I had some trouble with a scan tool that I was trying to use in my car (only works in other cars, but not mine).

I hooked up a scope to the power and there are very large spikes coming from the ignition coil or module. Im pretty sure they are as the frequency matches the RPMs x 4, and they follow in lock-step with the RPMs. They can be heard in the AM radio as a series of sharp "dits" in the speakers, so its unlikely its the alternator. The screen shot of the pulse itself makes it look like its coming from the coil. The width also grows and shrinks with the engine load.

Id expect to see some of the fuzz that comes after it from the spark, but not something nice and sinusoidal like that. The overall shot also shows a dip before each pulse, like a sizable amount of power is being drawn. How is the flyback pulse making it back into the electrical system? Wiring gauge too small? Faulty ignition module? Other than the noise, the car runs just fine.

 

Also, are the pulses the correct sign for the ignition coil? Occurred to me that if it was the coil, they should be the opposite sign (negative going pulses).

 

Install one of these. I use one to keep any ignition noise down for all my electronics.

http://www.jegs.com/i/MSD+Ignition/121/8830/10002/-1

 

Im going to try some more testing, but if the pulse is from the alternator itself due to the current surges from the ignition, then a big electrolytic can is probably the best way to fix it. Be nice to know for sure to make sure Im not just putting a band-aid on something actually wrong...

 

Here's a couple samples from primary and secondary of coil. They are DIS, but compared to DI for conventinal ignition the only thing differant is the enitail drop to peak is shorter.

HTH!

 

Where did you probe from? seems like your voltage is low to begin with, also have you tried running the car off of only the battery and/or alternator. Only two things that I can think of that would cause a spike like that, bad alternator or a secondary to primary leak, but since I feel that voltage is low I would also check that battery, well actually thats always the first step anyway.

 

The car has a u-drive setup, so the voltage is low at idle. It does it at all RPMs, though.

I did try a while ago to run the car with the alternator still hooked to the battery, but unplugged so it wasn't energized, and the noise was still in the radio. Im going to try again, but with the alternator entirely disconnected, just to make sure its not part of the problem. It definitely could be a leak in the coil. The coil is negatively switched, so the +12V becomes the cold end of the coil during flyback, with the negatively switched side becoming positive up to around 1600V or so. Its possible that there is a leak on the hot side thats getting back into the electrical system. Only a different coil can rule that out, but I've had multiple coils fail on the older HEI systems with visible internal arcing.