My tach won't work!
 

I finished reinstalling my motor, which included a conversion from T to E cams, as well as MFI to Webers. Apparently, I forgot to properly mark the wire(s) for the tach. The motor starts and runs fine, all other guages and lights appear to work, except the tach.

There is a purple and black wire not hooked up to anything. I must have assumed this was related to the MFI. Is this my tach wire? Any other thoughts on what wires I need to chase down in the engine compartment? The car is a '72T 2.4.

Thanks.

I'm pretty sure that's the universal color for the tach wire, although it's been a few years since I've looked that info up.

This might help you:

http://www.pelicanparts.com/911/911_Parts/911_electrical_diagrams.htm

-Wayne

Black/purple is the tach signal wire.

Great! Now where does this wire go? Coil, distributor, CD box?

Thanks.

Coil negative.

One end goes to the tach, the other end should go to the points (which should go to the coil negative, or vice versa)...

-Wayne

On later cars, the tach wire came from the CD box...

-Wayne

"Fried" tach?
 

I hooked up the purple and blach tach wire to the positive (+) terminal on the coil by accident. The tach moved a bit then nothing. When I tried to hook up the wire ti the negative terminal I got nothing. When I hook up an external tach, e.g., tach and dwell meter, I hook the positive lead to the black wire going to the distributor. This seems to work. Naturually, when I hook the purple and black tach wire to the black distributor wire, I get nothing.

So, is it possible to "fry" the tach by hooking it up incorrectly? Any other ideas or suggestions?

Thanks.

I'm sure it's possible to fry it, but hooking up the signal wire to the + terminal shouldn't do anything bad. Maybe if you hooked +12V to the signal terminal, that would be bad.

You should have brown to ground (-) (not necessarily "G" on the gauge), red or redish to +, and black/purple to the signal terminal...

-Wayne

Wayne,

I'm a bit confused by your reply. I did hook the black and purple (signal?)wire to what I believe is a 12v+ terminal, the + terminal on the coil. So, could this have damaged the tach?

Thanks.

The + side of the coil always has +12 V on it with the ignition on. I believe that the Black/Purple signal wire is looking for a square wave type of output, counting the points opening and closing. I don't know what would happen if you held 12V on continously...
:confused:

I don't know. I'm not as confused as you are as to what you did...

-Wayne

I'm not as think as you drunk I am.

If it's a CDI type system you may have damaged the tach as I believe that the voltage applied to the + terminal of the coil is higher than 12 volts......

Did you ever get the tach to work? I'm having similar trouble.

No, I haven't tried. It's "off season" here in Chicago, so I may just wait until Spring.