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Which is better when at a stop light with an automatic?

shmangshmang Member Posts: 297
in General
To shift it into neutral and put it back to D when green light comes or make it stay in D the whole time?

I know if you just put it in D and hold your brake, it will put a lot of force and the engine sounds different than idle, will that shorten the life of the trans. in general?

At the other hand, is it bad to shift more? (from D to N to D). every time I shift, the car shake(which is normal for automatic), so I assume it is just like more wear to the trans.

Please share your opinion with me. Thanks!
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Comments

  • brucer2brucer2 Member Posts: 157
    The transmission fluid temperature rises when you sit at a light in Drive, with your foot on the brake. Heat is the thing that kills a transmission, so I usually put the car in Neutral. (A long time ago I read in my dad's Plymouth factory service manual that it was advised to shift into N for stops of 25 sec., or more.) It is, of course, important to make sure the car is in gear before stepping on the gas. It's easiest on the transmission to: take your foot off the brake, put the trans into gear and then accelerate.
  • alcanalcan Member Posts: 2,550
    When an automatic transmission is shifted into Drive, the forward or low clutch engages. This clutch usually remains engaged until the trans is shifted out of Drive. Shifting neutral-Drive at every stop light increases the cycling of the clutch which increases it's wear rate and reduces it's service life.
    Regarding heat, the friction occuring when the clutch engages generates heat. Almost all automatic transmissions/transaxles are liquid cooled, sharing a common radiator with the engine. Normal transmission pan temp. is about 185 F. (coincidentally about the same as the engine thermostat setting). If the engine is overheating at stop lights, shifting to neutral allows higher engine RPM, higher water pump RPM, and improved coolant flow. Otherwise, leave the trans in gear. There is no benefit in shifting to neutral.
  • armtdmarmtdm Member Posts: 2,057
    Which drops the operating temp of the tranny and leave it in gear!
  • shmangshmang Member Posts: 297
    Thanks for all the information. I kind of agree that keep on shifting is not a good way to do (although I thought it might be good at first.)

    Regarding to change the trans. fluid, I also heard that sometimes, if you change it, it will start to leak, but that only happens if you didn't change it for, say, 50k or more. So, is it safe to say that if you change it every 30k, there is very little chance that it will leak, if it were not before?
  • hengheng Member Posts: 411
    My father in-law owned a NYC cab. In fact several over his hack career. The shifting to neutral trick didn't prolong the life of the transmission. He tried it both ways.
  • adc100adc100 Member Posts: 1,521
    ATF in a Corsica, at 30,000 (regular ATF) and went to syn at 50,000 miles, changed again at 125,000 (syn). Have had no leaks. Changed oil at 30,000 (regular ATF) on 94 Corsica and again at 65,000 (with syn). Had a cooling hose leak after that. Don't know if it was coincidence or what. I think it was a fluke and perhaps the hose was damaged when pulling the reservoir.
  • lspanglerlspangler Member Posts: 102
    alcan, your understanding of an automatic tranny are much different than mine. I have never seen a radiator share antifreeze with the engine and the torque converter is what allows you to stop at a light. at low rpm, the torque converter dis-engages the transmisison from the engine but because the engine speed is still spinning the fluid, it causes the car to creep forward. Trnsmissions use hydraulic pressure and vaccum to operate, which is why they are filled with a special hydraulic oil ( trans fluid )
  • brucer2brucer2 Member Posts: 157
    Just about all radiators in cars with automatic transmissions have two seperate sections. The lower one is used for cooling the transmission fluid. The fluids do not mix.
  • alcanalcan Member Posts: 2,550
    Most car and light truck radiators for the last 20 years or so are the crossflow type, with 2 side tanks. The transmission cooler is located in one of the tanks (usually the right one). Trans fluid is pumped through the cooler, bottom to top to reduce aeration, then back to the trans bushing/gearset lube system. As fluid passes through the cooler it gives up heat to the engine coolant which surrounds it. There is no dedicated portion of the radiator for trans cooling as was previously stated. Some systems use a remote fluid cooler only, and some have an auxiliary remote cooler on the return side of the rad. Btw, follow your trans cooler lines forward and see where they terminate.

    Automatic transmissions use an engine driven pump to develop hydraulic pressure which applies clutches and bands to control a compound planetary gear set (except Saturn, Honda, Acura).

    Clutch and band control (shifting) is determined by engine load from the throttle valve cable, manifold vacuum modulator, or TPS & MAF/MAP & coolant temp & RPM, and by vehicle speed from the governor or VSS.

    Transmission "special hydraulic oil" is required to apply clutches and bands, regulate the rate of friction element lockup for smooth shifts, transfer energy in the torque converter, provide cooling, lubrication, cleaning, etc, etc.

    Torque converters allow slip at low engine speeds, allowing the trans to remain in gear without engine stall. As RPM and centrifugal force increase, higher vortex flow is generated which strikes the turbine vanes, forcing the turbine and the trans input shaft into rotation. WHen appropriate speed and load conditions are met, a clutch locks the turbine to the converter housing to eliminate slip.

    That's my understanding of automatic transmissions, based on 32 years of rebuilding them and 21 years of teaching their construction, operation, overhaul, and diagnosis at the technical college level.
  • adc100adc100 Member Posts: 1,521
    Just got my 2001 Sentra back (400 miles on it). The torque converter was eating itself up. Dealer told me factory was sending a "new" auto trans. I picked it up tonight and the service sheet read "remanufactured" transmission. This is the first time I was told I was getting a "not new" trans. I bought a car with all new parts in it and now I have a major sub-assembly with some "old" parts." I'm not happy. Your opinion please. I respect your opinion a great deal.

    Thanks,
    Al
  • zandorzandor Member Posts: 67
    A lot of transmissions don't have drain plugs.
    After the pan is removed to replace the
    fluid, a worn out reused gasket (a lot of them
    are supposed to be reuseable), a defective new
    gasket, or an improperly installed gasket or
    pan (out of alignment, mistightened, etc.)
    could cause the trans to leak.

    As far a rebuilt trans goes, in theory some could
    actually be better than a new one. A properly
    rebuilt transmission should have all the parts
    checked out to make sure they're within factory
    new specs and the duds are replaced.
    In the case of a new trans, is there any guarantee
    they inspected all the parts? Perhaps they
    just pulled them out of the parts bin and tossed
    it together. In adc100's case, that's probably
    exactly what they did!
    OTOH, some rebuilds are sloppy and sometimes the
    new ones may get a good looking over. Like so
    many things automotive, we just don't know.
    Maybe the manufacturer knows, but they never want
    to tell you things like this.
    I'd probably twitch a bit if they wanted to put
    a rebuild in my brand new car. (if I had a brand
    new car that is!) A factory rebuild for my '98
    wouldn't bother me a bit.

    Mike
  • adc100adc100 Member Posts: 1,521
    Sort of how I feel. It's a crap shoot. I just feel I paid for a car with a-l-l new parts.

    Al
  • alcanalcan Member Posts: 2,550
    Al, I'd probably feel the same way you do. You didn't buy a remanufactured car and you probably didn't pay for it with remanufactured money, but there is a bright spot. First we should differentiate between "rebuilt" and "remanufactured".

    A rebuilt is disassembled, inspected, and defective parts replaced as required. When a unit is remanufactured, all components are inspected, measured for tolerances re original engineering specs, and replaced as required, all wear items are replaced, and any components or sub-assemblies which have had an update or modification partway through a production run (based on field reports of failures) will be changed. So you can actually end up with a superior, more reliable assembly. Hope this helps soothe the sting.


    Btw, check out this site for typical remanufacturing info:

    http://www.acdelco.com/parts/1350_set.htm


    P.S. I can feel the hot breath of our hosts coming to shoo us outta here and into the Transmission Trauma forum again :-)

  • adc100adc100 Member Posts: 1,521
    I appreciate this information.
    Al
  • shmangshmang Member Posts: 297
    hi, Alcan:

    As you are the expert, I want to get your opinion on the same question: Is it good or bad to shift to N while stopped at a light and shift to D before you go? Since you mentioned when car is stopped, it is slip instead of cut the engine off the wheel, does it mean it will have wear if you leave it in gear? or maybe D-> N ->D will make more wear/damage to the trans?

    Thanks
  • brucer2brucer2 Member Posts: 157
    On the Nissans we have the transmission fluid flows across the bottom section of the radiator. (This is where the lines from the trans goes. The return line is 16" to the right of, and 1 1/5 above the inlet line.)
    I have read, but do not have first hand knowledge, that some transmission's pumps do not engage if the trans is in Park or Neutral. If this were the case, I would feel better leaving the car in gear, when stopped.
  • shmangshmang Member Posts: 297
    So, does the engagement of transmission pump make more wear than the stress left it in gear when stopped?
  • brucer2brucer2 Member Posts: 157
    Just rather keep the fluid flowing.
  • alcanalcan Member Posts: 2,550
    All automatics that I'm aware of drive the pump from the converter hub or in some transaxles by a shaft running through the trans, also converter driven. If the engine's turning, so's the pump. Most newer units are using a variable capacity vane pump which will reduce it's output under light or no load conditions but there's still pressure output. Typical line pressures are:

    Drive or Overdrive = 50-70 psi @ closed throttle
    150 - 200 psi @ wide open throttle
    Reverse = 200 psi @ c.t.
    300 psi @ w.o.t.

    Fluid out from the converter goes directly to the cooler circuit, then back in to the trans to feed the gearset and bushing lube system. If the pump stopped working in Park or Neutral there'd be no pressure feed to the converter, cooler, or lube system.

    Seems like every time I turn around there's changes in technology but I can't see how an automatic could live very long without adequate cooling or lubrication.
    We have access to all the latest domestic product training (Chrysler and GM sublet their dealer tech training to us) but our info on imports is a bit sketchy. I pick up some things such as the dedicated cooler portion of the rad you pointed out from forums such as this one. Thanks for that info.
  • ahelmahelm Member Posts: 14
    Thanks, alcan, for sharing your wealth with us.
    A method of doing the fluid filter change was recommended to and tested on my F-250 E4OD by me recently.
    Unload drive wheels by placing axle on stands. disconnect supply hose from radiator and place in suitable container. Start engine, engage transmission and shift through all ranges observing when drive wheel stops turning. Stop engine and proceed dropping pan and filter. (Very little fluid remains in the pan.)
    My question: does this method cause any undue wear if done discreetly in a safe manner?
    Thanks for your help.
  • alcanalcan Member Posts: 2,550
    The idea's good, but when a trans loses pressure due to low fluid level the clutches/bands tend to release s l o w l y as pressure drops. Since most clutch and band facings are compressed paper pulp this could lead to glazing or deterioration of the facings. A less potentially damaging procedure is to first remove the trans pan, inspect for stray odds n' sods and expensive looking indescribable bits, then replace the filter and pan. Btw, it's normal to find a small amount of mixed casting sand, clutch and band debris, and tiny metallic particles from thrust washers. Now refill the pan, remove the cooler return line (usually the upper from the cooler) at the trans to ensure complete cooler drain, route it into a suitable large container, then proceed as you did. BUT have someone pour fluid into the trans at the same rate it's being pumped out until clean fluid comes out. This'll provide a complete exchange with no chance of clutch or band damage. Check fluid level after and correct as necessary by adding, or siphoning out through the filler tube (or re-removing the cooler line and allowing some to pump out).
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