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Old 08-12-2002, 10:21 PM   #1
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Transmission questions

My neighbor did his first travel trailer pull this weekend. He has a 2002 Ford pickup. His pickup has a tach so he noted some reading and sounds. I can relate this also to my Chevy Express van although I do not have a tach.

Example you pull in third gear. You start up a hill and as you depress the accelerator you hear the engine seem to increase rpms but you don't see your speed increase. You also note that you really haven't dropped down to second gear at that point. My neighbor noted that his engine picked up 300-400 rpm. As your speed drops and you depress even further you encounter a downshift as the transmission goes to the next gear down. At that point I'm in second gear and I can pull the gear selector down to the 2 position and no change occurs. My neighbor tells me his Ford may pick up 800-1000 rpm at that point.

I guess our question is what is that change we feel that is not a perceptable transmission downshift like from OD to 3rd gear, but a more suttle engine noise with a perceptable increase in RPM's. Is it the torque converter coming out of a lockup but not a downshift? Inquiring minds want to know....

Jack
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Old 08-12-2002, 10:50 PM   #2
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Lockup

Yes, I believe it is the torque converter unlocking.

My Dodge Dakota would absolutely not run unlocked except during a shift, but both my Chevy 1500 and a previous S-10 did just as you describe.

IMHO, It is better to press a little harder and get the downshift since running unlocked generates a lot of heat in the transmission fluid. I would want a transmission temp gage before I spent any amount of time in the unlocked state.
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Old 08-12-2002, 11:03 PM   #3
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Re: Lockup

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Originally posted by Pahaska
Yes, I believe it is the torque converter unlocking.
IMHO, It is better to press a little harder and get the downshift since running unlocked generates a lot of heat in the transmission fluid
I thought the heat we were trying to avoid was the heat generated due hunting between gears, for example allowing the trans to go from OD to 3rd back to OD etc. What you are saying is by getting the downshift you effectively have locked up the torque converter at the same time you downshift?

Its awfully hard to keep the trans from unlocking the torque converter since I don't feel that in the cases where have heard the converter unlock, we haven't dropped enough speed to do a manual or automatic downshift into second gear. Especially when you are running at 60-65 mph when this occurs.

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Old 08-13-2002, 07:24 AM   #4
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TC Slippage

There was quite a long thread on rec.outdoors.rv-travel a couple of years ago regarding running with the torque converter unlocked. I came out convinced that, whil it isn't as bad as the transmission hunting, slipping isn't real good because it generates heat.

The way I would interpret it is that the lockup is to improve efficiency. When the TC is unlocked, some power is going somewhere other than the wheels and it is obviously being lost in the transmission. The only way that that lost power can manifest itself is heat. Undoubtedly, that is why we have (or should have) transmission coolers on our tow vehicles.

I'm not suggesting that we shift down in every unlock instance (I don't); but, If the climb ahead is going to be a long one, that is probably the time for to downshift to minimize heat buildup. Most of our Texas Hill Country climbs are too short to worry much.

Somewhere in the shop is a transmission temp gage that I once used on the S-10. I should find it and get it installed on the Silverado.
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Old 08-13-2002, 08:19 AM   #5
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Internally the torque convertor is like two sets of fan blades. The engine drives one side, the other side drives the transmission. Set two household fans facing each other, turn one on, it will 'drive' the other. In a torque convertor, transmission fluid transfers the energy between the blades. That is why it gets hot when the clutch is unlocked (the lockup convertor orginally was designed to prevent the 3-400 rpm increase you mention, help the manufacturers meet the mandated mpg requirements).

Short periods of driving with the convertor unlocked will not harm the fluid, there are 10-12 qts. in the trans, sump, and convertor. But as John said, if it is going to be a long uphill pull, it will keep the temp lower if you downshift.

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Old 08-13-2002, 09:27 AM   #6
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I guess what is surprising to me is that it seems that the torque converter comes into play even if you are driving in 3rd gear and not overdrive. I always had assumed that it was only engaged once you got to speed and had selected the overdrive setting on your gear shift.

In essance I always tow in 3rd rather than O.D. to avoid hunting but still have to deal with the lock up issue.

Jack
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Old 08-13-2002, 10:36 AM   #7
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The convertor is the source of power transmission until you get to a steady speed and the clutch locks up.

Remember the reason for this setup, mileage improvement so the manufacturers can meet CAFE. It is better to have the clutch disengage and the engine increase 300-400 rpm with the convertor than it is to have the trans down shift and have the engine increase 1000-1100 rpm.

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