2 quick questions: Overdrive and stall converters

[Mac-MerC]

i read a lot of post and 2 things always seem to pop up that i have always wanted to question and since i know someone is gonna say something smart i might as well go ahead and ask

1. whats the deal with the O/D?.... (overdrive) why should it b cut off?..
im guessing since my merc is stock it doesnt matter if its on or not.. but if i get a tune i should make sure that its always off?...

2. these stall convertors...... can someone tell me what they are and what they do?....


i would appreciate it

if u have nothing nice to say dont say anything at all :D:D

[SC Cheesehead]

Actually, not bad questions.

You can use your O/D with or without a performance tune. However, if you're drag racing, you'll want to cut the O/D off.

[Pat]

Mac-Merc; There's nothing wrong with questions, some of the answers may require you to do a bit of reading.

1. Overdrive is like a forth gear. It's the only "gear" that is operated with a band and as such is somewhat weaker than the other three gears.

Turning off overdrive is just a matter of pressing a little button on the gear shift lever near the top left side, a lite will come on the instrument panel "O/D off". Turning it off makes the car's third gear the top gear. I usually turn mine off when racing, keeps the engine in the power band at higher speeds.

Try it at low speed, 30 MPH or so and notice the engine rev's a bit higher.

Conventional wisdom says that it's unwise to turn the O/D on or off at high speeds, wait till you stop or driving less than 30 MPH. That's my understanding.

2. For a tutorial on stall converters go to Precision Industries web site for a good explanation, or goggle "automotive torque converters".

The converter acts like a clutch in a manual transmission equipped car.

Stock MM' have a higher stall speed converter than usual. 03's around 2300 RPM, while 04's stall speed is about 2700 RPM.

The higher the stall speed the quicker the launch generally. But this stall speed must be matched to your engines torque curve.

It's an expensive modification and is usually done with other modifications.

Stall speed is not an adjustable parameter but manufactured into each converter.

My stall converter is made to 3500 RPM specifications because that's the RPM where 85% of my engines torque curve occurs.

I know this is a bit much but I just want to let you know that these are complex cars and require a lot of research to gain understanding. The Internet is your friend in the technical matters.

My first question was kind of silly, I read a lot but was confused as to what is a "tuner", a person or a machine. It's both. LOL, I was a little red faced.

Regards,

Pat

[Blackened300a]

The O/D band is the weak point of the tranny. Its just cheap insurance to shut it off when getting on the car. I also use it for slowing down when Im coming to a light.
The convertor is what puts the power from the engine to the transmission and then to the wheels. The stall determines at what RPM this occurs at. The higher the stall, the more into the power band your power will be put to the wheels.

Sorry for keeping it so basic, its early and Im half asleep over here :sleepy:

[Blackened300a]

Mac-Merc; There's nothing wrong with questions, some of the answers may require you to do a bit of reading.

1. Overdrive is like a forth gear. It's the only "gear" that is operated with a band and as such is somewhat weaker than the other three gears.

Turning off overdrive is just a matter of pressing a little button on the gear shift lever near the top left side, a lite will come on the instrument panel "O/D off". Turning it off makes the car's third gear the top gear. I usually turn mine off when racing, keeps the engine in the power band at higher speeds.

Try it at low speed, 30 MPH or so and notice the engine rev's a bit higher.

Conventional wisdom says that it's unwise to turn the O/D on or off at high speeds, wait till you stop or driving less than 30 MPH. That's my understanding.

2. For a tutorial on stall converters go to Precision Industries web site for a good explanation, or goggle "automotive torque converters".

The converter acts like a clutch in a manual transmission equipped car.

Stock MM' have a higher stall speed converter than usual. 03's around 2300 RPM, while 04's stall speed is about 2700 RPM.

The higher the stall speed the quicker the launch generally. But this stall speed must be matched to your engines torque curve.

It's an expensive modification and is usually done with other modifications.

Stall speed is not an adjustable parameter but manufactured into each converter.

My stall converter is made to 3500 RPM specifications because that's the RPM where 85% of my engines torque curve occurs.

I know this is a bit much but I just want to let you know that these are complex cars and require a lot of research to gain understanding. The Internet is your friend in the technical matters.

My first question was kind of silly, I read a lot but was confused as to what is a "tuner", a person or a machine. It's both. LOL, I was a little red faced.

Regards,

Pat

^^^^What Pat said^^^^^

I been using the O/D button as a means of slowing down the entire 4 years of owning the car and I never had a trans issue. Im sure Im not the only one who does this. With a S/Ced car or a higher powered one, it may be different.

[musclemerc]

Don't be afraid to post questions for fear of being belittled. The questions you ask are inportant ones and the answers are just as important. There is alot of knowlege to be gained here, these guys have all been there and done that!!! ;)

[ckadiddle]

If your MM is completely stock without a custom tune, try turning off the O/D when you are doing city driving. It will help somewhat with all the constant up and down shifting in city traffic at low speeds.

In general terms, the factory tune is designed to maximize fuel economy. I found it (all the constant shifting up and down) highly annoying in city traffic here in Orlando. Most of my driving is 35 mph and under going back and forth to work. Turning off the O/D helped lower my annoyance level.

If you never do another mod, buy a tune. It dramatically improves the shifting program and makes the MM more pelasant to drive in city traffic.

[Mac-MerC]

appreciate the info, there are a few ppl who act like asses when u ask a question so i just try to read up on subjects

and ckadiddle i do plan on buying a tuner... i should have had one by now but since i hit that damn deer last month that was $500 gone to my insurance detuctable. with christmas and all the other BS im going through im trying my best to have one by the end of january... if not then its the first thing being bought when i get my taxes

again i appreciate the answers

[TAKEDOWN]

I understand the part of having the o/d off while drag racing , but for everyday use driving city and highway its best to have the o/d on or off?

[juno]

O/D is a band that rides a drum. It can be burned up like any belt.
All other gears are...well..gears. :)
It is not meant to take much power, so it is not a good idea to upshift onto it at full power with a s/c car. A stock car is OK as the LEO's do it quite a bit.
While drag racing 3rd runs from about 85 up to 130-140 depending on your gears/tires so you want to keep it off to keep it in 3rd.
When you are on the street and in OD, when you punch her the car will downshift to 3rd or even second. During the second it takes to shift to 2nd, the tranny is driving the wheels through a very thin shaft. An N/A car may survive this a number of times but a s/c car will snap that shaft as the early pioneers found out.
You can turn O/D off at about any speed. I do it at highway speeds all the time. (highway speeds down here are 70-90 typically) If you want to punch the car on the street you should click it off and punch it as soon as it shifts into third.

Torque converters are a whole other issue.

[MrBluGruv]

^^^

If you happen to use your shifter to go to second with OD on, you may notice it blip the RPMs up and kick back down much like someone clutch kicking while shifting from 4 to 2. I believe that's the thing everyone will warn you about, apparently that's what's absolute torture to the trans. Otherwise it's actually remarkably well-built. In the time it takes to press that button to turn off O/D, it's worth it EVERY time, it's absolutely nothing to press it and THEN downshift to 2 even if you're caught on-the-spot with someone who wants to play. Do it enough, and you'll develop a rhythm as if the car just goes straight from 4 to 2 with no intermediate steps.

[BobC]

I have had interesting happenings with my OD. Either it turns itself off, or I inadvertantly push the button (I doubt this).

Either way, pressing the OD button will not engage the OD, or so the light says. The only way to get the light off is to stop, shut the engine down and restart.

Anybody know what's happening?

[Darrin]

I apologize because I didn't read every word of every post, so if there have already been corrections then forgive me.

The reason to run at wide open throttle with overdrive off is specifically because on the 3-4 shift there is the very distinct possibility that the forward clutch cannot exhaust fully before the overdrive band applies. What happens here is that if the overdrive band applies before the forward clutch has fully released then the transmission is put into a dead bind through the intermediate shaft. Do this enough and that intermediate shaft is going to twist and break. It also tends to heat up the overdrive band more than a little.

The breakage here hasn't got anything to do with the downshift. That is the first correction that I think is very importent. The second note is that in both 3rd and 4th gear the intermediate shaft is what is driving the direct clutch. The shaft is stronger than you think, but it can't handle a dead bind inside the transmission.

It's not as complicated as it sounds, but trying to explain how it works on a forum post with no illustrations is tough. LOL

Oh, and the stall speed on the 03's and 04's is exactly the same. The difference in the converters is that the 04 has a thrust bearing instead of a thrust washer and they are fully firnace brazed whereas the 03 doesn't have those features. The 04 is just simply a better converter strength wise as is the 04 transmission.

And yes, overdrive is brought on with a band, but it is a gear as well. The other gears are brought on through clutch packs instead of a band. Except for reverse. Reverse uses a band as well.

Darrin

[Darrin]

I understand the part of having the o/d off while drag racing , but for everyday use driving city and highway its best to have the o/d on or off?

Leave it on for daily driving. Because I have a great tune I never have to worry about my OD button ever. I don't even know if mine still works or not anymore.

Darrin

[fastblackmerc]

I have had interesting happenings with my OD. Either it turns itself off, or I inadvertantly push the button (I doubt this).

Either way, pressing the OD button will not engage the OD, or so the light says. The only way to get the light off is to stop, shut the engine down and restart.

Anybody know what's happening?

It's broke............... :D

[Darrin]

I have had interesting happenings with my OD. Either it turns itself off, or I inadvertantly push the button (I doubt this).

Either way, pressing the OD button will not engage the OD, or so the light says. The only way to get the light off is to stop, shut the engine down and restart.

Anybody know what's happening?

Sounds like either the switch has gone bad or the wiring to it has an intermittant short.

Darrin

[BobC]

Sounds like either the switch has gone bad or the wiring to it has an intermittant short.

Darrin

Thanks, I'll get it checked out.

[Mac-MerC]

^^^^What Pat said^^^^^

I been using the O/D button as a means of slowing down the entire 4 years of owning the car and I never had a trans issue. Im sure Im not the only one who does this. With a S/Ced car or a higher powered one, it may be different.


ok.... so how do u use it to slow down?... im lost on this

[Cordoba1]

ok.... so how do u use it to slow down?... im lost on this

Answer: Press it, and the trans shifts to third and "engine brakes" the car.

As a follow up, OverDrive is a fuel millage enhancer. If you're out on the interstate, leave it on, or you're not going to get 24MPG....

And as a third follow up -- A "TRUE" overdrive is a trans and rear end setup where the highest gear is tall enough such that the rear wheels are spinning at a higher RPM than the engine. (The wheels are "over-driven") By that definition, our OD's are not true OD's.

[Darrin]

I actually thought that overdrive was a gear in which the driveshaft turn faster than the engine. I haven't heard the definition where it relates to the rear wheel speed like that before.

Huh.

Darrin

[Rockettman]

Ok...I was always of the understanding that without a true "shift-kit" in the trans, that downshifting an automatic transmission would eventually kill it.
Is this old thinking?

[TAKEDOWN]

Leave it on for daily driving. Because I have a great tune I never have to worry about my OD button ever. I don't even know if mine still works or not anymore.

Darrin

Thanks Sir!