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DIY TCU Repair

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    DIY TCU Repair

    as title reads:transmission control unit/computer repair

    please add if anything is missed or left out, props to the original author.
    Last edited by hmchaos; 07-28-2010, 06:55 PM.
    " the only thing that stands between a Marine and God, is a Corpsman "

    "Don't shoot your mouth off unless your brains are loaded"

    if anyone needs to do this that is a great write up. i have a manual but i like good diy's nonetheless lol rep points for you sir

    Ride List
    1990 Accord dx coupe - my baby (dsm turbo set up in progress)
    2005 Mazda 6s - daily driver
    newest 2006 mazdaspeed 6


      good one..


        Sweet deal! I had a TV shop repair mine for $30. This is great if you already own a soldering iron. And it can be profitable too! Thanks for the cool story brah!


          maybe this can coincide with the 9293 A/T 4th gear light always on leaving you with only 2 gears. 2nd and a crappy 4th. great find tho very interesting.



            That's a great write-up. Nice find.

            Originally posted by xkjnboix View Post
            maybe this can coincide with the 9293 A/T 4th gear light always on leaving you with only 2 gears. 2nd and a crappy 4th. great find tho very interesting.
            Yes, of course.

            If you've checked the TCU code and have eliminated any other issues, your TCU probably at fault. The D4 light is equivalent to the S light on 9091 models with Sports Mode (The S and D4 lights are diagnostic lights, just like the Check Engine light is - if you've got a 1990, the codes can also be read through spot lenses on both your ECU and TCU.)

            If you continue driving the car with a faulty TCU, it will further ruin the TCU board by sequentially shorting out adjacent components along the board. This negligence will make it harder and harder to repair. It also presents you with an unwarranted fire hazard.

            The absolute, best, 100% fail-safe method of diagnosing a faulty TCU is by physically CHECKING THE BOARD within the TCU housing.
            My pictures/photos will return soon...


              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
              Last edited by Joshy; 09-09-2010, 12:01 AM.
              My pictures/photos will return soon...


                if the original capacitor wasn't touching the motherboard, does the new one need to be the same height off the board or does it matter if it's touching?
                Originally posted by scudweiser
                once again people, need i remind you that there is no road between the kitchen and the bedroom, and therefore women need not drive?


                  really simple fix if you have the patients to iron. you local radio shop should be able to do the same, just tell them exactly what you want done from these DIY.
                  "My FOR SALE thread - Click here"

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                    Originally posted by ingold View Post
                    if the original capacitor wasn't touching the motherboard, does the new one need to be the same height off the board or does it matter if it's touching?
                    It doesn't really matter. It can be touching the board - but the distance is probably to help dissipate heat.
                    My pictures/photos will return soon...