Anyone know why my tach pins at redline when ignition is "ON" ?

[SeaKaye12]

Hi,<br /><br />I'm in the process of renewing a 1977 Merc 850 SN4721196.<br /><br />When the ignition switch is turned to "ON" (with or without the engine running...) the tach needle pins all the way over to Max redline.<br /><br />Here's a pic to help ID the tach. On the back...on the bottom terminal...there are three wires. One is a hair-thin strand of bare wire...about 3" long...that goes nowhere. Maybe it should be a "GROUND"???<br /><br />

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<br /> <br />Anyone know what's apt to be going on?<br /><br />Thanks for reading, Chuck

 

[SeaKaye12]

Re: Anyone know why my tach pins at redline when ignition is "ON" ?

See the three terminals. The Top Left is labeled 12V. The Top Right is labeled "ALT" and the bottom one is labeled "GROUND"<br /><br />They are all connected to the three wire harness that runs from the front of the Mercontrol.<br /><br />However, Notice that there are three wires connected to the bottom (GND) terminal.... One of those three is much lighter guage wire....about three inches long...connected to nothing.<br /><br />What is it's purpose? I'm tempted to ground it and see if anything changes.<br /><br />Thanks for reading, Chuck

 

[AMD Rules]

Re: Anyone know why my tach pins at redline when ignition is "ON" ?

Member ZmOz had that same problem this past week. If I recall, the tach was getting 12V on the ground wire in error. The ground connection should be daisy-chained to all of your guages and also grounded through the controls. The ALT connection is for the sending wire. Is the sending wire hooked up on the powerhead? Check the voltage on the 'ground' wire coming from the controls, to be sure it is a ground and not 12V+.

 

[Dunaruna]

Re: Anyone know why my tach pins at redline when ignition is "ON" ?

Originally posted by AMD Rules:<br /> The ALT connection is for the sending wire. The positive is only required for the bulb.

Are you sure about the 12v connection? The bulb wire is not the far left 12v terminal, that terminal requires 12v ign to activate the tachs circuitry.

 

[AMD Rules]

Re: Anyone know why my tach pins at redline when ignition is "ON" ?

You're correct...that was a mistake on my behalf. I just went out and unhooked the positive lead (purple) from my quicksilver tach and sure enough it didn't work. <br /><br />I was always under the impression that the ground (black) and pulse sending wire (gray) were enough to power the tach circuitry. Learn something new everyday <grin>.<br /><br />Post above edited to avoid confusion.

 

[Dunaruna]

Re: Anyone know why my tach pins at redline when ignition is "ON" ?

Great!! Now my post looks stupid

 

[ZmOz]

Re: Anyone know why my tach pins at redline when ignition is "ON" ?

Originally posted by AMD Rules:<br /> Member ZmOz had that same problem this past week.

Yep, somehow the ground wire ended up being a positive. Switched it back to negative and it works great. How the hell it ended up positive I'll never know. <br /><br />My tach only has two wires, ground and signal. No positive...

 

[SeaKaye12]

Re: Anyone know why my tach pins at redline when ignition is "ON" ?

Yes; I'd read that post by ZmOz; either here or on the newsgroup...<br /><br />I decided to bump this thread anyway because of the fact that his tach has only two conections...and mine has three (three plus the bulb...)<br /><br />Nevertheless...I'll check the black terminal with a voltmeter and see... It's certainly worth the try.<br /><br />I'm so paranoid of the electical wiring because of the expense of switchboxes and the like if something I "try" fry's it...<br /><br />I'd still like to know what the phantom wire is...<br /><br />Thanks guys, Chuck

 

[MortenH]

Re: Anyone know why my tach pins at redline when ignition is "ON" ?

Check if you have +12V on the signal wire instead of the pulses. This sent my tach through the sky.. After I found the correct wire, everything was great.<br /><br />-M-