I have a '89, 5.8L and always wondered what the OD button on the dash actually does. What are the better, best and worst situations to use it. THX

 

BAsically it allows you to lock out the last gear in heavy usage situations. Sometimes in OD the TC will unlock to try and maintain that gear, that generates a lot of heat and if it runs like that long enough could overheat the tranny. By locking into a gear lower it will run hopefully without unlocking the TC and keeping the trans cooler.

That's my understanding of it anyway.

 

First you need to know how a trans mission works.
The lower gears, 1st-2nd-3rd-etc., are different ratios. 1st gear could be 2.15:1 ratio. What that means is the engine is spinning 2.15 times faster than the drive shaft. 2nd gear drops the ratio to, say, 1.86, 3rd drops it again to 1.25. An engine has to spin higher to make power at low vehicle speeds. Ever drive a manual trans and try starting off in third gear? If you have then you know how hard it is.
Any transmission, be it manual or automatic, high gear is a 1 to 1 ratio. What that means is when the trans is in top gear the engine and drive shaft spin at the same speed. Say your traveling at 60 mph and the tachometer says the engine is spinning at 2800 rpm. That also means the drive shaft is spinnning at 2800 rpm too.
Engage Over Drive and the final trans ratio drops from 1 to 1 down to, say, .80. Which is a 20% reduction in engine speed, the engine is spinning 20% slower than the drive shaft rather than the same speed. If the engine is running at 2800 rpm in high gear at 60 mph, then in OD the engine rpm will drop down to 2240, or a 560 rpm reduction. This reduction saves gas, reduces wear and tear on the engine.
An automatic will only shift into OD when the vehicle is at a certian speed and load. it will drop out of OD when you climb a hill or open the throttle to pass, or slow down.

By not using OD you can pull a trailer, climb a steep hill, etc, easier. You'll also use more fuel.
If you tow a trailer you shouldn't use OD while towing. It creates more heat in the trans which is not good.

I hope this answers your question.

 

Iggy,

Thanks for the reply...as a newbie I need to hear things like that. Is towing a light-duty trailer (500 pounds, empty...max of 2000 pounds) going to cause that much more heat to build up in OD? I have an '05 4X2, 5.4L, 3.55 LS, tow package...I didn't think it would matter that much...but your reply got me thinking...

 

I sometimes use it for passing and pulling a load. It drops it down a gear and takes it out of O/D I believe or that is what it feels like. Maybe I should read the owners manual some day. Nah. After 9 years why read it now!!

 

you are okay to pull a load in o/d as long as the transmission goes into o/d and stays there. if the transmission starts "hunting" between 3rd and o/d that is the time to turn it off because you don't want the transmission to be constantly shifting as you drive down the highway.

 

ive always wondered why its treated like this special thing. to me, its just 4th gear on an auto trans or 5th/6th on a standard.

 

Fourth on an auto is Over Drive. Some manual trans' that have 5/6 gears may have two OD gears ratios with 4th being 1:1.
I believe it's the Corvette or the Viper that can not achieve top speed in 6th gear. The OD ratio is too steep for the engine to pull the car. Drop it into 5th and it hits top end. Go figure.

 

ya, like was said above, use the overdrive button to lock out fourth gear when the tranny starts to switch back and forth between gears. for me, tis usualy hapens when im on certain inclines where the tranny just doesnt seem to be happy with either gear, so it keeps swithching back and forth. the constant switching is bad because it means more friction, and thus greater heat build-up in the tranny (a trannies worst enemy). it doesnt seem to be too big a problem for my 2000 5.4l f150, but it has often presented a problem when towing a 7,500 lb quad trailer with the '93 bronco 351ci. thats really the only time it is useful for your aplication. i do know, however, that with the newer trucks it is recomended that you lock out overdrive for all driving under somethin like 50-55 mph because it changes the fuel mixture, and causes al kinds of black sutt out the tailpipe. that caused a bunch of sut buildup on my chrome tip until about a month ago when i read a thread talking about it.

 

By locking out do you mean hit the button on the end of the shift lever or is there a means to force OD off as the startup default?

Thanks

 

You have to push the button every time you start the truck to lock out O/D

 

Thanks. The phrase lock out had me thinking there was possibly a means to have it start up with OD off.

 

It would be nice to have a choice to have it default one way or the other.

 

I don't think I saw it put this way yet but if it has, sorry for repetingg it.

It's my understanding that in a four speed auto, the torque converter locks up in third and fourth gear, but not in first and second. So the torque converter carries first, second, third gear untill it locks and forth untill it locks. Fourth gear locked is a tall gear and requires the engine to work hard to turn especially with a load. Third gear isn't as tall and can carries the load better stressing the engine less.

I hit the button when I see a cop sitting on the side of the road in a speed trap. It'll slow you down quicker too.

I'm certianly not an expert though. You might want to check out the How Stuff Works web site for a simple yet informative explaination.

 

the reason the soot build up stopped is because you are staying in a higher rpm range so the exhaust is moving faster and hotter out the tailpipe. this also translates into worse gas mileage. my preference would be to live with some soot buildup in the tailpipe than spend more money in gas than i already do.