Originally Posted by
steveo
these pcms don't really have failures like that, but still good to try.
tach output is filtered, probably your tach is spliced to coil negative, which is acceptable.
this is definitely a fundamental electrical issue.
im still thinking something is gettin' yanked at WOT. i can't correlate it to anything else.
other ideas...
give it a couple good hard stabs of throttle as someone is watching under the hood
unhook your alternator positive and do a wot run
add a chassis to ecm ground
jump positive power to coil positive (bypass ignition switch and everything)
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