I have replaced the TCU 3 times with a rebuilt unit, both solenoid and speed sensor. After test driving the vehicle for 30 minutes the car D4 light started blinking and now it won't start. Checked the codes again still gives me code 1,2,4,7 & 8. WHY THE SAME CODES AND SO MANY? My transmission shifts okay once it starts up and running. I was wondering if anyone had a similar problem. Any help on this is GREATLY appreciated. This problem is really stressing me out.

Can be a few things. 1) Check the shift solenoids on top of the transmission (front). Remove them and clean out the screens. This would mean your transmission fluid is over due for changing. 2) Maybe it's the Vehicle Speed Sensor (VSS), locate and change that. It's on top of the transmission back under the thermostat area. 3) It could be your Transmission Control Unit (TCU). It's under the floor carpet on your passenger side near the firewall. There is a metal plater covering your ECU and your TCU. The TCU is the smaller box. Remove it, open it and look for burnt items. If you're handy you can replace those items. If you are replacing the unit, make sure it is from a known good one and it matches your car exactly, ie: EX, LX, DX, SE, sport shift button present?, etc... on the circuit board.

Below is a picture of the TCU opened up, here are the transmission codes that your car can display if you are inclined to jump the OBD wire plug to get the reading.

TCU Codes:

CODE LIST:

1-Lock-up solenoid 'A' circuit open or shorted.
2-Lock-up solenoid 'B' circuit open or shorted.
3-Throttle Position Sensor circuit open or shorted.
4-Vehicle Speed Sensor open or shorted - No signal from speedometer.
5-Shift Lever Position Switch circuit shorted.
6-Shift Lever Position Switch circuit open.
7-Shift Solenoid 'A' circuit open or shorted.
8-Shift Solenoid 'B' circuit open or shorted.
9-Counter shaft or transmission speed pulse generator open or shorted.
10-Coolant Temperature Sensor open or shorted.
11-Engine RPM (Ignition coil signal) open or shorted.
12-(No code 12 used)
13-Main shaft speed pulse generator open or shorted.
14-Linear (line pressure control) solenoid open or shorted.
15-Kick down switch circuit shorted.

The first thing you need done is to have a tranny repair shop scan the "TCM" (not TCU) for fault codes.

Flashing D4 light is an error from the TCU (transmission control unit), so you got something going on with your transmission. If you want to pull the error code, remove the passengers side kick panel and look for a pair of wires with a blue connector at the end. Stick a paper clip or some other piece of wire into each hole of the connector to short the wires. Put your key in the ignition and turn 2 clicks. The D4 light will flash the code...short flashes=1 and long flashes=10. Add the number of short and long flashes to get your code. Errors are below:

0-Transmission control unit (TCU) faulty.
1-Lock-up solenoid 'A' circuit open or shorted.
2-Lock-up solenoid 'B' circuit open or shorted.
3-Throttle Position Sensor circuit open or shorted.
4-Vehicle Speed Sensor open or shorted - No signal from speedometer.
5-Shift Lever Position Switch circuit shorted.
6-Shift Lever Position Switch circuit open.
7-Shift Solenoid 'A' circuit open or shorted.
8-Shift Solenoid 'B' circuit open or shorted.
9-Counter shaft or transmission speed pulse generator open or shorted.
10-Coolant Temperature Sensor open or shorted.
11-Engine RPM (Ignition coil signal) open or shorted.
12-(No code 12 used)
13-Main shaft speed pulse generator open or shorted.
14-Linear (line pressure control) solenoid open or shorted.
15-Kick down switch circuit shorted.

Hope helps.

Flashing D4 light is an error from the TCU (transmission control unit), so you got something going on with your transmission. If you want to pull the error code, remove the passengers side kick panel and look for a pair of wires with a blue connector at the end. Stick a paper clip or some other piece of wire into each hole of the connector to short the wires. Put your key in the ignition and turn 2 clicks. The D4 light will flash the code...short flashes=1 and long flashes=10. Add the number of short and long flashes to get your code. Errors are below:

0-Transmission control unit (TCU) faulty.
1-Lock-up solenoid 'A' circuit open or shorted.
2-Lock-up solenoid 'B' circuit open or shorted.
3-Throttle Position Sensor circuit open or shorted.
4-Vehicle Speed Sensor open or shorted - No signal from speedometer.
5-Shift Lever Position Switch circuit shorted.
6-Shift Lever Position Switch circuit open.
7-Shift Solenoid 'A' circuit open or shorted.
8-Shift Solenoid 'B' circuit open or shorted.
9-Counter shaft or transmission speed pulse generator open or shorted.
10-Coolant Temperature Sensor open or shorted.
11-Engine RPM (Ignition coil signal) open or shorted.
12-(No code 12 used)
13-Main shaft speed pulse generator open or shorted.
14-Linear (line pressure control) solenoid open or shorted.
15-Kick down switch circuit shorted.

You will need to have the Tranny checked to see if it is a TCU or a bad solenoid in the Tranny.

Flashing D4 light is an error from the TCU (transmission control unit), so you got something going on with your transmission. If you want to pull the error code, remove the passengers side kick panel and look for a pair of wires with a blue connector at the end. Stick a paper clip or some other piece of wire into each hole of the connector to short the wires. Put your key in the ignition and turn 2 clicks. The D4 light will flash the code...short flashes=1 and long flashes=10. Add the number of short and long flashes to get your code. Errors are below:

0-Transmission control unit (TCU) faulty.
1-Lock-up solenoid 'A' circuit open or shorted.
2-Lock-up solenoid 'B' circuit open or shorted.
3-Throttle Position Sensor circuit open or shorted.
4-Vehicle Speed Sensor open or shorted - No signal from speedometer.
5-Shift Lever Position Switch circuit shorted.
6-Shift Lever Position Switch circuit open.
7-Shift Solenoid 'A' circuit open or shorted.
8-Shift Solenoid 'B' circuit open or shorted.
9-Counter shaft or transmission speed pulse generator open or shorted.
10-Coolant Temperature Sensor open or shorted.
11-Engine RPM (Ignition coil signal) open or shorted.
12-(No code 12 used)
13-Main shaft speed pulse generator open or shorted.
14-Linear (line pressure control) solenoid open or shorted.
15-Kick down switch circuit shorted.

The transmission control unit has gone bad, a very common problem. There is a capacitor in the TCU that goes bad and cause a short on the board. Do not get a used one, they are all bad. A number of people sell rebuilts on Ebay. The TCU is under the Front passengers feet, pull back the carpet and you'll see both the PCM and TCU.

google "accord tcu" or "tcm" if you like

Need more info, just ask
Dave

The transmission is controlled by the Transmission Control Unit (TCU).

Throttle position, rpm, and speed are input parameters to the TCU. The TCU has a default set of shift patterns. These shift patterns change over time based on your specific vehicle setup and driving habbits.

The shift patterns are only slightly affected by driving habbits over many driving cycles. But, changing the relationship between the throttle position and the actual speed being reported by the speed sensor can have an immediate effect on shift patterns.

While your speedo may now be correct, the realtionship between rpm and thottle position has not been corrected. That would take a gear change in the differentials.

The TCU can be reset to an inital learn base line. When this is done, the TCU will instantly learn the dynamics of your setup.

Any shop with an advanced ODB scan tool can reset the the TCU. The dealer or a transmission shop would be preferable.

You really should change your differential gears as opposed to resetting the TCU

This information was posted by ftgiles here

it is the Transmission Control Unit. you have to find the reason why the TCU is flashing the D4 fault code. not replace the unit. have your vehicle scanned for another code. your transmission or transaxle is giving you a warning that something is wrong .