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Old 04-23-2011, 04:42 PM   #1
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Default COPs and floating tach

So I've been having issues with my COP setup on my 94 where my tach floats up anywhere past 3k rpm. If I'm just slightly past 3k, the tach will bounce around between 3k and 6k. If I'm at normal highway revs (4k rpm), it will peg at the limit stop of the tach.

My COPs are wired so that 12v comes from one original coil connector and branches to all coils. The tach output is wired in the same manner. Ground goes to one connector as well but also has a branch which is grounded to the head.

I've tried most of the troubleshooting I can find for the floating tach issue. Checked and cleaned the grounds in the dash, under the brake booster, the strap from the chassis to the block plate, the throttle body side ground and the rear intake manifold ground (there's only one here on a 94 right? I ask because I also deleted my smog/egr system at the same time as doing the COPs so I don't know if I may have displaced one ground). I also added an additional ground from the drivers side chassis to the valve cover.

Other troubleshooting I've done is disconnected the tach signal from 2 coils so I only had signals coming from either 1 and 2 or 3 and 4 at a time. I also tried bridging the tach signal over to the tach pin of the other original connector.

At this point I'm a bit stumped. I had a perfectly fine working tach when I had stock coils and now I have this bouncing tach.

Could the fact that I'm feeding all coils from the 12v of just one of the original connectors be causing any issues? The car runs great, the tach is just screwed up.

Any suggestions or things I may have missed would be much appreciated. Sorry about the novel, I just wanted to be thorough.
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Old 04-23-2011, 05:31 PM   #2
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You have to do the 1k resistor mod in the diagnostic box or the COP harness iirc. I've got a 1k resistor in the diagnostics box
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Old 04-23-2011, 05:33 PM   #3
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Quote:
Originally Posted by djp0623 View Post
You have to do the 1k resistor mod in the diagnostic box or the COP harness iirc. I've got a 1k resistor in the diagnostics box
Already have a resistor in there. Technically isn't the resistor only needed if you have no tach at all?
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Old 04-23-2011, 05:37 PM   #4
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check to make sure your tach output is disabled in MS if that is what you are using. I could never get the tach to work with MS directly.
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Old 04-23-2011, 06:50 PM   #5
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Originally Posted by djp0623 View Post
check to make sure your tach output is disabled in MS if that is what you are using. I could never get the tach to work with MS directly.
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Thank you.

Well on the bright side, at least I have really good grounds on my engine now.
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