What is my tranny doing? - Fuelly Forums

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Old 03-12-2009, 03:25 PM   #1
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What is my tranny doing?

In an attempt to conserve gas I have been experimenting with taking my foot off of the gas pedal during the longer downhill portions of my commute to work that don't have an immediate uphill climb after them. At 55 mph my RPMs are around 2000. When I take my foot off, they drop to 1500 or so. However, after maybe 10 seconds (if I get a glide that long), it will do what seems like an upshift and the RPMs will drop to just above 1000. If I glide in anything other than overdrive, it will drop to 1000 first. It's only when I'm in overdrive and glide that it does these increments.

However, no matter if I begin accelerating again with the RPMs still at 1500 or after they've dropped to 1000, I'll start in 4th gear. Obviously I'd like it to be in overdrive right away. I'm not dropping speed that much. If i just loosen the gas pedal a little so that I just slow down the tranny won't downshift into 4th until 45 mph. But on these downhill glides I normally put my foot back on the accelerator at around 50 mph, and when I do it feels like it's shifting into gear from neutral or like a regular upshift.

I've heard on here that my tranny isn't shifting into neutral during these glides (maybe just the torque converter being locked/unlocked), but why the two different "stages" of RPMs? Am I really saving any fuel doing this? I normally have to have my foot on the gas for 2-3 seconds, even if I don't accelerate at all, in 4th gear before it'll shift into overdrive again. Would my injectors not be flowing any fuel at the 1500 RPM stage, or the 1000 RPM stage?

The tranny isn't broken, it performs flawlessly. I was just curious about these quirks and how they affect my FE.
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Old 03-12-2009, 04:28 PM   #2
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I've noticed the same thing in my '02 Escort. If you shift to neutral in a glide when you shift back into gear it also goes into 4th before shifting back into O/D. Probably another scheme to keep the car from getting better mileage.
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Old 03-12-2009, 05:10 PM   #3
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It could be the TCC unlocking and the transmission upshifting in two separate actions as you guessed above, or it could be a semi-engaged TCC. Modern TCCs aren't forced to choose between locked and unlocked; my 2002 GMC gets duty cycled so its locking status is measured in percentage instead of true/false. Your 1998 Ford TCC may have two or three levels instead of being a variable percentage.

However, for convincing it to use the gear you want when you resume stepping on the gas...you're probably SOL. If it's worth the investment, you might look into custom tuning (not pre-fabricated flash programs).
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Old 03-12-2009, 07:47 PM   #4
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It could be the torque converter, but then again it could also be the car entering DFCO that you may be noticing. The conditions you describe are also the same conditions where DFCO may engauge.

-Jay
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Old 03-18-2009, 05:17 PM   #5
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The lock-up might be in stages. My 2005 Passat has a 5 speed auto but has a strange combination of open or locked.
1st open only, no lock-up
2nd open or locked
3rd open or locked
4th locked only
5th locked only.
As a result my shifting is 1o, 2o.
Then from 2o it goes either to 2l or 3o
If to 2l, it then remains locked and goes into 3l
If to 3o, it has to go through 3l before going into 4th (only locked up) and then into 5th.

So what's that to do with yours? It might be that your engine braking in 5th locked up is greater than your engine and transmission controllers are programmed to accept and the transmission shifts to 4th to allow use under a greater slip of 4th open or unlocked.
When you start accelerating again you might start in 4 to reduce the jolt of no-slip engagement of 5th locked.
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Old 03-18-2009, 06:26 PM   #6
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If overdrive is a band and 4th is a gear it could be going into 4th from neutral to avoid damaging overdrive.
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Old 03-19-2009, 08:59 PM   #7
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Thanks for the info guys
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