Honda: Honda accord D4 light, TCU, transmission control unit

Question
Please help me?
92 Honda accord Lx - D4 light comes on solid car will not have any pep when first starting out. It seems to be starting in third gear.  The car will not shift after second gear. Manually of otherwise. This problem is completely intermittent. I have replaced the tcu with a used part.  Still same problem. i have replaced the upper and lower shift selenoids.The transmission fluid is fine. No codes when the wire on the tcu is tripped. Now here is something that may or may not have anything to do with this problem, but i have seen other people mention the same thing, my brake light bulb light on the dash is illuminated, but it's intermittent as well. the maintenance required dial that is wired to the speedometer has stopped working around 5k miles ago, right around the same time this problem with d4 light started. Also i have noticed that the problem seems to occur only after we have heavy rains.  Unplugging the tcu remedies the problem and over the past 3 months i have been swapping out the original and the replacement numerous times. The car then runs great for a few weeks and then it happens to rain and then the D4 light thing happens.  You can actually feel the transmission engaging and disengaging as you drive down the road. It has also happened while the car has been running in park. Please help?

Answer

Honda transmission con It sounds like you have the classic transmission control unit failure. Take the cover off of the control unit and inspect it for leaking or burned resistors and capacitors. see the images I have attached. Another issue that can cause these symptoms is a failing PGM-FI main relay. If the relay is failing it may drop voltage supplied to the TCU. A failing main relay would have a No start condition at times.

The maint. req'd indicator is not related and may be corrected by inserting the ignition key into the slot below the indicator.

There are two brake lamp indicators on the gauge cluster. One is for the hand brake and master cylinder float circuit. The other indicator is for the brake light bulbs. The tail lamps tend to leak water into the brake lamp harness or brake control units in the trunk.