Misfire Caused By Alternator
Had this truck in the shop the other day that was misfiring intermittently when accelerating. It had a code 41 field circuit control code for the alternator in the computer. Didn't correlate a misfire with an alternator problem but since it was there checked it anyways. Got out the DMM the alternator tested good, it was charging, ground looked okay, control wire signals looked okay so I went in a different direction. Checked spark line coming out the coil and had a test light on coil control wire and could see the misfire on the scope but also noticed the flashing of the test light interrupted when this happens so I am thinking input or computer problem. I know these coils go bad all the time, are cheap, and take out computer drivers so I put one on to be safe with no change, still misfiring. I check inputs next and found my crank sensor signal was glitching and dropping out. Again, another cheap part that goes bad all the time on these vehicles and when I saw the signal doing this jumped the gun and said get me a crank sensor, feeling confident that this would fix it. Now it's still misfiring so I go back to my crank sensor, the signal is still glitching and dropping out. Now I check the power and ground for the sensor and the same glitch is on the crank sensor ground. Now I'm checking computer ground, same glitch. Move back to battery and block ground and the glitch is there to. Followed the ground cable to the engine block, and then it goes from there to the alternator. Now I'm smiling and feeling stupid at the same time, I'm right back where I started. This time I take the belt off the engine and run it without the alternator spinning. The glitch is gone and the misfire is gone, the alternator was internally spiking the ground circuit, causing the crank sensor to glitch, and therefore causing the computer to momentarily not fire the coil. This spike in the ground circuit was missed with the DMM when I checked it the first time around - the ground looked perfectly good and steady on the DMM at 0.01V, I could only see the glitch with a scope. This job really reinforced the importance of checking powers and grounds before calling a part bad as well as the importance of using a scope to get an accurate view of what a circuit is really doing.
Ted from Tennessee
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1994 Dodge Dakota 3.9L Misfire Under Acceleration