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Go Speedracer, go!
Join Date: Aug 2002
Location: Indianapolis
Posts: 1,951
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dampening the tachometer
Is there a way to dampen the tachometer without having it rebuilt? Mine isn’t that bad, but it does bounce a lot if I rev the engine quickly. I have noticed that on an 88 it seems like it is dampened more than my 81.
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1981 SC ROW Coupe |
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08-02-2003, 10:39 AM
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Location: Port Macquarie Australia
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I have noticed a number of posts concerning high system voltages causing tachometer fluctuations, a problem I am currently rectifying on my 1988 930.
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2008 BMW 320i 2006 Mercedes ML320 2004 Mercedes ML270 cdi 1998 Porsche Boxter 1988 Porsche 930 Targa (sold) |
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08-02-2003, 12:38 PM
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Go Speedracer, go!
Join Date: Aug 2002
Location: Indianapolis
Posts: 1,951
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Ahh, didn't think about that. I have seen posts about this also. What is the alternator supposed to be putting out EXACTLY? Isn't it 14.2?
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1981 SC ROW Coupe |
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08-02-2003, 02:40 PM
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Location: Peoples Republic of Long Beach, NY
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an over voltage of 16v or 17v is NG for the tach.. and there is a damping that goes bad inside the tach ............Ron
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Ronin LB '77 911s 2.7 PMO E 8.5 SSI Monty MSD JPI w x6 |
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08-02-2003, 05:11 PM
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Location: Planet Eugene
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Ron -- you mean it is bad _while_ the high voltage is present? Or that the high voltage permanently damges the dampening?
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08-02-2003, 09:18 PM
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Location: Port Macquarie Australia
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My alternator was putting out 20-22v and the tachometer fluctuated wildly with rev changes, Idle seemed right but it magified RPM increases. I understand that your battery should be around 12-12.5 volts at rest and 13-14v with the engine running but I do not have a tech manual yet to confirm this.
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2008 BMW 320i 2006 Mercedes ML320 2004 Mercedes ML270 cdi 1998 Porsche Boxter 1988 Porsche 930 Targa (sold) |
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08-02-2003, 10:04 PM
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Voltage fluctuaction (bad battery, voltage regulator, bad ground) make the tach bounce. You would also notice your lights vary in brightness and other instruments do the same.
It does not damage the dampening inside the tach permanently. It might fry the entire unit though if you get up into the twenties. The tach is dampened mechanically by oil on the needle shaft. That's what I was told. I tried to fix my tach once that had the same symptoms. It would take a couple of swings to settle on any rev. No such luck, even with gear oil (very thick) it would still bounce. I ended up buying another used one that worked fine. You could send the unit to Hollywood speedometer for a rebuilt - about 100$ or buy another used one. Ingo
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1974 Targa 3.6, 2001 C4 (sold), 2019 GT3RS, 2000 ML430 I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs Porsche "Hammer" or Porsche PST2, PIWIS III - I can help!! How about a NoBadDays DualChip for 964 or '95 993 |
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08-03-2003, 12:45 AM
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I may be wrong on this one, but I would think that there's probably a capacitor that damps the movement on the tach. That being the case, you could theoretically change the value (increase) of the cap to increase the dampening. Not having ever dissected a tach to that level, I'm not sure though...
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David Dryden '86 911 Coupe '05 BMW X5 4.4i |
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08-03-2003, 05:58 AM
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1974 Porsche 911 Targa 3.6
