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Old 06-04-2009, 09:15 AM
John Bennett John Bennett is offline
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Lightbulb Repair the Automatic Transmission Control computer on a 1991 Honda Accord

How to repair the Automatic Transmission Control computer/unit on a 1991 Honda Accord for under five dollars.

I had to do mine, so I figured I'd write it up and share. Hope this helps someone.

Archived with photos at: http://flowthego.com/accordrepair

Car Symptoms:

- "Sport" light on dashboard stays on
- Clutch won't fully lock-up in 1st gear when shifter is in D4, in other words, clutch appears to "slip"
- Car will not shift out of second gear.

The Cause:

Bad 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 Fix:

Replace the bad electronic components in the TCU, or order a rebuilt unit.

How:

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

Photo 1, Pull the plastic off the passenger side door jamb. It should pop right out.

Photo 2. Pull off the kickplate

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

Photo 5. Unhook the two electrical plugs in the bottom of the TCU.

Photo 6. 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.

At your workbench, remove the Phillips head screws which hold the TCU together. The cover/box should then seperate easily. Inside we see the circuit board.

Photo 7. 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. Information about how to decipher the colored markings on resistors can be found here... http://en.wikipedia.org/wiki/Electronic_color_code

Photo 8. Edge view of the burned resistor (left), and leaking capacitor (right).

Photo 9. What it looked like underneath the capacitor.

Photo 10. At Radio Shack I purchaced 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.

Next I removed the old capacitor and resistor using my soldering iron. I cleaned away the old capacitor goo with rubbing alcohol.

Photo 11. Then I installed the new capacitor and resistor.

I put the TCU back together and reinstalled it in the car.

To clear any faults in the computer, I unhooked the negative battery terminal for 20 seconds.

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.

Last edited by 03LXV6; 06-28-2014 at 08:42 AM. Reason: updated url
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  #2  
Old 06-04-2009, 10:39 AM
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03LXV6 03LXV6 is offline
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Welcome to Driveaccord. This is the coolest first post I have ever seen.
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Old 06-04-2009, 10:46 AM
12153 12153 is offline
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John Bennett:

Most EXCELLENT information in your post! Good Show, Man!

You provide superb insight into fixing an ECU problem that would otherwise cost $350-$500+ for a "reman" unit. And you'd better have a really FAT checkbook to obtain a new one from HMC!

To further enlighten myself and others, what is the protocol on said ECU? My trusty '86 utilizes "OBD 0." You have ONLY its blinking LED to provide an error code, when the "PGM-FI" dash lamp is illuminated. And I'm THINKING (circa) 1994-up, everything else Honda uses is "OBD II" protocol. (I'm NOT a "Double E," just an old "mechanical" guy, so please excuse my ignorance!)

To my current knowledge, the earlier ECU PCBs are "PTH" (pin-through-hole) component style, whereas the newer ECUs are "SMT" (surface-mount technology). While I can repair much of the older "PTH" electronics, I am dead in the water on the "SMT" stuff.
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Old 04-27-2010, 08:15 PM
andy andy is offline
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Cool post. I did everything mentioned here and fix my TCU problem by just replacing the 330 μF capacitor. My Accord 1991 has been running smoothly for more than a week now! Thanks John.....
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Old 09-24-2010, 04:42 PM
mcw mcw is offline
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tried it

Ok I replaced my capacitor and that was all that looked bad but now my honda wont start at all what went wrong. It turns over but thats it didnt look like I melted any thing The car atleast run before. My cap was leaking and had to get same size that you used but mine had a lead on each end could that be the problem? Help me if you can.
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Old 09-24-2010, 05:19 PM
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Did you put the capacitors in the right way? The electrolytic capacitors are polarized, so the +ve lead has to go to the hole labeled + and -ve lead to the - hole........
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Old 09-28-2010, 12:54 PM
mcw mcw is offline
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made sure

I made sure everything was the way it should be but still nothing I turns over but will not fire up, I really dont have a clue anymore. does anyone know where I can get a reasonable price tcu everyone I come across cost at least 150 if not more and thats at my salvage yards around town just wanted a cheap fix, nothing else looks burnt so any help would be greatly appreciated.
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Old 09-29-2010, 03:32 PM
eb80871 eb80871 is offline
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Exclamation same tcu problem

it is a shame, that honda would not recall and fix the problem with the tcu R-41 resistor, im glad i took the time to search the net before going to a shop and being charged over 2 grand for a rebuild that still wouldn't fix the problem on my 92 accord lx, thank you all for the great info, i pulled my tcu today and changed r-41 which was burnt , i think it got burnt due to a short i had that kept causing the fuse that controls the shift lock,brake light and horn to blow i found that the previous owner had a remote starter installed and pull it out leaving the wires exposed, the one connected to the brake kept touching ground...
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Old 10-26-2010, 07:17 PM
TonyZ TonyZ is offline
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Great information. I have a 91 with the same problem. Pulled the TCU, found two capacitors leaking but no visual signs of anything else. Replaced the capacitors, put it toghether and it ran for two weeks before the S light came on. Car went into a limp/surging mode. Any ideas on what I should check?

Thanks;

Tony
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Old 05-22-2012, 08:12 AM
brandon t brandon t is offline
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thanks

your advise was dead on! i have a 91 accord that sport light was staying on and wouldn't shift out of 2nd gear. Took tcu out like in photo and you beat same problem. Had to take mine to a computer guru @ Discerner Computers were I live he only charged $25.00 n only thing i bought my self was resistors. Thanks a bunch for the page it helped beyond belief
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Old 11-28-2012, 12:00 PM
bigblock67 bigblock67 is offline
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R 41

Same issue on a 92 wagon. The R-41 is toasted. Will a 10 ohm resister work? It calls for a 15 ohm but not sure of the difference. Thanks
Don
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Old 11-28-2012, 04:06 PM
thegrouch thegrouch is offline
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Quote:
Originally Posted by bigblock67 View Post
Same issue on a 92 wagon. The R-41 is toasted. Will a 10 ohm resister work? It calls for a 15 ohm but not sure of the difference. Thanks
Don
needs to be the same
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Old 11-30-2012, 08:21 AM
bigblock67 bigblock67 is offline
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Angry R-41

Replaced 15 ohm risistor (R-41) and capacitor shown. Trans now shifts proper.(Is there an overdrive gear in D4?) It does not shift twice in D4. The
D4 light flashes constantly now. Pulled neg cable off battery for a while and no change. What gives??
Thanks

Don
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Old 11-30-2012, 03:59 PM
gypsy91 gypsy91 is offline
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For the resisters r41 r42. both of mine were burnt horibly bad. i got a 1/2 watt 15 ohm resistor for r41 and a 1/4 watt 15 ohm resistor for r42. is this correct???
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Old 12-01-2012, 04:43 PM
bigblock67 bigblock67 is offline
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You are correct for the R 41. I dont know about R 42. Where is it located on the board.
Don

Quote:
Originally Posted by gypsy91 View Post
For the resisters r41 r42. both of mine were burnt horibly bad. i got a 1/2 watt 15 ohm resistor for r41 and a 1/4 watt 15 ohm resistor for r42. is this correct???
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