Motors Wiring a 12v car clock : Help

Today i got some old car instruments. a 12v battery meter, a 12v clock and a tachometer.

But the only one i can get working is the battery meter, which is the main think i wanted so i could put it on the 12v line of my psu.

Though im trying to get the clock to work and i just can not. dose anyone have any idea with these kind of things? The tachometer is also 12v but im guessing it cant get that to do anything even hold a constant rpm?

 

digital clock or analogue clock?, digital ones will usually require 2x +12v power feeds (one permanent and one switched usually by the ignition) and an earth, an analogue one will generally only require power (permanent or it loses the time) and earth, the tachometer uses power and earth for main power and the tacho signal is a pulse (usually generated by the coil as it discharges or the ECU in an EFi vehicle)

 

The clock is analouge, and it has only 2 electric contacts on the back, but i feed 12v into it and nothing happens. It also has 12V printed on the back, so either its dead or im doing it wrong somehow

Edit : think the contacts where just a bit corroded. works fine now

But the tacho will still be useless except for looking good on 0rpm ?

 

pretty much, unless you know some clever bugger that can build you a circuit to drive it off the fan rpm wire

 

Something like that would be cool. I was thinking volume levels or something. I have lots of time on my hands so maybe i should make something of it

 

I just thought one way of doing it would be with a magnetic sensor on the fan, bit like a bicycle speedo setup, it would work pretty much the same as a crank sensor on a car which generates a pulsed signal, which may be able to drive a revcounter. For a volume meter you would have to have a circuit that would generate the right amount of pulses for the revcounter depending on volume level, bear in mind that the revcounter will be calibrated for a certain number of pulses per revolution displayed, 4 cylinder ones and 6 cylinder ones expect different pulse rates.

 

So just take the yellow wire from the fan for reading the fanspeed? i shall do some expermenting

Or do you mean something else. This is confusing stuff lol

Edit. Looks like this will be beyone me, so i shall stick with what i have for now.