tchdodg

Okay guys, I have a question. It concerns the transmission on my truck. It's the truck in sig, I need to update it. I just put on 285/75/16E's, so that may have some effect on your answers. I've read all the stories about towing/driving with OD on or off. Last night I took a little road trip and did a test. With OD off, at 65 I'm running about 2500 rpms. If I turn OD on at this point, the rpms drop to around 1800. Is this too much of a drop? Also, when I've got OD off and running about 65, the truck feels like it is "hitting a wall". Rpms climb faster than truck seems to accelerate. We are going to buy a travel trailer this spring, probably be around 8k loaded. I've heard these trucks love rpms, but am I going to hurt something running 70 with OD off? My current philosophy was if I can maintain 1800 rpms, I'd shift to OD. Now that I've added the larger tires, I have to run about 75 to do this and I don't think I'm going to pull a TT that fast. Anyway, enough rambling, you all please educate me. I'm sure you will and thanks in advance.

CarlJensen

These trucks will start de fueling at about 2200rpm or so. That's why many install a 3 or 4k gov. spring kit. Lets it rev past the old governor speed. I pull 12k often and I use OD if the engine does not strain. In hills the OD goes off. It can save you a chunk of fuel on level land when traveling. On mine, the sweet spot for fuel mileage is at 1800 to 1900rpm. I have the same gears that you do. I try to keep rpm at 1500 or better to ensure max plate pressure in the tranny and for economy.

tchdodg

Showing my ignorance here, but what is the "spring kit", where is it, and how hard is it to replace. I'll search it also.

CoastalDav

Your torque converter will not lock up with the OD off. Hence the wall in RPM's when the OD is off. The new trannys (48RFE) locks up in both 3rd and
4th in in tow haul mode. I can also turn my OD off. When I do, I get a big jump in tranny temp because of the slippage that is occuring not locked up.

I would use the OD. Tranny will run cooler with less wear.

Cummins recommends you maintain at least 2100 rpms when towing at 65mph.

http://www.powerspec.cummins.com/sit...ng/gearing.htm

Use the ISB tab. You would need a little bigger rear ratio to make that happen.

Otherwise I think your fine. You may consider putting a double deep pan on the tranny and add a tranny temp guage. With the tires you have my calculator shows 68mph at 1700rpm's in OD.

Dave

Russ Roth

He has a 24 valve and they will wind way past that stock. Mine is redlined about 3200. If it does not lock in third with the OD off (mine does) you could install a lockup switch and manually lock it if needed. I tow some and run OD all the time.

grantx5

Have to agree with Russ. I tow a lot with OD though not over passes, etc.

JohnX

One more vote to tow in OD. I never let it go into third until I get below 1600rpm...of course mines a 12valve, little different. I also have the 4.10 gears which help, but the 34 inch tires make rpm similar.

Russ Roth

Just use common sense and you will be fine. The shift logic on mine is very good just letting it do it's own thing although I overrule it on occasion.

tchdodg

Thanks guys, right now I just listen to the truck. If it sounds like it's straining in OD, I hit the button and put it in 3rd. I may look up that lock up switch, that might be a nice addition. Thanks again.

RowJ

Yup!
Lockup switch allows you to choose your gear with no concern for tranny temps, due to converter slippage!
It's a VERY simple DIY install and well worth doing! Can find it HERE ..... under "Installation Instructions".

Good Luck!

RJ

tchdodg

Did some searching on how the switch is used. Let me make sure I understand this right. My only concern is towing in OD to keep the rpms down. So, if I install this switch, leave it turned off until the truck shifts into OD, then turn it on, I can help my truck tow in OD better? Just have to remember to shut it off when I start slowing down or want to shift from OD to 3rd. Am I thinking right? Sorry for the dumb questions, but all my previous tow vehicles were standards. Thanks again for all the help. Looks like a worthwhile mod.

RowJ

Yup, If your tranny is factory, I would switch off the lock up when I slow down to 30 mph.
best to not downshift with switch on in a stock tranny.
if your towing and going to lock out od.... I would unlock converter before doing so. Not essential, but it is easier on the trans.
Going from 3rd to OD with it locked should not hurt anything, sinc eit is going to lock up immediately anyway. JMO.

RJ

GSP

"Your torque converter will not lock up with the OD off" Huh? The TC should lock up OD off.

xyzer

The only time IMO you need to run with the OD off is if it starts to hunt in and out....that is were the heat comes from locking in and out.... I tow heavy in OD untill I get to the hills then if I feel it drop out of OD I'll shut it off until the hills are gone....works for me! I think the manual tells you the same thing.

lifer

I have the mystery switch in my truck and there are times (allthough not very often) that I unlock my tc clutch and still keep it in OD. This gives me what acts like another gear because it allows the engine to rev 2-300 rpms more than when locked up and 2-300 rpm lower than taking it out of OD. I do not do this for a long time because of the heat build up of the tc slipping but it is handy to have this option. In the 93 and older OD there is no lock up clutch so as long as you keep trans fluid temps in check you are good to go
I do tow in OD 95% of the time with the tc locked.