6 replies to this topic

[splintercat]

want to put in a 1275GT 120mph 3 clock cluster in instead of the 110 set that i have, the only problem i have is the rev counter from the 120mph set up has three pins connectors at the back
male spade
male bullet
female bullet

the rev counter i have at the moment has just two
male spade
male bullet

the 120mph rev counter is an RV1 negative earth can anyone help me wire this in? looked at all the other questions in the forum but to no avail........cheers in advance

[dklawson]

If I understand you, you want to use the RVI tach from the 120 MPH setup. Is that correct? If you have an electronic ignition, do not plan on using the RVI tach. Very, very few RVI tachs will work with electronic ignitions and frankly, most are not worth paying to convert.

To fit an RVI tach plan on only using the power wire from your old tach. The switched power wire from your existing tach will connect to the male spade lug on the RVI tach next to the two bullet connectors. Obviously the RVI tach will also need an earth connection.

The RVI tachs work by monitoring the electrical current pulses passing through the ignition system. Therefore, they are wired in series with the ignition coil. The easiest method is for you to run two new, white wires from the bullet connectors on the RVI tach, through the fire wall and to the front of the engine. REMOVE the white/black wire running between the coil (-) terminal and the distributor. If there is a second white/black wire on the coil... it is the sense wire for your old tach. Disconnect that wire also. Connect one of your two new white wires to the coil (-) terminal. Connect the other new white wire to the distributor (where you disconnected the white/black wire). What you have now created is a current path that goes from the coil, through the tach, and on to the distributor.

Start the engine and observe the tach. If it reads incorrectly, switch the white wire connections (move the coil connection to the distributor and the distributor wire to the coil). If the gauge works but still reads incorrectly, consider having the tach professionally rebuilt and re-calibrated.

[splintercat]

Phew ...got all that mate and it makes sense to me.............. thank you for a quick reply n ill let you know how i got on bud

[Doz1971]

I must get round to making a circuit for converting RVI into RVC ones.... I'm picturing a FET and a handful of resistors... should be easy really...

Gate to coil with 10K to ground. 12V on the "Current sense" coil, other end of the coil to source via ~3 ohm watty resistor. Drain to ground. Diode across the coil to prevent ringing and havoc breaking out ....

[dklawson]

There are lots of projects out there where people have made driver boards to convert RVI tachs to RVC. Most/many focus on the use of circuits using the 555 timer.

I have made a circuit of my own on a breadboard but I chose the other popular route using the LM2917 chip. That chip handles almost all of the voltage sensing tach functions but by itself cannot drive the current requirements of the RVI tach movement. In my circuit it was necessary to add a second amplifier circuit to supply the required current. That in turn required a negative power rail for the amplifier. In the end the circuit became more complicated than I wanted to pursue as a board someone could make themselves so I left it at the breadboard level and never packaged it for use in a car.

The person who has probably spent the most effort on this is Theo Smit from Canada. Visit his web site (link below) to see his circuit and what he has done.
http://members.shaw....d/tachmod0.html

[Doz1971]

I'm not talking about going down that route, which is a complete conversion, just using a high impedance voltage sensor to drive a gob of current through the existing current sensing coil. That way it should work with electronic ignitions without all the hassle I went to to get it to work....

... Oh hang on .... No .... We're going to need to invert and shape the pulse. Poop.

[dklawson]

It's a bit more than converting the voltage pulse to a current. I tried using a MOSFET fired off a square wave with equal on/off periods to trigger RVI tachs. The tachs would work but would NOT read the correct equivalent RPM. The only way I was able to calibrate RVI tachs was to use a mock ignition system with a points distributor spun by a DC motor to supply current pulses to the tach as it would normally see.

RVI tachs wont work with MOST electronic ignitions because they have a different on/off cycle than points do. Once you get outside of the points equivalent dwell the tach does not read correctly. If you can figure out a way to take your voltage pulse as a trigger and control the current dwell through the RVI tach induction loop to match what points do you should achieve what you are suggesting.