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Honda Weak SparkPosted 5/7/2002By Wayne Pasicznyk
A call for diagnostic assistance came in on a 1992 Honda Accord. After the technician had replaced fuel-fouled plugs, the car started and ran but the shift interlock would not allow the shifter to be taken out of park. He manually released the shift interlock and did a test drive. The car bucked and jerked and misfired on acceleration and the speedometer worked intermittently. Now it was time to check for codes. Code 4 in the automatic transmission control module (TCM), located under the carpet, under a plate on the passenger side, indicated no speed signal. Since the shift interlock, speed sensor and ignition system share fuse No. 1, we started by checking the spark to see if it was weak on acceleration. The fuse was not blown and had a good connection, but spark was going weak on acceleration, causing the misfires. The technician replaced the ignition rotor, inspected the distributor very closely for signs of bearing failure and lab scoped the AC voltage signals from the distributor. Most Hondas have three pickups in the distributor, and all generate an AC voltage signal. We used a Fluke meter. The top dead center sensor generates about 300 mV AC, the cylinder sensor generates about 140 mV, and the crank sensor generates about 750 mV. On a lab scope, these signals will be a little higher and should show a nice AC sign wave. All the sensors in the distributor on this 1992 Accord were putting out the correct AC voltage signals. The next step was to check the dwell on coil negative. The dwell range is normally 10 to 12 degrees on a 4-cylinder scale at idle. Dwell should increase with rpm about 10 degrees per 1000 rpm. The technician found that the dwell reading was actually going down when the problem occurred, and he noticed the car would run OK until the speedometer and shift interlock quit working. I asked him to check the ignition switch since fuse No. 1 and the distributor are on the same ignition switch circuit. A voltage drop test on the ignition switch circuits revealed no problems. The technician verified the signals were correct from the distributor to the engine control module (ECM), also located under the carpet, under a plate on the passenger side. A voltage drop test of all the powers and grounds at the ECM also indicated no problems. There are three wires at the speed sensor on the transmission: 12-volt power source (yellow), ground (black), and 5-volt signal wire (orange). The ECM, speedometer head and cruise control unit all send out a 5-volt reference signal to the speed sensor. The speed sensor takes the voltage on and off to ground, making a square-wave pattern. The faster the speed, the faster the square-wave pattern. When the technician unplugged the speed sensor, the car ran fine. But when he replaced the speed sensor, it did not repair the problem. Testing the speed sensor, he found that reference voltage on the orange wire was only 2.8 volts when it was unplugged. When I asked him to recheck, he verified that it was indeed 2.8v. I asked him to check for a possible corroded connector, which could cause the voltage to drop on the speed sensor circuit. The connector on the left strut tower was OK. The only other connectors in the circuit are on the control units. The cruise control unit (located above the left kick panel) has an orange wire leading to the speed sensor. The tech unplugged it to see if the 5v reference came back - it stayed at 2.8 volts. The connectors at the TCM showed no signs of corrosion or water damage, and the powers and grounds were OK. The technician unplugged the connector with the orange wire from the TCM, and the speed sensor reference voltage increased to 5 volts! After backing the orange wire out of the TCM connector, he did a test drive and the car ran fine. He replaced the TCM, and the car was fixed!
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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