Transmission ?
 

Hello,
I am having a problem i have never seen before. First off my vehicle is a 1992 EX Coupe 2.2L FI 4 cylinder automatic transmission, i believe its a KA4AT. Any further info needed please ask i apologize if i left something out. The car does currently have a CEL 41
(oxygen sensor) which i have replaced but the light comes on still during extreme cold start. OK now on to the actual problem, when i am at highway speed say 65 MPH steady on a flat road, the RPM's will drop about 300, then a second or two later rise back up 300 RPM's it does not happen unless i am at highway speed and the rpms does not rise above normal speed/rpm than it would be. did that make sense example would be at 70 MPH im normally at say 3000 rpm, it will drop to 2700 then pop back up to 3000. it has gotten increasingly worst over the last week. Any suggestions on were to start would be appreciated, im not sure if it is transmission related or maybe an ECU issue? thanks in advance

When the RPM drops, do you feel anything in the car? I'm thinking that's the torque converter locking & unlocking. It feels a bit like the tranny shifting gears, only more subtle/softer/weaker.

I'm trying to decide between flaky tachometer signal, vs. the transmission hunting between locked & unlocked torque-converter.

Checked your tranny fluid? Also are you using ONLY honda AT fluid? If not then your tranny won’t like you much.

jim i do feel it in the car, i hear the exhaust sound change, its almost like its downshifting but not changing nearly enough RPM for that. thanks

g22ccd5 i did check its filled, its a little brown, probably can use a change, and ill be sure to use honda AT fluid, i just didnt want to change it right away if the symptoms seemed to not fluid related, IE low or burnt. thanks

So it IS locking & unlocking the torque converter.

That's partly controlled by rpm & road speed, as well as by the downshift cable. That's the cable from the throttle-body down to the trans. That cable should be adjusted so the lever on the trans begins moving IMMEDIATELY when the throttle begins opening.

Partly, it might be just the nature of the beast. Later years (like 1998) Honda started using more computer control for the auto-trans. Part of that is to prevent hunting gears & lockup like that.

thanks again Jim, i found that in the archives here and have checked, the cable does move at the transmission when i manually apply throttle. I let the car warm then i manually applied throttle and watched the cable to transmission move. i believe thats the procedure correct? so that slight rpm change is the torque converter than ? if the cable is ok, what would be the next thing to check ? it happens anywhere above 55 mphs, it has happened as high as 65mph. thanks again for your help.

I'm no auto-trans guru, so I hope someone else chimes in.

The T-C is supposed to UN-lock when you step on the gas. So check whether the cable is too tight. If you loosen the cable nuts just a bit, does it go slack?

For many MANY types of cars, this has been a common annoyance & part of the reason for more sophisticated computer control over the trans. But back in 1992 that stuff was for more exotic expensive cars.

I'd change trans oil (Honda ATF like others say). Drain & fill, don't have it "power-flushed". Depending on how dirty it comes out, repeat the drain/fill a couple times with 10-200 miles inbetween. The torque converter doesn't drain, so you can only drain about 1/3 of the total fluid. As you drive the fluid mixes into the torque converter & dilutes the old stuff out.

i will check to see what happens with the slack, i will otherwise still drain and refill thanks for all your help.

the cable adjustment seemed ok. tonight i am going to begin the 3 part transmission drain. i have bought 9 quarts of honda ATF and will be draining and refilling tonight then again friday and then again sunday if necessary. i also noticed it does not do the lock/unlocking after about 20 minutes of driving, i figured it was due to the fluid heating up and thinning? does that make sense? thanks again

Never owned an automatic in the 1992 era, so I don't know what makes sense. Fill the fluid based on dipstick level, don't just pour in 3 quarts. Maybe overfilling/underfilling can cause some of this??

Honda (unlike most other automatics) wants you to check ATF dipstick with the engine warmed up but TURNED OFF. Are you checking level with the engine idling?