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Thanks “29 Chev” for sharing your work here. I frequent SmokeStak also, and have been following your thread. Fortunately my 9800 works fine. My 7.5 V battery is 8 years old and about ready for a change. After quite a search I found a supplier here (S-E MI) that can get them (Ray-O-Vac #903). Your suggestion of five “D” cells will give the 7.5 V . but will it provide sufficient amperage?
Joe B ( 31 Ford)
Hi Joecb,
I cannot tell you for sure – it would depend on the coil being tested , the length of time it was being tested and how much the current being fed to it was limited by a resistance unit. I think that for a 10 – 15 test second test period one could probably establish whether a coil is firing reliably or not using 5 D cells which I believe is what the person looking to construct a single circuit to load test a coil was looking for – one would have to try it and see the results. If I am wrong in my understanding of that comment I apologize and of course a larger current source would be a good idea but that would add to the cost of a coil testing circuit. To determine if a coil under test is actually meeting a manufacturers specification such as is listed in the Mercotronic manuals is a different situation. One would need to first determine the specification and then construct the identical circuit that the Mercotronic (or another manufacturer) uses to test it properly with the correct battery source capable of delivering the proper current. Since we do not know the actual meter specs that was used in a Mercotronic that could prove to be a challenge. During my experimenting on making an electronic version of the vibrator used in a Mercotronic I was using a 12 volt automotive ignition coil that was capable of drawing over 4 amps at 13+ volts using a 10 amp 12 volt battery charger to supply power to the coil without having a current limiting resistor in the circuit. I was also able to fire the same coil with a good spark reliably using a widely gapped spark plug at a voltage level of 7.5 volts using a bench top variable voltage power supply that was only capable of delivering 1 amp of current by limiting the current using a 10 ohm variable resistor – the wire wound resistor I had was light duty and got hot quickly which is why I specifically stated to get one that is rated for the wattage I mentioned. As the resistance value of the current limiting resistor is decreased the current draw of the coil will naturally increase and that could drain the 5 D cells quickly to a point where they would no longer supply adequate current flow. The suggestion I made of using 5 D cells was just that – a suggestion of just one example of how a 7.5 volt potential difference level can be achieved for a reasonably low cost since the 7.5 volt style batteries that are used in a Mercotronic are no longer easy to find or inexpensive. Two sets of 5 D cells could also be connected in parallel to produce double the current and still maintain a 7.5 volt level. If the current was found to be lacking there are also commercial power supplies that can be purchased with adjustable or fixed voltage levels and capable of producing up to 5 or 10 amps if someone wants to spend the money. This would also be dependent on how many ignition coils someone is testing each day, week or month. In my experience most people who tinker with antique engines have high expectations and (like me) very shallow pockets which makes a compromise like having a less than adequate current source a reality.