I just bought the 03 Gmc Duramax with the automatic tranny. When I tow our holiday trailer should I just put it in drive and use the tow haul mode only? Or should I also use the overdrive lock out too? The manual really doesn't say when to use the overdrive lockout feature, it just tells you what it does. I thought I could just put it in drive and turn on the tow haul mode? With my old truck I just put it in 3rd and that was it, I never let it go into overdrive with the trailer on it. I was told that you never have a heavy load and be in overdrive, even on flat grade. What does everyone else do? The trailer weighs 5200 by the way. Thanks Jason
Ok so after a little bit of research the general concensus is that I just put it in tow haul mode with the trailer and forget about it for the most part. If the tranny is doing a little "busy shifting" going up hills or whatever then hit the OD lockout. Does this sound about right? Thanks Jason
any overdrive auto tranny I have ever had I have only hauled with over drive off and it should be the same for Chevy and I dont think using the lock out would hurt just an extra measure to keep it around longer
That is what I thought but I was in some other Duramax forums and they were all saying otherwise. That's my problem, I always thought that too but the manual doesn't really say what to do, just what it does.
Owning an 03 dmax and my dad having a 02 dmax, I can tell you from experience, and from many other's experiences, put it in Tow/Haul and forget about it. The alli does not hunt like others. The dmax has the power to pull in OD and there will be no problems. Plue there's a tranny temp gauge. You'll be fine in just tow/haul, I promise. John
Just throw it in tow/haul and forget about it. I have an 02 and it dosent have the option of turning OD off. So just through it in tow/haul and haul a$$.
Like he said....I have an 06 and I just put it in D and use T/H mode. These new trannies are so much better than the old auto tranny was. I had an old 12 valve dodge, with the 4 speed auto in it, and to tell the truth, I was never impressed with the truck, and I blame it on the old tranny. These allisons are pretty smart too. Actually, smarter than me, so I just let the tranny do its job, and it is always in the right gear and the engine/tranny brake is the best thing since sliced bread. Just let the truck do the job for you, and you can sit back and marvel at what a great job these trucks do when towing a load......Kong
Like they all said, put in in tow/haul and let the Allison do its thing. Sit back and enjoy the ride. My wife will even tow in the mountains now!
I have questioned the same thing. I live in California where we have "Cajon Pass" on I-15. It's a good pull. I use both tow haul and turn off the OD. Works great going up hill but, I don't like the way it slows on the down hill. The RPM's go up but, the speed doesn't pick up and I get worse MPG's. What works best for me is keeping it in OD on flat ground and then preparing for any climbs and then again shutting them off/on to slow or stop on the down hill.
The problem we have in CA is the speed limit 55. At 55 mph (OK 60) your just passed the point where the Alli shifts in T/H. Add 34.5" tires like me and it get worse... When towing I leave the T/H on most of the time. When approaching a hill that looks like it will cause a down shift I Lockout OD and force the down shift. This keeps my EGT's down to an acceptable level (My truck is not stock). Once I get out of CA I don't have to do this as much because I'm at a higher speed. "RPMs go up but, speed doesn't" That doesn't sound right...the TC is locked in high gear with or with out T/H and most of the lower ones with T/H. Are you referring to a grade brake down shift? If so yes you will get worse MPG at a given speed in a lower gear (In this case) but you will be working the brakes a lot in a higher gear. What I do on a looooooong down grade that would require braking (In CA). I set the cruise at the speed I want then lock out O/D. I've made it all the way down the west side of the Sierras into Sacramento with out using my brakes(other than for traffic). Makes for a nice relaxing drive also.
