1992 Merc 40 hp SN#0D105940 tach signal

[stockdragrace]

Gentlemen, I have purchased the correct 5 pin cable from control unit to tach that I am trying to install, I checked, and I have good 12v power and ground to the harness, but how do I check to see if I am getting a signal on the grey wire which is the one that is supposed to be sending the signal to the tach? I bought a used tach that came off a new boat, and have tried to set the switch on the back of the tach in all positions ie. 6p etc., but still get no readings at all on the tach, other than it goes to zero when the ignition switch is turned on? Not sure if I have a bad/incompatible tach or just not getting a signal from the engine.Your help as in the past will be greatly appreciated. Thanks, Brad

 

[rickdb1boat]

Re: 1992 Merc 40 hp SN#0D105940 tach signal

First thing to do is check to see if your charging system is working. Your Tach. works off the A/C pulses from the charging system. You can do this by taking a voltage reading across the battery with the motor not running and then again with it running. You should see an increase in voltage when running at fast idle( Approx.1500-2000 RPMs). If there is no change, your charging system has problems(Rectifier bad) and you will not get a signal to the Tach.

 

[stockdragrace]

Re: 1992 Merc 40 hp SN#0D105940 tach signal

Rick, Thanks for the fast response, just had the boat into a shop, for a general look over, and asked them to check charging system, and they had said it was charging, I'll check myself per your suggestion, but can I check at the harness connection for tach to see if I am getting a signal? I only ask because since I'm not sure is used tach that I am trying to operate might be the problem. Thanks, again

 

[ricksrster]

Re: 1992 Merc 40 hp SN#0D105940 tach signal

make sure that tach wire is connected at the same terminal on the rectifier as either yellow wire,.

 

[stockdragrace]

Re: 1992 Merc 40 hp SN#0D105940 tach signal

Rick, Sure enough, battery reads 12.7 volts when motor not running, 12.5 running at high idle and above, what now? Thanks, Brad

 

[ricksrster]

Re: 1992 Merc 40 hp SN#0D105940 tach signal

Sounds like the rectifier. Check all your wiring connections and condition of your wires. to check rectifier you can use an OHM meter. Connect the red lead of the meter to one of the alternater terminals. connect the blach lead of the meter to the postive terminal. You should have continuity. Do the same test on the other altrnater terminal. Now reverse the red and black leads. You should show no continunity.

 

[ricksrster]

Re: 1992 Merc 40 hp SN#0D105940 tach signal

Also Try red lead to rectifier case ( ground ) black to the alternater terminals. You should have continuity. Now reverse the leads, black to ground, red to alternater terminals. You should have no continuity.

 

[stockdragrace]

Re: 1992 Merc 40 hp SN#0D105940 tach signal

Thanks for the help gentlemen! Sure enough, rectifier was the problem, replaced it and tach is working properly, old rectifier looked melted by one terminal, when I checked the the battery while running the motor after replacing the rectifier, I now have 16.7 volts going into the battery, where I had 12.3 while running before.