Car: Connected microcontroller-based tach - dash tach quits

[Scruit]

I am designing a device that need to know the rpm if the car very precisely and instantly. I have a Z8 microcontroller and I wrote a simple routine to count the number of milliseconds between each pulse of the dashboard tach signal wire, and then display the calculated RPM on an LED numerical readout.

The I am using a standard IO port and counting on the leading edge of the tach pulse. low-to-high signals to compute rpm based on the current ms timer, then reset the timer. The input pin is wired up using a 470ohm resistor to pull down to vehicle ground and the tach signal wite connected directly to the same input pin.

When I hooked up the device to the tach signal wire I noticed that the RPM readout failed to 'see' the rpm pulses. I removed the pull-down resistor and it started working, but it read only half the correct RPM (but that could be my computations) and that the dashboard tach quit.

When I disconnected my device, the dashboard tach started working again.

What would you guys suggest? I was thinking about a much bigger pulldown resistor, like 10k or something.

 

[Nigel Goodwin]

Why do you need a pulldown resistor at all?. The existing tacho works, so presumably the signal is perfectly acceptable for that. I would suggest you use a scope on the signal to find out exactly what it is, then use an opamp as a buffer/amplifier to feed your Z80 circuit.

 

[Scruit]

Why do you need a pulldown resistor at all?. The existing tacho works, so presumably the signal is perfectly acceptable for that. I would suggest you use a scope on the signal to find out exactly what it is, then use an opamp as a buffer/amplifier to feed your Z80 circuit.

If I use the 470 Ohm pulldown resistor, the dash tach works but my project tach doesn't.

If I use no pulldown resistor then the dash tach stops working and my project tach works.

I need to get hold of a scope... :?