Tach Trouble

[jwclbi]

Hello, I have 1998 225 EFI Mec, trouble is the tach is intermittent. I removed the tach cleaned the connections, no different. Friend of mine has a new one in the box so I installed it with same result. Is there a "sending unit" for lack of a better term, somewhere on this engine. I looked for a couple minutes but couldn't really see much. Any thoughts would be great, thank you.

[gofastsandman]

Tach hot feed is usually grey.
Good ground?

Could be rectifier as well. I`m omc and not merc.

[pelican]

Originally Posted by jwclbi

Hello, I have 1998 225 EFI Mec, trouble is the tach is intermittent. I removed the tach cleaned the connections, no different. Friend of mine has a new one in the box so I installed it with same result. Is there a "sending unit" for lack of a better term, somewhere on this engine. I looked for a couple minutes but couldn't really see much. Any thoughts would be great, thank you.

that motor,it has 2 regulator rectifiers - back of the motor - swap the grey wire with the other rectifier - it's probably the rectifier


grey wire isn't "hot feed" - grey wire is tach signal
purple wire is ignition
black is ground

not uncommon for those to go bad

check charging system - make sure it's working properly

the high output charging systems,they use 2 regulator/rectifiers

[jwclbi]

Here is a picture

[flyingfrizzle]

Tach diagram:

http://boatinfo.no/lib/mercury/manua...3lit.html#/158

Gauge arrangement:

http://boatinfo.no/lib/mercury/manua...3lit.html#/148

Engine harness:

http://boatinfo.no/lib/mercury/manua...3lit.html#/172
http://boatinfo.no/lib/mercury/manua...-3lit.html#/98

The 225 3.0 is different than the 150-200s of that same year with the 40 amp system dual 20 amp rectifier boxes (looks like what you have pictured in your post) The 3.0 will have an alternator and the grey wire will come off of the box shown as #34&35 in the manual provided in the link above. Sounds like you could have a corroded wire or connector. Check all the connections in your harness to make sure they are clean and well connected also.

[gofastsandman]

Originally Posted by pelican

that motor,it has 2 regulator rectifiers - back of the motor - swap the grey wire with the other rectifier - it's probably the rectifier


grey wire isn't "hot feed" - grey wire is tach signal
purple wire is ignition
black is ground

not uncommon for those to go bad

check charging system - make sure it's working properly

the high output charging systems,they use 2 regulator/rectifiers

Tomato

[jwclbi]

Thank you for the information, but I can't find anything that looks like what the shop manual is showing, a little box with five wires coming out of it. The manual states there is two of them as well. It also says they located behind the coil mounting plate, bolted to the power head. I removed the four bolts holding the coil plate on and they aren't there. Am I blind??

[jwclbi]

Thank you flyingfrizzle! I was hoping I wasn't losing my mind!

[jwclbi]

Well here is photo of the alternator, I removed the only connection and it was very clean. I still can't find the allusive grey wire people keep referencing. The connection to the wiring harness is very clean as well along with the connections behind the tach itself. Any more thoughts?

[flyingfrizzle]

Originally Posted by jwclbi

Well here is photo of the alternator, I removed the only connection and it was very clean. I still can't find the allusive grey wire people keep referencing. The connection to the wiring harness is very clean as well along with the connections behind the tach itself. Any more thoughts?

I just realized sometimes the manual links go to the cover instead of the page listed at the end of the link. pg 173 has the diagram that will show you where the "grey" wire runs in the harness

I wont be coming from the alternator plugs, but from the ignition box #35 in diagrams. It shows a grey wire from both boxes but you want the one from the ignition ecu not the #34 fuel ecu. Hard to tell from the pic but it may be in the covered bundle your hand is on. I would start under the gauges and check the connections at the other end of the harness if you haven't done so yet then work my way back. Most bad connections are where the gauge harness plugs to the main harness at that end or the pin connector that plugs to the motor harness at the other end. The pins will get green and corroded sometimes and cleaning them and adding dielectric grease will help keep them from getting bad again. Sounds like you have done that part possibly, just make sure you check the outboard plug end and there are also plugs to the ecu's. Don't forget to check all associated grounds too. The tach signal is produced by the ignition ECM based on the crankshaft position sensor input. The crankshaft position sensor should has a good gap and read 1200 ohms. I would think if the position sensor was bad the motor would not run so you probably have one of two things.... One being the ecu that produces the signal is losing that channel (which I doubt because it is trying to work coming in and out) and the Second being a bad connection between the ecu and the tach. Intermitting issues are always hard to find. I would look at and check all the grounds on the motor as well, make sure they are clean and bonded well. It may fix the problem or keep another potential problem in the future from occurring.