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Old 10-13-2015, 01:29 PM   #1
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Why would I need to turn off OD?

We just bought a 91 Class C, 460 Ford on an E350 chassis. Over the speedometer (see pic top) is a hand made label that states "Turn OD Off". There is a button (see pic left) that will turn it off and I was told by the previous owner that he always turned the OD off because of this label. But he didn't know why, it just said to so he did. Now I have left it on while highway driving and OD seems to work just fine. So my question is, can anyone explain WHY this label exists and WHY would I want to turn off the OD?
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Old 10-13-2015, 01:42 PM   #2
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Just a quick guess, but I will suppose that the first owner was towing something heavy enough to cause the transmission to shift in and out of overdrive continuously.

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Old 10-13-2015, 03:12 PM   #3
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X2 Overdrive off when towing.
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Old 10-13-2015, 04:15 PM   #4
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Best would be to investigate what the owner's manual says for that chassis. Many modern engine/transmission controls allow OD when towing and manage the shifting better than old wife's tales.

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Old 10-13-2015, 04:16 PM   #5
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Agree, turning OD off keeps the trans in 3rd gear (1.00 ratio) to keep the trans from hunting between 3rd and 4th when towing or pulling slight hills. It also keeps the trans from getting too hot when towing. It won't really hurt having OD off when not towing and on flat ground, just makes the engine run at higher rpm and less mpg.

So run with OD on when you can, it is designed to be used under lighter loads and flat ground. If you find it hunting between, such as on a slight uphill, turn the OD off will keep it form switching between gears.
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Old 10-13-2015, 04:44 PM   #6
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I would respectfully disagree with turning it off when towing. It sounds more like somebody who was not willing to pay attention to his shifting. Possibly because they were clueless about torque and horsepower curves. The easy out was to leave the OD off. The better answer is to leave it on unless one needs the forced down shift usually proactively shifting when going into a hill and/or coming down one. If one is towing and needs to shut off the OD on flat ground they are probably towing too much.
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Old 10-13-2015, 09:15 PM   #7
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I turn my off on long down hills for more engine braking.
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Old 10-13-2015, 09:36 PM   #8
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Modern automatic transmissions are a lot smarter than their drivers are. The chassis manufacturers have a lot of master's degree engineers (and a few Ph.D's) working on these systems.

Just leave the selector in "drive" and don't try to second-guess the thing, All you'll do otherwise is burn more gas than you need to and cause excessive wear on the engine by causing it to rev more than it needs to.

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