Tach Problem

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Aug 4, 2011 | 11:25 PM

bleep

Thread Starter

Cruising

Joined: Aug 2011

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Tach Problem

78 tach is pegged fully to the right, both when and when not running. when running and the lights are turned on, deflects all the way to zero, then back to full right. When going from dim to brights and back to dim headlights, tach again deflects full left to zero, then back to full right. These are the only situations I've found to date when this happens. I know, could be a dozen different things, but anyone had a similar issue?

Aug 5, 2011 | 04:57 AM

hunt4cleanair

Le Mans Master

Joined: Sep 2002

Posts: 5,228

Likes: 895

From: Myrtle Beach SC

Quote:

Originally Posted by bleep

Its suggested that 90 percent of tach problems are due to the tach board and in your 78 is a piece of cake to replace versus that required for 75-77 vettes. Others will jump in but I had problems with my 78 tach, replaced the board and it works fine. We've had several threads on the topic so do a search and gain some background info. Wilcox offers some nice videos and instructions. I'm providing a link to an article that was published in Corvette Enthusiast on the subject.

Tach Boards

Aug 5, 2011 | 07:24 AM

bleep

Thread Starter

Cruising

Joined: Aug 2011

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thx. watched the Wilcox video on the tach board.

Aug 5, 2011 | 07:28 AM

bleep

Thread Starter

Cruising

Joined: Aug 2011

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ok, one more question. is there a way to troubleshoot the tach board to determine if in fact that's the problem, or does this end up being the most likely cuplrit and I just have to bite the bullet, change the board and hope that's it?

Aug 5, 2011 | 08:09 AM

BKarol

Melting Slicks

Joined: Sep 2004

Posts: 3,216

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From: Northeast

At the top of the page there is a "search this forum", type in a word or two and get past discussions. ie tach problems

Like this about checking the filter first before buying anything:

http://forums.corvetteforum.com/c3-t...azy-today.html

Aug 5, 2011 | 10:44 AM

0Willcox Corvette

Former Vendor

Joined: Aug 2006

Posts: 76,656

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From: Jeffersonville Indiana 812-288-7103

St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15

I think at this time he should pull the speedo cluster and take a look at his printed circuit.

With the information provided about the headlamps he may have a shorted circuit board.

Once he has this out he can then check the tach by powering it up on the bench as shown in the video. If he hardwires the tach with only power and ground then it should go to zero. If it fails this test then he needs a new tach board.

On another note, he could just pull the tach out and test the three clips to see what feed is hitting on each clip. If you find power on ground or signal clip then remove the housing and check.

If you need instructions on removing the housing go to this link: http://willcoxcorvette.com/repairand...lp.php?hID=146

If you need diagnostic help on the tach system see this file.
http://willcoxcorvette.com/repairand...lp.php?hID=290

Willcox

Last edited by Willcox Corvette; Aug 5, 2011 at 11:31 AM.

Aug 7, 2011 | 10:16 PM

bleep

Thread Starter

Cruising

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Thanks. I am experienced with a multimeter so will check the filter, then move on to the tach/tach board as suggested. Will post results when done.

Aug 7, 2011 | 11:58 PM

0Willcox Corvette

Former Vendor

Joined: Aug 2006

Posts: 76,656

Likes: 1,847

From: Jeffersonville Indiana 812-288-7103

St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15

Quote:

Originally Posted by bleep

Thanks. I am experienced with a multimeter so will check the filter, then move on to the tach/tach board as suggested. Will post results when done.

Unplug the filter.. and just bypass it. you can do this by unplugging the filter and plugging the tach right in to the distributor. Best test there is.

Make sure the filter is bolted to the block...