My tach won't work!

[Roger 911]

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.

[Wayne 962]

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

[dhoward]

Black/purple is the tach signal wire.

[Roger 911]

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

Thanks.

[dhoward]

Coil negative.

[Wayne 962]

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

[Wayne 962]

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

-Wayne

[Roger 911]

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.

[Wayne 962]

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

[Roger 911]

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.

[dhoward]

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...

[Wayne 962]

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

-Wayne

[dhoward]

Originally posted by Wayne at Pelican Parts
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......

[tfranko29]

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

[Roger 911]

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