12 replies to this topic

[WillMini]

hi, i have a 1978 mini 1000, and i want to hook up a tach. using this as a reference i wired the tach up which is from an MGB. having the femal go to a switched live, the male fo to the negative side of the coil and the spade connector in th middle of the body going to ground, and i left the other one with nothing as i was going to test it. wellll, i think there may have been a short and the wires melted as soon as i turned on the ignition. so i un hooked it all. then i went to restart it and the car wouldnt start. i then checked to see if there was a spark and there wasnt one. did i fry my coil? if the wire comming from the switched live somehow shorted with the wire from the coil would that kill my coil?


also, has anyone used an mgb tach in a mini?

this is it

Edited by WillMini, 28 March 2009 - 03:20 PM.

[WillMini]

oh and this is the reference i used to wire it.
http://www.lotuselan...abeledtacho.jpg

[dklawson]

What you have probably done is melt a wire to/from the coil, and you may have destroyed the tach but you are not likely to have damaged the coil.

What you show in the pictures is the Smiths RVI tachometer. It's old. It's from the late 1960s, early 1970s. It does not wire like a modern tach.

Start by removing the tach from your car and put the wiring back as standard. Turn on the ignition and use a multimeter to confirm that you have battery voltage between coil (+) and earth. Disconnect the wire between the coil and distributor (white/black). Again with the ignition on, measure between earth and the coil (-) terminal where you disconnected the white/black wire. With the white/black wire removed it should also be at battery voltage. Let us know what you find.

Assuming you have power at both coil terminals with the white/black wire removed, remove the distributor cap and look inside. You MAY have melted the wire between the breaker plate and the dizzy housing or you may have melted the wire going from the points out to the coil (-) terminal. Obviously if either of these wires is damaged you will need to replace them. Again, let us know what you find.


The RVI tach is a current pulse sensing and is wired in series with power to/from the coil. It does not connect like a normal, modern tachometer, nor is it compatible with most electronic ignitions. Here's a succinct explanation of the wiring.
Spade lug on case = earth connection
Spade lug on insulator plate (next to bullets) = switched, fused battery power supply (12V)
The bullet connectors....
For an aftermarket installation (like this) run two wires from the bullet connectors on the tach from the gauge to the engine compartment.
Remove the white/black wire between the coil and dizzy.
Connect one tach wire to coil (-), connect the other wire to the distributor.
Test the tach by starting the engine.
If the tach reads incorrectly, switch the two wires at the coil and distributor.

Fundamentally, each time the points open and close, current flows through a transformer inside the tach case (the two bullet connectors). The tach is NOT directly connected to the ignition system. Current for the coil simply flows through the tach and creates pulses (via the transformer) that the tach monitors and uses via circuitry to move the needle.

I'm not sure exactly how you wired up the RVI tach but any damage to your car is likely to be with the wiring to the coil or in the distributor, not the coil itself.

Do post back with your findings.

[WillMini]

hey, thanks alot for that! i found that i actually fused my points together, so with a little bit of sanding i was up and running. I used your method of hooking up the tach which worked, the car runs and the tach reads. BUT the tach is reading about half of what it should be. it idles at about 400 and revs to about 2000. do you have any idea how to calibrate it??

[dklawson]

Earlier RVI tachs than you have had a two transistors and a capacitor that got tired with age. Once replaced, there was a trimming potentiometer inside that allowed easy calibration of the gauge. Some of the later RVI tachs like yours have a single transistor and I don't think they had a potentiometer. Sorry I can't help further. This may be a job for Speedy Cables if you want to use this tach.

[Saxo-Fiesta-Mini]

will be reading the cam speed and not the engine speed u will need to adjust the voltage or possibly get a rev suppressor off a fez

[dklawson]

??????

[Saxo-Fiesta-Mini]

on the fiesta mk3 the ignition coil when wired to a tach reads half speed cam shaft speed

theres a lil rev suppresor unit that bolts to the wing that cures this as mine ready half speed i just lived with it

[dklawson]

That's interesting. Was your Fiesta tachometer a factory installation or was it an aftermarket part? If it was an aftermarket part did you check that it was set for 4-cylinder operation as most aftermarket units can be set to 4-, 6-, or 8-cylinder. If you install an aftermarket tach on a 4-cylinder engine with the selector switch set for 8-cylinder it will read 1/2 the true speed.

WillMini's Smiths RVI tach was designed to be placed in series with coil power. It was designed for a 4-cylinder engine (MGB) which uses an ignition system fundamentally identical to the Mini. No additional filters, suppressors, or external components should be needed. However, as I mentioned earlier, the RVI tachs do not work with most electronic ignitions.

[Saxo-Fiesta-Mini]

nah was the same with factory dials and mine was set to 4 cylinder not 8 lol

[dklawson]

Usually when suppressors or other bits are added to the coil wiring it is to filter out radio noise. I've not seen them applied to tachometers before.

WillMini, have you tried reversing the two coil/dizzy wires for the tach as I mentioned above? This will reverse the current flow through the tach's internal wiring/transformer. Sometimes that helps, sometimes not.

[WillMini]

i have tried switiching those wires, when it is reversed there is no change. Also i popped a plug out of the back and foind an adjustment and it made the revs go up, but it still only revved to about 2500 where it should have been at about 5000. i might try to replace the capicitors maybe. is this difficult?

[dklawson]

The capacitor inside is something like a 0.25 uF stacked Mylar item. It is NLA. However, I think you'll find similar parts available "close" to 0.25 uF. One of those will be fine. They are pretty easy to replace as when new they are relatively bullet-proof.

Google for RVI tach repair and you should find a couple of online schematics. I've come across a couple of web sites that discuss modern replacement components for the capacitor and the transistors.

This is one such online schematic.
http://freespace.vir...ndham/tacho.gif
I believe this is for the earlier RVI with the external induction/transformer loop.

This one is for a later Austin-Heley tach:
http://www.hazelden....uildingBJ8Tach/
You'll note that it's similar but has some differences in components to work with the Healey's 6-cylinder engine.