In my automatic pickup truck, I find that the transmission will upshift quicker if I take my foot off the gas for a moment after getting up to about 2500 - 3000 RPM. Otherwise, just pressing the gas pedal down further and further would cause the transmission to upshift slower, with the RPMs getting well over 4000 RPMs before a gear shift finally occurred.
Is this typical of older transmissions? My truck is only a '95, but it has over 115,000 miles on it (about 60% highway miles).
During the mid-90s I drove a 1980 Ford Granada and the automatic transmission was the same way – I had to let up off the gas to achieve an upshift. Heck, it was a lot like driving a standard sans clutch, because the Granada’s tranny usually would NOT upshift without letting up off the gas pedal.
So, the GQ is – what does this “behavior” in an automatic transmission indicate?
When my car is cold (91 Nissan Stanza) it will sometimes fail to upshift properly (well, I’m not a mechanic but the engine revs much too loud/high). Oftentimes I do notice that if I take my foot off the gas, allowing RPMs to drop, it will upshift. It only does this when very cold, however (as in winter, negative temps).
Guess I have nothing to add, except that I’m also interested in the question.
You don’t say what kind of truck…only that it is a '95. Older cars/trucks have vacuum modulators. It’s possible some newer models may still have them, but I can see a degrading or even broken diaphragm in the modulator causing an upshift problem.
But, being a '95, I think it’s possible your truck’s shifting may be controlled by the computer (as it reads RPMs and the VSS[vehicle speed sensor])…depends on the make. More components and systems are being computer controlled (i.e. shifting) in order to make fuel consumption more efficient (and thus, less emissions).
DP White, that was a good article, but it didn’t explain why letting up off the gas pedal produces an upshift.
The article DID say that an automatic transmission will upshift at lower speeds if the driver is accelerating gently, while the same upshifts will take place at higher speeds when the driver is flooring it. This doesn’t seem to really address my OP, but maybe I’m not wrapping my mind around this correctly.
It indicates that the cable from the throttle to the tranmission ain’t broke!
It’s perfectly normal. The transmission uses how hard the engine is working (usually from a vacuum line) combined with what you are doing with the accelerator (via a special cable that runs from the carburetor to the transmission) to figure out how to shift. When you let up on the accelerator, the cable moves, and the transmission does what it was designed to do. If you don’t lift up on the accelerator so quickly, then it’s usually the vacuum portion that causes the shift. If the cable is broken, the tranmission’s performance won’t be as good because it won’t respond so quickly to sudden changes of the accelerator. The fact that yours shifts proves that this mechanism is ok.
The granada’s vacuum portion of the shifter was not working correctly. Most likely it’s the vacuum modulator (previously mentioned) but it could be a vacuum problem in general. If the car ran a little rough it may have been a vacuum leak somewhere, because this would cause the spark advance in the distributer to get mucked up as well.
Hmmm. One way to find out is to call your local parts store (although my bet would be that a vacuum modulator for a Nissan would be a “dealer item”) or Nissan dealer’s parts department and ask if there is a listing for a vacuum modulator. If they say no such animal, my bet would be on a electronically shifted trans.
Sorry I am so rusty in my explanation, I haven’t touched much cars since I gave birth to my son. I also must rush off and to go my parents’ house to figure out why my father’s car isn’t starting.
Perhaps GaryT will be here later. His mechanical explanations are much more eloquent than mine.
Yeah, see, I’m not too sure about Nissans, but my bet is that that cable is probably an obsolete thing already (hence the computer reading RPM from the tach and the VSS)…and I’d also lean heavily that there is no carb but it is fuel injected and probably has a TPS (throttle position sensor) to indicate how hard you’re pushing the car/truck.
The “how” of the question will probably be different depending on the year/model. The “why” is a little more straight-forward. The transmission is trying to give you what you want (in terms of acceleration) as economically as possible.
At a lower gear, a given amount of engine torque (turning force) translates to a greater amount of wheel torque than at a higher gear. Thus, for the same amount of engine work, the low gear will lead to a larger acceleration than a high gear.
So, if you bear down on the gas, you’re telling the car that you want a large acceleration. The transmission does its part by staying in a low gear so that the wheel torque is maximized.
If you let up on the gas, you’re basically telling the engine that you’re not interested in having a high acceleration. So the transmission upshifts to save gas.
If you’ve been cruising along at 55 for a while in high gear and you suddenly stomp on the gas, the transmission senses your desire to accelerate and will downshift (so-called “passing gear”; it’s really just one gear down).
Yes, it was a good article. No it did not describe the particular parameters of your transmission and engine.
Now I am going to guess based on my long experience as a civil litigation trial lawyer. (And auto shop class in high school.) Between the engine and the “gearbox” is a nifty device called a torque converter, which you all learned about at the web site. One measure of an engine’s power is horsepower (550 foot pounds per second squared for each horse power), but another is torque, namely, how many foot pounds at a specific RPM. Different engines have different configurations. People who like manual transmissions like them because they personally control where the shifts occur. Vroom, vroom. The torque converter acts something like a clutch, and will tend not to allow shifts into higher gears while you are demanding more power, because your transmission configuration is probably interpreting this leaning on the accelerator as a request for more power/torque, not a higher gear (higher gears mean less torque for most automatic transmissions). Therefore, it will keep it in the lower gear until the demand for more power ceases, that is you let up on the gas. The demands on the system will then allow your transmission to shift into the “ideal” configuration for the power demand being made at that time, which is less power at a higher gear. Incidentally meaning better gas mileage. That’s my guess. But I never studied transmissions.
It’s pretty much what sford said. If you’ve got your foot in it, normally you want to scoot, and transmissions are designed to accomodate this. The signal to the trans can be via engine vacuum or a throttle pressure cable (a very few designs use both). The behavior described in the OP is typical of all automatic transmissions I’m familiar with, old and new.
Now, if it won’t upshift unless you back off the gas (like the Granada described), something’s not working right. Could be the vacuum modulator or throttle pressure cable, could be the governor, could be related to the electronic controls (on newer stuff). But normal operation is that the farther down the accelerator pedal is pressed, the later the upshifts/sooner the downshifts.
Just one more little data point, I’ve driven a Toyota Tercel wagon, with an adjustable transmission, from “normal” to “power”. Switching to power basically delayed upshifts to higher RPMs, making acceleration faster when you really wanted to move. Frankly, the car is so underpowered that if you leave it on normal, it’s a total slug, on power it is at least marginal.
I have had to start doing this as well (taking foot of gas to get it to shift) otherwise it takes a good 5-7 sec to shift (only first and sec. “gear”) But mine is caused by something called a “clutch piston” going bad don’t know what it is but it will cost a MINIUM of 1,700 bucks to fix…and I just had the trannie rebuilt 3 years ago, so it truly sucks!
