My wife's '89 700R4 developed a nasty "banging" (I'm pretty sure the clutches are slipping) in low gear. It happens when starting from a dead stop. The harder the gas, the more the bang (or slip). Other than that, it operates quite normally. It has 116K miles.

On a limited budget, a rebuilt 700R4 is out of the question, local auto stores even want $1000.

I have a 700R4 with 134K that I "was told" works, but I never saw it run, so I can assume it needs a rebuild.

I've heard the poor man's shift kit for the 700R4 is the low/reverse boost valve, and the servo, both can be changed with the unit in the car. If I buy just these, maybe (perhaps a long shot) I'll get some more clutch pressure in low & buy some time for me to learn how to rebuild the other 700R4 in my garage??

The Transgo Kit for the 700R4, everyone speaks highly of, the picture I saw on Summit seems to show the low/reverse valve and servo. Am I correct? At $129, could I just get the kit and install just these two parts on the sick 700R4 and save the rest of the kit for a properly rebuilt unit (I'm trying to get a few more months out of the sick 700R4)?

I could put in a fresh TH350 for $550 (don't do much travel). But to keep the computer happy, the electric speedo I think needs to function (VSS sensor), and there are kits to make the TH350 spit out electric speedo signals. Though my 9" TH350 had a 7/8" speedo hole, I don't remember the 6" being that small.

I was told by several that the 700R4 tailhousing fits the 6" TH350? One person said it didn't. Anyone know for sure? Logic says if it did, B&M and Hurst wouldn't sell so many conversion kits, everyone would just grab the longer driveshafts.

Basically curious to know if the low/reverse boost valve and/or servo might help me out for a few months. I believe these firm the shifting, but installed alone do not "release" the clutch packs quickly enough, so some burning occurs. But I'm just after a bandaid.

 

Is the shift really soft and slipping or is the shift harsh--you said both....you are talking about the shifts?

The .5 boost valve and vette servo will make it shift harder, that's what has been in mine for quite a while with good results. But if it's already a harsh shift, it may get worse!!!

AFAIK it is possible to use a th350 with the speedo.

th350 and 700r4 don't interchange tailshafts or tailshaft housings.

The boost valve and vette server are great for firming up shifts on a good working tranny. The effects on a tranny that isn't working right....who knows? It may just kill it.

 

That's what I figured too, it might kill it quicker. But it shifts normal. Not hard. But not really soft either, factory GM tranny's are designed to shift comfortably I believe. I say "positive" shifting leaning towards soft.

When low gear is "asked" to take the motor's torque from a dead stop, that's when I get the banging of the clutch plates. If I launch the car real slow, no banging of the tranny, so I figured better line pressure might secure 1st gear a little bit longer.

BTW, where does one purchase the .5 valve/vette servo? I figure the sooner they go in the better, maybe.

 

AFAIK, the boost valve and servo are only going to affect the shifts. A bang when you take off, could be anything driveline related. I'd take a real close look underneath that car.

 

Have you tried replacing the transmission mount? I had that problem where when you took off the transmission felt like it hit the flor pan ( it was, trans mount split in two). Maybe I am not understanding what you mean by banging but thats something to check if you havent allready.

 

I took a real hard look under the car. Torque arm, shocks, springs, axle, pan-hard, etc. Driveshaft is re-done with strong u-joints just a year ago. Everything is perfect. Rear gears are fine and fluid is good.

I know the "bangs" are from the tranny because it's obvious when driving, especially if you put your hand on the center console. Also no bangs in reverse at any throttle, just wheel smoke.

BTW, the rear bearing on the tailhousing has been toast (squeals badly when driveshaft is rotated) for quite awhile now (over 6 months), along with the failed rear seal.

I've had a lot of bad automatics in my days, slipping is usually obvious (engine RPM) and very quiet. In this case though, low gear (or whatever is responsible in the tranny for launching the car) shutters (very loudly, you can feel the slight tranny slips and re-grabs of the clutches, it only happens for a couple of seconds, longer if you really mash the throttle) and it's directly proportional to engine torque.

I honestly thought it was the rear gears at first, because the shutters are so loud. But me and a few other riders have felt the trannel tunnel by hand, it's definately where the sound comes from. Underneath the car is clear and clean, no scratches anywhere.

Maybe the torque converter is going bad?? It's definately torque related. I do have an extra one (from the old 134K 700R4). If it is, I suppose it should be pulled before metal is sent through a working 700R4? But reverse gives plenty of quiet torque with no shutters, so I guess that would eliminate the converter?

 

Yes the mount is strong as is the torque arm rubber, inner and outer (I know what torque arm banging sounds like, and what an unbolting tranny mount sounds like).

I described it as banging, because that's what it does. I know it doesn't sound like a typical hydraulic tranny failure, but very loud bangs come from the tranny when launching from a stop, very strange, I've had many 700R4's fail, but not like this. Launching VERY easy = no bangs, moderate launch = nasty bangs for a second or two, hard launch = VERY nasty bangs for over 3 seconds.

I would suspect a cracked flywheel or bad torque converter, but in Reverse, I can mash the gas and everything's quiet (except tire smoking).

 

What happens if you try manual "1", instead of Drive?

 

Tried that too (occassionally I manually shift it only because the motor is so wimpy and 1st is so nice), same exact thing happens. The only other gear I can "launch" in is reverse, which works good. And the 1-2 and 2-3 shifts are fine, even under good throttle (I don't use 4th, since converter doesn't lockup & I don't want anything to burn up, seems to be the connector or something in the VB, it's intermittent). Fluid is red, level is good, doesn't smell burnt.

I tried some "Lucas" brand transmission additive couple days ago. Tried a moderate launch last night a got 1 bang instead of a series of them, but I didn't stay on the pedal either, I need this car to last a few more months. The stuff probably did nothing, but I'm definately stumped, because other than this the car drives beautiful at any speed.