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Discussion Starter #1
ok got my new tach board in but before I install I wanted to try to eliminate a possible issue that might be ready to fry this one when I install .

I ran this by Wilcox and they agreed I may have an issue so ..if anyone has seen this or has a good theory lend me some help cause i have just enough electrical know how to do some damage...

tach is dead ...when key is turned on it goes right over from 0 to 7000 and stays ...when I put on a blinker it will jump back to 5000 then to 7000 again about every 3rd blink ...and when I hit my brakes ..same thing often it will jump.:crazy:

so have I got something trying to ground in the wrong place? or __________<--insert best guess here.Everything else seems to work ok (cept my clock) and no signs of tampering with that stuff ...printed circuit on the back of the speedo seems in good shape ..

if its a grounds i need to check where exactly are the grounds for the harness under the dash? ..please explain as if i were a blind one armed man ...or i will miss finding it.:smack
 

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Does it fluctuate anytime you add load to the cars electrical system. Turning on headlights. Highbeems. Stereo etc?? Or just the few you mentioned.
 

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It may be the OE grounds are corroded and high resistance. Which makes devices compete for path to the battery post, and highest user wins. Tiny users like tachs get screwed.

Have you ever been into the harness to be sure nobody hacked it for a stereo/tach/gauges? Lotta times the fix for ghosts like this is tied into non stock radio or gauge wiring.

If you share a ground wire for an aftermarket tach and radio you can end up with an audio tachometer. It's kind of a neat video game effect.
 

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Discussion Starter #5
ok guess i hadnt noticed before because i dont do much night driving but it does fluctuate when headlights go on , or highbeams get flipped on as well most times,

does the underdash harness have a ground wire somewhere that i should check?
 

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I don't have a C3, so I'm clueless from that angle...but I am an EE. Can you add a ground wire directly from the tach ground terminal to the chassis, not shared by anything else? Got a voltmeter?
 

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Discussion Starter #7
thats a thought i had considered if i had no luck running the issue down just wiring the tach on its own ...kinda feels like a bubba job ..but i guess a ground alone would probably take care of it maybe .....the crazy dash harnesses are 2x that of other models once you get to the 78-82 ....like 800.00 bucks :surprised
 

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thats a thought i had considered if i had no luck running the issue down just wiring the tach on its own ...kinda feels like a bubba job

It wasn't meant to be a fix, just to help chase down the problem. :thumbsup: You can always look for open grounds using an ohm meter.
 

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Discussion Starter #9
yea ill use it to trouble shoot but i always get the plan B last resort on standby =P ...anyone know where there is a list of grounds and there locations for the entire car...or at least the dash?
 

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yea ill use it to trouble shoot but i always get the plan B last resort on standby =P ...anyone know where there is a list of grounds and there locations for the entire car...or at least the dash?
Go to Dr.Rebuilds site and order the laminated electrical prints for your year model it is layed out just like the car and has correct colored wires and connectors. Well worth the money heres the link .http://docrebuild.com/oosoez.html hope this helps:cheers: Mark
 

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I think there is a tach 'filter' also. Basically just a big capacitor. It may have dried out after all these years and is no longer working. I converted my tach from mechanical to electric and used a Willcox board, but my signal is coming from the ECU not the distributor so I haven't had any noise issues.

Do you have anything in the car that isn't original, like a stereo? Bubba might have tapped into the power wire for the tach or something like that.

:cheers:

Rick B.
 

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Discussion Starter #12
Go to Dr.Rebuilds site and order the laminated electrical prints for your year model it is layed out just like the car and has correct colored wires and connectors. Well worth the money heres the link .http://docrebuild.com/oosoez.html hope this helps:cheers: Mark
wow thats nice i will buy a set of those asap ..but they dont have my 78s yet =( thanks for the link ill keep an eye out on that

and as for the tach filter i have the old one on it but ill do a replace on that just incase
 

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wow thats nice i will buy a set of those asap ..but they dont have my 78s yet =( thanks for the link ill keep an eye out on that

and as for the tach filter i have the old one on it but ill do a replace on that just incase
Give Docrebuild a call they had one for a 79 I needed but did not list them on there site they will help if they have one.:cheers: Mark
 

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Discussion Starter #14
i sent an email incase he had one ..does it give a good description of where the grounds are placed?
 

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i sent an email incase he had one ..does it give a good description of where the grounds are placed?
Yes it gives you every wire on your car. This is not the GM electrical diagrams but the actual wires as they are run in your car as you would see them when you look at them. Plus the different plugs and what they look like and all the wire colors. I cant say enough for the quality of these drawings it must have taken him years to complete these. They were made for the guy that cant read electrical schematics so he can trouble shoot electrical problems. If you do get them set down and study them a while before you put your hands on the car, you might see something you overlooked.:cheers:Mark
 
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