Tach needle fluttering, possible crank sensor? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat
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Old 04-16-2004, 12:16 PM   #1
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Default Tach needle fluttering, possible crank sensor?

Since I changed the timing chain and sprockets on our 1990 LE, the engine has been running fairly decent, still have extended cranking time to get it to start, 5-6 seconds and the tach needle flutters randomly. I can feel seat of the pants lags in the power flow of the engine too, however I cannot catch it acting up at idle. Cam magnet is new, recent repairs have also included a new IAC motor and different EGR valve. Air filter, plugs and wires are all about a few months old.

Is the crank sensor responsible for sending the signals to the tach and is there a proper adjustment procedure for the spacing between the sensor and the discs? In the past, I usually just eyeballed the clearances to make sure it wasnt contacting the sensor.

Thanks so much!
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Old 04-16-2004, 07:28 PM   #2
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Check the fuel pressure? Sounds like a Fuel Filter to me...
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Old 04-17-2004, 04:57 AM   #3
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Spec for crank sensor is .025" minimum clearance but so long as the rings do not touch it you should be fine. I have seen one or more of the magnets break off partially and the negine still run but not well.

Tach signal is created inside the Electronic Spark Control Module and comes out on pin "E" of the harness connector. In general, a fuel delivery problem wil result in smooth tach movement while an ignition problem will result in more jerky motion. Since you describe a "flutter" I would tend to look at the ignition system first. Of course the easiest way to troubleshoot the ignition is to have a spare assemby & see if it cures the problem since R&R is a 10 minute job.

If not there is a six page description and troubleshooting process under "Cranks but does not Run" in the factory service manual. Keep in mind that intermittants are often very hard to find.

I do not care for the Magnavox ignition (squarish coils and single coilpack rather than oval individual coils of a Delco unit), have seem many indications that it is marginal and if faced with replacing the module (over $100), would rather replace the entire ignition (bolt in)

As to the long crank, try cranking just a second or two at a time. If no start then turn key completely off and try again. This will give a second prime shot through the injectors abd was a known problem on some 89-90 3800s.
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