S light had been flashing off and on, most often when the car is really cold. Example it froze over night, car won't shift out of first gear, once the car heats for awhile and then you turn it off the S light would turn off. When you restarted the car it would shift and run fine. Went below freezing tonight and the S light is staying on and car is not shifting out of first gear.
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Re: 1991 Honda Accord - "S" light on steady
It is without question your TMU (Transmission Control Unit) It is located behind the passenger side kick panel. You can buy them on EBay for about 79.00 with exchange it will have a burnt diode and is a common honda problem.
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This is your transmission control unit telling you there is a problem. I have another post here (http://www.fixya.com/cars/t5554380-92_honda_accord_d4_light_flashing) with details on how to diagnose. This link says "flashing D4 light" but the issues it the same. 90-91 accords have flashing S lights for TCU errors, and 92-93 have flashing D4 lights.
Remove the pass side kick panel (Plastic Part) that would be near the right of the passenger's foot. Locate a small blue plug. You jump the terminals together inside the plug with a paper clip. THEN, turn your key ignition on to power up everything but don't engage the starter. Count the amount of times the check engine light flashes. It's like morse code. A solid longer blink indicates the amount of the first number... a quicker flash will indicate the second number. Here's an example: ___ _ _ _ _ That would be code 14. Let me know what code you get and write back... I don't have room to list every code for 90-91 Accord. I'll check back frequently.
Is the flashing steady or is there a pattern to it? If there is a pattern, count the flashes. This will give you error codes relating to the TCU/ECU (Transmission/Electronic Control Unit). If the flashes are steady then it could mean that the TCU itself is bad.
1 long = x10
x shorts = 1 So 1 long and 5 short=15 Please check the light and respond as to whether or not the flashing is steady or not. If not please include the code that is being flashed. Also, include model of the vehicle.
Mine did the same thing. turns out i needed to change the gear selector. But it can be a number of problems. You have to check for a code in your ECU, which is under the passenger side carpet. Its better if you have a mechanic look at it though. Hope this helps a little bit.
A green flashing "S" is similar to a check engine light. It is telling you that something in or on your transmission is not functioning correctly, such as the speed sensor or shift control solenoids. If it starts flashing and the speedometer stops working or the transmission shifts weird, take it to a mechanic.