Tach modification?
 

I am wrapping up installing a carbed 73 2.4t in my 68 911 Targa. My question is, I went with a CDI box (68 did not have one) and am wondering if my tach is going to work. Do I need to replace the tach or purchase some sort of adapter?

Thanks,
Alan

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206AA
1968 911 Targa 2.4t

Anyone?

It should work OK. First connect it to the points connection,
then try connecting it to the coil. It shouldn't damage the CD.

The orignial was connected to the coil w/o a CD.

Good luck
Loren
Systemsc.com

Thanks Loren.

--Alan

So I tried hooking up the tach as recommended by Loren and no dice. Has anyone else run into problems with using a 68 tach on a 73 motor with a CD unit? Do I need to purchase a 73 tach?

Any help would be greatly appreciated. ..

Thanks,
Alan


206aa
1968 911 Targa (2.4t)

The tach then needs to be modified for the CDI signal which is
a very narrow pulse. You should first try a later type tach that
was designed for a CDI box. Also, made sure that the wire
(black/purple) is OK from the engine compartment.

Good Luck
Loren

Alan,

The early battery-coil ignition cars had a ballast resistor in the coil & tach circuit. It may not be needed if the Black/Violet wire is connected to the points at the distributor. The 100 Ohm internal pull-up resistor in the CDI unit at the trigger input lead [Terminal 'C'] causes the tach and Speed Relay [ a.k.a. RPM Transducer or Speed Switch] to 'see' a nominal 12 Volt and ground waveform at the Black/Violet wire to the tach.

Are you using the relay/regulator/fuse/CDI-unit panel from a '73 car? If not, my suggestion is to make sure that your CDI-unit wiring EXACTLY duplicates the '73 ignition system, including the Bosch 0.221.121.001 coil! The grounding of the CDI-unit is very important, as is a braided ground cable from CDI-unit to distributor and coil. Even though the early CDI-units are called '3-pin' ... the fourth terminal, ground, is a very important part of the system!

I suggest a careful examination of the '65 and '73 circuit diagrams at this Pelican page:

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

Got it. Looks like I just needed to run it (purple/black) off the coil, same post as the blue wire from the CDI unit. Works like a charm now. Thanks for all of the help guys. .

-Alan


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206AA
1968 911 Targa (2.4t)