postive to negative ground with alternator

[Boobtube]

I'm doing both a switch from positive to negative ground AND replacing the generator with an alternator(actually, a dynamator). I wanted to keep the tach and have changed the two internal connections for this. But I see in some of the writeups, that the direction of the loop in the back of the tach is also changed in conversions. Is this still necessary if I am also changing to a negative ground dynamator? Wouldn't that automatically change the current direction in the loop wire? Besides, the loop is fixed into a plastic bracket that doesn't allow the loop to be moved and seems to allow only one way of mounting to the back of the tach.

 

[Tom H]

When you change from Pos to Neg earth the direction of the current to the coil changes direction, so yes, you need to change the loop on the back of the tach. And yes, the plastic piece is there to maintain the same direction so it does not get accidentally switched when doing any maintenance work on the dash. But when doing a deliberate switch of polarity you need to reverse to loop. You'll need to cut the wire and re-loop it thru the plastic piece in reverse direction and then splice the wire. . OR you can leave the plastic piece in place and make a reversed loop a few inches away and put that new loop thru/under the steel U clip, being careful not to nick the insulation on the wire. The insulation is typically thick enough and strong enough that the steel clip will not cut thru it. I have had such a loop on my tach for 30 years with no problem. Be sure the steel clip is installed with both its tips in contact with the mating tips protruding out from inside the tach. That steel clip is a critical part of this pulse transformer.

Tom

 

[Boobtube]

When you change from Pos to Neg earth the direction of the current to the coil changes direction, so yes, you need to change the loop on the back of the tach. And yes, the plastic piece is there to maintain the same direction so it does not get accidentally switched when doing any maintenance work on the dash. But when doing a deliberate switch of polarity you need to reverse to loop. You'll need to cut the wire and re-loop it thru the plastic piece in reverse direction and then splice the wire. . OR you can leave the plastic piece in place and make a reversed loop a few inches away and put that new loop thru/under the steel U clip, being careful not to nick the insulation on the wire. The insulation is typically thick enough and strong enough that the steel clip will not cut thru it. I have had such a loop on my tach for 30 years with no problem. Be sure the steel clip is installed with both its tips in contact with the mating tips protruding out from inside the tach. That steel clip is a critical part of this pulse transformer.

Tom

Thanks Tom, I'll switch that loop around. I'm still a bit confused as to how this tach actually works and why the loop direction is so critical. It would seem that a pulse is generated in either direction of current flow so why does the direction of the loop make a difference?

 

[mikephillips]

I suspect it creates a magnetic field in the metal loop it goes through that the tach uses. I can say that when rewiring the Tiger I got the loop backwards initially and the tach was dead. When I reversed it, worked fine. So it does make a difference. And rather than cut the wire, follow it to the ignition switch and remove the spade connector there, replacing it when the wire loop has been changed. Seemed better than to have an addition connection in the middle of the wire.

 

[Tom H]

The tach circuit produces a pulse (about 2 mSec) each time the points close and apply power to the coil. The circuit I triggered by applying a negative going edge to the input transistor in the tach circuit. The pulse transformer that the loop is part of will couple the edge of each pulse of current going to the coil. That edge needs to be the correct (i.e. negative) polarity or it will not turn that input transistor on. Transistors are polarity sensitive devices.

 

[Boobtube]

Ah. I see. Thanks.