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MG MGA - Electric tach

I tried to test my MGB electric tach, as I'd like to know it works before I fire up the motor for the first time. I tried the DC battery charger method from Barney's site. It hums quite a bit so I expect there is a fair bit of ripple voltage. I loaded it up through the ign coil and then thru the tach pickup. I get no response. Tried reversing the pickup wires and still nothing.
The ign switch was on, so there should be power to the instrument, from the battery.
Any ideas on what to check next? It was apparently working OK from the previous owner, negative ground converted.
My circuit was charger to coil, coil to tach pickup, tach pickup to charger
Art Pearse

Maybe using the coil as a load for the chgarger was a mistake, as it is inductive and would tend to smooth out the ripples. I'll try a bulb.
Art Pearse


Surely you will not get a reading until you fire up the engine and get the coil to pulse?

Steve Gyles

Steve, according to Barney, a DC rectifier gives out a 120 Hz signal because on a battery charger, the output is not smoothed, so it provides DC sine waved pulses, corresponding to 3600 rpm.

Art Pearse


Doesn't Barney's website say "If the battery charger will make the tachometer indicator needle move, turn the potentiometer to set the output reading to 3600 RPM"? This implies that some battery chargers won't.
Gene Gillam

Gene, right. Some rectifier circuits include capacitor smoothing, but it's not necessary for a battery charger (as opposed to a hi-fi amplier!)
I don't suppose my el cheapo charger has that, but the series inductive coil might have the same effect, to smooth the voltage.
If I try a resistive load, such as a headlight bulb, it might work.
Art Pearse

This thread was discussed between 16/11/2013 and 19/11/2013

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