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Tachometer for a 6-volt system, Possible??


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Have decided that I really want to know what kind of RPM's the engine in my '41 is putting out at highway speeds rather than depending on the mph to rpm chart in shop manual because I have 15" rims vs. the stock 16". I have tried to exercise due diligence by querying prior postings and have only come up with one by ZondaC12 relating to switching wires on a 12-volt tach in his '38 until the needle stopped jumping around. Nothing was said about how many wires and where they were finally connected.

I also dug deeply into the piles of junk on the shelves in my garage and found the cheap, plastic cased dwell/tach meter which has just two leads and which I hadn't used in 25 years. Per my note taped to the meter, I attached the red clamp to the dist. connection on the coil and the black to ground (for neg. grd. systems) and got what I would assume was accurate idle rpms, 550, and dwell degrees, 26. Since I don't have the meter's instructions, I don't know if the meter was for use only with 12-volt systems, and use on my 6-volt system would give questionable readings. The major drawback of my meter is that it only goes to 1200 rpms.

Given the example of the results from the use of the dwell/tach meter, and even though almost all tachs seem to indicate that they are for use with 12-volt negative ground systems (eg. JC Whitney catalog), is there a possibility they might work on a 6-volt system like my tach/dwell meter? Since I realized early on in my college career in my failed attempt to became an electrical engineer, I assume nothing when it comes to electricity and always ask questions.

There happens to be a couple of 6-volt Sun tach sending units on ebay, but no tachs from that era.


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Just FYI all I did was hook the $20 Pep Boys Sunpro special up like the instructions said, and I used very thick out-of-place-looking wire to make sure it had the best signal it could get. It usually does something, and cruising about 30 mph or above in 3rd it does register, but probably 600 rpms too low according to the chart! Haha I hate having to use that too!

I have seen a couple different websites actually that sold 6 volt tachometers. Both were like $120!!!! I hate that, I cant understand why the high price.

Northern Tool sells this: http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_524744_524744 I dug out the email I sent them....but it claims "works on all four-stroke engines". Hmm. Seems to be intended for small power-equipment engines, but I drilled the guy on this claim and he said you "wrap the wire around one of your spark plug wires" so thus you have to multiply the reading by the number of cylinders in the engine.

I havent cared enough because $40 is still a good amount of money to someone my age for something I dont really need, but I just might bite. I dont need it updating CONSTANTly, Im sure it will delay a little since its digital. But Id be happy with that, to be able to get one for a reasonable cost.

The one thing Im thinking, and someone correct me if Im wrong, but wouldnt wrapping the input wire for this thing around the main coil wire going to the distributor instead of one of the plug wires account for the multiplying issue?


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