How to test the tach?

[Quickstep192]

Today I noticed my tach stopped working. I recently converted to Pertronix points, but I'm not 100% sure that's the cause. How can I bench test the tach to see if it's the tach itself, or something upstream?

[Dave Colangelo]

the tach takes a square wave input from the sensor so all you need to do is generate a 0-12V square wave anywhere from one to a few hundred hertz and you should see the needle move up. To do this you will need a wave form generator. If you have access to an electronics lab ( at a local college or high school) there should be one sitting around. There are some iPhone apps that will generate wave forms but you will only get them at audio line level (somewhere around 1.7V peak to peak) so if you built a simple amplifier you could use use the phone to generate the signal.



or find a local pelican and see if he will let you stick it in his car....

Regards
Dave

[304065]

What car?

Dave even with a function generator an amp circuit is still required. (I have tried it with an 11v square wave from my signal generator and it's not sufficient current to drive a October 1970- tach).

[Dave Colangelo]

Originally Posted by 304065

What car?

Dave even with a function generator an amp circuit is still required. (I have tried it with an 11v square wave from my signal generator and it's not sufficient current to drive a October 1970- tach).

Interesting, I had an issue with my speedo (78 sc) and had no issue testing it with my signal generator (HP Agilent) without an amplifier. I guess it just depends how much your generator can drive. However now that I think about it the speedo my have a separate 12V input I'm not sure its been a while.

Regards
Dave

Sent from my Nexus 7 using Tapatalk

[304065]

Dave I suppose if you have an Agilent signal generator your problems are much greater than a tachometer! And the later tachs are +5v square wave. I think mine is running out of gas hence I need to fab an output stage for it.

[Dave Colangelo]

Originally Posted by 304065

Dave I suppose if you have an Agilent signal generator your problems are much greater than a tachometer! And the later tachs are +5v square wave. I think mine is running out of gas hence I need to fab an output stage for it.

Chances are I shouldn't tell you about my Tektronix scopes either... Good up to 700MHz is all ill say...

Back to the issue at hand. OP if you can get to an oscilloscope you can test the signal coming into the tach as well.


Regards
Dave

[Quickstep192]

A related question.......

When I wired up the Pertronix points, I connected the red Pertronix wire to a switched +12V source. I connected the black Pertronix wire to the black wire coming from the "C" terminal on the CDI and connected those to the purple wire going to the tach. Can I assume that the black "C" lead from the CDI provides the pulse for the ignition as well as the tach? If so, then can I also assume that if the engine is running that the ignition is getting the signal from the "C" lead? If this is true, then it would seem to indicate that the signal is good and either the wire to the tach is bad or the tach itself is bad. Sound reasonable?

[Dave Colangelo]

Without the wiring diagram in front of me I can say that if the engine does run than it sounds like the tach or wire to the tach is bad. If the engine runs than the system is pulsing the coil correctly and if I were a betting man the signal is being generated its just a matter of if it is getting to the tach.

Regards
Dave

[304065]

Originally Posted by Quickstep192

A related question.......

When I wired up the Pertronix points, I connected the red Pertronix wire to a switched +12V source. I connected the black Pertronix wire to the black wire coming from the "C" terminal on the CDI and connected those to the purple wire going to the tach. Can I assume that the black "C" lead from the CDI provides the pulse for the ignition as well as the tach? If so, then can I also assume that if the engine is running that the ignition is getting the signal from the "C" lead? If this is true, then it would seem to indicate that the signal is good and either the wire to the tach is bad or the tach itself is bad. Sound reasonable?

Pertronix red is switched +12V
black is points negative

***

BHKZ
a= achtung = 350V output
b= batterie = +12V switched
c= contact = points
d= ground to CDI case

***
Before 1978 Porsche tachs were driven off the points terminal. In the Kettering ignition days (1964-1968) the points terminal was also the coil negative, so the tach received a couple hundred volts up the line.

Then with the advent of BHKZ (CDI) in 1969, the points were used to trigger the "C" terminal on the box. Inside the box there is a 33 ohm pull-up resistor that was used to create a 420mA current in the circuit between power and the points when they were closed.

In either case, the points establish a ground when they are closed (during the dwell period) and then when they open the spark happens.

So we need to know a few things. What year car? what is the date stamp on the back of the tach? What are the labels on the terminals of the Tach? What type of CDI, three or six pin?

[Quickstep192]

I don't have all that info with me, but I'll provide what I have.

This is a '73 911T with a three pin CDI.

[Quickstep192]

And it's wired like the example in the link below.

http://www.pertronix.com/support/tips/pdf/porsche_cd_diagram.pdf

[dshepp806]

I believe there exists some threads out there that speak to specifics in terms of drive levels/Z needs and what the expected results should be on the O/P side of the circuit.

One needs to determine which input circuit they're dealing with,...it's associated Z,.....and drive capabilities of the function generator....scope would be mandatory.....

Best!

Doyle