The engine speed depends on the final drive gear ratio in the transmission, and the tire circumference, so it varies from car to car. Engine speed alone is not a valuable indicator for the transmission's condition. There's so many ways a transmission can fail.
First of all, your transmission is computer-controlled. If any slippage takes place in one of the clutch packs, the computer will default it to second gear and it will stay there until you turn the ignition switch off and restart the engine. At that point at least one diagnostic fault code will be stored in the Transmission Computer, AND there will be code P700 stored in the Engine Computer, which just means there's a code in the tranny computer. Simple code readers won't read transmission codes. They just do engine codes. You'd need to find a mechanic with a scanner that can access the Transmission Computer. If the Check Engine light is not on, chances are the transmission is fine.
What you can do is watch how the tach. Responds during up-shifts. You should see four drops in engine speed. The first three are when it up-shifts to second, third, and fourth gears. The last drop is more subtle. That's when the torque converter locks up. Chrysler was the first manufacturer to introduce lock-up torque converters in the mid '70s for better fuel mileage. If you don't know what the torque converter does, it's basically a big doughnut filled with fluid, and a fan hooked to the engine, and another one hooked to the transmission. One fan spins the fluid that turns the second fan. The slippage of that fluid is what allows the engine to stay running at around 800 - 1000 rpm when the car is not moving. Torque converters are more efficient at higher speeds so the engine will run around only 200 rpm faster than the transmission is going at highway speeds. THAT is where the fuel mileage is lost compared to a manual transmission. The lock-up torque converter addresses that loss, but it has to unlock when you come to stop so it doesn't snub the engine off. The computer tells it to unlock under a number of conditions, and you can use them to determine if it's working properly. The car has to be going at least 35 - 45 mph, the transmission has to be in third or fourth gear, and the engine coolant temperature has to be up to a specific temperature. On my Chrysler minivans lock-up doesn't occur until I've driven about two miles on a hot summer day, and up to seven miles on a really cold winter day.
The things that make the torque converter unlock include removing your foot from the accelerator pedal, or tapping the brake pedal, ... Both indications you're planning on coming to a stop, vehicle speed drops below the minimum threshold for it to be engaged, the engine gets too hot, as in when I drag around a tandem axle enclosed trailer that's bigger than my van, or when you approach wide-open-throttle. A torque converter doubles the torque to the driving wheels compared to a manual transmission, but that advantage is lost when it is locked up. Then it's just like having a manual transmission clutch. That double torque isn't needed or used when driving normally. It IS needed at wide-open-throttle when you're racing a freight train. That's why it unlocks at that time.
The easiest way to verify the lock-up clutch is working is to watch for that 200 rpm drop at highway speed, then hold the speed steady and the accelerator pedal steady, and briefly tap the brake pedal with your left foot. You'll see the rpm go up about 200 rpm, then drop back down about two or three seconds later.
If you see the rpm bouncing up and down as you drive, observe how much it changes. If the transmission is slipping, (which the computer would detect), or if it's down-shifting to a lower gear, the rpm change will be a real lot more than 200 rpm;... More like 1000 - 2000 rpm. If you see that 200 rpm change periodically when none of the things I mentioned are occurring, the most common cause is the throttle position sensor. A wiring problem for that sensor would be detected by the Engine Computer. It would set a diagnostic fault code and it might turn on the Check Engine light. It might not turn the light on. The Check Engine light only has to turn on when the cause of a fault code could adversely affect emissions. Inside the sensor itself, it is possible for a little chunk of the carbon element to break off and change the reading while that reading remains within the acceptable range. Even though that reading might be wrong and the computer responds accordingly to it, no fault will be detected if those readings stay between 0.5 and 4.5 volts. One more thing that can make the computer unlock the converter is a rapid change in throttle position. That's what the computer can think is occurring when there's dirt inside the throttle position sensor.
You can watch the throttle position sensor's readings on a scanner as you work the accelerator pedal, (you can do the same thing quite easily with a digital voltmeter), but the momentary voltage changes usually occur much too quickly to be picked up by either one. The computer WILL see those changes, but to you and me it looks like the voltage is changing smoothly. The "record" feature on most scanners often allows you to see those voltage glitches when you play the recording back slowly, but this is one case where it is common to just pop a new sensor on and try it, based on the symptoms, rather than do a lot of time-consuming diagnostic tests.
Tuesday, November 12th, 2013 AT 9:51 PM