Testing a tach
 

Is there any way I can test a tach out of the car?

You can use a jumper off the dizzy (Black/purple) and a ground jumper. I forget your year, I know it an early 911. The one that has the transducer thingy might be a bit trickier. But, I'd bet a jumper off the black/purple as it leaves the relay panel would do it.

I sent DByers a tach for just this purpose. Well, checking his output at the engine compartment, not checking the tach itself. He reports that it works fine jumped as I described.

With a 12 Volt power supply and this DIY calibrator, yes ...

http://forums.pelicanparts.com/uploads/tach_calibr.jpg

Hmmm, maybe Warren should be at Willow Springs come Nov. 15th. ;)

Agh - I should have been more specific...

My car is a '73; the tach I want to test is, I think, from an SC.

I'm looking for something easy to do - like connect it to an ext. battery and then run a wire to some terminal on the tach like the Black/purple wire. Definitely don't want to run around twon all day collecting components and then have to wire something up.

Can I just run a wire from the dizzy and then get + and - 12v to an SC tach with my car??

also, there are 2 terminals stamped "TD" it seems -- are those signals terminals?

and why are there 2 ??

http://forums.pelicanparts.com/uploa...1161063843.jpg

Randy, I see you bought the tach. The "TD" or tachometer drive signal can be attached to either terminal. I think it's repeated on the tach for the fuel cutout, but both are connected to the same point internally.

So the signal requirement, as I have written many times in the archives :), is an 11v square wave. It doesn't carry a lot of current, only 420mA, so make sure you have a clean signal path.

So if you don't happen to have a signal generator handy, you can wire up Warren's test rig, should work great.

"So the signal requirement, as I have written many times in the archives , is an 11v square wave. It doesn't carry a lot of current, only 420mA, so make sure you have a clean signal path."

Actually, it's only a 6 volt signal and it's just a "pull" to ground of less than
10ma. A simple test is to power the tach with +12 & ground and then use a
coarse file connected to ground with a jumper wire connected to the TD input.
Run the jumper wire across the file and the tach needle will move. The faster
the jumper is moved across the file the higher the tach will read. This works
only for low voltage signal driven tachs like the 911SC tach shown.

Hey -- That's pretty clever

Save those old wall wort transformers

Loren,

Thank you for the clarification. I was basing the 11 Volt Square Wave number off of older tachs, like the VDO 333.230/041/007 to 333.230/041/010, where it is picked up from the distributor.

At first I was wondering, where is there 6V in the car? But now I remember, the tacho drive signal in an SC comes from the TD pin of the Six-pin CDI, so the 6V must originate inside the CDI itself.

Thanks again, I will update this in the tach chart.