How do you calibrate a tachometer?

[ToddinTO]

Hello all,

I purchased a used Telex tach off ebay. I connected it to a 1964 Mercury 650 (signal line on tach to the yellow line off the stator), set the unit to 12 pole (it was on 8 pole previously). Started the engine, and at idle, the tach read about 3K!

I hooked my multimeter to the two yellow alternator lines and set it frequency measurement. It read 160Hz, which I calculated to be 160 cycles/second * 60 seconds / 12 pulses per revolution = 800 rpm. I adjusted the calibration screw on the tach until it reported 800rpm. Is this the right process/method to calibrate the tach?

At WOT the tach states that the engine is running at only 2000rpm, but it sounds much faster, so I think that there may an error in my calibration formula. Since the multimeter is measuring a full cycle, does the stator with 12 poles only generate 6 full cycles per revolution, and hence I should only divide by 6 not 12, which would give me an rpm of 4000?

Changing the input to the other stator wire does not make a difference. When the engine is running, voltage is about 14.5vdc at the battery so I know the stator is working OK.

Any suggestions?

Thanks

 

[jimg984]

Re: How do you calibrate a tachometer?

it could be a bad rectifier does it show charging when running dont check by taking a cable off the battery. use above search forums to find how to check rectifier

 

[Chris1956]

Re: How do you calibrate a tachometer?

Todd, I like your math, but do not know if it is accureate. The best way to check a tach is to compare it to a known good one. Not too useful, right?

I do not think the rectifier would ruin the reading, however, you can disconnect the stator wires from the rectifier and try it. Do not disconnect the DC side of the rectifier, or you will blow it.

 

[j_martin]

Re: How do you calibrate a tachometer?

Hello all,

I purchased a used Telex tach off ebay. I connected it to a 1964 Mercury 650 (signal line on tach to the yellow line off the stator), set the unit to 12 pole (it was on 8 pole previously). Started the engine, and at idle, the tach read about 3K!

I hooked my multimeter to the two yellow alternator lines and set it frequency measurement. It read 160Hz, which I calculated to be 160 cycles/second * 60 seconds / 12 pulses per revolution = 800 rpm. I adjusted the calibration screw on the tach until it reported 800rpm. Is this the right process/method to calibrate the tach?

At WOT the tach states that the engine is running at only 2000rpm, but it sounds much faster, so I think that there may an error in my calibration formula. Since the multimeter is measuring a full cycle, does the stator with 12 poles only generate 6 full cycles per revolution, and hence I should only divide by 6 not 12, which would give me an rpm of 4000?

Changing the input to the other stator wire does not make a difference. When the engine is running, voltage is about 14.5vdc at the battery so I know the stator is working OK.

Any suggestions?

Thanks

The only problem with your math is that a 12 pole stator generates 6 cycles per revolution. Idle is at 1600 rpm.

BTW, you can't beat a frequency counter and (correct) math for accurately setting up a tachometer.

Hope it helps
John