Tachometer problem Noise?

[tbanzer]

I have been trying to get the tach to work properly in my boat this season. The boat uses a 2000 502 mag with a Vortech supercharger. The original tach would operate till 4000 rpms then any higher rpms the tach would drop to 0. I purchased a new Gaffrig tach and this tach operated to about 4200 rpms and then just stays there even if the engine speed increases. I called Gaffrig and they sent me out a new tach that they tested prior to shipping. This tach operates up to 4300 rpms then as engine speed increases it drops to aboput 4000 and stays there. I have a digital GPS unit that has mph voltage, rpms and a compass. The tach on this unit read properly all the way up to the rev limiter. I ran a new wire from the coil up to the tach with no change. I then brought a seperate battery in the boat to run the tach on its own power source again no change to the tach operation. I was just thinking that I noticed that when the boat is running I can hear some noise in the radio that increases with engine speed. Any Ideas?

[h2oski2fast]

Sounds to me like a bad tach output. The tach lead probably comes from the computer and not off the coil. As for the noise in the radio, more then likely a ground loop. Put all the grounds for the system (head unit, amps, processors) directly on the battery terminal.

[tbanzer]

The tach lead does come from the coil. It starts as a white wire then goes to a four wire conector and changes to the standard gray color tach lead. Its got me baffled.

[BadKachina]

Just a thought but that Gaffrig tach has a switch on the back for 4/6 or 8 cyl.. Make sure you're selecting the right one. I assume you tapped into the same grey wire for the digital gps? Try disconnecting that and see if it makes a difference on your analog tach. One other thing I would check is your voltage on the pos/neg on the tach, make sure nothing is loose along the chain from gauge to gauge and that nothing is shorted. Your positive and ground usually run from the ignition and piggy back from gauge to gauge, you could have two terminals touching somewhere along the chain, usually a signal wire touching a positive or ground anywhere along the line will do wierd stuff to all the other gauges. I've also seen the bracket touching terminal ends and doing wierd stuff.

[tbanzer]

Just a thought but that Gaffrig tach has a switch on the back for 4/6 or 8 cyl.. Make sure you're selecting the right one. I assume you tapped into the same grey wire for the digital gps? Try disconnecting that and see if it makes a difference on your analog tach. One other thing I would check is your voltage on the pos/neg on the tach, make sure nothing is loose along the chain from gauge to gauge and that nothing is shorted. Your positive and ground usually run from the ignition and piggy back from gauge to gauge, you could have two terminals touching somewhere along the chain, usually a signal wire touching a positive or ground anywhere along the line will do wierd stuff to all the other gauges. I've also seen the bracket touching terminal ends and doing wierd stuff.I disconected the digital tach from any power or tach lead signal while running the analog tach with its own seperate battery. Im going to try a different coil as soon as I get the boat back. There is something in the tach signal that doesent bother the digital tach but wacks out the analog tach.