How Does Od Work?

Champy · February 7, 2009

Ok, heres the basics. Plan on towing a 5000lb travel trailer; 2007 Sierra NBS Crewcab; 3.73 ratio; 5.3L engine; tow pkg; auto; tranny cooler. I have read other posts saying to drive with Towhaul on and in 3rd gear. Now for the stupid question......

Without anything behind the truck, I can take off and count shifts into 2nd; 3rd; and 4th gears. Then once speed is stable the RPMs drop about 500-600 revs and it appears to shift into what I know as "Overdrive". So is the truck a 5 speed or a 4 speed? Is overdrive a "shift" or a control change while in 4th? It seems like everything would work alright for towing if I could just eliminate that last OD shift & RPM drop while staying in 4th, but the only choice is to go with 3 or D......what am i missing here?

Thanks

BuckFeverZ · February 7, 2009

It's a 4 speed. When it appears to 'shift' out of 4th gear, its just the torque converter locking.

matt3 · February 7, 2009

It's a 4 speed. When it appears to 'shift' out of 4th gear, its just the torque converter locking.

Than can you tow it D with tow/haul enabled or should you still put in 3rd with tow/haul?

BuckFeverZ · February 7, 2009

I always go 3rd and Tow/Haul mode since it will still shift into OD when in Tow/Haul.

66nova · February 7, 2009

It's a 4 speed. When it appears to 'shift' out of 4th gear, its just the torque converter locking.

Than can you tow it D with tow/haul enabled or should you still put in 3rd with tow/haul?

Yes, but it depends on the size of your load. If you look at your manual it will give you recommendations as to when (too much weight or hilly conditions) to down shift to 3rd and out of drive.

I haul my car trailer with nova in most conditions in D with T/H on. Rarely, do I need to downshift and I always monitor my trans temp when I'm towing.

T/H mode will only let it go in to D at highway speeds.

VMax2007 · February 8, 2009

It's a 4 speed. When it appears to 'shift' out of 4th gear, its just the torque converter locking.

Than can you tow it D with tow/haul enabled or should you still put in 3rd with tow/haul?

Yes, but it depends on the size of your load. If you look at your manual it will give you recommendations as to when (too much weight or hilly conditions) to down shift to 3rd and out of drive.

I haul my car trailer with nova in most conditions in D with T/H on. Rarely, do I need to downshift and I always monitor my trans temp when I'm towing.

T/H mode will only let it go in to D at highway speeds.

+1

Use "D" with T/H and let your truck decide if it can use OD or not. It is "smarter" than you are.

Z71kris · February 8, 2009

It's a 4 speed. When it appears to 'shift' out of 4th gear, its just the torque converter locking.

Than can you tow it D with tow/haul enabled or should you still put in 3rd with tow/haul?

Yes, but it depends on the size of your load. If you look at your manual it will give you recommendations as to when (too much weight or hilly conditions) to down shift to 3rd and out of drive.

I haul my car trailer with nova in most conditions in D with T/H on. Rarely, do I need to downshift and I always monitor my trans temp when I'm towing.

T/H mode will only let it go in to D at highway speeds.

+1

Use "D" with T/H and let your truck decide if it can use OD or not. It is "smarter" than you are.

or you?

VMax2007 · February 8, 2009

It's a 4 speed. When it appears to 'shift' out of 4th gear, its just the torque converter locking.

Than can you tow it D with tow/haul enabled or should you still put in 3rd with tow/haul?

Yes, but it depends on the size of your load. If you look at your manual it will give you recommendations as to when (too much weight or hilly conditions) to down shift to 3rd and out of drive.

I haul my car trailer with nova in most conditions in D with T/H on. Rarely, do I need to downshift and I always monitor my trans temp when I'm towing.

T/H mode will only let it go in to D at highway speeds.

+1

Use "D" with T/H and let your truck decide if it can use OD or not. It is "smarter" than you are.

or you?

I wouldn't go THAT far.

Z71kris · February 8, 2009

Bmart · February 8, 2009

". So is the truck a 5 speed or a 4 speed? Is overdrive a "shift" or a control change while in 4th?"

wow i have been wondering the same thing for so long... thanks for asking.. i thought i was alone on that question and was horrified to ask it haha.

hockeypro1411 · February 8, 2009

". So is the truck a 5 speed or a 4 speed? Is overdrive a "shift" or a control change while in 4th?"

wow i have been wondering the same thing for so long... thanks for asking.. i thought i was alone on that question and was horrified to ask it haha.

It's a "4spd w/ Overdrive".

mljenkins · February 8, 2009

Use "D" with T/H and let your truck decide if it can use OD or not. It is "smarter" than you are.

+1

I always tow in D. Trucks shifts into OD on the flat and downhill and have had no issues. I can't use CC if I'm towing uphill because it drops down into 2nd halfway up the hills and makes a big jolt, which isn't good if I am hauling my horses. But on the flat, a lot of times I turn cruise on and have had no issues with it shifting in and out of OD.

66nova · February 8, 2009

". So is the truck a 5 speed or a 4 speed? Is overdrive a "shift" or a control change while in 4th?"

wow i have been wondering the same thing for so long... thanks for asking.. i thought i was alone on that question and was horrified to ask it haha.

It's really not overdrive any more. It's just drive or D. Overdrive is kind of a old school term. It comes from when everything we drove was a 3 speed and 3rd was D. Third was always a 1:1 ratio and we controlled our rpm on the highway with rear gears.

Then manufacturers started putting gear ratio such as 0.70:1 (atleast for the 700R4 and 4L60E). A lot of cars came with D and OD on the shift indicator.

Now with lockup torque converter technology and 4, 5, 6, and 8+ speed transmissions plentiful overdrive is no longer overdrive. Otherwise we would have overdrive overdrives.

As Buck said earlier, the change in rpm you see when cruising is the torque converter locking and unlocking. A lock up are unlocked in 1-3, this gives our motor the ability to stay in its power band better. Once in D, the converter can lock up when we are at a constant speed (the converter is not slipping as much) this lowers engine speed and reduces heat. All hopefully helping reduce wear and lengthen transmission life and increasing mileage.

So, yes its a 4 speed or a 3 speed with OD in old school terms.

irengineer · February 8, 2009

Actually the term overdrive refers to a gear ratio that is less than one, such as .67 or .70 which are common 4Speed 4th gear ratios. The 6Speed trans's have two overdrive gear ratios, .85 for 5th and about .67 for 6th. In an overdrive gear state the input to the trans (turbine shaft) spins slower than the output shaft.

While in the past, 3 speeds and 4 speeds had a gear ratio that was 1:1, aka Drive or D or direct drive, meaning that the input and output spin at the same speed, now with a 6 speed "D" is simply the normal drive range, which has a combination of overdrive and non-overdriven ratios (1st gr =4.027, 2nd gr - 2.364, 3rd gr = 1.532, 4th gr = 1.152, 5th gear = 0.852, 6th gear = 0.667 and reverse = 3.064).

With a 3 Spd it was easy to design a shift lever such that every available range had it's own selectable position on the "PRNDL" (a generic term for the selectable range positions, pronounced "prindel"), PRND21 (6 positions), and with the advent of the 4 Spd another PRNDL position was created and was typically shown as another D but with a squarish looking "O" around the D PRN[D]D21 (seven positions) (I think Ford typically kept the D21 and had an OD button to cancel OD).

The 6 Speed presented a packaging issue for the shift lever in that there wasn't enough rotational room to add two more positions without the shift lever ending up on you right knee, thus the elimination of some of the 4 Spds PRNDL positions. Today's 6 Speeds have a "PRNDM_" layout for LD PU's and SUV's, PRNDM_21 for H2's, and PRNDM_1 for HD's.

The M position allows manual selection of a range, and that selection is analogous to the phsysical position of that same number had there been room to design a 9 position PRNDL. In M, the visual position to the right of the M (the "_") is where number shows up to define the sected range. That number defines the top gear allowed in that range, unless the vehicle speed is too great to command that gear and would overspeed the engine (if that were the case that top gear would be selected once vehicle speed was slow enough and once the top gear was reached the higher gears would not be available).

In M ranges, gears 1-M are commanded similar to D, but just not exceeding M). In HD trucks M2 starts in 2nd, similar to 4 Speeds which did the same when in the "2" position.

As for the torque converter, many of you know that the torque converter is this big doughnut shaped contraption that is filled with trans fluid and the outer shell is connected to the engine via the flex plate. Internally there are pump and turbine blades (think kind of like a jet engine) such that the pump blades which are fixed to the outer shell (engine) pump the oil towards the turbine blades and create a torque multiplication as the fluid hydro-dynamics "do their thing".

When you launch you vehicle from a stop the pump side of the torque converter is spinning at engine speed and the turbine side is not moving. This is the point of maximum slip and maximum torque multiplication (There are many designs for torque converters that provide various amounts of torque multiplication and "slip", each with their own advantages and disadvantages for a given application).

After the vehicle launches the slip becomes less and the torque multiplication / slip diminishes. When the torque converter is not providing any advantage but there is still slip between the pump and turbine sides of the torque converter a clutch internal to the torque converter is applied which mechanically locks the pump and turbine, reducing the slip to 0 rpm. This can occur in the top gear and appear to be another shift, as was mentioned in a previous post, since it does reduce the engine speed just like the upshifts, but typically it occurs in lower gears and is maintained to be locked in higher gears (except perhaps while a shift occurs).

Many of todays torque converters are controlled so that even when "locked" there is still some slip allowed / designed between the pump and turbine. This allows isolation between the powertrain and the vehicle chassis to help minimize a powertrain's natural noise and vibrations characteristics.

Perhaps more detail than most want to know and I'm sure others could expand on this, but I figured this could help a few with some tranny basics.

IRE.

music · February 8, 2009

Actually the term overdrive refers to a gear ratio that is less than one, such as .67 or .70 which are common 4Speed 4th gear.

...

That's a nice writeup IRE... and I have a question. It's been my understanding that 3rd locks the torque converter at 1:1. Are you saying that it allows some slip "at speed". If so, does "tow/haul" mode affect it?