 Tach Bounce |
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| 914Bryan |
 Oct 14 2011, 05:54 PM
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#1
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Member  Group: Members Posts: 190 Joined: 6-June 11 From: Orange County Member No.: 13,162 Region Association: Southern California  |
What can I do about a bouncing tach? When I shift, the needle wildly bounces around until the rpm settles in a gear. Then it either climbs or drops depending on acceleration or decelaration. Any ideas? (IMG:style_emoticons/default/confused24.gif)
74 2.0, stock fi and dizzy/coil |
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| Spoke |
Oct 16 2011, 10:20 AM
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#2
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Jerry Group: Members Posts: 7,104 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None 
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There's 2 basic reasons the tach bounces:
1) Points opening and closing causes transients causing bouncing almost anytime the engine is running even a constant RPM. This happens when you have points-issues. 2) The early model tach needles do not have sufficient damping to prevent under shoot and over shoot when the RPM changes. Change the RPM and the needle oscillates back and forth and finally settles to the correct RPM. See the general step responses below (step being from one RPM level to another). If the tach is not damped enough, it oscillates.  To damp the 914 tach, simply connect a 2200uF capacitor across the needle winding as shown below. This will transform the operation of the tach to be like tachs in today's cars which never bounce or oscillate. Attached image(s) 
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Posts in this topic
914Bryan Tach Bounce Oct 14 2011, 05:54 PM
Dave_Darling If your car still has ignition points, check them ... Oct 14 2011, 06:22 PM Valy
What can I do about a bouncing tach? When I shift... Oct 14 2011, 06:23 PM 914Bryan I was thinking points too..Side note, I was thinki... Oct 14 2011, 07:08 PM Vacca Rabite If you are running Djet, you need to keep the stoc... Oct 14 2011, 07:50 PM avidfanjpl Get a pertronix for that car. I only keep points a... Oct 14 2011, 11:47 PM Valy Just for reference, this is what your bad points/c... Oct 15 2011, 02:07 AM 914Bryan Awesome info guys! Thanks! Oct 15 2011, 05:42 PM Tom Valy,
That is some bad switch bounce! The... Oct 15 2011, 07:40 PM michael7810 My tach does the same thing. I have a new distrib... Oct 15 2011, 08:09 PM
mgphoto
My tach does the same thing. I have a new distri... Oct 17 2011, 10:59 AM 914Bryan Sooo maybe an Autometer monster tach instead of th... Oct 15 2011, 08:26 PM Valy
My dear friend,
you solved a symptom, not the... Oct 16 2011, 10:34 PM Spoke
My dear friend,
you solved a symptom, not th... Oct 17 2011, 07:02 AM Valy
[quote name='Valy' post='1556077' date='Oct 17 20... Oct 18 2011, 07:26 PM Spoke
The problem is that the bouncing fools the tach t... Oct 19 2011, 04:40 AM Vacca Rabite I don't suppose you could point out the cap th... Oct 16 2011, 10:41 AM Bartlett 914
I don't suppose you could point out the cap t... Oct 16 2011, 02:38 PM Spoke
My Tach jumps also. I don't remember which c... Oct 16 2011, 07:05 PM Spoke
I don't suppose you could point out the cap t... Oct 16 2011, 07:01 PM 9146986 Sometimes just putting a clean piece of thin cardb... Oct 17 2011, 10:46 AM larss I had about the same symptoms on my tach, the need... Oct 17 2011, 11:23 AM |
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