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Discussion Starter #1
I have not posted a Thunderhill "tour" for a while. This is a decent lap, though you will see the gears are too short as I had to pedal it a long way before the T1 braking zone.

You'll notice the tach is going crazy and gives up, but there is a good RPM display provided by the same source that collects the GPS and accelerometer data. It gets the RPM from the MSD just like the tach. After the lap on the video the car quit running right and we have been chasing the electrical problem. I'm interested in thoughts any of you may have. The problem is somewhat intermittent.

http://uk.video.yahoo.com/watch/5323369/14034729
 

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Vibration maybe? Or a firewall grommet that came loose letting some wire rub & ground to sheet metal? Is the coil in the driver compartment & a firewall jack used to run the coil wire through the firewall? In the past I have seen firewall jacks fry & intermittently ground the coil wire to the firewall.
 

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Vibration maybe? Or a firewall grommet that came loose letting some wire rub & ground to sheet metal? Is the coil in the driver compartment & a firewall jack used to run the coil wire through the firewall? In the past I have seen firewall jacks fry & intermittently ground the coil wire to the firewall.

Thanks for the suggestion. It is a dual MSD setup with boxes and coils in the driver compartment and a through the firewall fitting where the coil outputs are combined. This connection looks pretty robust, but your hypothesis would explain why switching from one MSD box to the other has no effect.
 

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On my dyno I can make the tach operate like that sometimes if I'm running an engine with it's own ignition system like say a Chevy with a factory style HEI. I have an MSD 6 AL on the dyno and with the other system running the engine the MSD can actually "fire" even though it's pickup is not connected to anything. The EM energy produced by the secondary of the HEI will cause the MSD to trigger. I'm wondering if the extra ignition system isn't staying "on" and thus driving the tach by picking up the secondary pulses of the other ignition system......

Or it just has a bad ground at the tach and that's making it "see" many more pulses than what are really on the tach wire....
 
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