I'm assuming its a 2 wire pick up? if so i cant see how engine grounding would have anything to do with it. A 2 wire or VR (variable reluctor ) pick up is nothing more than a little ac voltage generator, it produces an alternating voltage wave as the metal tab on the distributor shaft approaches then leaves the fixed metal tab on the pick up. The important thing is that the ac voltage it generates swings to a negative voltage at exactly the point where the tab on the shaft starts moving away from the tab on the pick up, making it useful for a reference.
This is generally called a "zero crossing point" and the circuitry used to trigger off of this shouldn't really care what the " ground " looks like, as it is only really looking for the voltage from the VR lines to go from a positive to a negative, the ground potential, or center point, applies equally to both. The amplitude of the ac signal coming from the VR sensor varies quite a bit, it could be .3 volts ac at idle, then 30-50 volts ac at higher rpms,
the circuitry triggering off of this doesn't really care, it just wants to see the voltage (current flow) swap direction. As long as the main DC voltage used to power the triggering circuitry is sufficient to turn it on, then all should be fine. MSD likes the main power and ground connections right on the battery, so barring a dead battery, there should always be plenty of juice (DC voltage potential) to get the triggering circuit operating, and, it doesn't care if the blocks ground is not quite what the battery ground is, because the 2 wires coming off the VR sensor aren't really referenced to
anything but each other. So my guess is that its simply a bad pick-up, or a bad pick up circuit in the MSD box.
if it has 3 wires, its a bit of a different story, these type can either pull a high voltage low, or a low voltage high, based on the proximity of the tab on the shaft to the tab on the pick up. Either way, if their "ground" this sensor is using is referenced to the block and not to the battery, it could cause problems because the "ground" the MSD is using is what i like to call " the mother of all grounds ", otherwise known as the negative battery terminal.
By far the easiest way to check for a bad block ground is to use a voltage meter, put the positive meter probe to the block, and the negative probe to the battery negative terminal, then watch the meter while the engine is cranking. Its not uncommon to see a 1 to 2 volt reading, what you are seeing here is the resistance of the main ground strap from the block to the battery. try to get this as low as possible with a good beefy ground strap, but there will always be a voltage drop, unless you are using a super conductor, copper wire has resistance, and the more current you try to flow, the more it matters.
Im ranting a bit here, so forgive me, but im an electronics guy, and i see/hear the " it must be a bad ground " argument thrown around as a catch all for anything and everything. Yes, a bad ground can and does cause all kinds of problems, but that doesn't mean its to blame for everything, if the block ground is even marginally bad, the starter would throw a fit. If the block ground was bad enough to cause issues with the MSD ( witch should be grounded at the battery, but lets assume its grounded to the block..), the MSD draws like 7-10 amps, the starter has to go through the same ground and it draws 200-500 amps, if the ground is that questionable, the starter is going to let you know. So if she is cranking just fine, but still no spark, chances are the block ground is just fine.
Jay used to deal in these type sensors, so I'm sure he could explain it better, or tell me where i could be wrong
