what is the point of over drive? i knwo it has the 4th gear in it but why not juss always drive in over drive? it still shifts through all the gears anyways.

 

when you are in town ware speeds are low i guess you can put it in D not OD and the car will be a bit snappier. and when on the open road when you arnt stop in go all the time OD is good for gas milage

 

won't recieve any more "snappiness" between D, and OD. I do it for preference, knowing when my car is shifting into OD, and i don't see much use of having it in OD around town based on the fact that the car will most likely never be going fast enough to require overdrive....4th gear in these cars is offically the overdrive gear.

The only advantage you might have between driving around in drive instead of OD, is if you're actually close to a speed where the car would want to be in OD, it might go into it, then if you're coming to a hill, the car may need to slip out of OD and down into thirdgear (D) and if you have your car doing to much shifting, too often, it could "potential" result in premature transmission failure....but i'd highly doubt that you'd see it in your lifetime.

 

What do we mean by "snappier"?

Snappier shifts = no
Snappier feel, when I hit the gas the car seems more zippy = yes



I leave it in OD all the time, unless (like 85 said) I'm going up a BIG hill, or (like z28 said) I'm in a good mood and blipping the throttle.

Drive gear = 1:1.00 ratio, which is the normal ratio for a 3-speed auto. 1:1.00 means for every revolution of the motor, the driveshaft turns the same revolution. (Motor = 2000 RPM, Driveshaft = 2000 rpm.)

OD gear (on our trannies) = 1:0.70 ratio. This means for every 0.7 revolution of the engine, the driveshaft turns 1 rev. This is why they say "overdrive", because the d/s is turning faster than the motor. (Motor = 2000 RPM, Driveshaft = 2857 rpm). So the motor stays at low rpm (saving gas) but still maintaining high road speed.

This is also why you can't "pull" in overdrive with a stock trans. When the motor's at low RPM, it's not operating in it's powerband (the RPM range where you make the most horsepower). So when you stomp on it, the tranny downshifts into Drive, which raises the engine RPM, which pushes the engine near it's powerband. This downshift controlled by the TV cable and the tranny's valvebody.

Then you've got a guy like me who installs B&M's 4-3 kickdown kit (see http://store.summitracing.com/defaul...part=BMM-70237 ), part # 70237. It prevents a kickdown from OD into Drive- I have to manually move the stick. Now- remember I said the downshift is controlled by the TV cable and valvebody? Well when you're cruising and stomp the gas down to pass someone, the downshift happens. What if the gas was already down to the floorboard b/c you're speeding? Guess what, the trans stays in 3rd b/c the tv cable and valvebody think you want a downshift. Not my trans! I highly recommend that 70237 part because it makes 4th gear "useable" on the highway- even just for passing that old 72 toyota that's barely moving.

 

I always just thought that OD kept the engine at lower RPM's so you could have a higher top speed.

 

to an extent, yes, unfortunately in our cars, if the governer doesn't stop you the fact that the OD gear isn't quite strong enough to hold the car in gear w/ the required amount of throttle to maintain the speed, that it'll be essentially useless anyway.....unless of course you've done the modifications such as TOMP, or upgraded to corvette internals...

 

True, I forgot about the 'Vette stuff! They say that GM's 3-4 WOT Upshift kit that the Corvettes and B4C f-bodies had will do the same thing as the B&M 4-3 kickdown kit.

There was also an article (or message post? I forget which) on how you could modify the existing TV sleeve with a drill, tap, set screw, and loctite- but personally I think it safer to just buy the B&M sleeve.

 

i know i'm a bit late but once again thank you tomp, cause i now know wtf i can't maitain 77 in OD hahaha it sux man think i may spend the ca$h

 

Doesn't Overdrive also remove engine braking? I know when I'm cruisin in OD which is all the time, I come to a slight hill and don't wanna catch the person in front of me I downshift to drive and the engine slows the car down.

Is it a good idea or bad to downshift to 2 and/or 1 when going down a steep hill to reduce the amount of brake needed? As long as your well within the speed that the gear can handle, not shifting to 1st when your doing 40mph.

 

BAAAAAAAAAAAAAAAAAAD

If you want to down shift like that buy a manual car.

 

not so much of a want to downshift, I want my brakes to last longer cause me be broke.

Roughly how much would it cost to put a t-5 in if I have none of the parts required? Just curious.

 

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As far I know the 1 , 2 (or L1, L2 or Low 1, 2, etc.) must be used when going DOWN a steep hill as Nocturnall said.

If I go up a hill (or flat way), I put the shift in OD (4th gear) and the tranny use the gear recquired: 1, 2, D, OD, changing from 1 to 4 (OD) if it is possible.

Or I can use exactly as a "manual tranny" (without clutch) changing the shift by hand from 1 to 4 as desired.

(The same 85 said about D-OD-D-OD-D-OD... going to hill or towing a big boat , motorcycles, trailer, camper, etc.)

Denis V.

 

Downhill engine braking is one of the key reason autos have low gear positions. And yes, the OD shifter position enables a one-way gear which eliminates engine braking. The main advantage to OD around town under 40 mph is that one-way gear.

Whether or not it's a good idea, however, is debatable, as it is my understanding that 700R4s don't route fluid through the transmission cooler when the TCC is disengaged... and the TCC won't engage going downhill.. so for a steep grade, it would theoretically be remotely possible to overheat the fluid through excessive converter shear... but this is only if my knowledge on the fluid routing is accurate.

 

The TCC doesn't care if you're going downhill or not; the TCC will unlock tho if the TPS thinks your foot is off the gas.

And TS, where'd you hear that about the TCC and cooling? I looked up the fluid circuit chart in my GM manual, and it seems that the convertor apply valve will always supply fluid to the cooler, regardless of the position the valve is in (controlled by the TCC solenoid). That, and I haven't heard this in the 10 years I've had my car. Granted, it might be new info, and if it is, can you point me to a reference?

I did an internet search and I did find something related to a 4L60E, throwing PCM code 1870- but that'd be a 4th gen with a 3.8 motor. Maybe this is a 4th gen problem that's getting pulled into the 3rd gens? Go to this webpage, press control-F, and type in "1870" to find the info. http://www.redrider.us/4l60e_perform...smission_p.htm

What might also have confused this is that when the TCC works, the trans does run cooler- because the torque convertor is mechanically coupled instead of fluid-ically coupled. That automatically will keep a tranny cooler, but it has nothing to do with the external trans cooler itself.

I shift by hand a lot, especially when I'm in a good mood... downshifting, upshifting, etc. Remember; get a secondary trans cooler, and keep a check on your trans fluid. Most 700r4 problems are due to the year of the trans - 84, 85, and 86 will have problems. 87-88 will have less problems. 89-up is pretty much bulletproof. Just remember when you downshift that you don't want to redline the motor!

 

yeh, if i was "broke" (which essentaily i am) and had to choose between replacing engines and componets, transmission and componets , or brakes, i think i'd go w/ brakes every time....at the same time, i've already done the other two.

 

The main time I downshift when I'm trying to "save the brakes" is more for going down steep hills; where I'm worried the brakes might heat up. Well, that, and going thru towns that have 25 mph posted, but I'm going downhill- keeps me from hitting the brake every two seconds.

 

I've done several searches, and I keep getting mixed results. I'm sure this question's been asked a hundred times, but as long as I stay under the speed limits in the owners manual, will downshifting hurt my transmission? My trans is a 92 700-R4 (all stock) with 105,000 miles on it.




Max..

 

According to the shop manual and the ATSG service manual, one can place the shifter in the 2 or 1 at any speed (although for 1 it states that downshift will only occur at 40mph or less). As far as D and OD go, it doesn't say anything. I interpret that as "avoid shifting while moving at speed", although I'm guilty of doing it once in a while.

My 90 just bought it. Its got 53,000mi. Original owner was a middle aged woman, doubt she drove it hard. I drove it reasonably hard, but not constantly shifting either. According to her mechanic it was brought in for regular maintenance.

The other day after making a turn the car won't move with the shifter in D or OD, period. Manually shifting 1-2 is fine. Reverse works too.

Bulletproof, yet I get the exception.

 

Shop manual is wrong. My trans will down shift when I put the shifter in the gear I put it in. It doesnt do it when it wants to or when I reach a certain speed. Same with everyother cam/bird i have been in also.

 

As far I remember, when I had an old Pontiac 1963 manual (if I understood o.k.), that manual said as follow:

"You MUST put in 1th gear ONLY @ 40 mph or less".


Denis V.

 

Actually if I dropped into 1st at 40 mph (or above, the trans will let me), I'll redline the motor. I'd make that your biggest concern- and that's probably why it's in the owners manual.

So downshift all you want- BUT REMEMBER TO CHANGE THE TRANS FLUID / FILTER YEARLY!!! Worn-out trans fluid kills trannies fast- and since Heat Kills Trans Fluid, and our trannies run HOTTER than every other transmission, you gotta change the fluid yearly if you want any decent mileage outta it.