Coil pack tach adapter on active-low signals

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Joined: Sat Jan 30, 2016 2:47 am

Coil pack tach adapter on active-low signals

Post by petik »

This is a bit of a unique question related to using the tach adapter on a non-megajolt application. Been using various RCP products for a while so when I ran into an issue on my chump car, came straight to looking at these products.

Problem: my ford focus svt chump car does not provide a tach output (regular mk1 focus does, but the ecu output on the svt for this is just dead - fun, right?). the car uses the factory coil pack with a dual input waste spark setup for the 4 cylinder. each input effectively fires on 2 of the 4 cylinders. i picked up an autometer tach which i used on my kit car that uses a non-svt zetec where i was able to use the regular ecu tach output without a problem only then to discover the svt doesn't output anything there. this is the tach: ... 0-468X.pdf
I could pull one of the ecu output to the coil pack and run it to the tach and get half the RPM reading on the tach. Selecting other cylinder options just makes the situation worse since 4cyl is the lowest setting and the higher ones just divide the value further.

So I went on a search and quickly found the coil pack tach adapter here and ordered a couple to play with. Hooked it up per diagram but the tach just didn't do anything. Started looking at this more closely and I think what I see is that the tach adapter expects the signal to the coil to be active high (that is, the voltage is going from low to high when the ecu requests a spark). However on the ford, the ecu actually pulls the line low on what is probably an open-drain configuration. This means that the two pulses going into the tach adapter likely erase each other on the output since one side is always pulling the other one up.

Toying with this a bit, I desoldered the two diodes on the two inputs I am using and reversed them. This does indeed fix the problem!

Question: Is reversing the diode in this configuration safe? Am I missing anything that could possibly cause an issue driving my coil pack?

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