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RVI tach question

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Taz

Taz

Jedi Trainee
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I am converting the tach in my car to a neg earth RVI tach. I know this has been discused before but I can no t find it. On the back of the tach there are 2 blade term. one is a ground to the case the other I would think goes to the + on the coil. there is also to barrel conecters 1 male 1 female not sure what they are for. do not see a alt light in this tach like my old pos earth tach. could anybody shed some light on this for me.

Thanks Carl
 
RVI tachs do not wire like modern, voltage sensing tachs. They are current sensing and are installed in series with current passing through the coil.

You are correct that the case goes to chassis ground. The second spade lug should be for switched power. I assume what you are calling barrel connectors are the regular Lucas bullet type connectors. These form an induction loop.

To wire the tach you supply power to the spade lugs as mentioned. The two bullet connectors are inserted in series to the coil power. You can either go 1) from the ignition switch, to the tach, from the tach to the coil (high side)... or 2) you can go from the coil (low side), to the tach, from the tach to the distributor. Current flowing through that wire creates an pulse inside the tachomter. If the reading is erratic, switch which bullet connector is high and which is low.

If you've outfitted an electronic ignition to your car... by all means try the RVI. However, if it doesn't work with your electronic ignition (a hit or miss proposition) then I wouldn't expend the effort trying to make it work. It will cost more money and effort than it is worth.
 
Thanks
I was hopeing that this would solve my problem with a very eratic tach. changing to neg earth and pertronic ing have made the factory tach usless.

Carl
 
Taz
I have a 67 midget; originally positive earth. My original tach was one of the older large diameter, 4",ones. (RVI 2401/00)
I recently converted my positive earth tach to a negative earth RVI tach(RVI 1433/00)3 1/2 "(small ones). I bought a later model, 1970 or so, negative earth 3 1/2" RVI 1433/00 on e bay for five bucks or so. Wasn't much, for sure. I removed the face plate and needle from my old tach, replaced the small tach face plate and needle with these. A little bit of care is important, here. Just take your time and you should be fine... some case modification was necessary... remove just enough of your old case in the back to allow the new "small tach" connectors to fit; everything else fit included the screws in the back of the case and the front face screws /ubbthreads/images/graemlins/cheers.gif. Fire it up. Mine worked just fine. /ubbthreads/images/graemlins/driving.gif The white wires in back... one comes from the coil and the other goes to the ignition switch... the tach is in series. The green wire is 12 volt power and the case absolutely needs to be grounded. Any of the black wires back there behind the dash will do.

Good Luck
Gunner
 
Gunner's transplant may work, however, RVI tachs don't always like working with electronic ignitions. Again, it's hit or miss. Replacing one RVI movement for another when electronic ignition is fitted may be frustrating.

If you find a later RVC tach, you can transplant its movement to the RVI case and expect success. The RVC tach is a voltage sensing unit that connects to the low (distributor side) of the coil with a single wire like modern aftermarket tachometers.

If you're more adventurous, you can fit a circuit board or entire movement from another brand of donor tach but this takes more work. If you're interested, download my PDF:
https://home.mindspring.com/~purlawson/files/Smiths%20Tachometer%20Conversion-R1.pdf
 
what years did the RVC tach come in. I bought the RVI on ebay thinking that all the neg earth tach were 1 wire tachs.

Owell anybody need a real nice RVI tach

Thanks Carl
 
Thanks for your discreption. If I do not find another tach Iwill try this with my Pertronix and see if it will work

Carl
 
RVI, RVC whats that.

Hate it when you guys talk over my head.

Elec. tac came in 4" then sometime changed to 3 1/2 inch diam?
 
RVI = Current sensing, has either two bullet connectors on the back or a place for a loop of white wire to be clamped to the back of the case. Phased out sometime around 1970 (I don't know the exact dates... sorry).

RVC = voltage sensing tach. Has fewer connections, ususally including one bullet connector on the back of the case. I have seen some large case ones used in other British cars. I've even seen some Smiths ones re-labeled for sale as "Lucas".
 
FWIW, My 1970 car has a RVI Tach, and it works fine with a pertronix unit. It does, however, stop at 4700 rpm. But I think it was doing that before a installed the pertronix. I'm going to switch to a RVC unit I got from Ebay for $5. (If it works)
 
If you decide to go with an oem RVI I have a real nice one from a 71. make ya a good deal on it.

Later
 
When the RVI units work, they will work well until the components inside start failing with age. As I mentioned earlier, it appears to be a hit-or-miss thing finding one that works with Pertronix and Crane ignitions.
 
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