3 wire off of front of control box

[morge]

I have a 72 Merc 650 with what I believe is the matching original control box - R-N-F lever with shorter high idle lever off the right side, electric button choke behind on top, key goes in rear - 3 position. <br /><br />My question is: what is the the three wire harness that plugs/unplugs on to the front of the control box? It has a green, white, and black wire. When testing with my meter, ign on, Black - white = 13V (battery charged up) but 12V black to green. <br /><br />Also, am I going to be so lucky so as to find out I can hook up my VDO tach to the green wire?

 

[Ken G]

Re: 3 wire off of front of control box

That is indeed the connection point for a tachometer if you have the proper plug. My manual shows brown, white, and black wires running to this connector.

 

[morge]

Re: 3 wire off of front of control box

Thanks a million Ken. Now more questions:<br />1/ Green wire to the tach and black and white provide switched power?<br />2/ I cannot find any support on the net for VDO gauges, it has two connectors for the gauge lights, then three locations for tach signal - each with a number below: 2,3, and 4. Is this for 2,3, and 4 cylinder? <br />3/ Why no negative or ground for the signal?

 

[Ken G]

Re: 3 wire off of front of control box

I know nothing about VDO tachs but here's my guess. The black wire in the control box is the ground and your tach probably grounds through the case to the mounting bracket. A jumper from the bracket to a good ground should work. <br /><br />The white wire should be your 12V source for the gauge light. Since it has two wires it may be switched for day/night use. <br /><br />The green wire is the tach signal. I believe most other tachs have a switch for the number of poles on the stator which corresponds to the pulses which the tach interprets. I think my Merc 500 and inline 6's use a setting of 12. You will have to experiment with yours but I'd start on the 3 matching the number of cylinders.<br /><br />Remember, all of this is a guess so if it goes up in smoke I was never here.