I haven't tested a tach using a battery charger myself but I have repaired Mopar tach's and tested them with other methods.
So here is what I would do:
-Connect a +12 volt source to the long stud (the normal +12v input to the tach). This powers the tach circuitry.
-Connect your 12 volt source ground lead to the tach metal case somewhere.
-Connect battery charger + lead to the tach input terminal where the gray coil lead normally goes.
-Connect charger - lead to the tach case somewhere.
If I had a Mopar tach handy I would have tried this before posting. Sorry.
EDIT:
Re-reading John's post I believe he is saying to connect the charger's + lead to both tach terminals (where your blue and gray wires connected). This will power the tach AND supply an input signal by going to both terminals. Connect the charger black negative wire to tach case.
Let us know.
No, I do not think so. The charger puts out a very noisy and ripply brand of DC. It is not clean enough to power the tach but just right for simulating the open and closure of points or the pulses from a solid state spark controller. That is all the charger is supposed to run with the black to ground as normal and the positive to the gray lead. A clean 12 Volts sourced from either a battery as in a car or from a bench power supply is needed to power the tach itself.
Almost all battery chargers have simple half wave rectifiers with a limited amount of filtering. The 60 pulses per second of positive half of the AC waveform is what appears to the tach as a running engine. This design of chargers had no filtering because it was not needed, the battery was the filter. It also had a secondary effect of vibrating the plates in the battery to possibly knock sulfated material off to rejuvenate it. I am not sure I buy that and with AGM technology, it wouldn't work anyway.
Craig
Craig
