Tachometer -- Where to start?

[bds85466]

I have a 1972 Glastron GT 160 that I bought used. I have a 1972 125 hp Johnson motor. The tach is unresponsive for unknown reasons. I'm not sure if it's just a dummy dial to look good or if it was at one time functional. I'm inexperienced when it comes to wiring these in and would like to know some basics regarding this. Any recomendations of ways to check my tach's functionality or capability to be wired in?

 

[gcboat]

Re: Tachometer -- Where to start?

Here's a really quick way to tell if it is indeed a real tach. Look at the back of it - if you have wires attached to it then there's a really good chance that it did work at one time. As far as troubleshooting is concerned, just check for loose or corroded connections.

 

[bds85466]

Re: Tachometer -- Where to start?

Any recomendations on how to pull the tach out?

 

[Silvertip]

Re: Tachometer -- Where to start?

Look at the back of the tach. There is a "U-shaped" backet held on with two nuts. Remove them and the wires and it will fall out the front. There are normally four wires involved. +12V that becomes active when you have the key in the RUN position. There is a ground wire that connects to the ground terminal on any of the other gauges or to the ground buss at the fuse panel. There is another wire that feeds the light inside the tach/ That wire would come from the NAV LIGHT switch. The fourth wire is the SENSE wire that connects to the stator at the engine. That wire would typically run through the big cable harness between the control box and the engine.

 

[eurolarva]

Re: Tachometer -- Where to start?

Most tachs work off of the regulator/rectifier. The first thing to go on an outboard is when accidently reversing the battery wires even for a second. If the rectifier is shot the Tach wont work. If your battery is charging then the rectifier is good. An easy test is to hook up a volt meter to your battery and test the battery voltage when the motor is off. Probably around 12.6VDC. With the motor running at around 1/3 to 2/3 throttle the voltage should go up to at least 13.5VDC and maybe as high as 16 or 17 volts. If this happens the charging system on your motor is working and your tach is probably the problem. If the voltage only stays around 12 volts or so then the charging system on your motor is not working and your tach will not work either. Note when starting a motor the excessive draw on the battery could drop the voltage by as much as a volt and the voltage will slowly trickle back up to the original pre start voltage. This would also indicate the charging system is not working.

 

[bds85466]

Re: Tachometer -- Where to start?

I took the tach out of the dash harness last night and found that it wasn't wired in to anything. There's wires going all over my boat from the previous owner and it's more of a question as to where to send the leads from the tach. If I remember correctly from last night, the tach has a "GRD" (ground), which would obviously go to the system ground, IGN (ignition), which I think would go to the ignition switch, and a SND (send) which probably should go to either the control box or someplace on the motor itself. I realize how the tach works and the general theory of the wiring. It's more a question of where to wire these leads into. I'm not even sure which wire would be ground. If i tracked from the motor back to the control box would that give me some idea of what was what?

I'm not worried about anything wrong with the electrical on our boat...the lights/radio work and it recharges just fine.

 

[ricksrster]

Re: Tachometer -- Where to start?

This should help:
http://www.downloadingfiles.co.uk/t...structionmanuals/99320 rev 10 Tachometer,.pdf

 

[bds85466]

Re: Tachometer -- Where to start?

Thanks for .pdf ricksrster.

So I would want to find the wire that goes from the throttle control to the battery and basically put the tach in the middle of that circuit. + Battery goes to IGN on my tach, and the SEND lead goes to the throttle control? The grounds obivously all go to a ground circuit wire?

 

[Silvertip]

Re: Tachometer -- Where to start?

Black is probably the ground wire. Red is typically +12V, the remaining lead is the send line.

 

[ricksrster]

Re: Tachometer -- Where to start?

IGN is 12V+ from the key on post of the ignition switch. Sis wire from one of the alternator posts on the rectifier. Black is neg. battery. Light connects to your nav lights.