Tachometer Tester

[Bob Ashlock]

From time to time, I see users raising questions about ways to determine if their tachometer is accurate, or in some cases even functional. Often problems occur due to mis-matched engine or ignition systems, that are quickly solved by my TACH-ADAPT product (Home) But, in helping users diagnose such problems, we occasionally find a tach that simply does not work and more often, one that has significant measurement error and really needs to be sent into a shop (like Palo Alto Speedo) for repair and refurbishment.

I've been kicking around the idea of making a small device that enthusiasts could use to bench-test most vintage Porsche tachometers. This little box provides a calibrated signal for 1000 thru 8000 RPM and will be simple to connect to the tach to do a quick test.

So my question is "would this be a handy tool for enthusiasts to use and share with their friends?". I'm thinking the cost would be around $60.

Please give some feedback: ("thumbs-up" or "thumbs-down" "waste of time" "too geeky" or whatever ... )

Thanks much ... Bob

[LJ851]

Interesting. Would it be capable of various cylinder engines?

[Bob Ashlock]

Yes, switch selectable for 4, 6 or 8-cylinders. Powered by a simple "wall-wart" transformer, or with clip-leads to 12V battery in car. Possibly could have internal battery for portability if that makes it more flexible.

[cashman]

My $40 Actron Multimeter has a function for testing RPM. Set the selector to 4,6 8 Cyl and then hook up to your master coil. I compare that reading to the tach in the car. Just my opinion (since you asked), I would not pay $60 for this single application device. The only difference is you cant bench test the unit and it must be in operation.

[dshepp806]

Your requirement is on the instrumentation end (it sounds),..so a (bench-grade, for me) function generator would be in order, at minimum...and a boatload of manufacturer's calibration data and a nice handy O-scope (sometimes)!

Best!

Doyle

[Dave Colangelo]

As an electrical engineer I see a place for such a device. It could also work on speedo's as well, or even all electronic gauges. If you want some help on the device I would be more than willing to help you out. I was fortunate enough to have an oscilloscope and arbitrary wave form generator on hand when I needed to test my gauges but I do understand that not every one has access to that kind of equipment.

Regards
Dave

[ChrisBennet]

My fancy timing light has an RPM readout so I use that for checking tachs.

[dshepp806]

How can you check the circuitry (even JUST the output) of a tach with a (fancy) timing light? This, I must hear about,.....

BEST!

Doyle

[jpnovak]

I have a function generator and Oscope to use. No need for me.

Honestly, most DIY guys are rather perplexed by electrical. Those that understand probably have a work around. How many times would someone use it. Even with access to such tools and having been through multiple restorations I have only used these for a tach input once. And that one time was me building a digital tach circuit to fix an old 911E tach. It is still functioning well to this day.

[Bob Ashlock]

I think Chris is saying that with a fancy timing light (one that has a digital display of rpms) you can compare that reading to that of your tachometer to get an idea of its calibration error.

I appreciate all you guys responding today. I think the wind is blowing against making a little product out of this thing. As useful as it is to me in my endeavors, most people would not often need it and low cost does not really even come into play as an incentive.

[dshepp806]

Understood, Bob. and entirely two different "things".

I like the fact that I can step into my lab and address a simple tach circuitry issue,..giving my instrumentation a break from what it normally does for a living (audio). So, I already have the "stuff" (and then some), so it's easy access. True: not everyone has these means, nor do they necc'y know how to use it (especially at the electronic levels). Nowadays one can get a scope and function meter REAL cheap (non-Lab grade) and "play"...however, not for everyone.

When it came time to get ALL of gauges reworked (restored), I just went ahead and let NHS handle everything, in that case. It wasn't any "call process" that was the challenge,..I just didn't want to beat my bezels up!!!!!!!!!!!!!!!1

BEST!

Doyle

[304065]

I think there is a market, particulalrly for those who haven't had training in discrete components (as in, everyone born in the last three decades)

I have a function generator, but it's only 10v p-p so I cant' get it to reliably drive the later tach. ("Later" to me means post October, 1970)

So Bob, you would need a timer circuit that generates a square wave, like a 555-- and then for an output stage you would need an amplifier to produce a 5V and 11V square wave for the later and middle tachs, and then a coil to produce the (nominal 24V) high-voltage for the earliest tachs that run off the coil negative. How about 200V with lots of ringing and noise on that line!

Building all this and then getting it to work is a challenge for the home enthusiast-- you also need pulse width modulation so you can use it for both six and four cylinder models (63% duty cycle vs. 50%).

A fun project and one that I would support IF you offer all the functionality that is required. Go for it!

[ChrisBennet]

Originally Posted by dshepp806

How can you check the circuitry (even JUST the output) of a tach with a (fancy) timing light? This, I must hear about,.....

BEST!

Doyle

Only eyeballs can check a tach *output*. But I can use the timing light to check a tach's calibration just as Bob surmised. I have an assistant rev the car to a fixed RPM on the tach while I compare it to what the digital timing light indicates is the actual RPM.

Or in other words, I have a Porsche designed tach signal generator that doubles as a car. ;-)

This method is a lot quicker than removing the tach from the dash, clipping a scope lead to the black+violet wire then taking the frequency and dividing it by 3. The battery in my scope gets discharged when it sits so I usually have to have it charging for a few hours before I can use it anyway.

[Bob Ashlock]

In response to 304065, you described my current design perfectly:

A precision square wave generator with output voltage levels selectable for compatibility with the early, mid and later style Porsche tachs (as well as most aftermarket and other auto manufacturers tachometers.) Selection of high-voltage pulse is included as well as switch selection to scale the output rate to 4, 6, or 8-cylinders. These are the basic features of the first prototype I have running on my workbench.

This small portable test box could be battery powered for convenience. I spoke of using it to 'bench-test' the tachs, but it also could be used to provide a signal direct to a tach still mounted in the vehicle by injecting the test signal at appropriate places to insure the vehicle wiring is correct. (I have helped numerous owners troubleshoot "bad" tachometers, only to find that mods to the ignition system or engine swaps created an incorrect connection to the otherwise good tach.)