Highway RPM's....Normal?
 

At 70 MPH on the highway my 91 Accord LX Automatic tach's at approximately 3000 RPM's in 4th gear. If I accelerate lightly from that speed I notice that the RPM's will drop by a few hundered RPM's as the car begins to gain speed. If I lighten up slightly on the gas pedal the RPM's will go back up by the same few hundred RPM's. The change is abrupt and repeatable and does not feel like it is slipping.

I have only owned this car for a few months but I have never experienced this before with any other automatic tranismision. Is this normal? What is the cause/reason?

It could be that your transmission is shifting back and forth between the overdrive and the next highest gear.

I thought that initially as well, but the RPM's are actually decreasing when you accelerate lightly and it does stay in overdrive (4th gear)....very strange. I've also counted the upshifts while acelerating from a stop up to 4th gear and then tried acelerating hard to make it downshift to 3rd. The RPM's when it downshifts to from 4th to 3rd match where they would be on the upshift from 3rd to 4th.

Does the 91 Accord have a "lock-up" mode in the transmission where the motor and tranny lock under light load? When this occurs it feels like the car is going into and out of a lock-up mode through the torque converter.

I am very intersted in this topic as well. I have a 94 Accord LX that is doing exactly the same thing. I agree that what we are experiencing is the opposite of what I would expect. At 70 mph, the "lock-up" should be engaged, resulting in a decrease in rpm's. If it is necessary to accelerate, say going up a significant hill, I would expect the "lock-up" may disengage, resulting in a slight jump in rpm's.

I know the 94 has a "lock-up" solenoid for the torque converter that is supposed to engage between 60-65 mph. So, I expect either the "lock-up" solenoid is bad, or it is being activated incorrectly. I have not had a chance to test the solenoid yet, but that will be my first step.

I may be mistaken, but isn't that what overdrive is?

no, torque converter lock up is totally different from an overdrive "gear".

agreed however i have no clue what "locl-up" is, but have heard of something to do with the torque converter and lowering of rpm's

the torque converter takes the place of a clutch, allows the car to not stall at stops and is basically a torque multiplier that has two haves that slip and one half spins at a faster speed than the output shaft of your transmission. the lock up is when the torque converter 'locks up' and stops the slipping. it's actually sort of like a jet turbine hooked onto your transmission, but powered by fluid instead of the force of air. www.howstuffworks.com probably has a really good tutorial on it :).