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Old 01-07-2010, 09:31 AM   #29
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I have always been told that overdrive in a automatic transmission has less clutches in the clutch pack than drive does so when you tow a load in overdrive the clutches will slip more causing extra heat and wear on the transmission. Excessive heat will always shorten the life of a automatic transmission.
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Old 01-07-2010, 09:35 AM   #30
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Our Silverado is an 04 2500 HD diesel. We have the 5 speed Allison. I can turn on/off tow/haul and OD. We have 95K miles on the truck with many, many tow miles. Towing with tow/haul on and OD engaged we get 12 - 14 MPG (I have seen more and less). Typically the tranny temp is 200 F or less towing. Even in AZ summer. And I definitely follow the Chevy tranny service schedule. As much as I love the magic Allison I would consider a 6 speed manual (if I could get one) in a new truck.
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Old 01-07-2010, 09:36 AM   #31
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Why shouldn't I use overdrive?

My tow vehicle is quite a bit older than many of those discussed in this thread, but it has served me well since 1998. It is a 1999 GMC K2500 Suburban with the 7400 VORTEC V8. It was special ordered with the 10,000 pound trailer towing package including the 4.10 differentials. Both my dealer and the owner's manual suggested that towing in overdrive was fine so long as the transmission wasn't hunting between gears. My Overlander is only slightly more than 1/2 of the allowable trailer weight, and I have towed it using overdrive most of the time. The only time that I lock out overdrive is at the first hint that the transmission is hunting. The transmission has been serviced every 30,000 miles -- and the truck is approaching 200,000 trouble-free miles for the transmission and motor.

Kevin
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Old 01-08-2010, 10:16 AM   #32
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Gotta wonder if that dealer is trying to sell you a new tranny?!

48K without a change? Whats the owners manual say about that? Much less if 40K of that was under load.
My dealer said a fluid and filter change @ 100k is all this tranny needs with our application. Much as Andrew said, our 1/2 ton usage is nothing to what this tranny can take in 350 1 ton v10 and higher applications such as motorhomes etc. Time will tell if it is all correct, just wish the leather was durable as the rest of the truck. Really, what a great balance of engineering and value. I have never owned a vehicle that does the job intended so well.
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Old 01-09-2010, 08:28 AM   #33
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My $0.02.

There are enough different approaches to building a transmission and designing the electronic controls for it, that the best answer is to:

Read The Owners Manual
Follow the Directions Inside.

What was true for a purely hydro-mechanical system of the 80's may not apply at all today. In other words, it's a little like debating whether the 8-track tape should be stuffed into your I-Pod label side up or label side down.
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Old 01-09-2010, 08:39 AM   #34
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In other words, it's a little like debating whether the 8-track tape should be stuffed into your I-Pod label side up or label side down.
You need a converter to do that...
YouTube - "l8track" 8-track iPod converter
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Old 01-09-2010, 09:13 AM   #35
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Just to also note the clarification regarding tow/haul and OD on GM vehicles. Tow/Haul changes the shift points. When accelerating with tow/haul engaged, the engine is running at higher RPM's when it upshifts to the next gear. When a load occurs the transmission downshifts to a lower gear sooner. That's all it does.

I follow the factory's recommendation regarding the use of overdrive. In my current van, towing in OD is allowed with the note about watching to avoid frequent hunting (up and down shifting) of the transmission. My previous van with a different model transmission stated no towing in OD and I followed that recommendation.

If you follow the manual recommendations, change the fluid, and if possible run an external transmission cooler, you will find that transmission issues won't be in your towing future.

Jack
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Old 01-10-2010, 10:10 AM   #36
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Thanks, Jack--you taught me something about my Silverado. I've just been using the Tow/Haul option, but not the "D" with a slash through it, which would lock out overdrive. I'll be doing that now too, and appreciate the counsel on this.
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Old 01-10-2010, 10:20 AM   #37
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Quote:
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Thanks, Jack--you taught me something about my Silverado. I've just been using the Tow/Haul option, but not the "D" with a slash through it, which would lock out overdrive. I'll be doing that now too, and appreciate the counsel on this.
Noticing your tow vehicle is the same vintage as mine, I would expect that GM used the same HD tranny which does allow towing in OD. I've got the 6 liter gas engine and a 4.10 rear axle. It's rated a 9,900 lbs. The GM tow package did provide an external transmission oil cooler. I do lock out OD in the hills of Branson though. Do you have a transmission oil temp gauge on your Silverado??
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Old 01-10-2010, 10:35 AM   #38
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The 5 and 6 speed transmissions almost always have only a single "overdrive" gear. The extra gears are in-between, and are created by simply engaging and disengaging the torque converter clutch in the different gears.
For everybody that noticed, but didn't say anything, thanks. Instead of being specific, "in most Ford transmissions I've seen", I just made a blanket statement. I hate when that happens.
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