tow vehicle

As a Ford sales guy and owner of most everything they make at one time or other, I respectfully disagree with camperboy on on towing in overdrive. If the tranny remains in overdrive without shifting down and up repeatedly the only thing you gain by defeating the overdrive is a hole in your wallet paying for more fuel. The overdrive is made to use not by pass unless really necessary.

regarding OD... maybe each truck is different? i tow with a toyota tacoma 6 cyl ... the truck's manual states to turn off the OD when towing.

[Bill]

Regarding the manufacturer's instructions re towing in overdrive - jniles is right - and so is CCTurtles. And most likely, so is Camperboy. The instructions apparently vary with manufacturer, model, and year. The owner's manual for my Ford Explorer, for example, specifically states that it is OK to leave the overdrive on, unless the transmission starts to "hunt" - i.e., shift up and down a lot. I have confirmed this on the web - but even some Ford mechanics weren't aware of it until I pointed it out to them. It is apparently a new rule.

Since the day that automatic transmissions were invented, the general rule has been to "turn the overdrive off when towing", and many people cling to that rule like a religion. But despite the sexy name OVERDRIVE (which was invented long before automatic transmissions came on the scene, by the way), there is nothing magic about "overdrive". It is just another gear, just like 2nd gear or 3rd or 4th gear. And you don't want the transmission to be shifting a lot between ANY two gears. If you turn overdrive off, and get into some mountains, and your transmission starts shifting a lot between 2nd and 3rd, then you are harming your tranny just as much as you would if you were in flatter country and your tranny was hunting between 3rd and 4th gears. In this case, you have already turned off 4th gear (overdrive), and now you should "turn off" 3rd gear by moving the shift lever to 2nd.

So the best advice is simply to be aware of what your transmission is doing! If it is shifting a lot, then turn off the overdrive. If the terrain gets steeper, and it again starts shifting a lot, then use the shift lever to drop down another gear. The goal isn't to adhere to some vague and poorly-understood rule - it is to keep the transmission from hunting. And by the same token, if you come down out of the Rockies and head east through Colorado, then by all means turn the overdrive back on and save some gas.

Bill

[Denny_A]

I will echo Bill - re. to Overdrive/not to overdrive question.

Since purchasing my 2000 4Runner last September, I've towed 7000 miles with it. I have read all of the OD discussions on this forum. The polling seemed to tilt heavily to towing with OD OFF.

The D3, with OD on, is the 4th gear with a freewheeling mode, methinks. So, from day 1, hoping not to get caught by the OD-Off police, I decided to develop sensitivity to engine noise/tones which represent pending downshift to 3rd gear, and trip the OD off before the vehicle did so. It did not take long to learn and become close to 100% effective. My wife also is very good at this event, to the point of reaching across the "T" handle and toggling the OD button during my ocassional lapse of attention. Also helps to revive me from my power nap.

My Garmin iQue 3600 GPS is another neat performance instrument. When the vehicle velocity (cruise control on) decreases 2 mph in a brief interval, downshift is about to occur. Most useful as a diversion during miles 200-400 during a 500 mile day!

So what Bill said makes sense. My experience "seems" to confirm the wisdom of manipulating OD manually to avoid excessive transmission hunting in order to maximize economy and minimize wear.

Denny_A

Since the to OD or not topic is as lively as ever, I'll throw the rest of the thinking behind why I tow with OD off out there for ya'll to chew on.

Aparently the OD (at least on my trans) actually locks up the torque converter, essentially making a 6th "gear". There are 5 physical gears in the tranny, I can count them shift as the truck gets up to speed.

Now I thought that sounded great, no slipping. However, it seems that along with this comes less fluid flow which in turn can lead to heat build-up. Heat is bad. So since my truck is happier crusing along at 67, just under 3000 RPM (takes some getting used to that) that's what I do. Perhaps my next truck will be different, and perhaps someone else has different information, but i thought I'd share this one theory.

Towing in OD (for now),
Camperboy

[Bill]

Camperboy -

That's close, but I don't think it's quite correct. In an automatic transmission, the process of shifting from one gear to another is actually a sequence of three actions:
o unlock the torque converter
o shift the gear
o relock the torque converter.
Note that locking and unlocking the torque converter is analogous to depressing and releasing the clutch pedal in a manual-transmission car. You can think of a torque converter as an automatic clutch.

But here's the rub. A torque converter is like an automatic clutch, but a clutch that doesn't fully disengage. It transmits some of the engine's power to the wheels, but since the clutch is slipping, the rest of the engine's power is dissipated as HEAT! This is OK for a short time, or at low RPM and low load, but once you get out on the highway, the torque converter MUST lock up. No slippage = no heat. You must not ever drive along for an extended period with the torque converter unlocked.

It is my belief (but I can't prove it) that the rule about turning off the overdrive comes from the fact that the transmission computer is programmed to perform the unlock-shift-relock sequence properly for the car itself, providing good comfort and economy. But it doesn't know how to change its operation to accomodate a towed load that could be as heavy as the car itself. As a result, it is possible for the computer to do it wrong, and end up with the converter unlocked for long periods of time. (My Explorer will do this - it drives me crazy.) The manufacturers avoid this situation by simply telling you to lock out the top gear altogether, and run in a lower gear (higher RPM). Therefore, under moderate loads, the downshift cycle never starts. The manufacturer could make the computer smarter, of course, but that's expensive. BTW, in some auto magazines, you'll see ads for a service that will reprogram the computer to optimize it for towing - but then you are no longer optimized for non-towing operation.

If you are driving along in 5th gear, and start up a little grade, and you feel the little lurch and the RPMs go up a little, that is the torque converter unlocking. You should immediately feel another little lurch, which is the transmission down-shifting to 4th (the RPMs go up more), and then a third little lurch, which is the converter relocking in the new gear (RPMs go down a bit). Because the sequence happens quickly and is desiged to be smooth, it may be hard to feel the steps individually - but if you watch the tachometer closely, you can see each one quite clearly. In my Explorer, 65 mph in 5th gear with the converter locked is 2100 RPM. If I start up a grade, the converter unlocks, RPMs rise to 2300. When the shift to 4th occurs, RPMs rise again, to 3100. When the converter relocks in 4th gear, RPMs drop to 2800. All this at a steady speed of 65mph.

To summarize, the torque converter should lock up in every gear, once the vehicle has come up to speed in that gear and you have come off the accelerator a bit. (I wouldn't swear that it will lock up in the lower gears, but it will lock in the upper ones.) NEVER run with the converter unlocked.

The web has a lot of very understandable information about the workings of torque converters. See, for example,
http://www.technicalevolution.com/tcc.htm
http://auto.howstuffworks.com/torque-converter.htm

Bill

[RockyMtnRay]

I heartily endorse Bill's superb discussion of automatic transmission operation, particularly vis-a-vis the criticality of driving/operating the transmission such that the torque converter stays locked up...and add the following amplifying tips.

First, 90 to 98 percent of the heat generated by an automatic transmission is created by the torque converter when it is unlocked. Even with a decent cooler, an unlocked torque converter will easily raise the transmission temperature by 20 to 40 degrees.

Second, you don't have to warm up the transmission by very much to dramatically shorten its life. This chart of transmission life expectancy vs transmission temperature very clearly shows that raising the temperature just 20 degrees (from the optimum 175 to 195) will cut the transmission's life expectancy by 50,000 miles.

Third, the torque converter in some transmissions (the one in my Toyota Tundra being one) will not lock up in lower gears in an automatic fashion as the one in Bill's Explorer does. To get my truck's transmission (a 4 spd, or really 3 spd with overdrive) to lockup its torque converter in 3rd gear (direct drive) , I must turn off the overdrive. If overdrive is not turned off, the torque converter will only lockup when in overdrive. Each manufacturer's transmission(s) is different...check your owner's manual and observe its behavior to know if you can get torque converter lockup in any gear but overdrive if overdrive is enabled.

Finally, if you frequently tow over mountain roads, a transmission temperature gauge is a very good idea. I'm currently in the process of adding one to my truck because nearly every one of my camping trips involves crossing multiple high mountain passes with long/steep grades. Retrofitting a gauge is not, however, a particularly simple process. Nor is it inexpensive...I'm doing a pan-install of the sensor (better but much more costly) and estimate that by the time I'm done, the total cost of installation will be around $300 (not counting several hours of my own labor). Anyone who's interested in the issues that must be addressed and the how-tos in adding a transmission temperature gauge is invited to send me a private message. I doubt there are many who want/need to go through the process but if I do get a goodly number of requests, I'll start a thread on the subject in the Modifications Topic Area.

[fcatwo]

Ray

I think you'll get far more interest in this subject than you anticipate and I for one would like to hear more about your installation and Toyota transmissions in general.

We have about 14K trouble-free miles of towing our lightly loaded (no air, microwave, awning etc) 2619 with an 02 Odyssey (sans 2nd and 3rd row seats) but are admittedly still near GCWR with just the two of us aboard. We have the Honda auxillery coolers and a trans temp gauge with the sender installed between the torque converter and coolers. The ODY has a cast aluminum trans case (no pan) and a magnetized, inset plug so a pan installation is next to impossible. Other Odyclub.com participants have similar installations and the question perplexing us is whether a few minutes of high heat will destroy the oil. I've seen 260f for a few minutes in the Idaho mountains and often see 200-240 in rolling hills on hot days. 90% of the time we'll be at 180f or lower however. It's doubtful the trans sees these temps because the fluid goes to the radiator and aux cooler before getting back to the pan. Any opinions on this? The fluid (with one partial change at 15K) is still pink at 28K. Might mention that Honda just announced a recall of 600K of their 02-04 Ody/pilot/Mdx 5sp automatic transmissions to check for damage of 2nd gear due to oil starvation.

Back to your Tundra: We like the Ody because it will swallow our tandem bicycle, tow the TM with gusto and get 17.6mpg while doing so. I'm concerned about breaking it however so have been looking at RWD's that will do at least the first two. The Sequoia is one candidate and I'd be interested in your opinion of it. I think I'll wait for the 05's before jumping to see if they put the 5sp automatic in more of their V-8's. The 4dr Tundra would be good if it weren't so long.

frank

[RockyMtnRay]

[quote author=fcatwo link=board=20;threadid=1712;start=msg13038#msg1303 8 date=1081976659]
We have about 14K trouble-free miles of towing our lightly loaded (no air, microwave, awning etc) 2619 with an 02 Odyssey (sans 2nd and 3rd row seats) but are admittedly still near GCWR with just the two of us aboard. We have the Honda auxillery coolers and a trans temp gauge with the sender installed between the torque converter and coolers. The ODY has a cast aluminum trans case (no pan) and a magnetized, inset plug so a pan installation is next to impossible. Other Odyclub.com participants have similar installations and the question perplexing us is whether a few minutes of high heat will destroy the oil. I've seen 260f for a few minutes in the Idaho mountains and often see 200-240 in rolling hills on hot days. 90% of the time we'll be at 180f or lower however. It's doubtful the trans sees these temps because the fluid goes to the radiator and aux cooler before getting back to the pan. Any opinions on this? The fluid (with one partial change at 15K) is still pink at 28K.
[/quote]
No, a few minutes of high heat won't significantly affect the fluid...the fluid must remain at high temperatures for a sustained period (probably in the hours) before it noticeably deteriorates. Similarly a few minutes of hot fluid won't significantly damage the seals, clutch/band actuators, etc. But repeated...or sustained...exposures to high temperature fluid will cause seal hardening and other damage.

Also, the fluid temperatures you're seeing in the output line are probably anywhere from 20 to 60 degrees hotter than the pan/sump temperatures, the amount of difference being dependent on whether the torque converter is unlocked, and, if unlocked, how much slippage is occurring within the converter. However, as you note, the fluid stays at that temperature for only a few seconds before it's signifcantly cooled in the cooler(s). Finally the fluid that contacts the seals and actuators is probably close, temperature wise, to the fluid that's in the pan/sump. Because of the varying differences between the pan and output line temperatures, I've personally got real doubt as to how much meaning anyone can derive from an output line temperature reading. But if, as is the case with your Ody, you have no other temperature measurement location, then you have to get what you can get.

Back to your Tundra: We like the Ody because it will swallow our tandem bicycle, tow the TM with gusto and get 17.6mpg while doing so. I'm concerned about breaking it however so have been looking at RWD's that will do at least the first two. The Sequoia is one candidate and I'd be interested in your opinion of it. I think I'll wait for the 05's before jumping to see if they put the 5sp automatic in more of their V-8's. The 4dr Tundra would be good if it weren't so long.

I believe there's another member here who is towing with a Sequoia, don't remember who though. Yes, the Tundra Double Cab (full 4 dr version) is quite long (too long to fit in my garage)...physically it's basically a Sequioa with a cargo box. I'm not sure that 5 spd automatic is better for towing than the current one...but what may be worth waiting for is the possiblility that Toyota will be putting a 5.6 L (or thereabouts) V8 in the Tundra and Sequoia in either the 2005 or 2006 model years (to compete with the Titan). The current 4.7L V8 is a bit undersized for a full sized truck when used for towing at higher altitudes...though probably fine at near sea level.

Now I need to do some further investigating to see exactly when the converter does and does not lock up. I really enjoy these technical threads. The things being discussed here are relavent to almost any auto trans.

Ray, I'd love to hear more about your temp sensor install. How you mounted the sensor in the pan. Where inside your cab did you mount the gauge? I had been looking at a pillar gauge pod with space for three gauges.

Camperboy