[jagarra]

Clark,

Looking at the schematic of the sending unit, looks like the batteries are used to pulse the tach head. The relay portion is switching the batteries back and forth as the contacts move following the points in the distributor. I really don't think an over voltage of .15 is going to make that much difference in it's operation. If you have a dwell meter, you could mock the whole thing up and compare the readings. If the Sun reads high with AA batteries in it, well then you may have to add some resistors in the leads going to the tach head to reduce the voltage.

After I posted this I did more research on the internet. Surprised how much this mercury battery is an issue. There are many sites out there with schematics of circuitry to build a board to regulate the voltage. Seems the problem with alkaline batteries is that they discharge at inconsistent rate, where the mercury discharged at a stable 1.35V rate. I did find mercury batteries in the UK, most were camera sized so they are a small package. There is a site called Tachman that does conversions to make the older Suns work. This seems like it is going to get involved and a bit convoluted. I have found where some posters on forums have said current AA batteries work OK, use the E series as they have more umph, just change them often. Found other sites where there are boards being assembled to address the voltage, current and to provide a stable discharge rate. Of course, you will need a new regulator to replace each battery, so 2 boards. If it were me I wouldn't make any holes yet and mock this baby up first as I suggested.