Hello everyone!
I thought I'd start a thread about adding a tachometer to a conversion.
In general, it shouldn't be too difficult. One just needs to be sure the signal put out by the ECM matches what the tachometer is expecting.
I am attempting to connect the stock tachometer on my Toyota pickup to the output signal from the VW ECM. Hopefully some of what I learn here can be useful for other conversions.
To start, I'd like to quote a friend:
The VW ECM has a tachometer output pin. On my 1Z's ECM, it is pin 2/68. Be careful not to confuse it with 8/68 (engine speed sensor).
To see what the output signal of this pin is, I connected it to my oscilloscope:
My Laptop can interface with a PC oscilloscope.
I connected the oscilloscope probe to 2/68 and connected the oscilloscope ground to the truck's instrument ground line.
At Idle, the output is:
A clearer picture:
The output is a square wave signal with a 50% duty cycle. The amplitude is 14V. Notice the markers: 4 pulses correspond to 7.53 Hz, or 451.8 cycles per minute. Since the engine was idling, one would expect 903 RPM. In other words, there are two 14V pulses per revolution.
When you rev the motor, the pulses move closer together:
Since the amplitude is right, now all I need to do is match the pulse frequency or adjust the tachometer's output with a resistor.
I thought I'd start a thread about adding a tachometer to a conversion.
In general, it shouldn't be too difficult. One just needs to be sure the signal put out by the ECM matches what the tachometer is expecting.
I am attempting to connect the stock tachometer on my Toyota pickup to the output signal from the VW ECM. Hopefully some of what I learn here can be useful for other conversions.
To start, I'd like to quote a friend:
Ok, so the tachometer is expecting a 12V square wave with a frequency relative to the amount of cylinders in the engine as an imput signal.The tach signal was one of the very first digital circuits in vehicles, long before computers came along. By digital I mean 0's and 1's. There is either a voltage or there isn't a voltage at that coil terminal. Minus any electronic component induced voltage ramp-up or decay it is a square wave. When the "points" are closed the path to ground is complete, no voltage. When the "points" are open the path to ground is incomplete and there is a voltage.. Presence of the electronic ignitor and lacking any points doesn't change how the circuit functions as far as the tach is concerned.
Most tachs work, as I understand it, essentially as a "voltage duty cycle" and display as a voltmeter. They measure the voltage at the negative coil terminal, reference it to battery voltage, and display the percentage as RPM's. So a tuned resistor would skew the displayed "voltage."
The VW ECM has a tachometer output pin. On my 1Z's ECM, it is pin 2/68. Be careful not to confuse it with 8/68 (engine speed sensor).
To see what the output signal of this pin is, I connected it to my oscilloscope:
My Laptop can interface with a PC oscilloscope.
I connected the oscilloscope probe to 2/68 and connected the oscilloscope ground to the truck's instrument ground line.
At Idle, the output is:
A clearer picture:
The output is a square wave signal with a 50% duty cycle. The amplitude is 14V. Notice the markers: 4 pulses correspond to 7.53 Hz, or 451.8 cycles per minute. Since the engine was idling, one would expect 903 RPM. In other words, there are two 14V pulses per revolution.
When you rev the motor, the pulses move closer together:
Since the amplitude is right, now all I need to do is match the pulse frequency or adjust the tachometer's output with a resistor.