Tach problem

I recently bought a "new to me" boat with a 1997 Merc 200 EFI. The motor runs like a champ but the tach doesn't want to work right. Most of the time it indicates nothing. Every once in a while it will indicate what appears to be the correct RPM, but at other times the RPM indicated is wrong (i.e. indicating 3000 RPM at idle) I also had an issue that the original sonar unit at the console would give the message "over voltage condition" when running at fairly high rpms. This unit was wired to the cranking battery. I've replaced that unit with a Hbird 998 which I wired to the trolling batt. I'm wondering if this tach/over voltage issue might be related and if anyone has an idea of what it could be? The cranking batt stays fully charged so no issue there.

Any help is much appreciated.

You are right on the money...you have a regulator rectifier condition. You are probably overcharging the system. Do you have a voltage gauge on cluster? Need to get that checked or test it yourself. Simple thing to do is volt meter on battery...should be around 14 to 15 volts under normal conditions. Probably need new rectifier though. This can not only be bad on your boats electronics but is a fire hazzard if voltage gets to high.

A bad rectifier can also affect your tach as the rectifier and tach get their feed from the same windings on the stator.

I bought 2 new regulators off ebay last week. They came in the mail Saturday and I swapped them out with the old ones. I've only run it in the driveway with the water muffs so far. The tach now works and I'm showing 13.5-14 volts across the battery at idle. Thanks for the help.

I installed the new regulators a few weeks ago and the tach started working under 4000 rpm. Unfortunately, anywhere over 4000 and it starts to jump around between 4000-6000. Any ideas?

Try this. On the back of the tach is a setting where you tell it how many pulses your stator puts out. In your case you have a 12 pole stator producing 6 pulses. Using a screwdriver move this setting back and forth several times stopping exactly on 6P (close don't count). Then try it again. What happens over time is the contacts inside get cruddy or whatever and a lot of times you can clear it up my moving that setting back and forth. They are very small contacts and that is why the setting must be exactly on.

Thanks Bill. I'll give that a shot.

Bill,

I finally got around to trying what you suggested as to twisting the pole selector on the back of the tach. I worked it back and forth about 10-12 times. Took the boat out yesterday and it seems to have worked. Thanks a lot. Probably the cheapest fix I've ever had on a boat.

i had the exact same problem and fix. your rectifer is converting the ac current from your stator into a pulsing dc current. your tach reading is based on the frequency of the pulses. Tach is usually the first thing to show a rectifier problem.

hate to bring an old thread back from its sleep, but having a problem with my tach on my 04 Tracker PT 175 w/ a 60 merc 2 stroke. Was running down the lake right at 4000 rpm's, looked down and saw the tach bump up to about 4400 rpm, (I did not move the throttle) then up to 5000 rpm, then back down to 4000 rpm. I did not move the throttle during this time, water was smooth, motor did NOT change rpm's to my ears, motor had a steady hum to it.

Did a search and it might be my rectifier, but can someone tell me what I am looking(the rectifer) for on my 60 merc, where is it, what does it look like, etc. a picture would be helpful as well.