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Tachometer 66 Sprite

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M

MCS

Jedi Warrior
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Hello,
I have two problems. Tachometer 1966 Mk3 Sprite

1. Missing the induction loop connector for the tach. Any ideas where I might find one, can I make one? Anybody have one they can send me. What's it do?
2. The tach is the Smiths positive ground and I am using the pertronix ignition, will this tach work with pertronix ignition.
Thanks Phil
 
Looks like you might have hit a wall.
 
I assume the inductor is only a piece of copper, or plated steel. It should make contact on both ends to the tach connections. You could of course make one, and there is no reason it shouldn't work correctly. After all it's only retrieving emf from the ignition wire and giving it to the tach electronics. We do have an electronics expert here somewhere, "Sarastro". He would know.
 
I suspect that your only source for an original connector will be to steal one off an existing unit or wiring harness. However, there is no real need to use the correct one, unless you're really picky about originality.

I don't have a lot of experience with these instruments--there are several kinds. Some are as simple as a little loop of wire on the back of the can, and others have the loop embedded in a piece of plastic inside the can with connector pins molded into the plastic and protruding from the back. I suspect you have the latter, it being a 66 car. If so, it probably have a combination of bullet connectors and 1/4" spade lug--so you can just use a separate bullet and spade connector to attach to it. If you can't find a connector that fits, consider soldering a short piece of wire to the existing connectors and using whatever other connector you can find. Be careful if the pins are mounted in a plastic piece-it's easy to melt this with a soldering iron. Polarity is important--so, if it doesn't work, first thing to try is reversing the connections.

The way these work is pretty simple: the loop is in series with the ignition primary, so every time a plug fires, there is a pulse of current in the loop. This creates a pulsed magnetic field which is sensed by the tach, and triggers a pulse in the electronics of the tach. The faster the pulses, the higher the speed. The important thing to remember is this: the loop is in series with the ignition system, so if it open-circuits, you lose ignition and the car just won't go. So, be SURE you make good connections!

As for the Pertronic ignition--can't say for sure, since I'm unfamiliar with this unit. I have a Crane, and the instructions for the Crane address this specifically. I suspect it should work--the electronic ignition just generates a pulse in the coil, same as the non-electronic one. It's just that the pulse is a somewhat different. The tach circuit has to be pretty versatile in dealing with a wide variation of pulse shapes, which is what happens in an ordinary ignition, so I'd expect that there is a better than 50-50 chance it will work.
 
The Smiths RVI tachs with the induction loop are current sensing. You'd think they should work fine. However, RVI tachs that work with Pertronix are the exception, not the rule. By all means make the part you're missing and try it. You have nothing to loose. If it doesn't work "well" when you first hook it up, reverse the direction of the induction loop on the back of the gauge. That may help, it won't hurt.

If you're not happy with the way the tach is working after trying this (or if you can't get it to work at all) don't waste a lot of time and money trying to make the tach work. As mentioned, RVI tachs that work with Pertronix are few and far between and even the places that repair them professionally don't hold out a lot of hope. However, you could look for a positive ground Smiths RVC (voltage sensing) type tach. These may be hard to find as I think most were negative ground. The RVC tachs are pretty much a 'modern' instrument and will work with Pertronix.
 
Interesting about the difficulties making the tach work with the Pertronix. It kinda surprises me and doesn't surprise me at the same time. I don't understand, for the life of me, why the tachs were made this way. It would have been simple to use voltage triggering instead of current, and that would have eliminated the possibility of ignition failure from a bad tach connector. Just one of the many incomprehensible decisions in designing these cars--part of their charm, I suppose.
 
Actually I've seen more ignition problems related to voltage sensing tachs than current sensing.

The voltage sensing tachs make a single connection to the distributor side of the ignition coil. If the wrong component dies in the tachometer you can get a short to chassis ground. This creates a permanently closed kill switch so you can't start the car until you disconnect the tach.

On the other hand, it's almost impossible for the current sensing tachs with their induction loop to cause problems because they aren't 'directly' connected to the tach. If the connectors on the white wire are intact, there's almost never an ignition problem from RVI tachs.

I don't understand why the RVI tachs have their problems and why some work with Pertronix and others don't. You'd think that power through the coil would be the same in each instance. However, somehow the equivalent of the dwell, or the peak current... something is different and this often makes it impossible for the RVI to trigger with Pertronix.

I REALLY wish that someone would start making tachs, nice and affordable tachs, with an inductive pickup like you use on a timing light. I'd like something that has a simple clamp or ring that goes around a single spark plug wire. You'd think that the tach makers would be all behind this because they wouldn't have to calibrate it for 4-, 6-, -8 (or other) cylinder counts. As long as it was for a 4-cycle engine, one tach would be able to read RPM on any engine without special components to allow multiple calibrations.
 
Hello,

Lot's of good information. I have been away from my computer for a few weeks. A family camping trip and then a week on the road working. I have not worked on my car ,it will be another week before I get to the tach. I was out for a 10 mile drive, nearly drove off the road. My brand new superlite front wheel was flapping in the wind. After I stopped for inspection I found two lug bolts broken. I guess 40 years of bolt stress is to much. Ordered a complete set of new bolts from Winners circle. Just happy the wheel did not come off completely. No damage just scared me. I installed the bolts today and used quarter inch spacers on the rear superlites, they fit better with the spacers. Managed too put on 30 miles today, no hood or interior but I loved it.
Thanks for the info
Phil
 
Ok time to beat the dead horse again. I have spent several hours trying to get this positive gnd tach (RVI) to work with pertronix. It will not work. I have read several articles about the RVI tach but nothing seems to help. I found a RCA tach from a midget in my junk box. Only problem is it is negative gnd. Does anybody have any ideas on how to switch an rca negative gnd tach to a pos. gnd tach? It is the smaller tach so I am hoping to fit it to my tach body once it is working. I really would like to keep the car pos. gnd.
Thanks Phil
 
RCA? Do you mean RVC? You'll find a lot more info on converting pos to neg ground than going back the other way. Most of the info out there is for converting RVI from pos to neg.

I should have been more clear in my earlier post. The RVI tachs almost never work with Pertronix. If you really want to keep positive ground AND run Pertronix, you may want to contact APT (www.gaugeguys.com) and see what they can do to help you. You may also want to contact Theo Smit and see what he can suggest.
https://members.shaw.ca/tsmit/tachmod/tachmod0.html

Browse all of Theo's pages. He's spent a lot of time and effort coming up with a universal tach board for our LBCs.
 
How easy or difficult was that process? I'm converting to neg ground, so I will need to do the same thing with my tach.
 
I have not done the pos to neg ground conversion on a tachometer. A web link detailing the process can be found at:
https://www.lotus-cortina.com/electric/convert.htm
However, this is simply a polarity conversion. The polarity converted RVI tach still won't like electronic ignitions.

Theo Smit's board replacement is a bit more involved but will address the Pertronix issue. However, I don't know what is necessary to make his board work with positive ground. Write him. I corresponded with him a couple of years back and he was very willing to answer questions.
 
Yes I mixed up the letters, should say RVC. I have read through the links you provided many times in the past. Flew into Calgary today and have tried to locate and call Theo Smit, seems he only has email. To bad because I am leaving Calgary early tomorrow for Ottawa. Will be back in Calgary the next day so if anyone has his phone can you PM me. I get out to Calgary several times each month.
I wonder what would happen if I wire up the midget tach using positive gnd. Maybe I would make smoke or maybe it would work.
Phil
 
I made the loop by cutting plastic from an old film canister
so you can do that part your self yes the older tachs donut like the "igniter" so I had to put the points back in
steve
 
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