Need Help " BAD" on a Eratic Tach Reading

I've got a 99 merc 225 efi and the tach has always been eratic since I bought the boat used. At idel it reads about 1800 rpm's and when you crack the throtle it runs way over to 6000 or so. I talked to a guy around here and he told me to check the voltage at the batt. when running and I did and it's showing 14.5 volts.He thought it may have been a rectifire. This engine has an altenator on it and Im not sure if he was thinking about a different engine. The Gray signal wire going to the tach run's into the ECU modual on the back of the engine. I replaced the tach with a auto meter marine unit and set the Dip Switches in the correct position. I need some help as to where I should go from here.

You really need a MARINE tach. Additionally, it needs to be in the "6P" (six pole) position.

It's VERY unusual to have an "output tach signal" on that engine that's not accurate....

If you're unsure, or would like to have it checked to confirm where your problem is, we're just a short jaunt up the road from you. http://www.bassboatcentral.com/smileys/wink.gif

The tach is a auto meter marine unit and the directions said to put the dip switch in position 3 and i did I also ran another ground just to check if mabey that was it. Any other sugestions.

Engine serial number would be helpful at this point.....

Also, when the directions said to "set dip switch #3", exactly what "input setting" did that indicate you would have?

http://www.bassboatcentral.com/smileys/popcorn.gif This post may help me because I'm having the same plroblem on mine.

2001 225 EFI. 0T303168

This post will apply to "nearly every Mercury 2 stroke carb, EFI or Optimax engine, utilizing an ANALOG TACHOMETER". http://www.bassboatcentral.com/smileys/wink.gif

O K , I Called Auto Meter and they instructed me to set all the dip switches in the lower position and then put the last one on the right if your are looking from the back of the gauge in the upright position. The directions said to put the # 3 dip switch up , but that is for a inboard I guees . The 2 stroke fire's @ 12 ppr's and then that is divided into 2 which gives you 6 ppr's and their is a setting for that in the directions. I hope this help's everyone!!!!!! I set mine in this position and she is right on. http://www.bassboatcentral.com/boardstuff2/beers.gif