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Old 06-11-2012, 12:43 PM   #16
colormebad
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Default Re: how do u tell when msd box goes bad??

I fix these boxes all the time and they can cause missing when a 14 pin ic that goes bad..But had more dist pickups & coils cause this problem than the box..Doing a bench test will only show the box is firing to the coil, wont show any other internal problems..You can run the small red wire straight to the battery and make a pass , just to make sure you havent got a 12V ing wire thats went bad & has dropped or dropping voltage...Had alot with only 7 volts or so on the red ign wire, car would miss at higher rpms..
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Old 06-13-2012, 02:29 PM   #17
diehard
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Default Re: how do u tell when msd box goes bad??

Radio interference. Some boxes can be jammed.
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Old 06-13-2012, 02:45 PM   #18
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Default Re: how do u tell when msd box goes bad??

I've had several go bad. Sometimes they die completely...no spark whatsoever. Other times I've seen high speed misfires or intermittent misfires.
A bad tach can cause issues...disconnect the tach and try again.

One thing that will definitely kill them is low voltage. If the battery is weak, charge it up!!!
I always keep a spare handy.
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Old 06-13-2012, 03:06 PM   #19
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Default Re: how do u tell when msd box goes bad??

Quote:
Originally Posted by BigHemi View Post
I've had several go bad. Sometimes they die completely...no spark whatsoever. Other times I've seen high speed misfires or intermittent misfires.
A bad tach can cause issues...disconnect the tach and try again.

One thing that will definitely kill them is low voltage. If the battery is weak, charge it up!!!
I always keep a spare handy.
big hemi can u describe a intermittent or high speed misfire??? thanxs
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Old 06-14-2012, 10:51 AM   #20
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Default Re: how do u tell when msd box goes bad??

The most recent failure I had it was misfiring bad enough that it would just start popping out the exhaust and fall on it's face. It didn't do it all the time though...2 passes at the track it did it, loaded up and came home to diagnose it. Changed a few things, took it out on the highway and it ran fine at WOT roll-on from 50-120 or so. Went back to the track and had the same issue...changed boxes and all was right again.

One time the car would misfire as soon as it got over about 4000 rpm. Not enough to fall on it's face, but would start popping out the exhaust.

Another time it would start misfiring, more like exploding (really F'n loud), out the exhaust at idle...also spitting back through the carb. Again, it was intermittent...it would clear up for a few minutes, then come right back.
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