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Discussion Starter #1
Hey guys, I'm starting to worry about this little quirk my '95 Cougar has in its transmission.

At some point between 30-35 mph, my car seems to stumble for a moment. It feels like a quick tap on the brakes, and it seems like the tach usually drops quite a bit. I'm thinking this is just the 2-3 upshift, but it feels very strange. After the stumble, if I hold speed and press the O/D OFF button, it'll stumble again, with the same tapping-the-brakes feeling.

If I'm driving with the O/D off, it doesn't do it. I can't figure out why the overdrive should have anything to do with it.

Anyone got any ideas? This transmission's only about 70,000 miles from its last rebuild, according to the records I got with the car.
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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The "stumble" you are feeling is the 4-3 downshift. During this shift, the forward clutch has to apply as the overdrive band releases. If the clutch applies before the band is released, you get a bit of a sag on the shift. Check out Jerry's thesis in the tech articles accessed from the tccoa.com home page for more info on the J-mod which will greatly alleviate this condition. The 4-3 downshift usually occurs at ~31 MPH while coasting in the stock tune. With O/D off you are never in 4th gear, so the overdrive band is never applied and thus you never have the shift sag.
 

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Discussion Starter #3
That explanation makes a lot of sense, thanks.

The weird thing is that this is happening while accelerating from a stop. Starting out from a stop, I'll be at somewhere between 10-25% throttle, tapering off to idle as I approach my target speed of about 33 MPH (city streets). Seems like it shouldn't be in 4th gear at all during this evolution, yeah?
 

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Discussion Starter #4
Oh, also, this used to happen at just about exactly 35 MPH, now lately it happens right around 30 MPH.
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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The 3-4 upshift usually occurs at 33-35 MPH, depending on throttle position. Take into account ~2 MPH speedometer inaccuracy and that puts the 3-4/4-3 shifts right in that neighborhood. Normally that shift won't sag; either way I'd do the J-mod to address as the known valve body problems, as many as they are with the 95 model.
 

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Discussion Starter #6
Okay, now it all makes sense. For some reason I assumed the 3-4 shift would be higher, speedwise. So, I'm already planning to try the J-mod but probably won't get to it before it starts snowing. Does this sound like something that's going to cause damage if I leave it alone until next year?
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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If it's normal operation then probably not. If it is indeed behaving oddly compared to stock and something is already broken, the J-mod won't cure anything and your only recourse would be a rebuild (or upgrade to a newer model year trans) - but like I said what you're describing sounds normal so I don't see the need for worry.

The primary function of the J-mod is to reduce heat and clutch pack wear as a preventive measure. A side effect can be quicker and snappier shifts (depending on how you drill certain holes), which some people enjoy.
 
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