Tach modification?

[206aa]

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

[206aa]

Anyone?

[Lorenfb]

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

[206aa]

Thanks Loren.

--Alan

[arringto]

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)

[Lorenfb]

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

[Early_S_Man]

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

[206aa]

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)