Another tachometer question

[Realdon]

Okay, about two months ago I purchased a 1982 Ranger 375-V with a 1993 Mercury 150hp XR6 (serial #0D254172) motor on it. I had no intention of getting it in the water this year but I did want to get it all ready for next year. I cut and engraved a new dash panel for it and carefully marked all the instrument wires to the existing gauges and even added a couple of additional gauges. After that work and much more I pulled it out of my shop and hooked up the hose to it and to my surprise it started and all the gauges appeared to work except the tachometer.

The tachometer is an OMC 0-6000 brand and may have been there for another motor. Although the seller was quite vague in some of his statements I am almost sure that he said that the tachometer worked and the speedometer did not work. I found that the water input for the speedometer in the motor was plugged so that somewhat verified what I thought that he said.

As I have all the wires securely attached back up under the dash it is somewhat difficult to see what wire is what but it does appear that the tachometer is wired as follows. A couple of wires going from the tachometer to the speedometer (probably for the light and power), one black wire from the harness to the tachometer (probably a ground), and finally a gray wire from the harness to the tachometer which I assume is the signal.

Now the question: Is there any way that I can start the engine (I am running out of nice days here in SE Iowa) and check the gray wire at the dash for a signal or should I even get a reading from the gray wire?

I realize that was a long post but I see many where all the details are not included. On the other hand, maybe I went overboard (pun intended).

Any and all, answers, comments, and suggestions would be appreciated.

Cheers,

Don

 

[Chris1956]

Re: Another tachometer question

The OMC tach may not work with the merc, depending on what year of OMC motor is was designed for.

You should check continuity on the grey wire from the dash to the voltage regulator on the rear of the motor. A '93 should have dual VRs, with the grey wire connected to one of them and the other VR will have a plug on the grey wire.

If you have continuity, either the VR is no good, or the tach is NG. Tachs need +12VDC and ground as well as signal. When running the grey wire should have some sort of AC signal on it.

 

[Realdon]

Re: Another tachometer question

The OMC tach may not work with the merc, depending on what year of OMC motor is was designed for.

You should check continuity on the grey wire from the dash to the voltage regulator on the rear of the motor. A '93 should have dual VRs, with the grey wire connected to one of them and the other VR will have a plug on the grey wire.

If you have continuity, either the VR is no good, or the tach is NG. Tachs need +12VDC and ground as well as signal. When running the grey wire should have some sort of AC signal on it.

You sure do know the engine. It does have two VRs and the gray wire on the lower one is plugged. I unplugged the gray wire at the VR and cut into it right before the tach and I do have continuity so the wire is intact. What I found interesting is that when I unplugged the wire at the VR I had 12 volts (actually 11.67v) from the wire from the VR to ground. Does that seem correct?

I am going to have to loosen all the wires under the dash again so that I can see exactly what I have up there. The wire that runs from the tach to the speedometer may be a ground now that I think of it because I doubt if the speedometer uses a hot wire except for the light in it. I checked the other black wire that comes from the harness and it is definitely a ground. I need to get up in there and see where I am getting a hot wire from. Of course the gray wire does have approximately 12 volts for some unknown reason.

More to come once I loosen the wires so that I can see what I am doing.

Don