Weirdest electrical problem yet?

[Bo]

Got my 72 911 back from the shop a few weeks ago, after having the 3.0 L engine installed (with all electronics), and I have the weirdest electrical problem I have ever experienced!

This will be a test of your collective brainpower:

The tach isn't working - UNLESS (and this is weird..) you remove the ground wire from the tach AND turn on the headlights.
No headlights on - not working. Ground wire on - not working. And when it DOES work, it only goes up to around 2500rpm (no matter how high you rev the engine).

I have tried the following:
1. Ground the tach directly to the chassie. No difference.
2. Check to see that power is delivered to the tach. It is.
3. Tried supplying power (and ground) from external power supply. No difference.
4. Tried no less than 3 (!) other tachometers. Same thing.

The signal the tach is supposed to work with comes from the CD-box, right? (wrong?)
I haven't gone there yet, but my mechanic told me he made sure the wire he connected to the box (harness) was the one going into the tach.
What else can I do?

The thing is that my speedo doesn't work either, but that COULD be a defect sender.. (havent got 'round to that yet).

I'm pulling my hair from my head! I love my car, but on days like these, she sure is testing my patience...

[Early_S_Man]

Are the tachs you tried from '78 or later cars with the magnetic trigger 6-pin CDI units? If not, that is your problem!

Yes, the TD signal from the 6-pin CDI-unit drives the tach.

[Bo]

Warren, I'm not sure I quite understand your reply - the tach I have is from a 76-77 (or something like that) car, and all of the electronics original 82 SC. Aren't they compatible?

[Early_S_Man]

Bo,

Generally, the tachs from '77 and earlier cars that used the 13-14 Volt point-triggered signal are not compatible with the TD signal from '78 and later magnetic-triggered CDI-units. Some late '77 tachs were more sensitive, and will work, but most early tachs are not sensitive enough to reliably use the lower-level signal generated by the CDI-units at the TD terminal.

[Bo]

Ok, that could be the explanation then...

Great, I have to buy another tach then - more money to throw into the Bottomless Money Pit (aka 911 ownership..).

Thanks for your reply anyway! Might have saved me a great deal of trial-and-error time.

/Bo