Transmission operation at highway speeds question...

[DennisG01]

I was driving a 1997 Tahoe (350 gasser - with I assume a 4L60 tranny) the other day and noticed at highway speeds when I let off the gas that the RPM's stay pretty much constant. My truck drops the RPM's whenever I let off the pedal.

Is this correct operation? Is this correct for both vehicles, or which one is wrong? If they are both correct, why the difference/what's the purpose?

[markrinker]

I have wondered that before as well. A 1979 Z28 I owned with 350 engine/turbo 350 transmission would 'freewheel' like you describe when you got out of the gas. Other Chevys I've owned stay engaged and engine continues to turn high RPMs even with foot out of the gas.

I have alway assumed it was a function of torque converter lockup. Early years, it is controlled by fluid pressure and tranny internal settings. Later years, its burned into ECM and controlled electronically.

Personally, I like the freewheeling because you could get out of the gas and not experience any engine braking. It was just like depressing he clutch. When you got back into the gas, the transition to acceleration was smooth and predicatable. Great for hanging curves at high speed where you don't want engine or braking deceleration at the apex.

[DennisG01]

But you know, the funny thing is that with my truck (where RPM's drop) the vehicle speed drops faster than the Tahoe (where RPM's stay high). You'd think it'd be the other way around!?!?

ON EDIT: Hey! Maybe that's why I've been getting less mileage lately! Should I check into wheel bearings or something? Maybe I'm experiencing higher than normal rolling resistance?

[ 03-18-2005, 06:41 AM: Message edited by: Dennis Galligani ]

[Marty Lau]

I by no means am an expert but with the the truck the converter is "locked up" so it would work much like a standard transmission that the engine slows as the truck slows until the converter unlocks, maybe the Tahoe's tranny doesn't lockup the converter?

[markrinker]

Yep. The Tahoe probably has a 4L60E, like my old Turbo-350 tranny. The heavier 4L80E stays locked up. My two Chevs with Turbo 400s both stayed locked up as well...'70 SS 396 and '74 400ci Chevelles.

However, its not that simple...most turbo 350 equipped vehicles I've owned or driven stayed connected on deceleration.

I wonder if it isn't an unusual problem with the converter or a valve in the tranny, that has some benefits and is easy to get used to. Too rare for it to be purposefully designed, I thinks.

[DennisG01]

Sorry, maybe my original description wasn't very clear.

The tahoe (4L60E) acts like a manual transmission (rev's stay high)

My Suburban (4L80E) lets the rev's fall.

For all of you with a 4L80E: Do your rev's stay up or do they fall when you let off of the pedal (say at about 65mph)? Mine go all of the way down to about 750 rpm.

[rjwest]

96 K3500:
Reducing throttle to a point where the engine is not providing power to the drive train releases the TCC. and rpm drops to about 750 rpm.

This does not appear to completly unload the
engine as the RPM will only drop back to an ilde rpm
( 600-650)when brake switch is made ( slight brake pedal movement ). Also: the fuel rate , coasting in gear is LOWER than the fuel rate coasting in nuetral. Makes me believe that the engine is never completly unloaded when throttle is at idle position.

[markrinker]

My bad. Now I really am confused.

Bottom line: Its a function of the torque converter unlocking, or staying locked, correct?

[Jcolg]

My 97, 2500 6.5 TD truck drops the rpms just like you describe. I purchased the truck in December and I noticed this on the test drive. All of my previous cars and trucks have been gas and I don't recall any one of them behaving in this way. I asked the dealer who sold me the truck about it, and he stated that was the way the transmission works. I assume it might have something to do with the diesel engine having higher compression then a gas engine. If the transmission stayed locked to the engine when you take your foot off the throttle, would not the extra compression tend to slow you down quicker then a gas engine would? Might be annoying if you just lift of the throttle briefly to see whats around the corner. Anyway the transmission seems to reconnect very smoothly when I get back on the throttle, so I do think the engine and transmission are behaving properly.

[DA BIG ONE]

Originally posted by Dennis Galligani:
Sorry, maybe my original description wasn't very clear.

The tahoe (4L60E) acts like a manual transmission (rev's stay high)

My Suburban (4L80E) lets the rev's fall.

For all of you with a 4L80E: Do your rev's stay up or do they fall when you let off of the pedal (say at about 65mph)? Mine go all of the way down to about 750 rpm. The 4L60 tq conv does stay locked at the speed you mention, thus requires more use of brakes than does the 6.5 w/4L80 at speed.

I have a 96 gasser tahoe w/4L60 and just did a full tilt rebuild kevlar, and all the corvette mods too making it a much better and more reliable tranny. Great tranny, but watch tranny temp while towing at speed.

Loss of mpg on a well mauntained gas tahoe usually points to the o2 sensors and, or clogged cat IMO.

[gunner 6165]

Both of the trucks I have are 4L80E equipped. So the pickup, gasser, the engine will hold RPM for about 2-3 seconds before dropping to about 1000 RPM until the truck fomes to a stop, then go to idle. I was told the reason for this is emissions. The 'burb does the same as yours,except my speed doesn't drop very fast when off the pedal. Might be something amiss with my truck though.

My wife used to have a 2002 Blazer (not Trailblazer) until a drunk decided to total it for us, :mad: :mad: it had the 4L60E in it and it would hold RPM and have engine speed follow truck speed down ala manual transmission. So I'd say it's normal. But I know the 4L60's are not a towing tranny, normal driving slowly kills those.

[DennisG01]

Thanks, everyone - it seems the consensus is that my 'burb is normal. Unfortunately, that still leaves me searching for why I am experiencing a loss of mileage. Oh well!

[3500HD]

after reading this thread thought i'd offer my .02 my truck acts the same 4l80e. problem I see is when it's loaded with crusie set @ 65 and I drop over a hill engine drops to 700 rpm and I coast down hill faster with no trans braking. when speed drops to cruise set point it revs up and pulls on. my question would be there is a large difference between 700 rpm and crusie rpm of 2500 and truck just acts like it's in nuetral until it locks back up

on edit done some research seems overrun clutch dose not engage when trans is in OD. rpm should be as a 4l60e when in 3rd hope this helps

[ 03-27-2005, 11:55 AM: Message edited by: 3500HD ]

[DennisG01]

Thanks,

I agree - I don't like the fact that the engine has to jump from 700 back up to 2500, either. I'll check that out about when the I only have it in 3rd instead of OD.

[JohnC]

In OD the overrunning clutch prevents the truck from driving the engine. In third it all stays hooked together. The 4L60 doesn't seem to have an overrunning clutch.

[oka065]

G/Day Guys, from what i have just been looking at the petrol 350 has a dual stator & the 6.5 dont. The dual stator reverses the flow in the torque convertor for engine braking when you remove your foot from the gas pedal.

Cheers

[DennisG01]

OK, I just checked out the "3rd stays locked up" idea with my truck. Mine DOES NOT stay locked up in 3rd gear. Once the TCC locks up at 50mph, and I continue to lightly accelerate to about 60/65 MPH , and then I let off the pedal and the RPM's dropped by about 500 - 600 rpm's.

Thoughts?

[JohnC]

Originally posted by Dennis Galligani:
OK, I just checked out the "3rd stays locked up" idea with my truck. Mine DOES NOT stay locked up in 3rd gear.
I was not referring to the TCC lockup. There is a separate overrunning clutch that allows the engine to drop to idle in overdrive only. It doesn't actually idle unless you touch the brake as the PCM feeds a little more fuel for emissions reasons.

[DennisG01]

Thanks John. That clears it up for me. I had never heard of the "overrunning clutch" before it was brougt up here.

[3500HD]

I agree john 5-600 rpm is not bad what I was refering to is the way mine drops into georgia overdrive when I drop over a hill in OD when loaded.