Tach Installation .....

[George Zeck]

Hi All --

OK, first I must admit, I'm getting around to the items I've ben dreading. I had my tach re-conditioned / swapped from pos to neg groung in the summer of 2007. When I finished up the interior - I was so excited to get her on the road again and threw the unattached tach in the dash hole and started driving.

Fast forward to spring of 2009. I can't find my paperwork on who did the tach, but I probably have better - you guys.

How in the heck do I install this thing?

Some facts: Electrical Tach
Neg Ground

I know it has something to do with the ignition coil, but where, I keep reading about some "white loop" for magnetic fields and am just lost.

Tx-

George

 

[Sarastro]

The tach fires by sensing the current in the wire to the (+) side of the coil. It does this by sensing the magnetic fields generated by a loop of wire in this part of the circuit. Depending on which tach you have, there might be a loop attached to the rear of the enclosure, with a pair of bullet connectors on the ends. In some, the loop is internal and you have a two-pin connector. When the tach is changed from pos to neg ground, you have to flip the loop--presumably, whoever modified the tach has done this. In any case, if you hook it up and the tach doesn't work, try turning the loop over or, if you have the internal one, reverse the connections.

 

[George Zeck]

Do I splice into the + wire of the coil? Is there a way to test the tach? Again, electrical. Maybe 2 years of driving and bouncing around in the dash did it in? Probably not, electric is my wakest point and that is a generous commentary.

Tx-

George

 

[jvandyke]

White wire from ignition (key switch), loops through tach and goes to + (CB) side of coil. - side of coil (SW) goes to distributor. If you've been running without the tach I assume the white wire off the key is going straight to the coil, maybe there's enough slack to loop it through the back of tach first, or add some wire. Just converted my '63 to - ground and did the tach myself. Don't know how a '64 differs though. The direction of the loop matters though, so maybe use connectors so you can switch the "coming and going" wires around in case it doesn't work the first time? The internals of the tach have been done already it seems. Or is this an internal loop tach? Quick look at the back will tell.
Here's the insides of mine, not that you'll have to do this.

 

[nomad]

George: Your car is a 64 that originaly had a electric tach. Right?
If so I would bet the little loop is still in the coil feed wire and all that would be necessary is to cut the wire on each side of the loop and swap ends. The white loop fits inside a square piece of plastic that is held on to the tach by a strip of brass and a small knurled nut.
KA

 

[jvandyke]

He had it reconditioned and swapped to -earth but of course the white wire wouldn't have been on it then, and isn't now. I can look and tell you which direction the loop goes on a -earth car if you need me too. I think all you have to do is take the white wire off the ignition switch (the one that goes to the coil, there's three I think, you can use a continuity test between the coil end white wire and check which of the three it is at the switch end). Take it off there, add a wire from it, looping through the tach's back and then back to the ignition switch. Just need to know which way to run the loop, right?

 

[dklawson]

George... it seems like only yesterday we talked about this but I guess it's been several years.

Look on your tach's face for the letters "RVI" or "RVC". If your gauge is original it will probably be an RVI (impulse) type. For general wiring information on this see this link at the Speedy Cables web site:
https://www.speedycables.com/page34aaaa.html

The link shows the generic installation, yours will be a little different since the Midget will have pre-existing wiring. Succinctly, switched power in on the insulated spade lug, the case connected to a ground wire, illumination should be obvious.

On the back of the early RVI tachs there is a loop of white wire held in place by a bracket and an thumb-nut (picture in the link above). On OEM installations (like yours), a white wire leaves the ignition switch, goes to the loop of wire on the tach, then leaves the tach and goes to the "high" side of the ignition coil (the side of the coil that is NOT connected with the white/black wire to the distributor). That's all there is. Power to the coil passes through the looped wire. Each time the points open and close current is interrupted and creates the magnetic pulses Steve mentioned.

On aftermarket installations there is no supply from the ignition switch so the loop of white wire is connected IN PLACE OF the white/black wire between the coil and the distributor. This is the configuration shown in the link above at Speedy Cables.

If after you wire up the tach the instrument reads "funny", switch the two white wires on the tach so the current is effectively flowing the opposite direction. If that doesn't help you'll need to contact whoever rebuilt it. However, if you have fitted an electronic ignition, the RVI tach is unlikely to operate properly with it.

 

[George Zeck]

Doug -

As soon as I saw your name, I remembered the prior post. That's great. Will mess with it during the week and check back in.

Thanks -

Geo

 

[Sarastro]

The picture at the Speedy Cables site surprised me a little, since I thought that the loop was in the line to the + side of the coil, not the lead from the coil to the distributor, which is what the site shows. I checked the wiring diagrams and they show it in the lead to the + coil terminal. I guess it doesn't matter much, since the currents are ALMOST identical on either side of the coil. The only difference is that the high-voltage return is through the lead to the distributor, and I'd be reluctant to feed this current, even though it's very small, through the tach pickup. The voltage is quite a bit higher at the distributor side (a couple hundred volts), so be careful if you do it this way.

Whatever's most convenient probably will work just fine.

 

[dklawson]

Steve, why exactly wouldn't the current be the same on either side of the coil? Where would this difference in current be going?

The Speedycables site is only showing the "aftermarket" wiring of the induction loop. The Smiths datasheet it was taken from was for an accessory tach, not an OEM gauge. Obviously the BMC cars that had RVI tachs as standard would be easier to wire by pulling one side of the induction loop off the ignition switch and running the other to the high-side of the coil's low-tension circuit.

 

[Sarastro]

The catch is that the "cold" side of the high-voltage winding is connected to the low-voltage terminal that goes to the points. So, this terminal carries the HV coil current as well as the low-voltage current; the other LV terminal doesn't carry the HV current. It's gotta be a really small difference, though.

I don't know exactly why the cold side of the HV coil isn't simply grounded to the body. I suspect it's for safety--if it were grounded through the coil's body, and you unbolted the coil and thereby disconnected the ground, you'd have high voltage on the body of the coil.

 

[dklawson]

Thanks Steve.