I have 2 issues and I am not sure if they are related or not. The car is a 1991 accord with the 4 cyc engine and auto trans (mileage about 160k). About 2 months ago we had the dealer check over the car, they recommended that we replace the battery since it was 'weak', we did that. My kid now has the car at college and has run into two problems about 2 weeks ago:
The alternator light comes on for a few mins then goes off. So far no dead battery.
The trans will sometimes drop out of 4th or 3rd at highway speeds with out being under load.
I asked my kid if the trans acts up when the alternator light is on. He said the trans will act up even when the alternator light is out.
My kid had the car checked out at AAA garage, they connected electrical system up to there diag equipment and it showed everything was working ok. They also test drove the car and could not recreate trans problem. I think they may have disconnected the battery, see below.
About 4-5 years ago the trans was doing the same thing. I found that if you disconnect the battery the trans would work ok for a few weeks. I tried to diagnose the problem my self and couldn't really find the cause of the problem. After some googling I found these cars have a problem with the trans computer going bad. I ended up putting a rebuilt trans computer in and everything was working ok.
Any ideas on what my next step should be? Is the alternator getting ready to dye?
Should I just go ahead and order a rebuilt alternator and trans computer?

Thanks

 

Verify that the battery connections are clean and tight. Have your son look at the cables and verify that they are not corroded(green). Check the grounds that come off the battery cable. One small one goes to the chassis, the larger one will go down to the transmission, they should also be clean and tight connections.

Have him make sure the D4 lamp and CEL lamp illuminates when he turns the key to II(ON) before starting. When the fuel pump is priming these lights will come on as part of the prove out. When the main relay drops out and the fuel pump stops priming he should hear a click along with the D4 and CEL lamps going off. If neither lamp illuminates then most likely the bulb is blown. He will need to replace the bulbs or pilfer them from a different spot on the dash cluster so he can then read any potential codes.
If the lamps do work. Have him reach over to the passenger side kick panel, hanging out in space over there should be a two wire blue connector. Have him use a metal paper clip to jump the connection. Turn the ignition to II(ON) and have him observe the blinking of codes from the CEL and/or D4 lamp.

If no codes are present have him verify the transmission is properly full of fluid. With the car on a level surface, the trans dispstick should have fluid in the hashmark area. If he lives in a hilly area I would suggest filling it to the top of the hash mark.
Have him also verify that the cable from the throttle body to the transmission lever is not pretensioned nor any slack. The lever should begin to lift as the throttle butterfly opens.

When the car drops out of 4th or 3rd is it a downshift? Or is it going into a Neutral state?
If downshifting I would verify the VSS is working properly, along with the throttle cable adjusted correctly and the TPS does not have a dead spot.
If it's going into Neutral, verify fluid is full and that the fluid has been changed regularly.