Tach Send for Tachometer 4 wire VS. 3 wire

[OldManMirage]

I have a question here I hope someone can answer for me.

My Volvo MD7A has a tach send that comes off the camshaft I believe. It is two wires. The tach doesn't seem to care which is which, it is a 4 wire tach that uses the "W" and "G" for the send.

I'm looking at a new panel upgrade that has a 3 wire tach. I believe I can just connect the "G" wire to the negative ? The "W" would go to the "Tach Send."

And thoughts on this ?

[OldManMirage]

The silence is awkward.

[OldManMirage]

Am I not asking the right question ? Or is my explanation poor ?

I guess I can go trial and error but I thought some pretty good mechanics were on here and would have come across this before.

[Wotname]

I don't have direct knowledge of the Volvo MD7A tach circuit but I do know a little bit about some electrical/electronic systems.

From your description it would seem the old Volvo circuit had a two terminal tach sender on the engine (neither of which is connected to the frame of the sender) and these was connected to two terminals on the tach meter and the polarity of these two wires didn't matter.

Also the new tach meter only has three wires which presumably are 12v power positive, 12V negative and the tach sender wire.

If both of the above assumptions are correct, I would approach the wiring this way.
Connect the tach meter power wires (+ve and -ve) in the normal manner and then connect the tach meter sender wire from the tach meter to either of the terminals on the engine sender and the other one on the sender to engine ground (i.e. 12V -ve). If that doesn't work , reverse the wires on the terminals of the engine sender. If still no joy, perhaps the new tach isn't compatible with the Volvo engine sender.

What is the make / model of the new tach meter?

[OldManMirage]

Thanks for the reply Wotname, that's pretty much what my thinking was.

I don't know the brand of the new tach, I haven't bought the panel yet. There is a company in California (I think) called ACDCMarine that builds them.

You understood exactly what I was trying to say. Thanks again. I suppose the smart thing to do would be to email them also.