Tach does not work with MSD in my 67 911

[1QuickS]

I upgraded my Permatunes for MSDs and as expected the tach does not work. I have twin plug ignition with switches to isolate each MSD and am using the tach trigger wire from one MSD.

What is the most typical work-around that has been developed for this.

Thanks!!!!!!!!

[Richy]

Questionable upgrade changing to a product made in China .

[1QuickS]

Thank you for that.

[Tom F.]

This?

https://www.pelicanparts.com/More_In...8aAmSKEALw_wcB

[edmayo]

Paul, I would contact Bob Ashlock, I think his 'tach adapt' might solve your problem.

[1QuickS]

Possibly, I thought there was another, domestic work around.

[1QuickS]

Thanks Ed, that is what I was looking for.

[1969911T]

Have a 1969T, went to an MSD 6AL-2, called their tech support and had to buy an adapter(PN 8920) now the tach works.

[Xavier PJB]

I run a MSD 6-AL on my 68 and I needed a MSD Tach Adaptor 8910 or 8920 (do not recall which one is best for Porsche) to make my rev tach works.

[304065]

Hi Paul! At last I can return the favor of technical advice.

Here's the story with the early tachs from 1964 to November, 1970. We'll take the Kettering-ignition cars first (before there was BHKZ!)

A 67 911 has a simple ignition circuit consisting of switched +12V to the coil 15 terminal (positive), then a wire from the coil 1 terminal (negative) to the points. The points are grounded through the distributor and from there through the transmission ground strap to the body.

When the points are closed, the coil charges to the first time constant. When the points open, the magnetic field in the coil primary breaks down, inducing a current in the coil secondary which comes out of the coil 4 terminal (HV) to the distributor, through the rotor to the plug and to ground.

Now, the tach in a 67 911 has a /1 terminal on the back that is connected through a black wire with violet stripe to the /1 terminal of the coil. Practically speaking, when the points open, WHAM! the tach input sees about a 200 volt dirty RF pulse on that terminal. This is enough to make it through the input circuit resistor on the tach board and turn OFF the first transistor Q1 which starts the Capacitor C2 charging and the needle starts moving. Successive pulses pin the needle in proportion to the arrival time and there you have your tach indication.

Ok, long story longer. The MSD box outputs a 12 volt square wave with a 50% duty cycle, if my memory serves. That's not enough to trigger the early tach.

You could do as I did, and have North Hollywood convert the tach to run on MSD (mine also runs on Kettering ignition as well because that's what I use these days, having given up exotic ignitions in favor of something I can diagnose without an oscilloscope) or use Ashlock's box which is an excellent solution.

Or. . . you could go down to the local ACE hardware and buy yourself a doorbell transformer, and connect one end to the MSD output and to the points, and one end to ground. This would energize the transformer's coil when the MSD signal was HIGH and when it breaks down, would give you a nice dirty pulse that would probably fire the tach just fine. This is, in fact, what is hiding inside of the expensive plastic box pictured above.

This also is a solution for 69- cars using BHKZ (CDI, for those who like that term) - the Factory and VDO didn't modify the tach to run off an 11 volt square wave until 10/70, so tachs before that use an "intermediate unit" -- the one that says "Einbau Senkrecht Klemmen Unten" on the side (great advice in more than one area of life, too). Inside those little unobtanium aluminum cans there's a coil of wire that charges up just like a . . . coil of wire. . . and fires off a signal to the tach in the same fashion.

My apologies for the simplicity and duration of this explanation.