Overdrive lockout for towing up hills?

[traveller]

My transmission temp on my 2003 Duramax is going into the red zone when towing my 12,000# 5th Wheel up hills. I am using tow/haul but not locking out overdrive. Would my trans temp decrease with overdrive locked out? I guess I'm not sure what the most efficient method is for an uphill climb while trying to keep the trans temp low. Any help would be appreciated. Thanks!

[DennisG01]

Are you positive the converter is locking? What speeds are you taking these hills at? I've pulled boats of that weight (which actually feel heavier than 5er's) and have not had problems. The stock programming does a pretty good job at controlling things.

Someone help me here - what speed will the converter lock-up at when in tow-haul? Is it around 35?

[DmaxMaverick]

The torque converter will lock in T/H at about 20 MPH in 2nd gear, and never in 1st. In T/H, the OD should never be an issue with lockup (can't get into OD at 20 MPH). The OD lockout does nothing but prevent a shift into 5th (or 5th and 6th if you have a 2006+). If you are towing up hill in OD, or DOD, it may help lock it out, but that depends on your speed and the grade of the hill. Of course, if it's really steep, you ain't towing in OD with 12K in tow in any case. I frequently tow very heavy on steep mountain roads, varying speeds from 15 to 30 MPH. At these lower speeds, the TC doesn't get to lockup much and the temps do climb significantly higher unlocked than when locked (which is to be expected). The only time I've ever seen the temps higher than "normal" when highway towing is up steep high speed grades (55 MPH+) in the California summer. It has never been "too" high, though, just higher than my normal temp.

All that said, you may have an issue with the tranny or cooling stack (all those radiators and cores behind the grill). I've not heard of any heat issues with the 2003's, except CA smog equipped trucks (they can run a bit higher under certain conditions due to the EGR and cat). If you have a fresh spin-on tranny filter, recent tranny service (fluid full and clean), and no other heat issues, I'd suspect the stack. Check for debris blocking airflow (like a plastic shopping bag, bugs, etc.). If you haven't done a tranny service lately, do one and get a look at the condition of the fluid. If the spin-on filter is overdue, it can cause higher temps, but usually will cause shifting issues first.

Other than that, we need to know more info on the conditions. What is the grade %, speed, RPM, distance, ambient temp, engine temp, mods you've installed, trailer profile, tire size, A/C use, at what time/distance does the tranny temp begin to climb, etc? Just about anything you can think of during the pull.

[DennisG01]

Wow, about 20MPH? I never knew it was that low. Not trying to hijack the thread, but will it stay locked - even under full throttle?

[DmaxMaverick]

Wow, about 20MPH? I never knew it was that low. Not trying to hijack the thread, but will it stay locked - even under full throttle?

Except for brief un/locking during shifts, it will stay locked, even at full throttle. It is programmed this way, to minimize heat generation, maximize power transfer, and economy. Heat generation is lost power to the wheels, which translates to less fuel energy reaching the ground. Energy used to heat the fluid (outside of normal) is wasted fuel. This is the parasitic loss associated with auto trannies, and why manual trannies generally don't get as hot, even with a fraction of the fluid and little or no cooling system.

Excessive heat means poor heat transfer under normal operation, or greater friction in the system somewhere, be it fluid or mechanical friction. Neither would be good indicators.

[Robyn]

If you are seeing temps into the red zone on the tranny, an auxilliary tranny cooler added up front would be in order.

You should not see temps that high except under extreme conditions and certainly not highway running even with a large trailer in tow

RC

[DmaxMaverick]

If you are seeing temps into the red zone on the tranny, an auxilliary tranny cooler added up front would be in order.

You should not see temps that high except under extreme conditions and certainly not highway running even with a large trailer in tow

RC

Perhaps true for the 4L80E, TH400, etc. But it's not the nature of the Allison tranny. They come out of the box with an aux cooler. The cooler is as large (and the correct design) as any aftermarket cooler I've seen added for the others. If the tranny is getting that hot, something else is wrong. Adding more cooler capacity would likely be covering up the root problem, if it helps with the temps. Also, I wouldn't rule out a faulty temp sender, or gage. Many questions need to be answered before throwing parts at it.

Just for reference, I tow very heavy, and often travel with friends who tow near the same weight. We're talking about near or slightly above GCVW, 20-23K+. Most of the time, where we go involves steep CA mountain grades, like The Grapevine, Tehachapi, The Siskiyous, Questa, Pacheco, and others. Many times, when the ambient temp is 100°+. None of us with GM/Allison trucks (one of them has the 496 gasser, poor guy brings up the rear) have ever had tranny heat issues (although the Ford and Dodge guys do all the time).

[Roy W]

I've found that if terminal towing speed never gets high enough for the trans to shift into overdrive when in tow-haul (about 56 mph on my speedo) then it's to my benefit to lock it out. The reduced gearing (1:1) on the level gives me the extra power to "drive ahead" and accelerate up grades, minimizing any lugging or excessive transmission downshifting, plus it increases hold back on the downhill side. I once drove at 55 mph for 400 miles with overdrive locked out and transmission temperatures stayed on the cool side (160 degrees) and fuel mileage didn't appear to suffer at all. If driving on the interstate at 65 mph in overdrive and I see a steep grade coming up that I know I can't pull in overdrive, I then lock out overdrive and get a little running start on the grade and sail over the top. Remember that the increased rpm as a result of locking out overdrive also increases fan speed, resulting in more airflow through the cooling stack and resulting in increased transmission cooler heat transfer. I've never pulled any of the long grades like you have out west, but here in the east we have a lot of 6-8% grades that are on secondary roads that you have to pull without the benefit of getting a "running start". If I'm towing under 56 mph, I've got overdrive locked out.

[socaltoolguy]

We are full time RVers and have towed all over the US. We have never had a trans heat up problem. I think the problem you are experiencing is due to trying to keep up with someone that is going too fast. Settle back and let the truck work the way it was designed and don't try to be the first to the top of the hill. I use tow/haul all the time and just let the trans do the shifting as necessary. The computer is much smarter than the driver. I'm also 65 years old. That probably has something to do with it also. Good luck and take time to smell the roses.
Ray

[rhsub]

As far as I know there are three causes of transmission overheating
1 Poor heat transfer at cooler. Cooler is dirty or obstructed, lines going to cooler damaged/obstructed flow
2 Torque converter clutch (tcc) not locking up, most of the heat generated in an auto trans comes from slippage (fluid shear)in the tc, have vehicle with trailer road tested with a scanner that is capable of reading tcc slippage, also check accuracy of guage when on rt with scanner
3 Internal slippage in the trans causing friction heat. Not likely your cause because this will cause a major failure fairly quickly
Good luck
Ron

[Philsauto]

My transmission temp on my 2003 Duramax is going into the red zone when towing my 12,000# 5th Wheel up hills. I am using tow/haul but not locking out overdrive. Would my trans temp decrease with overdrive locked out? I guess I'm not sure what the most efficient method is for an uphill climb while trying to keep the trans temp low. Any help would be appreciated. Thanks!

I don't have much to add to what has already been said, but I have a 2001 LB7 and likewise tow about 12,000lbs of 5th wheel. I installed the OD lockout button and I really like it for hills. I tow at 55-60mph (more relaxing) and never see my trans temp clear the half way point on the gauge. I think you have a problem or you are really pushing it.

[carco]

I have a 2004.5 LLY w/Allison. This set up has a history of problems w/overheating on long hard pulls. I gross about 22 to 23500# depending on the trip and long hills will cause very high coolant and trans temps. I mounted a 1" tube cooler made for Allys under the cab w/ a fan and temp sender of 190 degrees. Last trip to Colorado last Aug trans stayed under 210', Engine still warmed up to 240 to 255' degrees if pushed too hard.

[afp2007]

Earlier models of the Duramax with the Allison had a temperature gauge, but the newer ones, like ours can display the temperature on the instrument cluster. When towing with tow/haul mode is engaged, what should the transmission temperature range be?

(Dealer has yet to supply us with an owners manual, but it is on order)

Thanks

[carco]

180' would be ideal all day long. There is an old rule of thumb something like for every 10 degrees over 200' shortens trans expected life some large percent, sorry I forget the exact rule but suffice it to say heat is a trans enemy. I would prefer not to see over 210' @ any time however most all trans if used hard will see over this temp.

[afp2007]

180' would be ideal all day long. There is an old rule of thumb something like for every 10 degrees over 200' shortens trans expected life some large percent, sorry I forget the exact rule but suffice it to say heat is a trans enemy. I would prefer not to see over 210'

+++++++++++++++

Thanks Bob

[rat4go]

I read somewhere that most newer ATF formulations (like Dexron III) are good to 250F, but beyond that, you start losing significant fluid life (additives break down). If you're running Transynd or an aftermarket synthetic, you may be good to higher than that.

I agree that heat is not a transmission's friend, but 180-210 seems conservative to me with the newer fluids (I'd agree with these numbers if you were running the old, original Dexron or Dexron II fluids, but the newer fluids are supposed to be better)

[rustyk]

There's no reason not to run synthetic in ATs. BTW, the latest spec is Dexron VI...which will almost certainly always be synthetic due to the stringent specs.

It's what I run.

Here are the approximate temps/issues (with Dexron II/III): (IMHO as a recovering oil guy, these are pessimistic, unless the temps cited are routine and not intermittent)

"The approximate life expectancy at various temperatures is as follows:

. 175°F - 100,000 miles optimum street temps
. 195°F - 50,000 miles max street temps
. 212°F - 25,000 miles fluid fatigue
. 235°F - 12,000 miles varnish forms
. 255°F - 6,250 miles seals start to harden
. 275°F - 3,000 miles
. 295°F - 1,500 miles plates start to slip
. 315°F - 750 miles seals and clutches burn out. Oil forms carbon
. 335°F - 325 miles
. 355°F - 160 miles
. 375°F - 80 miles
. 390°F - 40 miles
. 415°F - Less than 30 minutes
. Above 295°F, the metals inside the transmission will warp and distort. This is why rocking in snow, mud, sand etc. should not exceed a few minutes."