I recently posted on some problems with a Smith's impulse tach.

At first I thought it might be a bad capacitor, but later learned that the tach was missing the iron core bracket. It won't work without one, and I was lucky enough to get a replacement.

My question concerns the direction of current travel. This was a positive ground car, but it's been converted to negative ground. That seems to be important because reversing the loop wire is part of the conversion, as is reversing a couple of connections within the tach (which has been done).

The Hayne's manual has schematics for both types, + and - ground, cars. I'll try to outline the circuit:
Starter solenoid - wire to ignition switch
Ign switch - wire to impulse loop on back of tach
Impulse loop
Wire from impulse loop to + terminal on coil
There is also a wire from the - coil terminal to the distributor.

The circuits appear to be the same for + and - ground cars with one exception.
The positive ground shows the wire from the ignition switch entering the loop from the LEFT side, while the negative ground version shows the same wire entering the loop from the RIGHT side.

It's a big deal to get this right, or it won't work and could possibly damage the innards of the tach if it's connected backwards (so I've read). I thought about just trying it one way, and if that didn't work try it the other way, but I'd rather not risk messing up the tach. Part of my confusion is because I don't know from what view the schematic is drawn. Are they showing the loop from the back or front? - I don't know.

To add to my confusion the loop kit came with directions and an all together different wiring diagram.

For a negative ground car they show the impulse loop in series between the distributor and coil. The wire from the coil CB terminal to the distributor seems to have been eliminated. Instead they show the wire going from the distributor to the RIGHT side of the impulse loop, then the loop (with little arrows drawn in to show current direction?), then the same wire goes to the CB terminal on the coil.

I was hoping to get a clue from their diagram, but don't understand electricity enough to do so.

I'd just as soon retain the stock wiring, so how should I connect the loop? I'd guess the wire from the ignition switch goes in the right side of the loop, and the wire (same wire) on the left side of the loop goes to the + coil terminal.

Thanks.

“For a negative ground car they show the impulse loop in series between the distributor and coil.”

Not sure of the answer so maybe I’ll add to the confusion. Any tach needs a pulsed input to it. On a negative ground conventional coil car, it is coil (-) that provides the pulse.
A car coil is a step-up transformer. Any transformer doesn’t like DC voltage into it and won’t step-up the voltage. The points to ground pulse the voltage and mechanically chop or pulse the voltage into the coil.
If your English car is now set up the same way as a GM car, the coil (+) is a steady DC voltage and won’t pulse the tach.

Again, not sure of the answer but sounds like the line between the distributor and coil has something to do with the pulse signal. That is, assuming this is coil (-) and the points are inside the distributor. On a GM car you would attach to this point but your tach sounds like it requires you to cut this line and put the impulse loop between the coil and distributor. This impulse loop sounds like another coil.

Thanks for the reply John. I checked the coil wires tonight, and the (+) side goes to the ignition switch and the (-) side goes to the distributor. I guess the second wire is similar to the wire whcih came out from the bottom of Chevy points distributors and went to the (-) coil terminal.

I thought about it and tried what I thought would be the right way. No reading on the tach.

Then I reversed the wires from the (+) coil terminal and ignition switch at the loop on the back of the tach, and it worked. Don't know if it's accurate or not, but now I do know how Moses felt when the Red Sea parted.

Originally posted by Glenn1018:
Don't know if it's accurate or not, but now I do know how Moses felt when the Red Sea parted. Yeah, but heard the Israelites had to walk though the mud afterwards.

Glad I've got a Chevy too, or I'd be walking in the mud right behind them.