914 Tachometer not working Options

[vin man]

I've done some searching and have not yet found a thread that I'm sure has been covered at some point.

The tach in my 72 does not work. The car was converted from fuel injection to carbs long ago. I believe the reason the tach does not work is because it is no longer getting a signal from the computer (no longer have an ECU in the car).

How does one use the factory tach when I no longer have FI? In other car projects, I just ran a line from negative side of the coil to the back of the tach. But I don't think that will work here, will it?

[timothy_nd28]

20 bucks in Radio Shack parts and a Iphone (or similar), yes you can. I built my own signal amplifier a few years back, and it still works great.

You need to amplify these following frequencies: 33.3, 66.6, 100, 133, 166, 200hz

Each frequency correlates to engine rpm. 33.3 hz is 1000 rpm, 66.6hz is 2000 rpm, 100hz is 3000 rpm and so on.

The iphone has many good apps that you can download which will simulate these frequencies, but the amplitude of these frequencies is the problem as you will only get 1.4 volts from your phone's headphone jack. The tachometer needs to see atleast 30 volts before it will start moving. Buying a few transformers and a very standard JAN2222a transistor, you can build a circuit that will amplify the 1.4 volts to around 60 volts, enough to drive the tachometer.

If you are interested, I can get you a schematic.