Towing in OD

Next week, I'll be helping my friend move from Denver, CO to Lexington, CO. I'll be driving her family's truck towing a 6x12 Uhaul enclosed trailer, maybe 2000-3000 pounds total. The truck is a '98 Dodge Ram 4wd, half-ton, with the 5.9 automatic. The truck is totally stock and the trans has not been rebuilt. I believe it has ~150k miles. Obviously, I'd like to be able to tow in OD because of the gas mileage concerns. However, I don't want to grenade the trans in the middle of nowhere.

I've spent a lot of time driving one of these as my dad has a practically identical truck. I've actually towed with it before, in OD, and have had no problems. I was towing a car trailer, total weight about 3000 pounds. The truck would downshift but only on big hills and generally pulled itself fairly well. However, it has had the transmission rebuilt and a ginormous external cooler installed.

So, if I leave it in OD and it pulls itself well without downshifting, should I be okay? I know the constant downshifting builds heat, but is there any other strain put on the transmission just by towing the larger load? Anything I'm not thinking of?

Some of it depends on the manufacturer recommendations and speed and terrain. On my Land Rover, there is no mention of not towing in OD, so I use common sense. On the flats, I let it roll out in OD, but if I am coming up on a hill, I will drop it into 3rd and let tue torque converter lock up as I approach the hill to keep the heat down or the stress in OD

The 4th gear section in those 518 trannies is usually the part that fails and causes it to need a rebuild.

Remember, its not about how its accelerating, its about the load on the transmission. I can tow anything in any gear with complete confidence as long as i know im not putting any real power through it.

My recommendation is leave it in 4th when it doesnt take much load to keep speed or accelerate very gently. If you get to the bottom of a hill that you KNOW will end up downshifting to third anyway, downshift it yourself at the bottom and keep it in third until you know you wont need much throttle once you get back into 4th.

If you drive it like that, you have nothing to worry about.

Even if the owners manual should say not to tow in overdrive (like mine does) that does not mean it's to prevent the transmission from overheating. Tucked away elsewhere in my owners manual it explains the no overdrive when towing is to maximize engine braking.

Don't know yours, but you can likely find a lot of good information on the boards for that truck. Look for messages that mention temperature gauges, so the results are known, not guessed at.

Before you tow, watch the trans temp gauge to see where it stays after an hour of driving at speed. When you start towing, watch and if it goes higher than the unload temperature drop it out of overdrive. If its shifting more than normal, like its "hunting for a gear", drop it out of overdrive. Thats how I operate.

I’ve towed a car trailer with a 1998 f-150 with a V6. When towing in overdrive I noticed I got 2-3 MPG worse then when I was towing in just drive. I think I just had to use more throttle with the V6 to maintain speed when in overdrive.

OD was the first thing to fail on my two Dodge trucks. I did very little towing with them, usually less than 1000 lbs.

I towed 3900 lbs in OD with my Liberty. No problems and the temperature light never came on, even in the hills of PA on the way to Pocono. Of course it has Amsoil atf.

It's not just the weight of the trailer, its the wind drag. How tall is it? The extra drag is like adding alot of weight, the faster you go, the worse it gets.

i killed one trans towing in OD - 2k empty car hauler on flat land. thought i was ok since it wasnt hunting, overheated it so bad the case burned my fingers. had a big trans cooler on it too.

so now if i am in an auto with OD i leave in 3rd. dont care how flat the road is.

Overdrive builds more heat in the trans because you are running the power through a gear set. In 3rd, all you have done is locked the input shaft to the output shaft. One big lump of rotating mass, instead of the output shaft rotating faster than the input.

Having said that, my Dakota tows a flat deck trailer with my Neon roadracer in OD until I get to the rolling hills, then I'm on the OD off button like an old gambler at a slot machine. Uphill direct, downhill OD. My engine temp goes up in direct, even at 100k, but with 3.91 gears, its running over 3k rpm.

In my truck, you will build more heat in third then letting it use OD. That's because you've also unlocked the torque converter. Very little heat is generated by engaging the already spinning planetary gears of the overdrive at the back of the transmission. Gas mileage in my truck (Toyota T100 3.4 V6) with OD locked out reduces by about 10 mpg, maybe more. Seriously.

Spitfire on a landscaping trailer, with sundry extra parts, tows through the mountains of New England and PA without any trouble, though I might be floored holding 70 mph up the mountains. At no time did the overheat light come on, and the fluid never scorched.

Dropped out of overdrive and the truck is screaming holding 60 mph. Throttle nearly wide open to keep up with the engine rpms. You can watch the gas gauge moving as you drive. A roughly 90 mile test drive with just the trailer took almost an entire tank of gas.

I used to tow a 4000 lb boat and trailer combo with my 98 Explorer. I always towed in OD. The truck had a tranny cooler from the factory. Not just the loop in the rad but an actual seperate cooler.

Much depends on the vehicle and load. My father tows a travel trailer with his V8 Tahoe and stays out of overdrive just because his mileage is better. The engine is at higher revs but it isn't working as hard.

I tow everywhere in O/D with my '03 diesel Dodge Ram 2500, dropping out just on hills or when I want to pass. But this truck is built and equipped for towing heavier loads. The tranny is known to be the weak point in these trucks, but I've got 120k on this one with no problems, as yet.

I think im hearing a lot of self-diagnosis here from people who havent rebuilt their own transmissions and dont explicitly know what's going on in them.

The point about engine braking is entirely secondary to why the manufacturer doesnt want you to tow in 4th. A manufacturer is not going to sell you a car for tens of thousands of dollars with a book inside telling you which thousands of dollars worth of parts are going to fail. They are going to gloss that over and give you a layman's excuse for why not to do it.

Anwyay, first thing is that not all transmissions are the same. Not all transmissions have the same power in front of them, the same weight behind them, and the same driver mixing it all up for MAXIMUM FAIL. Comparing towing across platforms is always apples to oranges. An oversimplified rule of thumb like 'Dont tow in 4th' is about as specific as youd ever want to get trying to give advice for every platform and situation, and even then it is oversimplified generalization.. you know, the kind of stuff you'd find in an owners manual

Now, a few analogies are in order.. As far as transmission temperature, consider what causes heat in the transmission. The pump creates some heat. The converter creates some heat. the slippage between clutch elements when they are ENGAGING creates some heat. The friction between metal surfaces such as bushings on shafts, thrust bearings, gear surfaces like in the planetary gearsets, all those create some heat.

But lets parse that down and see how much of that is reactive to LOAD.
Does the pump create more heat when there is more load coming in the input shaft? No. The converter isnt even driven by the input shaft, its driven by the converter case, which spins at engine rpm. Unless rpm changes, the pump doesnt change. The amount of resistance to flow out of the pump changes somewhat depending on which paths are open, but the more paths are open, the less resistance there is! The pump is not a major contributor to heat.

How about heat from friction between bushings, bearings, and gear teeth? This can definitely go up or down depending on which elements are spinning or held. But is it load dependent? Not really.. there is effectively no thrust load in the longitudinal axis of the transmission. Sure, we've got some side loading on the gears that can change slightly with load, but its also very minor. MUCH less force there than in a manual transmission which has MORE gears spinning ALL THE TIME in a fraction of the heat sink (fluid) which only gets splashed over the gears and doesnt run through any sort of cooler AT ALL and we NEVER worry about overheating our manuals. So, heat from bushings, bearings, and gears contributes to heat generally but its not really load dependent.

How about the converter? Oh, the converter definitely generates a lot of heat. Probably most of it. And, it IS load dependant how much heat it generates. But how does it generate heat? Not much from the thrust bearings. Most of it comes from fluid shear. The amount of fluid shear going on is connected to the difference between input and output speeds. So, when you are at a stop and you floor it, the input goes up to the stall speed, probably between 2 and 3k rpm, and the input is at 0 rpm. This is when there is the most fluid shear going on and the most heat being generated. Now, say you are rolling at low speeds in 2nd or 3rd gear and accelerating gently like you normally would in urban traffic. The input speed is somewhere between 1000 rpm and the stall speed of 2 or 3k. The output shaft speed is probably somewhere between 1000 and 2000 rpm. There is fluid shear in this situation, but less than you would find leaving a stop. Now lets get up to highway speed. Generally speaking, when you are moving down the highway, the output speed of the converter is nearly the same as the input speed. In fact, when the converter clutch is activated, they ARE the same. If you unlock the converter in 4th, you will find the stall speed is only a few hundred rpm over the output speed (where it was when the converter was locked). Therefore, with only 2-400 rpm of difference, heat from fluid shear from an unlocked converter in 4th is not that great in the short term. If you keep the converter in a shear situation for long periods, like gently accelerating in 4th with the converter unlocked while towing from 50-80mph, that probably ends up being a substantial amount of heat. Now, if you drop it into 3rd, in some cars lockup will be disabled. In some, it wont be disabled. But even if it is, the output speed is usually higher than the input speed, or very close to the stall speed. You will rarely ever operate with more than a couple hundred rpm difference between input and output speeds on the converter when in 3rd at highway speeds, meaning the converter is not making a lot of heat.

What about the clutches themselves? Do they generate heat? Yes, they do, but only under one circumstance: When they are slipping. They should slip for only a short period while they are engaging, and then should lock up and generate no more heat. But, if you force them to slip with sheer torque, they will generate a LOT of heat. This isnt common and its usually not possible to slip the clutch with sheer power unless other things are already wrong, such as them being overheated to begin with by fluid temps. The big thing you need to know here is that these clutches are just like the clutch in a manual car in that they generate a lot more heat when shifted under power. In a manual car, you let off the gas to shift. Even in the strange, rare instances where you DONT let off the gas to shift, you have the good sense to let out the clutch faster so it has less time to slip. But, an auto tranny doesnt shift any faster just because you're putting power through it. At least, most dont. So, what im saying here is that the best/worst way to generate heat with the clutch packs is to shift them under power, such as letting the trans downshift itself out of 4th gear when you've loaded it up like crazy and its still slowing down. Do this repeatedly (i.e. gear hunting) and you are really making some heat in that clutch pack. Once it gets too hot, it will start to slip, and once it starts to slip, it usually doesnt stop. Its a self-fulfilling cycle of meltdown.

So what do we got so far: 1. The only heat you need to worry about comes from running the converter with a lot of speed differential across it. This means more load in a higher gear makes more heat than less load in a lower gear. 2. The clutch packs are most susceptible to being shifted under power repeatedly. The easy solution to this is to give it only moderate throttle while shifting it to 3rd manually.

Another thing about Trans Temp. What a trans temp gauge can show you is NOT the temperature of the components most susceptible to heat. It will only show you the temperature of the fluid, which is not much more than an averaged out temperature of the entire unit. Not only is it averaged out, its also affected by your engine coolant temps, unless you bypass the stock cooler. If you understand how fluid flows through a clutch pack in a trans, you know it doesnt flow very fast at all. A little bit is sprayed and splashed around and eventually finds its way out and back into the pan via centrifugal force and gravity. It does not fill the clutch pack top to bottom and then flow directly back to the temperature sender to give you a good reading. It also doesnt cool the clutch pack like your engine cylinder is cooled by a water jacket. It just kind of splashes or sprays onto it, finds its way through it, and then drains off. Not like total submersion. So, when the clutch starts to slip, it doesnt send that heat into the fluid nearly as quickly as it generates it. So, the fluid that passes across it heats up, but not as much as the clutches do, and it also doesnt cool them down that fast. Then, that fluid gets mixed in with all the other fluid and cooled down before it gets to the temp sender.

So, a trans temp gauge can only tell you a few things: 1. Your engine coolant is getting hot. 2. Your trans cooler is clogging up. 3. You may be low on fluid. 4. You're using the torque converter the wrong way. It really is not going to tell you that your clutches are slipping until its too late. Probably the most important thing it can tell you is 'what's normal', so that when you see a temp that's NOT normal, you can be cautious.

Well E36 M3 that was too much typing. Dont operate your converter with large speed differential for too long. Dont shift under power repeatedly (I.e. shift it into 3rd YOURSELF, under light throttle). Think of the trans gauge as an idiot light. And if you feel it slipping, BACK OUT, or you're fugged.

Vigo wrote: . MUCH less force there than in a manual transmission which has MORE gears spinning ALL THE TIME in a fraction of the heat sink (fluid) which only gets splashed over the gears and doesnt run through any sort of cooler AT ALL and we NEVER worry about overheating our manuals.

You'd worry about it if you'd ever paid to replace 5th in the Getrag trans behind your Dodge trucks Cummins engine.

Vigo wrote: I think im hearing a lot of self-diagnosis here from people who havent rebuilt their own transmissions and dont explicitly know what's going on in them.

I think I can summarize this... no particular gear is going to cause more heat/wear/problems. What does cause the issues is repeated shifting between 3rd and 4th.

My advice: Use 3rd (Drive) anyplace but a flat, level freeway. Once you're up to a solid cruise speed on the highway, feel free to shift into overdrive.

It was beat into my head that continual up/down shifting is what causes transmissions to overheat when towing.

To make shifts less harsh, the programming is designed to have a rather slow engagement of a given clutch pack, this is done through pulse width modulation. While the clutch pack is being applied, it of course slips. So while the transmission is shifting and under a heavy load, the heat buildup can be very rapid. If the torque converter clutch engages and disengages as well, the heat buildup during that same type of slip period gets added to the other heat and soon you get fried tranny.

Add to that the fact that there is a constant small amount of slip in any clutch pack (usually 1-4%) under load and it's easy to see how a transmission can overheat quickly.

With the Trooper, I leave it in O/D and I DO NOT use cruise control. Why? CC makes it 'hunt' for gears all the time. In most rolling hills, I'll leave it in O/D since the torque converter locks in both 3rd and 4th. Also, if you have to tap the brakes the TCC will disengage as soon as the brake lights come on, if this happens under load of course the TCC will slip on reengagement.

When on steeper grades, I leave it in 3rd (since that is where it will go anyway) and try not to let it downshift to 2nd, since in 1st and 2nd there is no TCC engagement.

On really steep grades (such as some of these skinny little roads around the hillclimbs), I stick that bad boy in 4 Low and thus save the tranny from getting roasted by continual 1st and 2nd gear torque converter 'slip'. And of course I run an oversized tranny cooler, the fluid gets changed ~25K intervals (come to think of it, it's due again).

But back to the OP's question: I have seen a lot of those 504 Mopar OD's get cooked at high mileage. The question: what's cheaper, crappy gas mileage or a very long tow and a tranny rebuild? Oh, BTW: the 504 tailshaft and O/D unit are available as a reman unit, you pop the tailshaft off and stick a new one on in its place.

I tow my 7000lbs enclosed trailer using 99 2500 V10 dodge with a 4 speed auto.

I tow in OD when ever possible. In fact I can tow in OD at 1600 with that giant gas motor. However I always take care to enure the torque converter is locked-up. If the tranny starts hunting at all I will drop to 3rd manually. I try to never run without the converter locked-up for extented periods. I have to run that way at times when climbing hills, but try to find a road speed to minimize that.

joepaluch wrote: I tow my 7000lbs enclosed trailer using 99 2500 V10 dodge with a 4 speed auto. I tow in OD when ever possible. In fact I can tow in OD at 1600 with that giant gas motor. However I always take care to enure the torque converter is locked-up. If the tranny starts hunting at all I will drop to 3rd manually. I try to never run without the converter locked-up for extented periods. I have to run that way at times when climbing hills, but try to find a road speed to minimize that.

That sounds about the same way I tow with my 6.0/4L80E. Not the 1600 rpm part, but the rest of it...6.0's are revvers, not torquers.