Pertronix Flame Thrower / Bosch Sport Tach - 0 Reading

[74SBBaja]

Hi,

I am running a Pertronix D186504 Flame Thrower. I am testing a new Bosch Sport tach 7906. I am receiving a 0 reading on the tach. I have wired black -> negative, red -> positive, green -> negative of coil.

Is anyone else running this combination? Did you need to install a filter that the Bosch user manual states?

Bosch support said:
"In order for the tachometer to work it needs a 5 volt square wave for the reference signal. You would need to contact the ignition manufacture for specifications of the tach reference signal and their recommendations if a filter would be needed with the specific ignition system being used."

Thanks.

[Alstrup]

that tach is for newer cars/engines with magnetic pick up or VR sensor. It will not work with points or points replacement sytems.
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[74SBBaja]

The tech at aircooled.net said the tach was bad. Perhaps it needs a filter?

https://www.amazon.com/GlowShift-Tachometer-Gauge-...amp;sr=8-5

[Alstrup]

Points and points replacement units deliver 12V. So I´m pretty sure it wohnt work with those. But if you use some sort of filter between the units it might be another matter.
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[74SBBaja]

Ahh. Thanks for the info. I bet a square 5v wave. Wonder if a small resistor setup would work.

[richierich]

I have the bigger 5" Bosch one and nothing was required for that
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Blah.....

[Dale M.]

This document ...

https://www.boschperformance.com/sites/default/files/0002-002-2504.pdf

Says green wire goes to negative terminal of coil on page 3

PDF file was downloaded from this page....

https://www.boschperformance.com/products/sport-ii-2-58-tachometer-black-face-fst-7906

As usual confusion reigns...
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"Kellison Sand Piper Roadster" For Street & Show.
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All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ...

[74SBBaja]

Yes, I have read that document. It is confusing. Even though this tach is for a "12v" system, the tech at Bosch told me that it requires a 5v square wave signal for the trigger.

I did try using a 10k and 22k resistor inline with the green trigger wire with no luck.

I am not sure how they expect to get a 5v trigger from the negative of a 12v coil.

I am going to call them back and pick there brains for a more detailed explanation.

[Dale M.]

[Ohio Tom]

5V is just the minimum.

Resistor is not likely to do anything as the input impedance is high anyways.

[74SBBaja]

I called Bosch. A different tech told me that they are not even suppose to tell customers this.

The original tech emailed me back and said that they don't test or support non oem ignitions. Yet, on the product page it states:

Functions with breaker points, electronic, and most aftermarket ignition systems (not magnetos)

https://www.boschperformance.com/products/sport-ii-2-58-tachometer-black-face-fst-7906

I am going to try to call Pertronix again when they open.

A bit frustrated.

[74SBBaja]

I was able to contact Pertronix. The tech told me that their product looks for 0-12v. He recommended Autometer.

The Bosch tach can't read below 5v.

The Bosch tech said I might be able to get it to work if I can find a way to create a perfect 0-5v wave.

Looks like I will be stopping here.

Thank you all for your help.

[74SBBaja]

Quick Update.

Bosch is working with me to fix this issue. They have been very helpful so far.

We hope to have a resolution in a little while.

[Paul Windisch]

5 volt square wave is electronic stuff, and will not likely work in an analog environment. 5 volt reference circuits are what new (like, last 30-40 years, OEM digital fuel injection) vehicles use. Crank and cam sensors, throttle position sensors, MAP sensors, they all use a 5 volt reference. The control module (Engine Control Module, etc) provides a 5 volt circuit for use with the 5 volt sensor. There are usually 3 wires; 5 volts, ground (provided through the ecu), and a signal wire. There isn't anything on an old VW that provides 5 volts.

For a crank signal, as would be used for a tach, there is 5 volts applied to the sensor. The sensor usually is in close proximity to a toothed reluctor wheel. The pulsing of the teeth (and also the voids between them) causes a 5 volt ON/OFF signal that can be read on the signal wire. This is the square wave theory; it doesn't ramp voltage up or down, it's just on or off. The amount of times it turns on and off in a fixed amount of time indicates speed. But again, you don't have a reluctor, nor do you have a clean 5 volt signal. Unless Bosch provides some work around, you will likely find yourself using a different tach.
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[Ohio Tom]

[Paul Windisch]

[Dale M.]

IS tach new, used, donated?...

Wire tach up to known good working points distributor system as a test..... So far everything points to failed unit....

Dale
_________________
“Fear The Government That Wants To Take Your Guns" - Thomas Jefferson.

"Kellison Sand Piper Roadster" For Street & Show.
"Joe Pody Sandrover" Buggy with 2180 for Autocross (Sold)
============================================================
All suggestions and advice are purely my own opinion. You are free to ignore them if you wish ...

[Paul Windisch]

Well, after looking up the OP's tach model, it appears it should just work, not only from the installation manual, but also from the Q&A section of Summit Racing, where several people commented that they hooked it up to the negative post of the coil, and it worked fine.

Perhaps you just got a bad one.

Double check the wires are going to the proper places, and double check that it is set for the correct amount of cylinders.
_________________
*ASE Recertified Master Automotive Tech*

1984 Mexican Beetle
-1914cc
-L3 Heads 35x32 valves 52cc chambers
-0.040" deck for about 9.1:1 Compression
-Engle W110 cam
-CB Super Stock 1.1:1 Rockers
-Stock Heat Exchangers w/ Tri-Mil Muffler
-Dual Weber IDF 40s w/ 26mm venturis
-034 SVDA Distributor

2013 Chevrolet Volt DD
2005 Pontiac Montana SV6

MAHLE Service Solutions
Applications Engineer

[74SBBaja]

Bosch is stepping up and sending me a replacement tach. It will take a few weeks to get it. I will up date this thread with the results in case anyone else runs in to this.

Thanks again for everyone's help.

[74SBBaja]

A quick update.

I am still waiting on the replacement tach from Bosch. I am told that I should receive it in a couple of weeks. We will see.

Take care,
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74 SB
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