Trouble with tach signal

kcolbey · Post by **kcolbey** » Tue Jul 05, 2016 1:00 am

I tried to do a ton of research and looking around before asking for help, but now I am stumped.
I have an RCP mk1 in a Honda CRX. There was an aftermarket tach that was installed in the car previous to when I got it. I tapped in to its signal line and plugged it directly in my pulse ports. I got really weird readings, like 3000000 then 0, then 1000, then 0 and on and on.
My first round of research told me to scope out the original signal, so I did. It is a 12V square with coil spikes that are (only guessing from memory) 70V. I figured to make a circuit with a zener diode to condition the signal to 5V. I drew the circuit me and a friend made. Please forgive the paint drawing, and my limited knowledge of all things electrical. I think I drew an accurate schematic.
The result we got was a decent square wave with very nice 5V peaks. However, the low point is now maybe 1.5V. The scope of the current signal is included. The device is still erratic, and it is not getting a good tach signal it seems. We added the second resistor pictured to give the diode a little more voltage & amperage. That seems to help (when we first plugged in the circuit, we weren't getting a signal at the device). However, I can't keep lowering the resistance because we're starting to get to where we might short the diode out soon.
I tried a different input, to see if that would fix it. I tried to play with the pulses, but that does not seem to work either.
What should I try next? We had thought about adding a resistor between the diode and ground to see if that might help us convert 12V to 5V by making another voltage drop down the line.

Post by **brentp** » Wed Jul 06, 2016 3:38 pm

Hi,

Looks like you're on the right path for figuring out a clean square wave from your tach signal.

See our CoilX adapter and write-up here; that might give you some ideas on how to improve your circuit:
https://wiki.autosportlabs.com/CoilX

Let us know what you find out!

kcolbey · Post by **kcolbey** » Thu Jul 14, 2016 2:06 am

Thanks for the words of encouragement. I think I'm going to try a CoilX and see if that will work (press the easy button!).
One question, can I have a 5V output from my RCP mk1 to power this or will I need to separate off another 12V hot and drop to 5V?

Post by **brentp** » Thu Jul 14, 2016 2:18 am

Keep us posted!

You can use the 5v output of MK1's vref to power this.

-Brent

kcolbey · Post by **kcolbey** » Tue Jul 26, 2016 1:11 am

Update: I got the CoilX installed and it still was not reading a signal. After using the scope, it seems is that it was dropping the signal at D2 diode. I tried to jumper it out, and it works! It is a little noisy from what I was looking at, it would be idling at 1200RPM then jump to 4800RPM and come back. I think the random multiple variables can be filtered out. I'll have to play with it after my next race, which unfortunately is a ways away.

Post by **brentp** » Mon Aug 22, 2016 6:38 am

Good idea with bypassing the diode! Looks like you're on the way to a stable reading.