Flex Plate Blues

[nvken]

Hi guys, just a little recap of what occurred on my recent vacation to the Northern Left Coast. After leaving Crescent City, Ca. going up hwy. 199, I was climbing a 5% grade,towing my 15. 5k fifth wheel in 3rd gear, (auto) the trans shifted to 2nd and BANG , all forward motion stopped and the noise continued until I turned it off. My first guess was, that the flex plate had let go.



I was right. Had the whole works towed and repaired in Crescent City. $$$



I was running Smarty Jr. on SW2, the transmission has a TransGo kit, billet single disk converter and all the other recommended billet pieces.



I am aware that breaking the flex plate (stock) is preferable to breaking the input shaft (stock), but I am surprised that the flex plate let go at this power level.





2006 3500 Big Horn DRW Auto QC Silver, 4x4, Billet single disk TC, Transgo, lots of billet parts, PacBrake, Max-Brake, Smarty jr. , Quad boost fooler, FASS 150/150, gauges.

 

[HBarlow]

The well known aftermarket parts suppliers like DTT and probably Goerend and others offer a billet steel flexplate.

When all is considered, you have a mildly modified engine producing increased hp and tq, a very heavy fifthwheel in excess of factory recommended trailer or gross combined weight rating, and probably 3. 73 standard differential gears. You were probably pulling very hard in third gear and losing rpm. When the 48RE made a forced downshift, probably at or near full throttle, it was more than an OEM flexplate could handle.

You were putting heavy stress on it. Things break under those conditions.

Did you put an OEM flexplate back in it? A billet steel would probably be a better choice.

 

[GCroyle]

I'm gonna offer my . 02$ on a topic that I know squat about, automatics. I have no clue about your power level or what the stock flexplate can take or your transmission, but I'm going to draw a parallel to manual transmission possible problems.



Angular misalignment of trans to engine.



I repeat, I know beans about auto's OK!



But a clutch disc can have a similar failure if some foreign object or other source of creating an angle between the engine and trans creates a lot of flexing at the clutch disc or flexplate. Or some other source of angular misalignment, were all of the bolts tight between flexplate and convertor? Any damage to front seal?



Might be worth checking, have someone look at the old flexplate for evidence of long term crack propagation (right word, trying to say spreading) any rust dust showing up in the crack edges? What parts were replaced to fix it?



Cresent City, NW CA on the coast, correct?

 

[RDeFayette]

Stock flexplate cannot take the locking action of an aftermarket billet converter.

If you run an aftermarket billet converter you must run a billet flex plate. If anyone has an auto rebuilt with billet components it is best to go billet flexplate, converter, input and intermediate shaft with stock output shaft.

That way the weak/sacraficial link does not require trans removal to fix (also won't trash anything else).

 

[Wingate]

were all of the bolts tight between flexplate and convertor?

This question warrants investigation. . I would examine the bolts and holes very closely... With that being said, when I overload my truck with 20-25K lbs... if I am forced out of 3rd gear, I chose where and when, I do not rely on the trans control to force the full torque backshift... as others have said, this is extremely bad, even for stock power levels... it shocks the entire drivetrain like a heart defibulator. . NOT GOOD!!. . In tow/haul, my 06 model will force a backshift at 45 mph... if I were to pull down to about 50, I go ahead an back out of the throttle and let it backshift, then roll back into it hard and hold 2nd gear until the load eases up...

 

[HBarlow]

If the flexplate bolts get loose the flexplate will make a terrible racket. I've had that happen.

In this case, if I understand the OP, his flexplate sheared the center out. A mechanical failure resulting from too much stress caused by too much power applied against a load too heavy then subjected to a power downshift.

I have always managed my shifts also just like I would a manual. If/when I see the rpm is going to drop to 1600 with full throttle applied I back off the throttle, cause a downshift, and put my foot back in it.