Need a pointer... 1975 Merc 1500 switchbox/rectifer/tach

[calvinator]

Hi all -

Been working on my new-to-me 1975 Merc 1500. Running well, but I want to get a tach on it for more tuning. I have a 1976 Mercury orig tach to go with it. I have the orig Mercontrol box with the tach harness on the front. All should be compatible, but nothing is working.



I did some troubleshooting, and I know there should be a brown wire (Merc tach wire) coming off the rectifier, that goes to the main wire harness, and back to the remote control box, to my tach. I don't have that.



Then I get out my Service Manual.... and there is a single page for 1975 motors and there is not supposed to be a brown tach wire coming off the rectifier.



But looking at the switchbox wiring, there is a brown wire coming off switchbox that goes back to the main wiring harness. There is no signal on that wire. Something must be unique about 1975!







OK, I am thinking I should jump a wire from the rectifier yellow/red to the switchbox brown connector. Then I will get a tach signal through my wireharness. Seem right? I just don't know what is in the switchbox and if this is a bad idea.....

THANKS

 

[Chris1956]

Re: Need a pointer... 1975 Merc 1500 switchbox/rectifer/tach

Well the old Merc tachs worked off the brown wire on the switchbox. New aftermarket tachs work off the yellow stator wire.

Your answer depends on what kind of tach you have. How may wires does it want? If two wires, it likely runs off the switchbox. If three wires, you will want +12VDC, ground and stator signal.

 

[calvinator]

Re: Need a pointer... 1975 Merc 1500 switchbox/rectifer/tach

Well the old Merc tachs worked off the brown wire on the switchbox. New aftermarket tachs work off the yellow stator wire.

Your answer depends on what kind of tach you have. How may wires does it want? If two wires, it likely runs off the switchbox. If three wires, you will want +12VDC, ground and stator signal.

I bought a 1976 Merc Kiekhaefer OEM tach. Designed to work with my Mercontrol tach connection. The tach wants +12v, ground, and brown wire tach signal.

But the brown wire on the switchbox (atleast my 1975) has no source, just a destination back to my control box.

I want to run a wire from rectifier to switchbox - as the tach source. Then I can use the existing brown wires all the way back to my control box.

I'm just looking for a sanity check. Seems weird to me that the 1975 motor has no wiring from rectifier to the tach. BUT, post 1975 all the wiring diagrams have a brown wire off the rectifier, through the wiring harness and back to control box.
Guess it is time to experiment !

 

[emckelvy]

Re: Need a pointer... 1975 Merc 1500 switchbox/rectifer/tach

There are special Merc tachs that were made to get input from the brown wire on the switchbox. Only that special tach will work with the switchbox signal.

Newer tachs are designed to work off the pulses of the 12-pole alternator, and the tach signal wire connects to either of the yellow stator wires, at the rectifier.

So, 1 of 2 things are possible: your tach is the correct one for stator-signal and it's bad; or, the tach is meant for a 12-pole alternator and needs input off the stator.

Maybe try switching the brown wire to the rectifier. Note that you shouldn't run a jumper from the rectifer to the brown wire on the switchbox, the wire needs to be completely disconnected from the switchbox and only connected to one of the stator leads at the rectifier.

I don't know what it would do if you jumpered stator output to the switchbox, but it probably wouldn't be a good thing!

Last thought, maybe the wire is broken somewhere between the motor and remote control. You could check continuity with a meter.

Anyway, give that a shot and see what happens. HTH...........ed

 

[mercurymang]

Re: Need a pointer... 1975 Merc 1500 switchbox/rectifer/tach

Calvinator, I would be very greatful if you could post up some pics of the inside of that mercontrol box. I have the same one and am contemplating tearing it apart to debug an issue with the neutral swich.

 

[calvinator]

Re: Need a pointer... 1975 Merc 1500 switchbox/rectifer/tach

Calvinator, I would be very greatful if you could post up some pics of the inside of that mercontrol box. I have the same one and am contemplating tearing it apart to debug an issue with the neutral swich.

Ha - I had it open last night, but didn't take any pics. It is real simple inside. I look at the neutral switch, it is straightforward.... Just a small button switch with a spring on it. When in neutral, the spring doesn't press the button. When you shift into gear, the Mercontrol gearing rotates against the spring pressing the button. Should be an easy troubleshoot.

 

[Chris1956]

Re: Need a pointer... 1975 Merc 1500 switchbox/rectifer/tach

Calvin, Move the brown wire from the switchbox, to the rectifier yellow lead. Either one is fine. If the tach still doesn't function, either the rectifier is bad, the tach is bad or the wiring from rectifier to tach is bad.

 

[calvinator]

Re: Need a pointer... 1975 Merc 1500 switchbox/rectifer/tach

Calvin, Move the brown wire from the switchbox, to the rectifier yellow lead. Either one is fine. If the tach still doesn't function, either the rectifier is bad, the tach is bad or the wiring from rectifier to tach is bad.

Thanks - that will be my plan for tomorrow. I'll report back some results....