I asked this question in the towing forum but maybe someone can explain how the automatic works as I don't understand the following: with OD off at 60 to 65 mph the rpm is between 2500 - 2700, with OD on normal rpm is about 1700. No problem so far I understand that but when she shifts on the hill it seems like there is a half gear where it revs at 2200 rpm and until the hill gets steep it pulls fine then when the going gets harder it shifts again and revs at 2500-2700 rpm which is the rpm when OD is off. So what gear is it in when it is kicking down to 2200; OD on 1700 or 4th, OD off 2500 or 3rd, OD on 2200 or 3.5? I'd like to know exactly what is happening when I tow these hills as it may be better just to leave the overdrive off or does it matter if you run 2700 rpm all day in 3rd.

 

The 1/2 gear you feel between 3rd & 4th, is the torque converter locking up.

If you are just cruising along with the O/D off and very little load, the torque converter will lock up to increase fuel efficiency. If you are still under load, the transmission will stay in 3rd to raise the torque curve.

If your transmission is going in and out of O/D while towing you need to turn the O/D off or you'll kill your transmission. If you want to be informed about the condition of your transmisison while towing, you should install and axiliary transmission cooler and temp guage. The less time your transmission spends above 200 degrees, the longer it will last. Ideally, you should try to keep the transmission temperature around 175-180 degrees.

 

You mean the torque converter unlocking.

 

jefflaws,

It sort of depends whether you're accelerating or decelerating. I was thinking, in my mind, about accelerating because this is when I most often notice.

Accelerating, it would lock, but decelerating it would be unlocking.

If I'm confused, maybe you could explain the function of the torque converter more thoroughly

 

When accelerating the TC will unlock to multiply the torque. The TC only multiplys torque when it is slipping. When climbing a hill the engine revs will go down. If they fall enough the TC will unlock to help get up the hill. If the demand for torque is even grater (rpm falling even lower) it will slip into 3rd gear.

Regards

Jean Marc Chartier

 

Thanks for the replies at least now I know why it seems to shift twice to get to 3rd when towing on a hill. So to ask the question that has probably been asked many times before, when towing should I leave it in OD and as long as the tranny just goes through its normal sequence of shifting just leave it there, or would I be better off and run just in 3rd. Not sure which is better, the tranny probably likes OD off but then the engine revs at 2500-2700 a 1000 more than normal. Whats the best overall?

 

Thats backwards, it will unlock at tip-in.

 

George;

As long as the truck will pull OD, it is OK to leave it there. If it starts cycling between third and OD, you should lock it in third. It is also a good idea to anticipate a downshift and do it manually (like at the bottom of a steep hill).

When towing, I have noticed that my tranny temp gauge reads lower in 4th with heavy throttle than in 3rd with light throttle and the gas mileage is slightly better, too.

Ian