I have researched this site and others and there are many opinions. Seems like it really boils down to exactly which motor and tranny you have and how much weight you're pulling and over what terrain.


I am running a 2012 E350 with a dry 6200 lb travel trailer behind it. Once the trailer is loaded, probably closer to 7500 lbs. Pretty much flat terrain.


I took it out for a test run prior to a long road trip. At 65 mph the motor was spun up to 2500 rpm with OD locked out, but then I would engage OD and the motor would drop down under 2000 rpm and seemingly pull fine. No shifting back and forth. Just running fine. Am I burning up something in the tranny like this?


So the real question I have is...what is the FUNCTIONAL value of the OD off mode?


IS the function of the OD off JUST to keep the tranny from toggling back and forth between OD and 3rd, OR IS THERE some sort of clutch plate or something that is getting worn heavily by allowing the OD gear to pull all that additional weight load?


Thank you!

 

Typically, switching the OD off is just to prevent heat build up from too much shifting. As long as it's happy in OD, you are fine.

 

Agree with Snowdog79, the only use for locking out OD is to prevent "hunting" behavior of the transmission, where it repeatedly shifts into and out of OD on level ground.
If it's just shifting out of OD at the start of each uphill section, then to me, that's normal and no cause for alarm.

It seems that some engine/Trans combinations are much more prone to this "hunting" behavior than others.

I would leave it in OD and unless you notice excessive shifting, enjoy the drive.

 

No, you are not hurting the trans. This is what it was designed to do.

The are two functions. One is to prevent frequent shifting in and out of overdrive. If that isn't happening, then you don't need to turn off overdrive.
Second, with OD off there is more engine braking available. That can help when descending a grade.

Sometimes you'll need more engine braking and then you can pull the shift handle to 2 or even 1. Don't worry about speeds when you do this. If you are going to fast for the trans to make the downshift the computer will prevent the shift until you slow enough to let the shift occur.

 

I have heard about this from local sources that I didn't feel comfortable with regarding their knowledge, but haven't been brave enough to try it. If Mark says that is the way it works, I feel much more confident about downshifting when descending a steep grade. I usually use the uphill side to slow me down enough that I'm able to downshift my E4OD to a lower gear before beginning the descent.

 

One thing I do note when traveling down a steep grade and relying in part on the transmission to slow me down is the engine fan does not come on and the transmission temperature does climb (not into the sky, but it does steadily creep up if the grade is long).

When running in a lower gear climbing, the engine is also working and the fan does come on and temperature drops for both the engine and transmission.

Steve

 

That's very true. It's also not a problem unless it gets above 250F. But if the trans is working THAT hard to slow you down, if you didn't downshift you most likely would overheat the brakes to the point that they don't work anymore. Then where are you?

 

When running very heavy, I have always encouraged folks to use both brakes and downshift when the downhill is long and steep.

Here is a true story I know of first hand. The owner of a large Ford V10 chassis Bounder motor home went down a long grade nearby relying only on his transmission to slow him discovering, when he reached the bottom, his transmission had gone to heaven.

Deciding against better judgment it must have been a fluke, while camping in the same area, he repeated his actions and trashed a second transmission, following which he decided maybe it would be best to use both the brakes and the transmission to slow his coach on long steep descents.

I think the idea of using the transmission alone comes from seeing/hearing the big trucks or motor homes with exhaust brakes, something I came to love in our diesel motor homes of years gone by. Just my two cents about an interesting subject.

I have debated adding an exhaust brake to my truck, but have read some statements the transmission in my 02 might not be up to the task when the torque converter is locking to increase the effect of the exhaust brake. Perhaps I should start a separate thread, but would be interesting in hearing from anyone who has added a true aftermarket exhaust brake.

 

Luckily for me, I have no such concerns - ZF-5!

 

a little late to the party here, and forgive the hijack....

how does one overheat a transmission going downhill? You said he was only using the gears to slow the coach going down hill, but how did that overheat the trans?

was he shifting gears on the way down?

i didn't realize that trans temps could go up on a decline (so i learned something), but is it possible to overheat the trans by just holding a gear on the way down? Assuming it will hold speed in 3rd (or 2nd or whatever), can you cook the trans just because you weren't also using the brakes?

trying to learn.
thanks.

 

Something has get hot since it takes energy to slow a downhill decent. Brakes get hot, transmission the same way. I watch my transmission temperature on the decent and it gradually goes up. The grade has to be steep and long though. It is not instantaneous. If the engine drops back in RPM and is just along for the ride, the fan doesn't come on.

Regarding the motor home. He just shifted down and let it do all the work. On that one, I only know that it happened and that the motor home was a very large Bounder. I know the grade and the owner of the coach, but was not with him so do not know more particulars. It is the only case I have ever known. Maybe there was some other problem.

The rise in temperature is easy to document, however, with my truck just by watching the gauge.

 

It IS possible, even likely.

When coasting the torque converter is almost always unlocked. When going downhill with the trans shifted to a lower gear to get engine RPM up, the torque converter will be unlocked and will have several hundred RPM slip. That's normal, and that's how a torque converter works.

When it slips, either power on or engine braking, the ATF is sheared inside the converter. This shear generates a lot of friction (yes, ATF can have friction) and friction causes heat. Rub you hands together, that's friction, and it makes your hands hot.

You can install a manual lock switch to keep the converter locked on a descent. This will greatly reduce the heat generated. One thing you must know, is that if you try to lock the converter when it has a lot of slip while in engine braking, the fluid flow in the converter is in the wrong direction to get it to lock. ALWAYS lock the converter BEFORE starting engine braking.

 

thanks for the explanation.

i was wondering if the heat build up was from converter slip or if there was some other mechanism at work. I also have always wondered why the converter doesn't stay locked when you are coasting down or downshifting to reduce speed.

two answers at once !
thanks

 

That's info I've never seen posted before.

Thanks, Mark.

Stewart

 

Mark, I do some towing through the mountains in TN and NC and there are a number of long downhill grades. I also plan on a trip out west in the next year or so and of course there are many possibilities there.

Is the manual lock switch you refer to a Ford item or aftermarket? If it is something that would help in these situations is it worth considering?

Thanks in advance, Tom