TCU/TCM Fix 

The resistors R41, R42, R43, are overheated or burned and capacitors C27, C28, C32, C33, C5 leaking or shorted. Some resistors will be burned off to the point where no one can determine the values. A common sense approach is to consult a working TCU and read the color code on the resistors or start with the highest resistance and go down. Find the best wattage resistor or the best voltage capacitor that can fit.

The problem originates with the electrolytic capacitors leaking. Often, the capacitor short circuits frying other components such as your resistor. Another words, the burned resistor is a result of another component failure, namely the capacitor. The fix is to replace the bad parts. See the values below.

Parts values: R41 is most likely 15 Ohms; R42 is 15 Ohms, both appears to look like a 1/4 to 1/2 Watt;   Capacitor C28 105`C 220uF 35V Nichicon H9146. Replace all of them or else the problem may return. If the problem returns, it's likely not your solenoids, but the parts on the circuit board.

sample: 
Before it was new and now it has fried.

Won't shift and no "D4" or "S-light": 

The electronic solenoid valve will likely need to be removed and cleaned if there are no diagnostic light and the transmission shows the symptoms above.  The solenoids may be restricted with metal particles or debris causing shift malfunctions. The old fluid should then be removed and inspected for dark color and debris then replaced with a Honda ATF (using the recommended fluid or the Honda ATF-Z1.)  This section will hopefully point you to a TCU, thermostat, TW sensor or a  valve body or a solenoid problem.

If there are no diagnostic light and the car doesn't move on the right gears or does not shift into first, second, third, fourth or reverse gear, inspect the two Shift Control Solenoid Valve (A,B) (location 2) which is located a feet below the two Lock Up Control Solenoid Valve (A,B) (location 1) in the circled areas on the image above, near the bottom of the radiator. 

    
The solenoid assembly can be removed with a ratchet and a 10mm socket and the inlet and outlet of the assembly cleaned with a carburetor cleaner while engaging the solenoid with a battery. The gasket can cleaned with soap and water (DO NOT SOAK in solvent cleaners) and be reused but you should check for leaks after having shifted thru all the gears and have test driven.

Troubleshooting: Connecting the battery to the solenoid test. The solenoid produces a click when battery voltage is applied to the terminal 1 and/or 2 while the solenoids are still attached to the transmission. The same click should occur under fluid pressure. If they don't work under pressure then they need replacing. 

Troubleshooting: Wiring. To troubleshoot the solenoid wirings see Troubleshooting: Proper operation and check whether the solenoids are clicking okay. If not, then the proper way to check for a faulty wiring is to check for the solenoid resistance at the TCU harness and ground. How to do this? Read the wire color on the solenoid. Go to and disconnect the TCU wire harness at the TCU. Locate that wire color. Measure for the resistance to ground. The resistance should show (correction:) 12-24 Ohms.

Troubleshooting: Resistance test. Measure the solenoid's resistance using an Ohmmeter. Their resistance should be between 12 to 24 Ohms. If their resistance are not within the spec then they need replacing. 

Troubleshooting: Thermostat. If the thermostat temperature is too low the car will refuse to shift because the temperature sensor (TW sensor) tells the TCU not to shift on a cold motor. To troubleshoot, try a hotter temperature thermostat, check the resistance of the TW sensor or check for low coolant.

Troubleshooting: Proper operation. Warm up the transmission after a drive and put the car into reverse and you should hear or feel a click on one of the Shift Control Solenoid Valve. Put the car in neutral will feel another click. Put it in D and another click is heard or felt, except it will be felt on the other shift solenoid. Use a mechanic's stethoscope.

The activation of the solenoid is triggered after receiving signals from each sensor fed into the TCU. A disconnected TCU will still allow the car to move but will do so in 4th gear.

Troubleshooting: Checking the solenoid and TCU for proper function.

Tip: The proper way to test the solenoid function is to do a pressure drop test on the selected solenoid pressure ports (see "Troubleshooting the solenoid and the modulator fluid pressure" several paragraphs below.) 

To troubleshoot, a current measurement can be taken. What this does is attach an ammeter at the solenoid connector and observe the reading for trouble. If there are no ammeter a test light is fine if you just want to know if the TCU is putting out the right power to the solenoids at the right time. The driving test results should point to a malfunctioning TCU, or PGM-FI problem based on how the gear engages and the activity on the ammeter. It can even point you to an external problem such as a bad thermostat or a low coolant level. Look for the following when operating the ammeter:

• The gear should shift when the reading in the ammeter or the test lights comes on.
• To find out when the readings is expected to come on, see below:
  
  1. How the second gear engages: The second gear engages when solenoid B is on and solenoid A is on. When the solenoid is off fluid is restricted from flowing back into the oil (fluid) pan.
  
  2. How the third gear engages: The third gear engages when solenoid B is off but the solenoid A is on. This engages the 2-3 shift valve, thereby supplying the third clutch with pressure.

A troubleshooting tool called an AMP connector (left.) (Right) the ammeter on the solenoid helps determine whether the TCU is providing the right current at every shift stage which may help locate an intermittent faulty TCU or a faulty solenoid circuit. A test light can be used in their place.

The solenoid must be able to restrict the flow of fluid when it is off.  If the solenoid is old or dirty it may not have the ability to stop fluid, therefore may not have the capacity to control the 2-3 shift valve correctly, in this case slipping on 3rd and sometimes fourth. But keep in mind that if there are debris suspended in the fluid the debris may clog the 2-3 shift valve or the modulator valve causing the same symptoms.

Enlarge
Shift solenoid valve Honda Accord 4 Cylinder 1990-1997, Honda Odyssey 1995-1997, Honda Prelude 1992-1997 Notice that the pintle tips are slightly worn, but [not] (edit 3-19-07) enough to be a concern. The job of the pintle is to block or pass the fluid under pressure and/or in motion. 

Other possible scenarios: The modulator valve (which hydraulically feeds the solenoids) may be stuck open, flooding the solenoids with more fluid than they can exhaust. The solenoids may also be restricted with metal particles. The symptoms are erratic shifts, no kickdown, wrong gear starts, early shifts, skips second on medium to full throttle, or skips 3rd. To troubleshoot, follow the test below.

Secondary valve body: The secondary valve body is located inside the transmission. The modulator valve maintains line pressure for the shift and lockup solenoid valves.

Troubleshooting the solenoid and the modulator fluid pressure: 

Step 1. Attach a 0-100 psi pressure gauge to shift solenoid A and B for the modulator pressure ports. Place shift lever in park and start the engine. Check the pressure on both parts. It should be 70-95 psi. If it is over 95 psi the modulator valve is stuck. 

Line pressure testing: Remove the bolt and attach a pressure gauge. The modulator valve maintains line pressure from the regulator, to the pressure to shift control solenoid valves.

If there is no pressure or low pressure at the line pressure port then the ATF strainer is clogged, the pressure regulator is stuck or oil pump worn or binding. 

Step 2. With the engine running connect one end of a jumper lead to the positive terminal of the battery and the other end to BOTH solenoid terminals in the connector. The pressure should drop quickly to under 6 psi. If the pressure is above 6 psi or drops slowly, replace the shift solenoids. See Acura, Honda TSB # 226 for details.

Visual Troubleshooting: Checking fluid level.

Check the fluid level with the engine at normal operating temperature and parked on a level ground. Shut off the engine then remove the dipstick (yellow loop) from the transmission and wipe it with a clean cloth and insert the dipstick all the way down but don't screw in. The fluid level should be between the lower and upper marks.

Visual Troubleshooting: Checking fluid condition.

Drain the fluid and inspect the fluid color or for any small suspended particles. Inspect the magnetic pickup on the drain bolt. If there are suspended particles and the magnet has picked up more debris than it can hold then there could be an internal seized valve problem. If there are debris or the fluid color is black then the old fluid should be removed and replaced.

How to remove the 92-93 (or 90-91) Accord automatic transmission.

Tools required. 

Steps:

1. Remove the hub cap but put the tire and nuts back in.
2. Punch out the spindle nut lock and loosen the spindle nut.
3. Raise and support the vehicle and block the tires.
4. Drain the transmission.
5. Remove the all the damper fork nuts and all the brake hose nuts on the damper fork and the on the shock absorber.
6. Remove the damper fork.
7. Remove the driver side and passenger side front suspension flange nut inside the engine bay and slide the upper arm out.
8. Read more...

NC/NM speed sensor, code 9/15

Symptoms of a faulty NM speed sensor: Located at the end (passenger side) of the transmission on some models, check for code 15 or transmission jerks hard when shifting. Symptoms of a faulty NC speed sensor is the lock-up clutch does not engage. The NM or NC speed sensor (code 9) should be within 400-600 Ohms at 20-degrees C. If they are not within spec replace them. If they are within spec check for a possible short to ground but first unplug the harnesses at the TCU.

How to replace the NC speed sensor:

1. To locate the NC speed sensor on the back of the transmission follow two wires connected to the sensor. The sensor is located on the transmission between the engine firewall and the transmission towards the tire's mud guard on the 90-93 Accord.  The NC speed sensor appears to look like a fuel injector (not the best of description.)

2. Reach inside with a 10 mm socket wrench and unbolt 1 bolt that holds the speed sensor in place. 

3. Carefully twist the speed sensor as you pull it out. 

4. Replace the O-ring.

How to replace the NM speed sensor:

1. To locate the NM speed sensor on the front of the transmission follow two wires connected to the sensor. The sensor is located on the transmission between the engine radiator and the transmission towards the tire's mud guard on the 90-93 Accord.  The NC speed sensor appears to look like a fuel injector (not the best of description.)

2. Reach inside with a 10 mm socket wrench and unbolt 1 bolt that holds the speed sensor in place. 

3. Carefully twist the speed sensor as you pull it out. 

4. Replace the O-ring.

Problem: My car won't automatically shift into third but the car always work normally when I shut the engine off. What is causing it?

Probable Solution: Fix the electrical problem or the transmission, or replace the fluid. 

To find out if the cause is from the electronics shut off all the electronics, including the ECU while coasting in gear for 5 seconds. How is this done? Bring the car up to 50-mph in D3 and release the gas. Turn off the ignition (DO NOT remove the key.) When the rpm decreases and reaches 1300-rpm switch the ignition to ON. If the shift problem persist then the problem is inside the transmission. If the problems goes away for a while (but only after disconnecting the battery) then the TCU is most likely at fault.

Problem: My car shifts harshly... when it shifts it jerks, a bad shift shock. What is causing it and how do I fix it?

Only for the computer controlled transmission. The transmission detects pedal dept via two points. One at the TPS and the other at the transmission. If the deeper you press the gas the less harsh it becomes then the problems lies on the out-of-adjustment at the transmission throttle cable.

Probable solution: Using a wrench, adjust the transmission throttle cable in small increments and test drive. Tip: The more the slack the harsher the shift. If this does not solve the problem then it could be a shift timing or other problems as expressed below.

Extra thoughts from the author about harsh shifting: The harsh
shifting found on these electronic transmission is notorious. Where is
the problem coming from? From speculation, it could be coming from the 
clutch (which could have been damage from using generic ATF.) Or the problem 
could be coming the TCU. The TCU, which in effect is fed data from sensors 
(such as the transmission NM/NC speed sensor) throughout the transmission 
and the engine. The sensors tell the TCU when to shift at the correct 
moment. If any of these moments are off a bit due to bad sensors readings 
then they could produce a jerk as it shifts. Such as we don't normally 
shift when we are at the wrong rpm, car speed or gear. Doing so will 
produce a harsh embarrassing shift. Back in the days of the mechanical 
shifting, none of these problem occur. The current problem is that there 
are too many sensors and some of them could be failing to work correctly 
and the TCU is blinded by their conditions. The TCU is often only acting on
bad advice. Troubleshooting may require the car hooking up to a
computer for analysis making sure the sensors are working as they
should. Many of you don't have access to these sophisticated system.
So, the best advice is to remove each sensor, clean them and test them
using a multimeter and put them back in and test drive. And repeat the
procedure for other sensors. The software could be rewritten to fix its 
behavior but is not solving the problem. The internal parts of the 
transmission itself may be misbehaving. Inside the transmission is a 
clutch fluid shock absorber which is somewhat controlled by the throttle 
pedal. This absorber may not be of any usefulness if the clutch is already 
possibly damage from using the wrong AT fluid. Replacing with a new good 
transmission may be the best bet in trying to solve it. Your luck will be 
needed since people often will put various kinds of random fluid in 
there. The harsh shifting problems isn't mission critical that is why 
we cannot expand the topic further.

Added June-10-09, last edit on Sept-12-10

Problem: My car [electronic controlled transmission] won't automatically shift into third and there is a check engine light or sometimes no check engine light. What is causing it?

Solution: Fix the engine problem. Remember, a weak engine due to ignition or PGM-FI problems will not allow the vehicle to shift to third or fourth nor does a faulty TPS, shift solenoid, thermostat or coolant temp sensor.

Problem: My car won't shift into fourth and there is smoke in tailpipe. What is causing it?

Probable solution: Timing is off on vehicles with twin cams.

Problem: My car won't automatically shift into second (non computerized only.) What is causing it?

Probable solution: Possible shift valve seized. Rebuild required. (effectively, a rebuild simply requires sanding the shift valve.)  

Problem: My car won't go in reverse (can't backup.) What is causing it?

Probable solutions: On a car without an electronic transmission, replace fourth gear selector or the affected gear(s). The reverse gear is activated by fluid pressure. Check for an internal clogged fluid. Cars with electronic transmission don't require the shift solenoid to go in reverse.

Problem: Cannot remove the key even in park. What is causing it?

Probable solutions: Make sure the P light is on. See key stuck at  ignitionswitch.html#stuck for details.

Problem: My car won't start in park. The starter will not engage (no sound, nothing - except a main relay click) but all other electrical such as headlights are working fine.

Probable solutions: If the car starts only when the P indicator come on or when the car is on neutral then clean or adjust the console switch located on the shift selector (next to your seat.)  For the manual transmission, clean, inspect or replace the clutch switch (or the starter relay on some models.) If there are any alarm starter disrupt, bypass them sufficiently to allow maximum current to flow.

Problem: My 90-94 Accord (and some Acura Integra) takes off real slow and won't shift properly, usually happens with a no start. The "S" (D4) light is also blinking, can this caused by the main relay?

Yes. This can happen when the ignition is ON and the ECU/TCU power supply from the main relay drops out momentarily. This could happen in conjunction with a no start condition. The symptoms will usually cause the transmission to shift into the wrong gears (such as 4th.) Instead of 1st, they take off in 2nd, 3rd or 4th gear. This can happen for several different reasons, caused by either internal transmission problems, or external control system problems. External control systems can also cause wrong gear starts. Here are two common external causes:

If you have the conditions above, you are probably looking for an electrical problem. If you don't have the symptoms above, you probably have a valve body or solenoid problem.

Probable solution: The solution is to replace or desolder and solder your main relay (using the best possible method) and check your ignition switch and the culprit clattering relay. To fix the main relay see Main Relay Fix to find the Honda accord main relay location see mainrelaydefine.html. If the intermittent problem persist, there could be an intermittent TCU external power supply problem, or other problems. Check all power supply inputs to the TCU. The TCU power supply ports should come on at the same time as the ECU power ports.

An interesting personal experience from a reader.

Solution: Bad TPS

My name is Jerry and I am an Electrical Engineer by trade and a very avid DIY'er on my vehicles.

I have a 1991 Honda Accord that I bought for my daughter and it developed transmission shift problems.

It was displaying no trouble codes but had very similar symptoms to a bad TCU.

SYMPTOMS:

Starting off in D4, the car would stay in 1st gear, not shifting.
Starting off in L1, then manually shifting to L2 the car would shift into 2nd but manually shifting to D3 and D4, the car would just stay in 2nd gear.

I searched the web for advice, found no real help although I did find about a dozen cases just like mine where people were asking for help...no answers found.

Your web site being the only help, I followed your troubleshooting steps and found them very thorough.

REPAIRS:

Unable to find the real problem, and out of frustration, I replaced the caps in my TCU...just for safety. NO FIX

I then, borrowed a factory service manual from a friend (a 13yr experienced Honda dealership mechanic) and started troubleshooting this tough dog. He, by the way, had never seen this particular problem, "that was not caused by either the shift solenoids or a faulty TCU)

Focusing now on the solenoids, I attached a voltmeter to each solenoid wire to test for voltage. Using the factory manual's truth table, I determined that each solenoid was working...just not being told to go to the next state to shift to 3rd gear.

I removed all 3 speed sensors, tested individually, all OK
I then bought a used TCU, $60, thinking that it must have some internal issue with inputs.
NO FIX

Then, going on my friend's suggestion, against my own opinion, I bought new solenoids for $200. The logic seemed sound at the time, he did after all have 13 yrs experience on me at real dealership.
NO FIX

The night I installed the new solenoids, and took her for a test drive I was VERY disappointed. After spending $250 and still no fix, I was frustrated enough to let my temper get the best of me and jammed the accelerator pedal to the floor!

MY EUREKA MOMENT!!!!

The car immediately shifted through all 4 gears (it was tached up pretty high at this point) and then drove normally.
The check engine light also illuminated (remember NO LIGHTS before)
When I released the pedal the car then downshifted to 1st gear.
Repeating the above gave the same, repeatable results, WOT, it shifted normally, pedal at rest, downshift to 1st gear.

I got back to the shop, shorted the service connector, got 7 flashes of the light, looked it up, and found a code for TPS trouble (throttle angle sensor as Honda calls it)

I tested the TPS with my ohm-meter and got some very strange readings.
The engine was also running very well, seemed normal!

The TPS sensors are not sold individually by Honda, sold only as part of the throttle body assy $430. Although I did find 2 places on the internet that sold them individually for $39 and $79 respectively, my friend had a blown engine that served well as a parts donor for the TPS.

Bottom line, this fixed it!

I bought myself $250 worth of education and in the end, a $30 used TPS solved my problem. No hard feelings though, still cheaper than a visit to the Honda dealership, which by the way would have installed a $430 throttle body assy plus maybe 1-1/2 hours shop labor at $66/hr.

I thought this might be a good case for you to share with the Honda community. Please feel free to use if you wish.

I kept pretty detailed charts of the testing measurements, if you'd like them I can send over in pdf format.

Best Regards, and thanks again for your web site.

Jerry Ginn

An interesting personal experience from a second reader.

Solution: Bad TPS.

Hi,

I was Honda certified master technician up to 1995, then moved into computer. I know ins and outs of electronic problems in Hondas, but I did not easily find my shifting problem on my 91 accord.

The AT would shift wrong, too early, too late, not at all, and the shift interlock would not release so I could not take out of Park

I did the following that did not fix it.

1. Replaced all caps on the AT control module circuit bard.
2. tested all wiring from/to AT module, battery, ignition switch,.
3. removed passenger seat and tested wiring.
4. numerous other items fixed (burned out brake light, other problems.

Finally, I removed the TPS sensor connection. Immediately, the shift interlock would open and the transmission shifted correctly all the time, (though the 3-4 shift was slipping with any extra torque from the engine.

I replaced the TPS with a new replacement, and the car runs SOOOOO good now.

Shifting is precise and correct. Car runs better. Idle is much much better. Car runs smoother. Much more power

Many good points to this item and When I felt the old TPS and compared it to the new one, I could see the problem.

The old one has a wiper that is warn out. If it is warn out on mine at 245K, it is warn out on other peoples car. They really need to consider to replace it even if it doesn’t seem to be a problem.

The benefits of replacing it on a high mileage car like mine means it should be replaced as a standard maintenance item at… (I think) 200K miles.

Highly recommended.


Best,

C E T

Only one case I'd found on a Legend where it'll shift up to second but will not go above second. Turns out the ignition timing was off. Very odd connection - that you wouldn't have guessed.

You also might want to make sure any single plug or an injector isn't firing, or some head gasket problems. Again, this is an odd connection I'm making.

Answered by author (9-20-14):