MT Prop Tach Signal Advice?

GMN

New Member
Joined
Jul 14, 2010
Messages
2
I am having trouble getting my MT Prop Tach signal working. It has an active Hall Sensor pickup that returns 6 ppr at 10.25 volts, however its low signal is about 0.45 volts. I got no indication at all in the Sensor Debug screen when I first tried it on a Standard tach input. Because the low signal never actually hit zero volts, I modified the signal with a pull-down resistor and now it produces a 6.25 volt hi and 0.02 volt lo. I switched to a Low Voltage input and still get no indication of anything at the ADC. Looking at the signal with a scope, it appears to be a reasonably good looking square wave. So what's going on here? I looked in the sensor file to see if the tach input had also selected a pullup or pulldown, but the tach inputs are not even in the sensor file. Is the built-in implementation providing a pullup and expecting the tach pickup to provide the ground? Is this then not compatible with the voltage being supplied through the MT harness?

I think one of two things is going on. Either I am still not producing an acceptable signal, possibly due to a conflict of pullups. Or I may have toasted an ADC input during my initial testing. This already happened when I first hooked up the fuel quantity senders (pins 20/21) because my old BMA unit used pullups on their resistive senders and I hit the ADC's with 12 volts. Even after removing these and returning to a purely resistive sender, the two inputs were unresponsive. Moving to a different pair of ADC inputs solved the problem. So perhaps I have toasted my tach input channels too?

Anyone gotten their MT tach signal working yet? Please advise.
 

dynonsupport

Dynon Technical Support
Staff member
Joined
Mar 23, 2005
Messages
13,226
First off,
All our GP inputs can handle 15V without damage, in most cases. They can't read above 5V, but they aren't damaged by it. But when you have a device set as a fuel input, it has a 200 ohm pullup to 5V, and if you put 12V on that you are putting 7V across 200 ohms, which probably will burn up that resistor since it's not designed to take 1/4 watt for very long. I'd expect the pin could read voltage, but it would not have a 200 ohm pullup working anymore. This all depends what the external pullup to 12V was.

On the Low V RPM input, you should be able to put 15V on these with no issues. As long as the voltage goes below about 1V, we should be able to read it. There is no pull-up on these inputs, they are about 1K impedance to ground and that's it.

So your RPM signal sounds like it should be working fine.

You do need to have an input configured for the sensor debug screen to work, so you do have RPM turned on, right? Also, this screen for RPM is a counter, so you must be toggling high/low to see anything.

We'd be happy to look at you EMS module and fix it for you if you are concerned it might be in there.
 
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