Hello,

what are the symptoms of a torque converter? And what can cause it to fail?

Thank for your feedback

 

Well, being able to put the trans in gear, and not stall the engine.

But I'm guessing you're REALLY asking, what are the symptoms of a torque converter FAILURE. In which case, usually it's strange noises, or metal in the fluid. Or in the case of a lockup one, failure to lock up.

The usual cause is ... failure. There's a sprag in there that can fail (it can sort of "strip"), which in a non-lockup one, is about the only thing that typically ever goes bad. It makes noise and sheds metal into the fluid. Lockup ones such as the 700-4R can fail by the lockup clutch material wearing out, or the hydraulic circuits that work it, having problems such as leeeeeks.

 

ok, then I am not on this problem, in fact, I have a problem on downshifting on 2-1, with a little clunking or bump or litle jump(sorry for the term I am french), only in D, in OD no problem !

For example when I arrive at a stop, when I brake gently => problem detected and if I brake hard, it is not detected....

I have changed the mount trans, yokes are good...check the tv cable, flush fluid trans........ where could this come from? Gouvernor ?

 

when in D the over run clutch is engaged and a little clunking or bump when downshifting is normal

 

Ah ok "the over run clutch" is engaged only on D and not in OD ? if it is normal, I could try a same vehicule for compare..

Perhaps a problem resolved after 1987 on the TH700R4 ? I don't know the evolution of TH700R4 ...

 

Correct. That clutch is not actuated in OD. These 2 clutches are stacked up in the same drum. The overrun clutch both operates the forward clutch AND at the same time "locks up" a sprag, which otherwise would be free to free-wheel in OD when you take your foot off the gas. Having it "locked up" allows the engine to act as a brake in non-OD. The clutch itself isn't likely to be the culprit of any clunk or bump; rather, it makes it more likely that you'll feel whatever imperfect shift timing might happen to be going on in your transmission. By "timing" I mean the relative application and release of clutches during a shift: in the 700, one clutch (the band actually, which is just a differently shaped clutch) has to let go to get out of 2nd and another (3-4 clutch) has to apply, and vice-versa to downshift. If the band lets go too early or the clutch applies too late during a shift, the engine RPM flares up during the shift, and if the band releases too slowly and the 3-4 clutch applies before it's fully released, the car is effectively in 2 gears at the same time, which tries to instantly stop the car. This creates that familiar "stutter" kind of feel as it shifts and also causes a HUGE "clunk" from the transmission, drive shaft, and rear end all quitting propelling the car (applying torque in the "drive" direction) to braking the car (torque in the opposite direction) for a fraction of a second. This is one of the hardest things to get "perfect" in this transmission; ideally they should "overlap" just enough to keep the engine RPM under control, but not so much that it "clunks" on an upshift. It's EVEN HARDER to make the trans get upshifts right, AND downshifts too. So, while maybe not "perfect", in a properly-working 700, some degree of mis-timing of that shift is natural during a downshift. It shouldn't be harsh or aggressive or anything like that; just, not as smoothly and perfectly timed as the upshift. The overrun clutch transmits more of that to the driver when it's engaged.

Yes newer transmissions had more refined valve and fluid passage control of things, improving all of that. The 4L60E and 4L65E use the same clutch arrangement, and similar refinements continued during its production run as well.

 

ok thank for your detailed feedback ! .. and a new adjustment of the cable TV will not correct this sensation ?
I have already done but that never change anything ... with minimal adjustment only because I know, it can be dangerous... for transmission

 

Probably not. It's internal to the transmission. A new servo (the thing that applies the band) might help. Get a "Corvette" one.

 

https://transgo.com/product-details/...eDOeQFPbVFVjN-

This one for example.

There is this kit also, perhaps it can be benefit ? => https://transgo.com/product-details/700-jr-shift-kit/

 

Hello, in fact, little rectification of my problem, it is in décélération on the dowshifting in 3-2 not 2-1 only in D. Perhaps it is the same explication, and that mean be as a a feeling of disengagement of the transmission, disengagement of the clutch, like a jolt and which goes up to the transmission shaft behind.

 

As described above, the shifts between 2nd & 3rd is the one where the band must let go and a clutch operate, or vice-versa, therefore is the one most likely to be affected by the servo in your situation. Not the 1-2 or 2-1 shifts. And yes, as also previously detailed, it's more likely to happen in D than in OD, because the "overrun clutch" is engaged in D only, not OD, which prevents the "low roller clutch" (a sprag) from decoupling a portion of the behavior when in D.

1st gear: forward clutch only is operating
2nd gear: forward clutch stays operated and band applies
3rd gear: band releases and 3-4 clutch applies: note that one clutch (the band) must release and the other (the 3-4) apply simultaneously
4th gear: forward & 3-4 clutches remain applied and band operates

As the car coasts top a stop and the transmission downshifts, the inverse of each upshift event occurs. Specifically, as it shifts down from 3rd to 2nd, the 3-4 clutch lets go and the band applies. In OD the low roller clutch (sprag) almost eliminates engine braking during that downshift, but in D the overrun clutch is applied and thus maintains the hard coupling through the transmission including engine braking. That's why you feel the imperfection in the 3rd-2nd downshift more in D than in OD. Since UPshifting in either D or OD is far more important to transmission operation than DOWNshifting, the timing of all the valves and orifices and accumulators and whatnot that control the clutches and band, is optimized for UPshifts; and DOWNshifts end up being just whatever they are. Furthermore, since the car is supposed to be driven in OD all the time except in a few particular circumstances, what the transmission does in D (rarely used) is somewhat sacrificed in favor of behavior in OD (almost always used). In other words, you're largely wasting your time worrying about it.

 

True ! but your answer is reassuring and makes you smarter

Thanks a lot !