Tachometer/Petronix help

[9112.2t]

My car is a 75 911s with a 1971 2.2"T" engine
I have a 2.2 that HAD a Marelli distributor. I replaced it with a Bosch 2.7 distributor that was machined down to fit, with a petronix igniter.

I hooked up the petronix red "+" wire to the fuse block & the black to the purple/black-along with the "-" side of a Bosch Blue coil.

Now my tach reads up to 1000 rpm then drops dead?
I need a little help.
Thanks Bob.

[Walter_Middie]

Bob,

Are you using a CDI box? The original Bosch 3 wire or something like an MSD?

If using a CDI, make sure it is grounded.

On my 1975, there is a white plastic connector for the +12 volts switched through the ignition and the tach signal – just follow the red wire from the CDI unit to the white plastic connector. The red wire from the CDI unit also contains a small canister about an inch in diameter and two inches long. I believe my manual refers to this as an RPM sensor for the tach. If you are using MSD, remove it.

Your tach's trying to "listen" for an 11V square wave with a 63% duty cycle, and what it's "hearing" may be a lower duty cycle. That means the tach doesn't have enough time for the capacitor in your tach to charge up and move the needle high enough.

[9112.2t]

First Thanks for the help, the Bosch 3 wire has bin cut & is not in the circuit, also No MSD that leaves CDI. I cant find the white RPM censer? I am posting a couple pic's and If anyone sees the problem please let me know, I am just trying to figure it out.
Thanks Bob.

[Walter_Middie]

Your tach is designed to work off the CDI box. If you are running a "Kettering" ignition (points and coil only), then you are sending a 11V square wave with a 63% duty cycle to the tach (I think I had it backwards in my post above).

The RPM sensor isn't white - the connector is. The sensor is silver. It's just a filter - don't worry about that.

[9112.2t]

Ok, I know dumb qustion how do I fix it? It looks like just points and coil olny "Kettering" ignition?

[Walter_Middie]

how do I fix it?

The easiest fix, and the best for performance, would be to connect the CDI box to your ignition - assuming that the CDI box is still working.

[9112.2t]

Is there a way to test the CDI to see if it still in working order?

[9112.2t]

OK, I have hooked up the CDI and Petroniox just like the diagram in the posting, now NO Tachometer at all? My car runs great but no tach. The car will die if I disconnect the "C" wire from the circuit, Meaning just the Black wire to Purple-Tach wire.
I really want my tach. I've seen a man here on Pelican selling an adapter for $60 & just go back to the "Kettering" circuit & leave it at that, Is this the answer?
Any help is appreciated Bob.

[Walter_Middie]

Did the tach work with the Marelli distributor in it? And it wasn't using the CDI box? I wonder if you have an older tach? Is the tach original to your car - 1975?

[9112.2t]

Yes the tach was working, sort-of it wasent reading right. The CDI was not hooked up. The po cut the wires and left stubs, in my opinion sujesting that when the original moter was removed this could be reused if a replacement 2.7 was found. I believe the tachometer to be original to the car.
The puzzle continues.
It's late and I have to lay down my back is hurting, I'll try again tomorrow.
Thanks again for the help, Bob

[9112.2t]

Any other ideas?

[Walter_Middie]

Morning bump for more ideas.

My original 1975 tach works great with either points and the CDI box or Pertronics and an MSD box. I'm concerned that you were using the tach with just points and coil - no CDI box. That makes me think you either have an older tach or some signal conditioners in line with the tach signal.

[9112.2t]

I think I am ging to try a Tach adepter From Bob Ashlock another member here and see whare that takes me???????
I domt know what else to try. Thank you Walter for the bump.

[9112.2t]

OK, I have installed the tach adapter, nothing. This is all hooked up with the 3 pin just like in the prior post & as per the tach adapter. I have ohm-ed the black/purple wire to the tach and have continuity to the tach, that makes me think I have burnt up my tach, Son Of A b***H.
I think I am in the market for a used tach part number 911.641.301.29
230/085/001
12v. 24900jmp
6zyl/4t8kt
12.74
I am going to try and take it apart the tach just to see if it's something I can screw up
Any input will help.
Thanks again Bob.

[9112.2t]

Anybody?

[Bob Ashlock]

Following up here. Sent you an email. I too am concerned that the tach may have been damaged when it was hooked-up to the coil (-) lead with the Pertronix.

[josborn10]

Hello 2.2,

Did you ever get an answer?

J