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Old 01-15-2019, 03:09 PM   #1
e30davie
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Megasquirt Tach output - coil driven tach (M20)

Hello all,

Just wanted to see if anyone has successfully used the megasquirt tacho output to run the oem tacho on a tacho that was originally driven by the coil.

My car is originally an L-jetronic system (m20b23), so the tacho is driven off the negative side of the coil. I'm not exactly sure what signal this is. is it just grounding the cluster tacho output every spark event?

my understanding is the tacho output of the megasquirt is a 5v pulsed signal.

I see reference to people using a relay to convert this signal. Not a normal clickly clicky relay i hope?

Is there any reason i cant just put a transistor circuit on the megasquirt output and pull the tacho input to ground?

for reference, im running ms2 extra, wasted spark on 6 x LS1 coils. distributor is still in place (to fill the hole) but not connected to anything. Everything is run off the MS2. Everything runs great, but having no tacho on the dash is a little annoying.
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Old 01-15-2019, 03:42 PM   #2
e30davie
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Ok well i might have found my own answer. Need to use the control side of the relay as it is an inductor. and when the tacho output turns off it simulates the callapsing of the coil and voltage spike, and this drives the oem tacho. Gonna grab a relay and try this tonight.
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Old 01-16-2019, 12:54 AM   #3
Jaxx_
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I have heard that you can use a standard tach out--as long as it is attached to S12C on the megasquirt board up the line.. I forget the specifics. It looks like this thread on the msextra forums might have your answer:
http://www.msextra.com/forums/viewtopic.php?t=41954
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Old 01-24-2019, 05:04 PM   #4
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Would this be the case for a M20B25?
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Old 01-24-2019, 05:25 PM   #5
e30davie
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My understand is the m20b25 ECU is motronic and has a tacho output to the dash cluster. and the megasquirt output can be used the same way. I dont see too many people complaining about the m20b25 tachos not working with megasquirt so it should just work.
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