transmission overdrive

[islandboat]

This is not a question about whether or not to tow a 6000lb boat in overdrive. This is a question about controlling the lockup convertor in a 96' Tahoe. I tow about 65 mph and the engine rpm in "drive" is only about 1850. That is too low in my opinion. If I drop the transmission shifter into "3rd", the engine rpm is over 2700 rpm at 65mph. If I leave the shifter in "drive', and hit a small hill, the rpm will hold at about 2300 rpm for a while and then shift back into overdrive and the engine rpm is then too low. I can't let it keep shifting in and out of overdrive but the rpm is good when it is in 4th gear and out of overdrive.<br /> Question; Has anyone heard of a transmission control device that can disengage or control overdrive and still let it shift into 4th? I have no button on the shifter to control overdrive like some Ford trucks. I do not want to raise all of my shift points (at full throtle) like some of the high performance chips will do.

 

[BUBBLES II]

Re: transmission overdrive

Don't understand. Why do you think 1850 is too low at 65? What rpm does truck do without boat?

 

[craze1cars]

Re: transmission overdrive

I know exactly what you're talking about, it's extremely irritating to not be able to prevent the torque converter from locking up on Chevy's.<br /><br />It is very likely you can find what you're looking for from companies like B & M that build performance transmissions. Contact Summit.com or Jegs.com for instance, they carry B & M products and some of their competitors. Most likely its a simple chip replacement or computer reprogram. Jegs and Summit both have excellent technical advice hotlines that will help you find what you're looking for.<br /><br />A good performance transmission rebuilder (not just your normal Aamco and local tranny rebuild shops) can likely help you too in your local area if you live in a metropolitan area. Make some phone calls to transmission shops and ask your question of them until you find one who understands what you're trying to do.

 

[tommays]

Re: transmission overdrive

i had a 1995 GM and in the manual you were not supposed to TOW in Drive only in 3rd<br /><br /><br />tommays

 

[CATransplant]

Re: transmission overdrive

2700 RPM in your "3" position is just fine. I don't see a problem with that. I wouldn't let the thing keep locking and unlocking the torque converter, frankly, on hills. I'd just use the "3" position and shift back to "D" on level ground.<br /><br />My GMC Jimmy has a "tow/haul" button. It doesn't lock out the torque converter lockup, but it does raise the shift points. It seems to work really well.<br /><br />The torque converter lockup is controlled by a solenoid. A competent mechanic should be able to add a switch to the wire leading to the lockup solenoid so you can switch the lockup out when trailering.<br /><br />If it were me, I'd visit a the dealership service department, explain what you want to do. I'll bet they can add in the switch for you pretty quickly.

 

[Silvertip]

Re: transmission overdrive

You are confusing Overdrive and Drive. Overdrive is 4th gear. The torque converter is not a gear. It's a clutch device that eliminates the slippage in the torque converter. Remember, the torque converter must have some slippage or you would not be able to stop at a stop sign without killing the engine as there is no other clutch. The "3" position is Drive range or direct (not overdriven). At 65 MPH and 1850 RPM you are in Overdrive (4th gear) and the torque converter is locked and this is simply not acceptable for towing that size load. You are forcing the tranny to shift repeatedly (not only from OD to 3rd, but the torque converter from locked to unlocked). The torque converter locks and unlocks depending on engine rpm, engine vacuum, and road speed, all of which is determined by the ECU. People frequently mistake the torque converter locking and unlocking as an actual shift because they see the tach increase as it unlocks and drop when it locks. To make this simple, you need to tow in "3" with that load. Overdrive does not come into play at all in that gear. Only the torque converter lockup/unlock. If you add a device to allow constant unlocking of the converter, you would overheat the fluid, cause it to foam, and destroy the tranny. Just tow in "3" and let the torque converter do its work. 2750 RPM at 65 MPH is "loafing" for that engine and its at a better part of the torque curve.

 

[QC]

Re: transmission overdrive

I agree with Upinsmoke on this. IMHO, the tranny is best when locked up. On my trucks I always manage the load/throttle so that they don't unlock. Any reason this is bad UIS?<br /><br />The only exception I would make is I would use OD for some slight downhill operation or other light load conditions. If you don't want to think about it, keep it in 3rd.

 

[islandboat]

Re: transmission overdrive

Upinsmoke, you are correct. I did misuse the terms overdrive and convertor lockup. I should know better than that. And I know better than to tow in overdrive/lockup with the transmission shifting in and out of overdrive. It looks like I will do what CAT says and just tow in "3rd". 2700 - 2800 rpm, sounds like the transmission is just asking for on more upshift. Too bad I can't get the rpm down to 2300-2400 rpm, saves a lot of fuel. 3rd gear lockup would be nice, but I can't find a company that makes a device to program that mode into the transmission. I suppose there is a reason no one makes a devise to do that. All of our Allison's pull full loads well in overdrive - lockup, but this little 700r4 isn't that strong.

 

[craze1cars]

Re: transmission overdrive

As I stated in my previous reply, several companies make manual converter lock-up controls for 700R4's. You can use them to lock the converter in any gear you choose with the push of a button, as long as you're over 30mph. So these kits will allow you to do exactly what you're wanting to do...lock the converter in 3rd gear for towing at a good RPM with no converter slippage creating heat, and prevent it from hunting in & out of lockup. Here's the B & M kit for $159.88:<br /> http://store.summitracing.com/defau...earchinresults=false&N=0&target=egnsearch.asp <br />If your setup has an electronic speedo cable you'll need their alternate part #BMM-70248. I believe TCI also makes a similar product. There may be others, you just gotta search a bit and ask some questions of the performance builders and vendors.

 

[Silvertip]

Re: transmission overdrive

Trophy, the converter does lock up in third gear. It will release going up inclines, especially with the load you are towing. I think you are trying desperately to make a problem where there isn't one. If you put a device such as Craze1cars suggests (they are available) you run the risk of using even more fuel as the converter will not unlock resulting in lugging the engine. Again, I'll suggest towing in third and let the tranny do the work its designed to do.

 

[Mark42]

Re: transmission overdrive

Just one quick note on locking converters: An unlocked locking converter will make more heat than the old nonlocking converters. This is because todays locking converters have more built in slip to help get vehicles going with the taller fuel oriented gears, but give good efficiency when locked. <br /><br />Just remember that whatever mods you make to your trans, be sure the converter is locked in its "normal" operated mode. Otherwise it is generating a lot of heat that has to be disipated under the hood of your vehicle. Not good for the trans, not good for anything under the hood. Also not good for MPG.

 

[Silvertip]

Re: transmission overdrive

Amen Mark!-- he was warned about that earlier.

 

[craze1cars]

Re: transmission overdrive

I agree these are all very valid points. If such a device is installed on your trans, you gotta understand how to use it properly or you'll do more harm than good. The objective of such a device is to allow you to leave the converter locked more often at highway speeds...not unlocking too often and generating heat. When properly used under the right conditions, they can help your tranny run cooler and more efficiently. BUT, you gotta know when the trans needs to be locked and/or unlocked, and hit the right button at the right time...you're effectively taking some of the "automatic" out of your automatic transmission.

 

[TilliamWe]

Re: transmission overdrive

Listen to Mark & Smoke!! <br />A GM transmission running in Overdive with the torque converter unlocked is a burnt up trans waiting to happen. It WILL burn itself up. You CAN NOT tow in Overdrive with the converter unlocked, period!<br />You can run in Drive (3rd gear) with it unlocked, but locked would be better.<br />If your truck turns 2700 rpm at 65mph in Drive with the converter locked, that's what you have to live with. If that's too fast of an engine speed, slow down. Cause you have already stated it won't tow in Overdrive without unlocking the converter & downshift. That will kill your tranny. <br />craze, I think I understand how to operate a GM four speed auto (ever see a guy manually shift into drive and rest his foot on the brake to unlock the converter just to help a Lumina sedan go aroung a curve under more power? ) better than 95% of the world, but I would never want to have to control the converter lockup all the time. Way too busy!!!!

 

[islandboat]

Re: transmission overdrive

O.K. I have a better idea of what I need to do from everyone's comments. Thanks. I will borrow a diagnostic tool for my truck and hook up the boat. The diagnostic tool will tell me whether the convertor is applied or not while running down the interstate. Seems like a little extra trouble, but it will give me the info that I need before making any modifications or spending money that may not help the situation. If the convertor is applied in 3rd and I can run about 65 mph, problem over and I will live with the higher rpm (2700) on the highway. If the convertor is not applied, I will order the lockup kit. Heat was a major concern, (already have a factory aux cooler, the heat index today was 96) and if lockup is applied in 3rd, no sense in making any changes. At least now I understand that towing in overdrive with lockup not applied is the wrong thing to do, even though the rpm "seemed" correct.

 

[QC]

Re: transmission overdrive

Trophy,<br /><br />That truck will run all day in 3rd in lock-up. I am not sure where you are, but it sounds a lot like AZ, CA & NV. Anyway, I have pulled all three of those states in up to 117 ambient with no issues. You are heavier than me (around 4800 lbs), so disregard my OD comments.<br /><br />I use OD to run in flat no wind conditions and downhill. I put the cruise on and listen. When it starts to work at all, I hit the tow button and it drops into third or it unlocks, if it unlocks I use the shifter to force into third and listen for it to lock.<br /><br />Forget what you know about RPM. High torque-rise engines allow you to lug down to 1500 or less with a fairly decent load on them. My attitude is if you can keep it locked you are not hurting it. Let the electronics unlock it and then force the shift with the shifter if you have to. No biggee.

 

[TilliamWe]

Re: transmission overdrive

trophy, you don't need a tool to tell you that the converter is locked up. Put the shifter in drive (3rd gear) accelerate up to a normal, safe cruising speed, 55 to 65mph. Then, with your right foot still applying light pressure to the gas pedal, use you left foot to apply light pressure to the brake pedal (usually just enough to turn on the brake lights). The converter should noticably unlock. Release the brake pedal, and it should lock up. You should be able to hear & feel it. Good luck

 

[islandboat]

Re: transmission overdrive

Ya'll were right. I was about to make a problem where there was none. I will tow in 3rd with the convertor lockup clutch applied. And the convertor does lock up in 3rd and overdrive. Tried it today and it works just like I was told it would. Problem solved and thanks for everybodies help.