Burned-out tachometers
 

This is an interesting electrical problem. I own a 1934 Ford pu with a 59AB flathead. I wanted to install a simple tachometer to see what rpm I am really doing. Over the last 6 months, I have purchased ( and taken back) 7 tachometers from Walmart, Ebay, Pet Boys, Advanced Auto and other stores. All were made in China.
I typically start the engine, hook up my 1960's shop dwell/tach meter, then connect the new tachometer to see if it will work. About 50% of them registered well off from my shop tach. The rest worked OK at first, but burned out within a mile of driving after installation.
The coil had 1.5 ohms resist. The ballast resistor, 1.5 ohm. About 8.8v on the + side of the coil when running and about 6v on the distributor side. I can live without a tach, but the problem is driving me nuts! Any suggestions?
Thanks, Bill

Re: Burned-out tachometers
 

Re: Burned-out tachometers
 

Westach
Westach | 6 Volt & Tachometers

Re: Burned-out tachometers
 

I had a similar problem. I have an 8N tractor with an 8BA. 12V system. I had a 6V coil and ballast resistor. I bought a marine tach with hourmeter. Would not read over 1000 rpm. Thought maybe it was bad. Bought a China one on Amazon, same thing, sent it back. Then I bought a new VDO tach, same thing. So I said maybe it doesn't like a 6V coil so I bought a new 12v Bosch Blue coil and removed the ballast resistor. Same thing, could not get it to go over 1000 RPM no matter what I did. Settings checked and changed multiple times "just to be sure" but most tachs seem come out of the box set up for V8. I finally gave up and bought an alternator with tach drive and an NOS Stewart-Warner mechanical tach. It works great now.

No help for you I understand, but just to say I feel your pain.

Re: Burned-out tachometers
 

I have been running the Westach on all my cars including my Model A with no problems. They will make them 6 or 12 volts.

Re: Burned-out tachometers
 

Original post never said, but based on his voltage readings, must be a 12 volt car.

I don’t know why the tachs keep failing. They are designed for 12 volt negative ground. Is your car negative ground?

Could the charging system be exceeding normal voltage? Modern electronic devices are sensitive to voltage spikes.

Im just guessing here.

Re: Burned-out tachometers
 

i've found that attempting to measure voltage at the coil, at either stud of a canister style coil, while the car is running, is problematical at best. IME, i get good, consistent results only when testing at the ignition wire, with the ignition switch in the "on" position, and the engine quiesced.

of course any electrical device should match that of the nominal voltage of the battery, and if a modern device, should match the polarity of the electrical system too. as modern solid state devices must be manufactured to match polarity. or possibly be manufactured to work with both + and - polarities and a commensurate price increase.