Changing from a 911 641 301 00 (early 70's ) tach to a 911 641 301 86 (911 SC MY 86),the early tach has a different rear plug but both share a 3 wire connection.
Bowlsby (thanks Jeff ) pic shows wire 11 as being red/white as 12V supply ,wire 13 as being earth (ground) brown x 2, wire 12 black /purple I presume is signal from CDI box via relay board ?.
Click to view attachment
Late tach is #2 to 4 across the top and #5 to 8 across the bottom ,#2,6 and 7 are the only used, help please as me and VDO instruments have never been happy together .TIA.

Not quite sure what you're asking...but if you wired it as per Jeff and it's still not working, it's possible because the tach needs a low-voltage signal.

*IF* that is the case, then this *may* help. It allowed me to use my 1974 914 tach with the Microsquirt system.

https://www.pelicanparts.com/More_Info/PELT...tm?pn=PEL-TA-V3

Do you have MSD? Notorious for a poor tach signal for old analog tachs. MSD has a red box adaptor that some still have issues with.
Bob Aslock's Tach Adapt is a better solution.

I just put my /6 tach ('76 IIRC) in my 914 and it doesn't work.
I have MSD, before I had a Autometer Monster tach that works fine.

Thanks Greg and Mark and yes I should have been a bit more clear,I'am running a Ashlock tachadapt with the jumper reinstalled to suit low voltage for the 86 MY tach,the tach I'am replacing was high voltage and worked off the adaptor but was unstable from the get go until it failed completely.
I'am running a 911 Permatune blue CDI from a 75 MY car ,Pertronix ignitor II and correct Permatune ($$$) transformer coil.
What I'am trying to sort is connection to pins 2,6 and 7 so far I have this ;
Pin #2 > black purple stripe
Pin #5 > red white stripe
Pin #6 > brown (earth )
With Pin #2 my wire is yellow as output from the adaptor and the black/purple is the input to the adaptor if that makes sense.
I'am thinking this is correct ?.

I'm only seeing three connections on the tach in the pic. So my guess is:
2 - tach signal - typically black with purple
5 - power - typically red with white
6 - ground - errbody knows that one

Thanks Perry,what I'am thinking also after further research,your engine loom continues to give good service,cheers mate and stay well .

Quick test is to shove a 4 cyl tach in there, switch the Tack-Adapt, and see if it works.
I'm dealing with a engine harness on the wife's car where a PO managed to melt the black/purple wire. Don't forget to test all the basics.


Bob Ashlock is/was pretty quick to answer questions, send him an email.

Cheers Mark,sold my last 4 cyl tach about a year ago,yes I will send Bob an email,wonderful guy ,way back I contacted with a question on his adaptor and he also helped me setup my whole ignition system....super helpful guy.