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tachometer questions

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Z

zimasprite

Jedi Hopeful
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Hey guys,

I'm trying to get my tachometer to work with the crane ignition system. I'm done a bit of reading in past posts but still am not sure how to go about getting it to work. The white ignition wire/loop has continuity to the distributor, the green has power when grounded to the black. No reading on the tach whatsoever. It's an RVI style tach.

1) Will this tach work? If so, how should I hook it up different?
2) If another tach works, what kind and how should I hook it up?
3) Are there any aftermarket options like VDO, etc that work?

Thanks again - Drew

p.s. Primer is here, new wheel cylinders are here, interior panels about done! Getting there....
 
I know more about the Crane than about the electronic tachs, but I'll hazard a suggestion.

First, you need the current from the positive supply (assuming negative ground) to go through the tach to the coil. The Crane literature shows a direct connection, with a vague picture of the tach connection (see this picture). But the tach is current operated, so you need the coil current to pass through it.

If it still doesn't work, try reversing the connection at the tach--the Crane's current waveform is very different from that of the standard ignition, and, without getting into a lot of reasons, I'll just say it might be necessary. If you do all this and still can't get it to work, you might be out of luck. A lot of the old electronic tachs didn't work well with newer electronic ignitions. I don't know what year your Sprite is, but if it's fairly early, you might try a tach from a later car.
 
OK, I looked at the ignition/tachometer today. The diagram has a connection from the - of the coil to the tachometer. I'm assuming this ISNT the RVI type of tach. The RVI has a loop that is continuous with the positive side of the coil on to the ignition module. Do I need an RVC type? I also did try reversing the wiring, but the connectors are not compatible. Can't hook a male to male and female to female.

Thanks for any help - Drew
 
Yes, the Crane instructions and the figure in my last posting assume a non-RVI tach. Which is what just about everyone else uses--I have no idea why Smiths chose that strange loop-coupling system, but there it is anyway.

Did you check the instructions regarding Smiths tachs in the Crane installation instruction book? I rechecked my book--it seems that the standard installation should work, as long as you have a negative ground car. They do recommend reducing the coupling to the tach, which can be done only if you have an external loop. It's true that the tach could fail to trigger if it has too much signal, as well as too little.

Another thought--is there anything connected to the +12V circuit between the tach and the ignition? Any capacitors connected to the coil, for example, to suppress radio noise? These could definitely interfere with tach operation, but probably would do so with the standard installation, too.

Only other suggestion is an electronically knowledgeable friend with an oscilloscope. He could probably sort it out in a minute.
 
One more thought--did you check for ballast resistance? The XR700 requires a ballast resistor, either external or a so-called ballasted coil. If you don't have that, there will definitely be too much current in the circuit, which could prevent the tach from operating and will eventually fry the module from overheating. If the coil has about 4 ohms of resistance, which you can check with a cheap multimeter (about $7 from Harbor Freight), you are OK. If it's much lower, you need an external ballast resistor.
 
I did do a google search last night and find the XR700 instructions. I'm planning to reduce the loop # tonight and see what happens. The tach now, with double loop, doesn't register a thing. It does have the external loop. If that doesn't work, I'll try the RVC. The coil does have an external ballast resistor. I bought this thing from someone and it's a fire trap right now. I need to do some housekeeping to get it safe. I'd likely say that nothing else is in the 12V line since nothing is hooked up on the car now. That'll be a fun few days/weeks figuring out the wiring. Hmmm...does the wire not work/is old and corroded or does the switch/guage not work....I can see it now! I'll grab my multimeter and start plugging away.

Thanks - Drew
 
As mentioned, the RVI tachs are current sensing and have to go in series with power to or from the coil (regardless of points or electronic ignition). The most common RVI tachs use the external induction loop on the back of the tach. There are anecdotal reports of people improving the number of turns of this external loop, changing the direction of the coils, and/or changing where each of the two white wires connects. In short, you've got several variables to try to see if you can make your RVI tach work with your ignition.

I don't have any Crane manuals to refer to. However, if you want to try the RVI tach, make sure it's in series with coil power. This means that one white wire must go from the ignition switch to the tach, a second (after the loop on the gauge) goes from the gauge back to the high side of the coil. Alternatively, you can have one white wire go from the low side of the coil to the loop on the tach, then back to the distributor (or in your case... distributor connection for the ignition module). The important thing is that the RVI tach whites wires have to carry the power to or from the coil. The white wires are NOT powering the tach, you have a separate power supply wire on the gauge to worry about.

There's a lot of documentation out there about RVI tachs not working with Pertronix electronic ignitions. I don't know if that automatically applies to the Crane units.

The later type Smiths gauges are the RVC type and they are voltage sensing tachs like most modern instruments. They are MUCH easier to install and should work with your Crane without any issue.
 
I tried to modify the # of loops with no avail. Looks like it's time to get a RVC and try it. Power is there, the loop is there...must be the gauge.

Drew
 
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