Weird Tachometer Problem

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May 28, 2009 | 10:37 AM

hap1971

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Instructor

Joined: Mar 2006

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From: Jacksonville FL

Weird Tachometer Problem

Guys,
I'm running a 75-up style tachometer in my 71 Corvette.
I'm using a CS144 alternator out of a 1997 Deville, and I have the adapter cable w the resistor installed. I have an 8 gauge wire running from Bat+ to the battery itself, with a 14 gauge fusible link at the battery.
The factory Bat+ wire is also intact.
MSD 6A box w MSD tach adapter.
Power to the tach is from an ignition on wire from the fuseblock, and grounds for the tach and the filter are to the intake manifold.
If I unplug the regulator on the alternator, tach works perfect. (Without alternator charging of course)
If I plug in the regulator, the tach reads about 25% of engine speed, and eventually quits reading altogether. Also, if I turn on a blinker, the tach needle jumps about 500 rpm with each blink.

I have no idea what is causing this, any suggestions?
The last modification I did was adding the 8 gauge wire to the Bat+ to the battery, but it has worked correctly since then.

Thanks,
Richard

May 28, 2009 | 10:50 AM

7T1vette

Team Owner

Joined: Jan 2006

Posts: 37,637

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From: Crossville TN

Your tach filter is 'dead'. It should be "smoothing-out" those pulses from the other equipment (if it is functional and if it is wired in correctly). You can do some searching in the archives for the diagram of the tach filter's contents and make a new one yourself; or you can just buy a new filter.

May 28, 2009 | 04:05 PM

RunningMan373

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From: Bay Area CA

May 28, 2009 | 06:22 PM

7T1vette

Team Owner

Joined: Jan 2006

Posts: 37,637

Likes: 3,114

From: Crossville TN

There you go! Two resistors and two capacitors from Radio Shack ($3-5 max), remove the guts from the cannister on your present filter, wire the new filter and install it in the cannister (I don't know if it needs to be 'potted', and put it back in the car. Great info in the above post.

Jun 22, 2009 | 12:28 PM

Dins74

Instructor

Joined: Apr 2009

Posts: 124

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From: LA CA

7T1vette

Regarding the Tach filter - I have unbaribale motor noise (wine) in my 74 w/a new audio system w/ inline filters everywhere. The wine gets louder with the RPM. Could the Tach Filter be the suspect

Jun 22, 2009 | 12:38 PM

RunningMan373

Melting Slicks

Joined: May 2007

Posts: 3,246

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From: Bay Area CA

'74s didn't come with electronic tachs, so no tach filter, unless someone changed to a 75 or later tach, and included the complete set up, including the tach filter into yours. Look to see if you have a tach drive distributor. if theirs a cable coming off horizontally from the tach shaft, you have a mechanical tach, so no filter.

Jun 22, 2009 | 01:18 PM

C3 4ME

Le Mans Master

Joined: Oct 2005

Posts: 6,247

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From: Glen Allen, VA

the first thing stated is he's running a 75 and up style tach in his 71. I too would try the tach filter first since it's inexpensive, and then go from there.

Jun 23, 2009 | 12:55 AM

chvet73

Pro

Joined: Jul 2004

Posts: 605

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From: Alta Loma CA

I have 76 tach in my 73. Run same MSD. My tach runs from the MSD tach output on the box. With that output I'm sure you should not use a tach filter.