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Tachometer question 700 1992

posted by  someone claiming to be Kaz on  Fri Apr 16 08:48 CST 2004 [RELATED]

As I drove my 1992 740 turbo today, crusing at 70 mph, the tach was at about 2800 rpm which is where it runs for that speed. To my surprise I saw it jump up to 3500. I did not increase the gas petal nor did the car's speed change. (the tach change was a false indication) when I let off the gas a litte the tach fell to a reasonable 2500 and then when I went back to 70 mph it returned to 2800 rpm. It did this twice more then behaved it self. When the tach jumped up to 3500 from 2800 the engine ran smooth. What drives the tach? I assumed it was the engine crank sensor. Any ideas anyone. Thanks.

Mike Kaz








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Tachometer question 700 1992

uid 18083's pic
posted by  lucid on  Fri Apr 16 12:42 CST 2004
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last visit: Fri Oct 24 12:31 CST 2014
[RELATED]

What drives the tach?

Pulses from the Negative terminal on the Coil. Terminal #1 (not the Positive #15).
--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.








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Tachometer question 700 1992

posted by  haroon on  Fri Apr 16 11:19 CST 2004
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last visit: Thu Feb 20 11:25 CST 2014
[RELATED]

check the alt brushes, sometimes it can be a sign of a failing alternator.








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Tachometer question 700 1992

uid 14228's pic
posted by  ladeadhead on  Fri Apr 16 08:59 CST 2004
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last visit: Tue Nov 6 17:34 CST 2007
[RELATED]

Actually, the tach receives a signal from the positive post on the coil. If it is jumping like that, maybe the contacts are dirty. Clean the out and see what happens.

Eamon
--
'Where's your common sense?' 'It left me when I was 5'







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