Hi all:

Still trying to find a way to hook up a lil express 8-cyl. tach to the 6-cyl Cummins, I talked to an automotive electrical guy who suggested to identify first where the signal is from on the Cummins (on a gas truck, it would be the coil: what is it on the Cummins?)

Second, is to find what the resistor value is, so that the 8-cyl. tach can have the appropriate wired to it.

Does anyone know if this is the way to try and solve hooking up this tach?

Thanks!

 

The source is the PCM. I have no idea what he's talking about with the resistor.

 

The source for the PCM is a crank speed sensor, correct?

Wannadiesel: I don't know what the guy means with a resistor. All I got was that if I could find the resistor value for the Cummins and the resistor value for the lil ex[press, then he would wire (parallel or serie???) the appropriate one. What resistor is he referring to, I have no idea... Anyone??

 

Yes, the crank sensor is the source for the PCM, 2 pulses per revolution. There is a plug under the dash that has the tack hookup wire in it and the output is a square wave. I built my own with a basic stamp 1 and a display. Dave's seen it, gets peoples attention.

As for a resistor, what is he talking about? Most gas tachs have a switch that allows for 4, 6 or 8 cylinder operation with it hooked to the coil. An 8 cylinder would have 4 firing events per revolution, which would mean 4 pulses per revolution. A 6 cylinder would have 3 per revolution and a 4 cylinder would have 2.

Next question is most gas engines us 12 volts on the coil. The tach output on our trucks is only a 5 volt pulse, I don't know if a gas tach would pick it up or not, I think it should but I'm not completely sure.

 

Thanks for the explanation. Some further research - surfin' - led to a VW site where an electrical engineer describes a conversion: most of it has to do with differences in ohm values of resistors, some nano and microfarad values and clean soldering, all this taking place on the board inside the tach. Most of it is beyond my comprehension, but it is very do-able if you know what to do... Might be a dead end for me...

 

I know there is a resistor in the wire and someone on here put what size of resistor is need to make on tach work on some models. I just cane not find the link or size if someone would please help.

 

A tachometer is a device that takes an input that may be widely variable, but whose constant feature is that it is timed to engine speed. It uses some feature of that input to generate uniform pulses, then a simple circuit (integrator) to count those pulses.

A resistor in the input line to change the time base makes no sense at all.

Changing resistors and capacitors in the tachometer circuit of course is changing it's calibration. The parts change would be in the integrator portion of the circuit.

I like the basic stamp digital tach idea.