Check engine light, obd 1 says code 12 on 1993 Toyota Camry

My son's car has a ck engine light on (the car is running great in every respect). We brought it to a car parts store who looked up the code using obd 1 diagnostics. The owner of the store said it was a code 12 (something to do with rpm signal not responding to engine). We disconnected the negative side of battery for 18+ hours, but light still comes on. Can you suggest any other solution?

Asked by for the 1993 Toyota Camry
Hello Bevy, my answer might be late for your question but just saw it today. Ok, Munkyrench is right, code 12 has to do with the distributor. In it there are two caption colis, and in my case the one that had the problem was the upper one. At the beginning the engine worked with no problem (no vibrations)like yours. I disassembled the distributor and saw nothing. After several months the engine started a strong vibration. I dismounted the distributor and found that the upper caption coil was totally unwinded. There is no spare part to replace it. You must buy a new distributor or from Toyota you can buy half the distributor. I bought half the distributor, reassembled it and the warning light went off. Good Luck.
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There have been a lot of problems with he coil in the distributor on these models. Good chance, but have it checked out to be sure, since you normally have driveability problems when the coil starts going bad.