`A Bucking Bronc of a Bonneville

[1964 Bonneville Ragtop]

First a quick bio background of my trusty steed: 1996 Bonneville SE, naturally aspirated, original drive-train with lots and lots and lots of miles (243k !). Rescued at 170k from sheer neglect and abandonment after cracked intake, etc issues by former 2nd owner idiot
Slowly with time and lots of TLC brought her back into fine shape and she has treated me very well since Dec 2005....
Recently however a head-scratcher of an annoying, if not concerning problem of bucking has shown up: Only when under load such as climbing even mild road grades, or passing gear auto downshifts but with this one caveat: It's ONLY in 4th gear above 50mph. When locking out OD by manually downshifting into 3rd gear and remaining there for remainder of highway trips, the bucking is non existent even past 70+mph. Present once again when back in OD-4th.

I'm theorizing a number of possible problem origins from a throttle position sensor to faulty trans shift sensors, valves or
OD clutch failing...Okay now boys; rack your Pontiac history encrusted brains for similar occurrences will ya now?
Gimme some solid intel here...

I don't just want to throw part$ at it, of course, but no useful DTC codes other than a small evap leak someplace.
Any true blue knowledgeable transmission guys out there on line?

If a trans rebuild is in my future, my budget dictates I would have to pick up a used 4T60-E with low as possible miles someplace - any "buyer bewares" with that scenario?

[RJolly87]

I would rule out a misfire first. If there was ever a time it was going to do it, that would be it. My Regal has a misfire under those conditions, and sometimes it can feel like I hit something it's so violent. I drag the brakes a touch just to disengage the TCC, and it isn't as bad, and when I downshift, it goes away.

Start with the small stuff first.

[1964 Bonneville Ragtop]

This is always solid advice, even as postulated with Occam's Razor : the most simplest basics is often the most likeliest.
Thanks bro.

[94SilverSSEi]

I have personally had 3 cars that have done this that I thought for sure was a bad torque converter. One car ended up having a bad coil, the 2nd car had 2 bad coils and the third just needed plugs and wires as they were 21 years old and original.

So those would be the easiest things to rule out first. It is possible the torque converter could be having an issue but misfire is very likely.

[1964 Bonneville Ragtop]

Okay my Bonneville boys, heres what I finally found by a lot of frustration over weeks of bucking in overdrive, finally misfiring in all gears with difficulty starting and finally no start this weekend: a FAILED crank position sensor , which often fails progressively - then totally. And may or may not set a DTC code till the very end.

While many here correctly say its a misfire, it isnt always plugs, wires or coils.
Lets take notes: the Crank Position Sensor can wreak havoc with driveability - and our diagnosis- as it fails progressively .

I do appreciate everyones selfless generous help with all of our questions on this forum
and I'm glad to be a Bonneville owner ever since my first one : a 1964 Silver B convertible way back in 1969.

[bobc997615]

I've only have had crank sensors fail by totally shutting down the engine at an inopportune time...only to have the engine start some time later. Very hard to diagnose but never had one that caused the "bucking".

[RJolly87]

I can see it if it was intermittently dropping signal, but usually when they fail, they just pack up when warm and stall the car.

Glad you got it fixed though.

[1964 Bonneville Ragtop]

Thanks Randall !

~ best regards and Bonneville carefree motoring!
Geo