Tach test

[hemi rt]

Any way to test a 3 wire tach without hooking it up to the motor?

 

[Scott Danforth]

Signal generator would work. Would need to generate a square wave output. Could rife up a set of points to a variable speed drill and use a mechanical tach to compare.

At that point, a few wires with aligator clips to the - terminal on your cars coil and battery would be faster

 

[sam am I]

W/O motor?

Few ways.....On the bench, I've used the flyback voltage pulse generated off a common relay coil.........Drive a FET gate or BJT base with a pic (or sig gen) programmed to output pulses at 20Hz-200HZ for example. With the tach powered up, hook the tach's input to the drain/collector coil node where the other side of the coil is connected to 12V . Can use to cal/check mostly any tach pretty easily

 

[sam am I]

Little more details FWIW and if anyone's interested, fairly easy, quick and cheap motorless way to bench check/cal those old Tachs.......

Build this




Go to http://onlinetonegenerator.com/



Tach sig input signal will look like this ( this was at the 67Hz and 400 Hz, see below examples)






8 Cylinder 4 Stroke Tach Example 1

Engine's coil gives 4 pulses per rev (4 P/R)

Want Tach to Read/Check 1000 RPM (1K R/M)

1000 R/M * 4 P/R = 4000 P/M or 66.67 P/S or 66.67 CPS (C/S) or 66.67 Hz

Set signal generator to 66.67 Hz sine wave for tach reading of 1K RPM





8 Cylinder 4 Stroke Tach Example 2

Engine's coil gives 4 pulses per rev (4 P/R)

Want Tach to Read/Check 6000 RPM (6K R/M)

6000 R/M * 4 P/R = 24000 P/M or 400 P/S or 400 CPS (C/S) or 400 Hz

Set signal generator to 400 Hz sine wave for tach reading of 6K RPM





6 Cylinder 2 Stroke Tach Example 3

Engine's stator has 12 poles and gives 6 pulse per rev (6 P/R)

Want Tach to Read/Check 1000 RPM (1K R/M)

1000 R/M * 6 P/R = 6000 P/M or 100 P/S or 100 CPS (C/S) or 100Hz

Set signal generator to 100 Hz sine wave for tach reading of 1K RPM

 

[mjf55]

sam i am, any need for a flyback diode across that relay coil?

 

[sam am I]

Nope, ya want/need to let her sing like the car's/boat's coil/stator would........FET's Vds = 200V.

Mistakenly wrote IRF540 (above), should be IRF510, either works tho.

Also Vgs threshold on the FET is 10V (to turn it on), as such, you'll most likely need an amp on you puter audio out, I have an pioneer home amp on my puter audio out that allows me to crank up an output voltage high enough..........


Or (adding three things), if ya want to drive it straight w/o amp and/or use smart phone app (i use/like this one and run direct with my ooooold galaxy 5) and audio out jack

 

[sam am I]

Had a couple of requests for a few more examples............Also, using **smart phone app and direct connected circuit in post #6


4 Cylinder 4 Stroke Tach Example 4

Engine's coil gives 2 pulses per rev (2 P/R)

Want Tach to Read/Check 1500 RPM (1.5K R/M)

1.5K R/M * 2 P/R = 3000 P/M or 50 P/S or 50 CPS (C/S) or 50 Hz

Set signal generator to 50Hz sine wave for tach reading of 1500 RPM






6 Cylinder 4 Stroke Tach Example 5

Engine's coil gives 3 pulses per rev (3 P/R)

Want Tach to Read/Check 7250 RPM (7.25K R/M)

7.25K R/M * 3 P/R = 21750 P/M or 362.5 P/S or 362.5 CPS (C/S) or 362.5 Hz

Set signal generator to 363Hz sine wave for tach reading of 7250 RPM




**My Galaxy spazzes out if I get it too close to the pulse (tach in) lead..........I just normally sit it a foot or so away, works fine.

Also, both the tach's I have shown/used here have a 8-6-4 switch on the back I've had to change for the appropriate engine type.