manually up to 65 mph, overdrive seemed to engage. Upon stopping the transmission also engaged when placed in Overdrive and pulled through all gears??
I street tested this again and everything seemed normal for a little while, but then the harsh slam occurred again and no pull when Overdrive is selected with the shifter.
Fluid is clean and red, doesn't smell burnt. T/V cable is still intact and seems to be adjusted properly. I haven't dropped the pan yet. I am not sure where to start with diagnosis, governor, servo, torque converter? Any ideas?
 

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I am not a trans person! Please take my opinions with a grain of salt or not. I own a classic car and restoration shop and we install about 8-10 700r4 transmissions a year. Two things I have noticed with this trans: It does not like tall tires unless you have adjusted the gear ratio accordingly. When having our 700r4s built we always add one more disk to the clutch pack in overdrive. All I know is he uses thin plates in that pack to get room. Hope this helps.

:confused:
 

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I would first drop that pan and see what you've got for junk in the pan. If it need that rebuild, go for that extra clutch count and add bigger valves in the pump to get a little more pressure for clutch holding power.
 

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I am a little confused:

You say the trans is like neutral with shifter in Overdrive. You move the shifter from park to first with no change.

Your next comment is how the truck pulls good in reverse and forwards gears ?

If you place the shifter in OD position. The truck does not move? From that exact point: what do you do to make the trans engage and move the truck?

You describe actions from the trans that point to a couple of mechanical malfunctions. I am trying to figure out which one
 

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Crosley it sounds like he has broken the input sprag.

OP, when you start it up cold and put it in the D4 position, and it sits there in neutral, if you pull the shifter down into MANUAL 3, 2, or 1 does it then engage and begin to move forward? If the sprag is broken, it may be just strong enough to drive the wheels so when you say you jacked up the back tires off the ground and it seemed to work normally, that would explain why.

I'm sorry you are having problems with your transmission. It is the WRONG transmission to be in a 4X4 truck. Are you by chance towing with this transmission?

My guess would be broken pump, or broken input sprag. Either way, the trans must come out.
 

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Crosley it sounds like he has broken the input sprag.
I do understand that. The posts confuses me, or I mis read it.

is it a flipped fwd sprag, cracked fwd piston, split center support.

Does he have to shift all the way to 1st gear for the trans to move the vehicle? Or simply shift to "3" for the vehicle to move?
 

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Discussion Starter · #7 ·
Blazer has 35 inch tires , 4.10 gears in Dana 44/Corporate 14 bolt. Axles are from a military three quarter ton truck. Blazer came feom Chevy with the 700r4. When the Trans was installed, it seemed fine and drove fine for 20000 miles. Had firm shifts due to the build, "bulletproof ".
With shifter in Overdrive, nothing, like it is still in neutral. Neutral is neutral. D,2,1 pull, reverse pulls. I have not towed with this transmission installed.. When I jacked it up and manually shifted all the way to overdrive everything was back to driving normal. Then road test caused flare up and loss of overdrive again.
 

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Tennblazer, I have been told that I can be too direct some times so I will choose my words very carefully here... It makes me laugh when I see the words "bullet-proof" and "700R4" in the same sentence. That's like saying my "top fuel dragster MOPED".

What price says above is probably the best route for you to go. I would drop the 700, and then covert back to a much stronger 3 speed transmission. Further, even if you go to the 3 speed it would help the transmission immensely-as in a much longer life-if you geared the axles to give the transmission a mechanical advantage over the larger tire diameter.

A 700, paired with big tires, and numerically low gearing (anything under 4.10:1) is a recipe for a very short lived transmission. IN MY PERSONAL OPINION the 700 is one of the weakest transmissions GM has ever put behind a V8. Towing, big tires, off-road use, hard driving, increased horsepower or torque will all drastically reduce it's longevity. I am honestly surprised that you got 20,000 miles out of the transmission.

If my guess above about the sprag being broken is accurate, and you end up replacing it, please note that GM started using a better sprag in the 4L65E's (mid to late 2000's) and that it will retrofit into your transmission without problem. If your sprag is broken it will have damaged all 5 pieces of the sprag assembly so you might as well buy the entire later assembly. It will have a plastic cage, and elements that are longer to provide more contact surface area. The only way to confirm a broken sprag is to remove the trans, disassemble, & inspect.

Let us know if you have any further questions.
 

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Discussion Starter · #10 ·
Thanks to all for all the information and input, not exactly what I wanted to hear, but it's kind of what I was expecting. I was under the impression that the gear ratio/tire combo was pretty close to stock with a very small difference in RPMs at 65 mph, 11 RPM difference according to the charts I found. From the factory the Blazer came with 3.42 gears and 29 inch tires.
I agree that the 700R4 is one of the weakest transmissions around, I almost went with a TH400 last time around, but keeping the 700R4 meant I could keep the same transfer case adapter, and drive shafts.
This transmission has been mostly street driven as a daily driver, sometimes hard, some beach action, but nothing too extreme. Engine is the stock 350 with headers and a K&N filter, I haven't ever got to my engine rebuild because the transmission has died time and again while the stock engine lives on.
I was hoping someone would say that it was something simple like the servo or the governor. If this trans comes out there will not be another 700r4 in the truck. I keep looking at TH400 or SM465 to replace the 700r4.
I'm not sure where to go with the Blazer at this point, but it sounds like it will be an expensive fix no matter what I choose.
 

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they make a 350 swap kit it comes with ouput shaft adapter block filler tube and dust cover to make a direct swap from a 700 r4 would require a dissaembly of the 350 trans but should be done with a rebuild any ways
Not saying a 700r4 cant be built to last but it gets expensive and tv cable adjustment has to be perfect or it will toast one fast to do a 400 swap u need the t case also since the out put spines are larger
 

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TennBlazer, you've broken the forward sprag assembly in the input drum. I wouldn't drive it any more until the trans has been repaired.

I've read some comments here about the 700r, well as is , it's pretty weak but never fear, you're luckier than you think. That case will accept heavier upgrade components from 4L60E and the 4L65E transmissions.

If you can remove the transmission, you can fix it yourself even if you've never been inside a transmission before. There's a great course on the 4L60E on youtube that will walk you thru your transmission. A rebuild kit with new seals, gaskets, band, frictions and steels can be purchased for around $200 online if you do some shopping.

Check out youtube video "Project 4l60e Part 2 Lesson 5" for a detailed look at the assembly that is broken in your trans. The 4l60e is just a 700r with a few heavier parts inside.

If I were you, I'd upgrade the transmission during the rebuild. Some specific items to change would be the planetary set. I'd go with a 5 pinion planetaries from a 4L65E and resell the 4 pinion units that are more than likely currently inside your trans. (Even though it's supposed to be already upgraded) Also a switch to the molded pistons for the forward, 3-4 and overrun clutch pistons in the front input drum. Those 700r lip seal type pistons tend to blow out the lip seals easily under pressure. You can spend about $50 and get a good aftermarket 3-4 clutch pack that upgrades from 5 or 6 frictions up to 9 frictions in some kits. Green frictions are good but red is best and cost more money. Toss the old Sun shell and buy a "Beast" Sun shell, it's heat treated, thicker and will take the abuse.

I wish you luck, I'd lose the shift kit if it was me, shift kits and 4x4's don't mix well. There's zero benefit gained and the result is often a broken sprag as unfortunately you've experienced now. Best, Chris B.
 

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I saw the title of this thread and had to chime in. I went back into my 700R4, for the 3rd time over a two year period, last summer. The second trip inside was largely my own fault for creating problems with my original build. The third trip was to perform further upgrades. Now my transmission, aside from 5 pinion planets and 300M steel shafts is about as strong as a 700R4 can be. It lives behind a 420hp/450ftlb SBC. I'm not shy about pounding on it when I choose. Some of the parts I used to build it were very pricey and special, like the Sonnax Smart Tech input housing at $500. It eliminates the cheesy snap ring and flimsy backing plate that contribute to early 3-4 clutch failure, using a bolt on super hard backing plate and also adding capacity. I now run an 8 disc 3-4 clutch with thick frictions and steels. I also have the highest apply area servos for 2nd and 4th gear so no more worry about burning the extra wide carbon band. Along with an excellent pump, built by a local company who does a great job based on the pump I got, and a 9.5" 3,500 stall converter(which helps keep pump speed and pressure up as the load is applied), the price tag for this build comes in at about $2,400. That's with me doing the build myself. That's just a clue to what it takes to build a really tough 700R4. And I still wouldn't put it in a heavy vehicle for extended high load, low speed operation. It just isn't the right unit for that. The 4L80E is a much better choice for a heavy vehicle.

I hope this unit of mine will hold up to my use and another 120hp for long enough to be worth the work. When it has issues again, I'll go back in and do it again. At least the hard parts(expensive parts) will be good for reuse. The wear parts aren't the pricey parts. I love that I built my own this time and became familiar with it, even if it did take three tries. No off the shelf unit, with the exception maybe of a few that sell for $4-5,000 is built to this level.
 
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