Best way to pick up Tach signal from Car

Jim Fouch

I just bought a 2000 GMC Safari w/ a 4.3 V6. It has no tach. I'm thinking of making one using a SX28.

Any ideas the best way to pick up the tach signal? I'm thinking of using a 3.9V Zener Diode to chop down the signal to someting the SX can handle.

I'm also thinking of adding a gear display, but I don't think it's going to work too well when the torque converter is not locked up.

Another Idea is to get an interface chip that talks dirrectly to the OBD-II port, read & display the info with the SX.

Jim

Paul Baker

I would be concerned about the zener diode interfereing with the operation of the ignition coils (I assume thats the line you'd be tapping), the best would probably to using an inductive coil to pickup the signal from the line in an electrically isolated method. Another method would be to AC couple the power line of the battery (but would likely need more signal conditioning). Or use the OBD-II interface.

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Jim Fouch

On the interface I did for my motorcycle, I used the Zener approach, but I was using the signal that is sent to the bike's display. Since the Van does not have a tach already, I'll have to pick it up somewhere else.

ChrisP

The zener through a current limiting resistor approach shouldnt have the least effect on the ignition, the additional draw is tiny compared to normal operation. I'm playing with using an opto as the interface to the tach signal from the coil ground, clean things up and electrical decouple.

Jim Fouch

On my current set up on a Honda motorcycle (2004 CBR1000RR) I get a strange side effect that the needle (for the stock TACH)·bounces a bit around 5-6K. I'm thinking that the 12-18" of wire that go from the bike's wiring harness to the Mico are creating an attena effect. I'm also thinking of changing to an opto.