RPM Question

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  #1  
Old 08-06-08, 02:31 PM
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RPM Question

hello and let me pick your well of knowledge on something.
here's scenario:
- my truck normally idles at 475 rpms
- normally, it does 65 mph at about 1800 rpms
- it is automatic, thus there's no direct connection between the gear box and engine

now, thing i have noticed - and i don't get it!
i was rolling down a 1.1 mile 9% decline. so, engine should be, pretty much, idling, correct? at 500rpms. to my surprise, it was still at 65mph/1800 rpms ratio.

why is that? why it is not going down on rpms?
 
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  #2  
Old 08-06-08, 03:02 PM
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Uh, I might know.
There actually could be a direct connection to your engine if your torque converter is still locked up.

Try touching the brakes lightly to unlock the converter.
 
  #3  
Old 08-06-08, 05:32 PM
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When all is working properly the torque converter clutch is disengaged when throttle is closed(at idle). Still could be getting some increase in rpm from the trans pushing the engine. Torque converter is a fluid coupler, and does not care which part, rotor or stator that is driven. To prove this put the vehicle in neutral and see if the rpms go down.
 
  #4  
Old 08-06-08, 05:36 PM
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Gravity is the culprit.
What speed did you enter the decline at?
 
  #5  
Old 08-06-08, 06:42 PM
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And w/o knowing the truck? Hard to tell. Rented a Uhaul Ford V-10 that had a hill decent thing in it. Jeez that sucker screamed during the move here, but prob saved the brakes from burning out.
 
  #6  
Old 08-12-08, 02:55 PM
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let me place a combined response
1. 99 silverado 4x4
2. decline was entered at roughly 35 mph. it zips and speeds down that hill quite nicely, i am at 65 midway
3. i was always told never to engage neutral while driving/rolling. matter of fact, i have researched this before, and could never find a definitive answer, if gears in automatic can be engaged/disengaged during regular drive. would be nice though...
4. i'll try brake pedal tomorrow morning. it's "uphill" now.

speaking of which - if truck accelerates so nicely by itself, that means - trannie is not creating any decelerating moment on the engine, so, apperently, convertor is not locked?? 'ts my guess. i still don't get it - why rpms stay high?
 
  #7  
Old 08-12-08, 06:39 PM
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Converter is simply a fluid coupling, when accelerating, engine power turns the rotor, which causes the fluid in the converter housing to turn the stator, no mechanical connection just fluid. When decelerating it is reversed, the driveshaft turns the transmission, which turns the stator, causing the fluid to rotate the rotor, speeding the motor up. The only time there is a mechanical connection(converter lock up) is in drive or overdrive, above 40 mph, with the throttle off idle, and the brake pedal is not depressed. As far as shifting gears while driving, as long as you do not go into reverse, park, or a lower gear at too fast a speed nothing will be damaged.
 
  #8  
Old 08-13-08, 01:25 PM
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1. as of wire twister post, i do not quite believe fluid connection will hold engine rpms at exactly the same ratio as driving. fluid is fluid. LOWER rpms, i can see that, but not exactly the same. that's why you can't push-start automatic.
2. touching brake pedal does not change anything
3. rpms go up proportionally to the rolling speed, like as if engine is directly connected to the trannie. i rolled down another hill yesterday, and started with low speed, at about 700rpm, and let her roll untill she reached 60 - rpms religiously went up by themselves to 1600.
it all just looks like as if gears are being engaged by themselves, depending on rolling speed.
why in the blazins do i care? because it is more rpms=more gas used, of course.
i have almost decided to give neutral a shot on my way home today - and haulted the idea. it's 4x4. they have special towing protocol to set trannie into the true neutral mode for towing, or it'll get damaged. rolling down the hill and being towed - same difference on my book. nahh, i'll pass.
 
  #9  
Old 08-13-08, 02:02 PM
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If yer foot is off the gas, the injectors aren't shooting gas in, so yer not burning fuel. I'll bet putting it in neutral you'd go even faster. Even my wife's Mazda 4WD Tribute (ok, mine, til I get a new truck) gives some engine braking going down the mountain passes. So there is still some connection from trans to engine.

As long as the engine is running, putting it in neutral for a mile or so wouldn't hurt it
 
  #10  
Old 08-13-08, 03:49 PM
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You cant push start an automatic because they no longer have a rear pump in them. When engine is running front pump builds pressure to operate clutch packs, if engine is off no pressure. In a trans. with a rear pump the trans. output shaft turns the rear pump and builds the required pressure to operate clutch packs. This is why some older cars with automatics could be push started. There is absolutely NO direct connection from engine to trans. I suggest you look at a service manual for automatic transmissions. As far as putting trans in neutral, 4x4 or not neutral is neutral. What you put in neutral for towing is the transfer case not the trans. As long as the motor is running so the front pump is pumping fluid to lube trans. it will NOT hurt to put in neutral.
 
  #11  
Old 08-14-08, 10:39 AM
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ok, here's interesting thing.
i found how NOT to make rpms go up.
if i press 'tow/haul" button, which, to my knowledge, diconnects overdrive mode, RPMS DO NOT GO UP.
i did it yesterday off a short hill and today off my fovorite 1 mile 9% dicline. matter of fact, today, i pulled over right in front of the slope, made sure there was no one behind me following that hill, and just let her roll down.
of course, i had to bring truck to at least some rpms to get moving, but after i pressed that button - that was it, rpms just set there, while i went up to 65 mph.
thus, there's some sort of a direct coupling between trannie and engine in overdrive mode, that keeps engine rpms proportionate to the transmission/wheels rpms. and it is diconnected in no overdrive mode.
not that i am fixing to save a bunch of $$ on this, but it was most peculiar observation. at least, to me. 700 rpms less rolling down the hill does make difference for one who rolls down the rocky mountains, though. i did it 12 times, and some of the downhills there you roll for good bit. cha-ching!!
 
  #12  
Old 08-14-08, 10:54 AM
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Per www.4x4review.com/vehicles/chevy/99silverado.asp

"Transmission
The M30 4-Speed automatic transmission sports a new deep transmission oil pan that allows for cooler running and more fluid. Additionally, a larger, redesigned torque converter produces 15 to 20% less heat than the previous model, improves fuel efficiency, and reduces the overall transmission temperature for greater durability and life expectancy.
The slush box also has a Tow/Haul mode that can be activated by depressing a switch on the end of the shift lever. This mode reduces frequency of transmission shifting and improves predictability of shifts when pulling a heavy load. It also allows for towing/hauling while in overdrive which is a real treat when your making long highway trips."


Is it possible you were in Tow/Haul mode before? That would make sense to me. In tow/haul normally I would think the converter would be locked to allow compression braking going down hills, which would give the conditions you described. I'm guessing there's probably a lighted indicater of some sort that shows in tow/haul though, so maybe not. Just guessin here I guess.

And not to beat a dead horse, but it's fuel injected right? If yer foot is off the gas, unless I'm really mistaken, it's not using any more gas. It's based on a lot of factors, but the throttle position tells the computer not to shoot any gas through the injectors.

Where the heck is our resident GM mechanic anyway? I'm sure he would have the definitive answer.
 

Last edited by Gunguy45; 08-14-08 at 11:14 AM. Reason: forgot to add
  #13  
Old 08-14-08, 04:13 PM
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Originally Posted by Gunguy45 View Post
Per www.4x4review.com/vehicles/chevy/99silverado.asp

"Transmission
The M30 4-Speed automatic transmission sports a new deep transmission oil pan that allows for cooler running and more fluid. Additionally, a larger, redesigned torque converter produces 15 to 20% less heat than the previous model, improves fuel efficiency, and reduces the overall transmission temperature for greater durability and life expectancy.
The slush box also has a Tow/Haul mode that can be activated by depressing a switch on the end of the shift lever. This mode reduces frequency of transmission shifting and improves predictability of shifts when pulling a heavy load. It also allows for towing/hauling while in overdrive which is a real treat when your making long highway trips."


Is it possible you were in Tow/Haul mode before? That would make sense to me. In tow/haul normally I would think the converter would be locked to allow compression braking going down hills, which would give the conditions you described. I'm guessing there's probably a lighted indicater of some sort that shows in tow/haul though, so maybe not. Just guessin here I guess.

And not to beat a dead horse, but it's fuel injected right? If yer foot is off the gas, unless I'm really mistaken, it's not using any more gas. It's based on a lot of factors, but the throttle position tells the computer not to shoot any gas through the injectors.

Where the heck is our resident GM mechanic anyway? I'm sure he would have the definitive answer.
though i am not questioning the source, driver manual states that in TOW-HAUL mode, overdrive is disconnected and shifting points for transmission are changed to permit longer acceleration between shifts.
when button pressed, light comes on the dash saying 'tow/haul". unless really towing something, i'd never drive truck in that mode, as trannie kicks like an old mule in it.
there is no way computer shoots no gas through injectors as that will stall the engine. it just does not shoot enough to propell the car, but still shoots enough to run the engine. ergo, 800 rpms less is how many "shots" less? say, times 20 minutes rolling downhill in rockies?
what is interesting is that even if i start down that hill at 5mph (1st gear), rpms will go faithfully up to 1700 at 65mph all by themselves. i am, also, assuming, that gears will upshift all by themselves into overdrive. now, if it locks TC in overdrive, what is causing them gears to upshift all the way through 2nd, 3rd, and then into OD? now, keeping in mind, that this trannie is ECM and TCM operated, it just looks like TCM shifts gears based on wheels rolling speed.
 
  #14  
Old 08-14-08, 04:41 PM
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Exactly why it is called an AUTOMATIC transmission, output shaft speed, and consequently wheel speed do have input in trans shift point. As to the extra fuel use, when your foot is off the throttle only a small amount of air is entering the engine, the computer injects fuel to make the proper 14.7 to 1 air to fuel ratio so regardless of engine speed when going down hill fuel use is equal to idle. If the computer was adding extra fuel withouy extra air the result would be black smoke from exhaust, fouled spark plugs. and a service engine soon light, accompanied with a rich mixture code stored in ECM.
 
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Old 08-14-08, 04:54 PM
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Ok Ok..I wasn't arguing...just an opinion based on my understanding. Like I said...figured there was an indicator of some sort when in diff modes. Everything on the 'Net with a grain of salt, right?

Still think I'm right about the engine though. If I have a manual trans, I can put it in gear, and roll down a hill. The engine will turn, but not be happy about it! LOL ... Just think the fluid coupling gives it a bit of slack. Maybe idle speed amount of fuel, but not sure why.

Again, not arguing your observed results. Just hoping for clarification. I have a durn (better word deleted) Suzuki that drives me crazy cause it slows going down hills when I don't want to. I know it's something in the trans, but can't get a good answer as to why or how I can eliminate it. It's like it downshifts (what it feels like) but my RPM drops for a sec and my speed does too. Someone explain THAT????

Ahh well, unexplained mysteries of the universe.

One more time, where the heck are the GM guys...
 
  #16  
Old 08-14-08, 05:01 PM
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I had a very simmilar problem with my 98 sierra. I thought it was the transmission and had it checked out and no problem was found. After lots of testing and diagnostics I found out it was a bad ignition coil. changed the coil, plugs and wires and that fixed my problem. Its something where the computer alters your rpm`s when it detects repeated misfires or detonation to protect engine. You might want to look into that. Good Luck.
 
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