Pertronix and Tachometer

[Graham]

I have searched for this without luck.

Does anyone have a Pertronix points replacement installed along with a Flamethrower coil AND have a working Tachometer?

I have read many posts where the tach did not work properly, but never what exactly the solution was.

[WANT '71 280SEL]

I don't know why a Pertronix and Flame Thrower coil would incapacitate a tachometer.... I don't think the two issues are related.

Regards,

David

[Mike D]

Probably because the older tachometers were triggered by electrical impulses (on/off) and the Pertronix is electronic (pulses). Don't know for sure but it stands to reason. The "negative" terminal on the coil probably receives a minute electronic signal which disrupts the "short-open" pattern the tach is looking for.

[Graham]

Originally Posted by WANT '71 280SEL

I don't know why a Pertronix and Flame Thrower coil would incapacitate a tachometer.... I don't think the two issues are related.

Regards,

David

They are related. The vdo tach was not designed to work off the 12v coil negative as most cars do. It worked off the line between the points and the MB switchgear which had a ~3V pulse and the coil negative was grounded.

I had a response from Pertronix:

I have had many different people make this work. It can be a little bit of work but you can make the tachometer work with the Ignitor.

First the tachometer wire will hook to coil negative. Sometimes you can just hook up the tachometer wire to coil negative and it works. But most of the time you are going to need to install a resister in the tachometer wire that is connected to coil negative. This is the tricky part of the install. For years I just recommend an 11.5K ˝ watt resister and that seems to work. However recently I have had to use different size one’s to get the tachometer to work. I have used a 10K ˝ watt and a 7K ˝ watt and a 9K ˝ watt. So, as long as you’re willing to work normally you can make it work.

[mercmad6.3]

Yes the resistor is the trick. go to www.m-100 .cc where this issue was resolved some time ago.

[WANT '71 280SEL]

Interesting. I'm glad I know this before my Pertronix install!

Regards,

David

[Graham]

Originally Posted by mercmad6.3

Yes the resistor is the trick. go to www.m-100 .cc where this issue was resolved some time ago.

Sorry, what was that link again?

OK, I found it by trial and error

I found one reference to using a 50Kohm resistor, but it also seemed that those engines used a higher voltage at points, so perhaps that makes a differnce?

No idea, but another 350SL owner on BW says he has his Pert/Flamethrower running with an 8.2Kohm resistor.

I have a coil on order, but it's freezing here, so I have about 5 months to find some 1/2+watt resistors

[350SL4spd]

Hey Graham,

Did you ever get this sorted? At the point on 350 that I'd like a working tach...

Thanks!