tach jumping
Thread Starter
Instructor
Joined: May 2006
Posts: 144
Likes: 0
From: Stockbridge GA
tach jumping
I have got my vett on the road and when the elc fans kick on my tach jumps around at idle. the problem get worse when the lights and a/c fan comes on and I even think the engine skips a little bit ( hard to tell ). when the fans come on without lights the tach jumps then settles down. I have checked, and even shortened a few grounds. I am running the fans threw a relay and using a 140 amp alternator. I even put a MSD noise filter on the ignition box.I am starting to wonder if this is a problem of more than just proper grounding ( adding more noise filters somewhere else maybe?) or if there are any tricks to properly grounding these fiberglass cars. any help on this will be greatly appreciated.
Pro
Joined: Jul 2004
Posts: 605
Likes: 0
From: Alta Loma CA
You did not say what distributor you have. But I have an MSD and a GM HEI. For months I had a similar issue. I also had changed to a '76 tach. Finally the car would not start. That was cured by a new ignition module in the distributor. Once that was replaced the the tach was fine. Bottom line as near as I can tell was that a failing ignition module was creating a weird tach signal too. So I would consider that as a possibilty.
Thread Starter
Instructor
Joined: May 2006
Posts: 144
Likes: 0
From: Stockbridge GA
I will try a new ignition module. but I am not getting my hopes up since it is a brand new HEI but on that same note I might try bypassing my summit ignition box ( also new ). Has any one had problems with these units?
Thread Starter
Instructor
Joined: May 2006
Posts: 144
Likes: 0
From: Stockbridge GA
triple checked my grounds. I even tried to isolate some circuits. The only way I can get tach not to jump around at idle is unhook the alternator. When I do this I can run my A/c lights and cooling fans all at once and tach stays smooth and car runs smooth. Any one have any ideas on how to fix this.
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
Your tach probably "sees" a voltage drop when the fans kick on.
Cut the +12 V power wire going to the tach and insert a 1N4007 serial diode ( 1 amp should be enough if this wire just supplies the tach ).
Then add a 20V or more / 1000µF capacitor between the tach +12V ( after the diode, near the tach positive terminal ) and ground.
This capacitor will supply enough energy to the tach when the voltage drop occurs, and this energy won't go back to the battery, thanks to the diode.
If you need a diagram or if you have questions, feel free to ask...
Cut the +12 V power wire going to the tach and insert a 1N4007 serial diode ( 1 amp should be enough if this wire just supplies the tach ).
Then add a 20V or more / 1000µF capacitor between the tach +12V ( after the diode, near the tach positive terminal ) and ground.
This capacitor will supply enough energy to the tach when the voltage drop occurs, and this energy won't go back to the battery, thanks to the diode.
If you need a diagram or if you have questions, feel free to ask...
Le Mans Master
Joined: Oct 2000
Posts: 8,897
Likes: 1,357
From: Santa Ana CA
Be sure to get the polarity correct on the capacitor or it will explode with the force of a shot gun shell. Also, you have be sure the diode is wired in the proper direction or no electricity will get through it.
Last edited by toddalin; Apr 17, 2010 at 03:32 PM.
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Thread Starter
Instructor
Joined: May 2006
Posts: 144
Likes: 0
From: Stockbridge GA
problem solved. It was the summit ign box. The car even runs better. I have been trying to get my timing right and tack to stop jumping for a while now took the box out of the mix and every thing is ok
Le Mans Master
Joined: Oct 2000
Posts: 8,897
Likes: 1,357
From: Santa Ana CA
I have a Mallory HyFire IV CD box triggered off the points. When Lars did a tune-up, the car ran terrible and missed terribly to the point of stumbling/dieing when you jumped on it and the secondaries kicked in. We by-passed the Mallory box with Mallory's provided jumper plug (a nice feature!) and the car ran fine on the points.
Later I cleaned the corrosion off of the Mallory's fuse and holder and it ran fine on the box once again, and still does.
Maybe try cleaning all connections and fuses/holders associated with your Summit box and give it another try before you junk it.
Also, are you running a ballast resistor, and should you be?
My Mallory system says to omit the ballast resistor and run the points directly to the + ignition wire.
I kept the ballast resistor, but have a wire with a plug wired in parallel that by-pass it. When I install the Mallory jumper, I simply unplug this by-pass wire and I'm back through the resistor.
I have infact used these by-pass features a couple times over the course of years.
Last edited by toddalin; Apr 18, 2010 at 01:15 PM.