Picking up an RPM pulse safely

Hutchie · Post by **Hutchie** » Tue Nov 26, 2013 5:36 pm

I'm still installing my RCP into my MR2 -and am now getting close to finishing, but I was wondering if anyone can advise me regarding my RPM input.

I have an RPM feed that was originally wired in by the race technician that built the car, with the sole purpose of supplying a data logger.

When I connect my multi-meter to this feed it reads about 17v, but given that the voltage is obviously pulsing I don't know what it peaks at.

I know the RCP's circuits are protected to 40v so my question is...
Is it safe to connect this feed and see what happens or could I risk blowing the circuitry? Should I get someone to accurately measure the peaks? or is there some other approach I should adopt to ensure I get a "safe" RPM feed?

Many thanks in advance - really excited about getting out on circuit to test some of this wizardry

Phil.

GTIspirit · Post by **GTIspirit** » Sat Nov 30, 2013 2:46 am

Check out this thread.
http://www.autosportlabs.org/viewtopic.php?t=3711
RCPro inputs can handle up to 40V. But, the signal still needs to be appropriate for the RCPro. I found out that even though my tach signal is 35V, the shape is no good for the RCPro.

The red signal is my tach signal. It is not compatible with the RCPro. The blue signal is from the distributor hall effect sensor. It is compatible with the RCPro.

My guess is that the RCPro pulse input registers anything over around 3.3V as high, and anything less than say 2V as low. The red signal is just too noisy, where it trails off is a couple volts and the RCPro gets erroneous readings. The blue signal from the hall effect sensor is a nice digital high/low signal and works quite well with the RCPro.

So what works to drive a tach might not work for rpm acquisition on the RCPro. If your tach signal is over 40V then Brent has a solution for you with the rpm opto isolator adapter. If it's less than 40V, the tach signal may or may not work, you might have to tap into the distributor or other digital rpm signal on your engine.

Hutchie · Post by **Hutchie** » Sat Nov 30, 2013 9:38 pm

Many thanks for this GTISpirit - really appreciate it!
looks like I need to get a scope to look at what the signal(s) look like and save myself a lot of heart ache!

All the best.

Phil.

Post by **brentp** » Sun Dec 01, 2013 2:23 am

This is what we will have for safely interfacing to coil pack primaries:

https://github.com/autosportlabs/CoilX

We will be offering built modules in our online store within the next few weeks.

SaabPower · Post by **SaabPower** » Thu Jan 30, 2014 4:55 pm

While looking into existing progressive shift lights and tachometers, I noticed that instructions mention tapping into the line from the ECU to the injectors to pick up RPMs. Has anyone done this successfully? My car is a Saab 900 with an ignition coil and distributor that has a hall effect sensor on it. Maybe I could tap into the hall effect sensor. I don't have a scope to see how many volts the coil puts out. I'm okay with buying the CoilX but if I can just tap a single wire that'd be great.

Post by **brentp** » Thu Jan 30, 2014 7:19 pm

The injector flyback may not create that high voltage pulse that CoilX was designed for, so we can't say for sure if it will work. Are you able to tap into the ignition coil (-) signal?

CoilX is now available here: http://www.autosportlabs.com/product/co ... t-adapter/