Thread: DIY TCU Repair
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Old 08-06-2010, 03:06 AM  
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I added information and copied the pics to my web server, just in case the info is lost. Good to have it here, too.

How to repair an Automatic Transmission Control Unit (TCU) on a 1990-1993 Honda Accord.
"For five dollars."†
† Possibly more (or less), depending on which components need replacement.

Article originally written by John Bennett

Edited by Joshy

I gathered this information from many places on the web. I hope that sharing my experience with this issue is helpful to you.

Car Symptoms:
1990-1991 Only:
- 1990 1991: "S" light on cluster is continuously illuminated (without Sports Mode being activated).
- 1990 1991: "D4" light on cluster is NOT illuminated, or flickers, when in drive (D4), and instead the "S" light flickers, or becomes constantly illuminated (without Sports Mode being used).
1992-1993 Only:
- 1992 1993: "D4" light on cluster is continuously illuminated, in either P, N, D3, 2, 1.
- 1992 1993: "D4" light on cluster is NOT illuminated, or flickers, when in drive (D4).
General Symptoms (NOT year-specific!)
- Speedometer and throttle seem to be erratic, due to incorrect engagement and disengagement of the automatic transmission clutch lock-up solenoids.
- Clutch won't fully lock-up in 1st gear when AT shift selector is in D4 - Clutch appears to "slip." (The tachometer will spike without the transmission shifting gears.)
- The transmission will not shift above second gear in D4 or D3. (Your transmission is stuck in limp mode.)

The Cause:
Poorly manufactured capacitors were installed Transmission Control Unit / Computer (TCU) in this year-range of Accords. As they age, the capacitors leak goo onto the TCU circuit board, causing it to fail. The transmission control computer defaults into "limp mode" which causes the symptoms above.

The Solution:
Replace the bad electronic components in the TCU (CHEAP and EASY!), or order a rebuilt unit (CO$TLY).

Step-by-Step Repair:
Remove the Transmission Control Unit (TCU) from the car. It is located under the carpet in the floorboard where the passenger puts their feet.

Step 1:
Carefully pry the kickpanel off the passenger side door jamb. It should pop right out.

Step 2:
Carefully remove the passenger side cowl trim.

Step 3:
Pull the carpet back to expose the cover plate for the Engine Control Module / Automatic Transmission Control Unit (ECM/TCU). Use a 10mm deep socket to remove the four nuts indicated by the arrows, pictured.

Step 4:
Unhook the two electrical plugs in the bottom of the TCU.

Step 5:
Pull the right side of the assembly out and lay it flat on the floor to expose the back side of the unit. Remove the three 10mm bolts shown by red arrows in the photo below to remove the TCU from the plate.

At your workbench, remove the Phillips head screws which hold the TCU together. The case should then separate easily. Inside you'll see the circuit board.

Step 6:
The red arrows show the leaking capacitor and burned resistor on my unit. Your unit may have more extensive damage, or less.

The leaking capacitor on my unit was a 330 microfarad (μF), 10 volt capacitor. The burned resistor was a 15 ohm.

Step 7:
Determine which component(s) you need to replace. For capacitors, check the microfarad and voltage ratings (μF, v) listed on the side of their casings. For resistors, check the colored markings and use them to determine their ohms and tolerance ratings.

Information about how to decipher the colored markings on resistors can be found here...

The below photo shows an edge view of the burned resistor (left), and leaking capacitor (right). See the residue and charred casings?

The below photo shows what it looked like underneath the capacitor.

Step 8:
Collect your new TCU components. Check local electronic stores, online electronic component sellers, eBay, or recover them from old electronics - computer towers contain MANY capacitors of equal μF and voltage ratings.

At Radio Shack I purchased a pack of 15ohm resistors for 99 cents, and a 470μF, 35volt capacitor for $1.39. This is not the correct capacitor, but it was the closest value capacitor they had.

Step 9:
If you haven't done so already, remove the old capacitor(s) and resistor(s) using a soldering iron. I cleaned away the old capacitor goo with rubbing alcohol. A soapy solution should work as well. Don't use paint thinners (acetone, xylene), because they'll dissolve the circuit board's protective coating.

Step 10:
Install and solder new components onto your TCU board.

Step 11:
Put the TCU back together and reinstall it into your car. Be sure to properly tighten the ECU and TCU to their protective plates, and seat the plate on the car properly. This is important because the boards must be grounded to the chassis properly.

To clear any faults in the computer, I unhook the negative battery terminal for 20 seconds. This should not be necessary, as the ECU and TCU have most likely been disconnected from the vehicle for over 10 minutes.

Originally Posted by John Bennett
I kept my fingers crossed as I cranked the engine. Hooray! The "Sport" light stays off. The clutch locks up. It shifts as it should. I'm on the road again. A month later I achieved my goal of 200,000 miles.
Hope this helps you.

John Bennett
My pictures/photos will return soon...

Last edited by Joshy; 09-08-2010 at 11:01 PM.
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