Weird transmission "stumble" - TCCoA Forums
 
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post #1 of 7 (permalink) Old 10-26-2011, 03:49 PM Thread Starter
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Weird transmission "stumble"

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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post #2 of 7 (permalink) Old 10-26-2011, 05:00 PM
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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.

-Brandon
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post #3 of 7 (permalink) Old 10-26-2011, 05:06 PM Thread Starter
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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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post #4 of 7 (permalink) Old 10-26-2011, 05:12 PM Thread Starter
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Oh, also, this used to happen at just about exactly 35 MPH, now lately it happens right around 30 MPH.
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post #5 of 7 (permalink) Old 10-26-2011, 07:48 PM
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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.

-Brandon
97 Laser Red Thunderbird LX 162k, Stage 2 4.6L 2v N/A | 300 BHP (255 RWHP, 290 RWTQ) | 13.95 @ 97.58 | Build details | Pics at the Lorain Assembly plant
98 Black Mark VIII 160k, stock daily driver
07 Redfire Fusion V6 SEL 178k, for the wife
Gone but not forgotten: 96 Mark VIII, 94 Cougar XR7, 93 Mark VIII

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Once you have tasted flight, you will walk the world with your eyes turned skyward, for there you have been and there you long to return. -Leonardo da Vinci
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post #6 of 7 (permalink) Old 10-27-2011, 02:29 PM Thread Starter
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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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post #7 of 7 (permalink) Old 10-27-2011, 02:34 PM
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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.

-Brandon
97 Laser Red Thunderbird LX 162k, Stage 2 4.6L 2v N/A | 300 BHP (255 RWHP, 290 RWTQ) | 13.95 @ 97.58 | Build details | Pics at the Lorain Assembly plant
98 Black Mark VIII 160k, stock daily driver
07 Redfire Fusion V6 SEL 178k, for the wife
Gone but not forgotten: 96 Mark VIII, 94 Cougar XR7, 93 Mark VIII

TCCoA's resident pilot since 2014
Once you have tasted flight, you will walk the world with your eyes turned skyward, for there you have been and there you long to return. -Leonardo da Vinci
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