Jumpy Tach
Thread Starter
Drifting
Joined: Apr 2003
Posts: 1,323
Likes: 2
From: Castle Hills Texas
Jumpy Tach
Well after the last cold snap, my tach needle has started getting sort of jumpy. I figure it is either the filter or the circuit board. Any idea which is more prone to failure? This is in an 80 by the way.
Thread Starter
Drifting
Joined: Apr 2003
Posts: 1,323
Likes: 2
From: Castle Hills Texas
Thanks gents. How does one get to the backside of the tach? I have had the lens off and replaced the plastic around the speeo and tach that have the "Brights" and "Brake," etc. lens but I don't see how the entire housing comes out.
Melting Slicks
Joined: Sep 2004
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From: Northeast
Pro
Joined: Apr 2005
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From: Batavia IL
You know I had a jumpy tachometer in my 79 before and once I was making a long roadtrip in it and the alternator failed to charge. Well I only had about an hour left to go and figured to try to reach home before the motor failed due to lack of electricity. long story short the car died while I still had ten minutes to go. I killed the battery too while I was at it. After replacing the alterenator and battery, I started the vette back up again and noticed my tach was not jumpy any more and worked like new again. been that way ever since. Strange huh.
Thread Starter
Drifting
Joined: Apr 2003
Posts: 1,323
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From: Castle Hills Texas
I had read a tale similar to yours about a faulty alternator but this is my daily driver and I have been driving it about for a couple of weeks with no issues yet. That is why I ruled that issue out as a probable cause. Thanks for the heads up on the tip just the same. 
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Once my tach was starting to act a little strange. Started to look around and noticed I could move the filter by hand. Got it tight and everything was fine.
Thread Starter
Drifting
Joined: Apr 2003
Posts: 1,323
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From: Castle Hills Texas
It is odd as it doesn't jump all the time. When it does, it will bounce in about a 1K (IE, if holding at 2K RPM it will randomly jump between 2k and about 3k or so) range and it will read high. Other times it will work as advertised for a while. I know my tach filter is not attached to anything but it has been this way for some time. I will bolt it down to the intake and see if that helps, though I don't think I will see a change. I have a spare ICM I could try as well.
Pro
Joined: May 2003
Posts: 601
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From: Nottingham
Check the filter is ok, and GROUNDED! That's important, without the ground it won't filter out the spikes. It should be securely and cleanly screwed onto bare metal on the manifold.
If that doesn't work, then the on-board filtering stage on the board itself might be going, so then it's time to replace some resistors/caps, or the whole board itself.
Be careful who you buy from, there are apparently some crap ones out there. Redline gauges do a good quality one.
Good luck mate!
If that doesn't work, then the on-board filtering stage on the board itself might be going, so then it's time to replace some resistors/caps, or the whole board itself.
Be careful who you buy from, there are apparently some crap ones out there. Redline gauges do a good quality one.
Good luck mate!
Safety Car
Joined: Feb 1999
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From: Central NJ
Thread Starter
Drifting
Joined: Apr 2003
Posts: 1,323
Likes: 2
From: Castle Hills Texas
This is what makes it tough, conflicting suggestions, not that they won't all possibly work. I will start with the tach filter I guess and just systematically go to town to solve it.
Drifting
Joined: Nov 2003
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From: Northern NJ
St. Jude Donor '04 & '05
Just bypass the tach filter to test it. If the problem goes away, then it's likely the filter or related. However I'd advise you not leave it w/o the filter for any period of time as it is there to suppress any voltage spikes which can, and eventually will, damage the tach PCB. 
I hear all the time how people have removed them and it works fine, and they are not needed. My logic is that GM put the filter there for a reason and if they could have saved the $$$ because it wasn't needed, they certainly would have.
If it's not the filter then it is quite possible that the tach board has some bad/failing components. This is pretty common and many times is easily repaired with a couple $$$ worth of electronic parts. Let me know if you get to this point and want to pursue it further. I can likely point you in the right direction.
Good Luck!
I hear all the time how people have removed them and it works fine, and they are not needed. My logic is that GM put the filter there for a reason and if they could have saved the $$$ because it wasn't needed, they certainly would have.
If it's not the filter then it is quite possible that the tach board has some bad/failing components. This is pretty common and many times is easily repaired with a couple $$$ worth of electronic parts. Let me know if you get to this point and want to pursue it further. I can likely point you in the right direction.
Good Luck!
Last edited by ACECO; Feb 1, 2007 at 12:51 PM.
Thread Starter
Drifting
Joined: Apr 2003
Posts: 1,323
Likes: 2
From: Castle Hills Texas
Just an update for anybody interested, since the the temps have gone back up a bit here my tach has been jump free for a couple of weeks. I haven't even touched it yet. Of course, now that I have posted this, it will get screwy again....
