944 Tachometer Test and Calibration

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Tom
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Carmagic wrote: Mon Nov 03, 2025 4:37 am Is that 9v battery trick only applicable for the 944 tach, or can that also be used for any other tach ? I have a few tach sitting around and would be good to know if these are still OK.

Thanks for posting this
Engelbert

Hard to generalize with confidence, but I'd say it has a good chance of working on any tach that has a 12v, ground, and ignition pulse input. The DME cars drive the tach with a digital pulse, so very little current is needed. Not sure if the older tachs need more power to twitch, but I doubt you could hurt one trying. With each ignition pulse, the needle is propelled upward a twitch, so the idea with the screwdriver is to create a crude pulse by dragging it over the stud threads (where is 'pulses' by breaking contact as it skips from thread to thread). If the tach does not have a stud connector like the 944, you can connect the input line to a spare bolt and do the same thing. Will be curious to hear if it works. :)

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spacecad3t
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Well, I should have taken a photo, but my '87 NA Tach did not look exactly like yours Tom, and only has 1 adjuster, assuming the 2nd is for the boost gauge?

I took a stab at just turning my adjuster left (reduce speed) a tiny amount, well I think my RPM increased? lol... I need to setup an Arduino and pull my cluster again...

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spacecad3t
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So I went back into my cluster and got this photo of the single potentiometer. Tom, I think in your video, turning left reduced speed and right increased it? I found the opposite here. Turning right decreased the low end RPM reading.
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Tom
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spacecad3t wrote: Fri Nov 07, 2025 7:52 am So I went back into my cluster and got this photo of the single potentiometer. Tom, I think in your video, turning left reduced speed and right increased it? I found the opposite here. Turning right decreased the low end RPM reading.
Good info, thanks! Yes, it was the opposite on my turbo tach. I assume the second pot is for the boost gauge...

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Luke
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Many thanks for your video and Arduino code @Tom . I used it to help diagnose my tachometer after It decided to stop working a couple of weeks ago. I used your battery test to prove the fault was with the gauge itself, then tested all the components on the little tachometer circuit board, and found a shorted 47uF capacitor, which I replaced, and brought the tach back to life.

I used your Arduino code to prove that it was fixed before I go through the effort of getting it back in the car.

However, I've always had an issue with the tach "bouncing", i.e. it overshoots where it's meant to go, then undershoots and finally ends up settling in the correct place, and replacing this cap doesn't seem to have improved that. I've tested all the electronic components on the board, and can't find any issues, so I'm starting to wonder if it's a mechanical issue with the hairsprings, etc.

I've uploaded a video of it here:
(I updated the Arduino code to cycle back to the start and move every 3 seconds)

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@Luke

Hope you don't mind, but I edited your post's "mention" to Tom to make it active (ie: so that it shows in red typeface). That way he will receive a notification. :angel:

Activating that function is accomplished by using the drop-down list that will populate as soon as you type the first letter or number after entering the @ sign.
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Tom
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My guess is that bounce is from mechanical wear/issues. For fun, I called on CarBot to write code to smooth it out. If your tach is really bad, it may get too sluggish with this solution, but my guess is you could use this approach to find a happy medium if nothing else...

https://www.carpokes.com/viewtopic.php?t=5219

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Luke
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Tom wrote: Mon May 18, 2026 5:51 pm My guess is that bounce is from mechanical wear/issues. For fun, I called on CarBot to write code to smooth it out. If your tach is really bad, it may get too sluggish with this solution, but my guess is you could use this approach to find a happy medium if nothing else...

https://www.carpokes.com/viewtopic.php?t=5219
Yes, that's my suspicion too. I've messaged a couple of people locally who are breaking 944s, to see if they will sell the tach or cluster. I think they're around £150 new 😬

Thanks for the carbot thread, very interesting reading. As carbot said, I think any electrical "dampening" we try to add will also impact the actual responsiveness of the guage. If I had all the components lying around, I'd give it a test for sure, would be interesting to see the result!

I still need to order some of the 911 style guage surrounds from you!

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