OMC dual station tachometers

[allen.st]

I have a 1976 OMC 800 w/351. My tachometer when engine is running multiplies my RPM's by 2. Tach shows idle at about 1500 RPM but a tune-up tach attached to #1 plug wire shows 750 RPM. I have checked and rechecked the wiring so I finally pulled the upper tach and wired it up directly to coil and ground, same results. These tachs appear to be older BorgWarner Medallion with only a light, ground, and - to distributor. Can the wrong coil double my RPM's like that?? I really need these to work at least close so I dont shell that old stringer <br /><br />could both tachs be bad and still read the same RPM's?<br /><br />Thanks

 

[allen.st]

Re: OMC dual station tachometers

Don S. Have you ever seen anything like this?

 

[rodbolt]

Re: OMC dual station tachometers

may be the tach setting switch is set to 4cyl instead of 8. have you ever seen it work correctly ? or they could be the wrong tachs.

 

[allen.st]

Re: OMC dual station tachometers

Thanks rodbolt but I checked the settings on them and the are set to 8c which is correct. I even set one of them to 4 and 6 and the RPM did increase like i expected, so the appear to be doing what they should. As far as being the correct ones they are the factroy tach's that match the rest of the gauges. My guess is they both have the same problem and that is why they both read the same, unless there could be something about the coil that has them messed up. With one still hooked to the wire harness and one directly to the coil they both read double the RPM while one on plug wire #1 read correctly. I was hard to set the idle until I put the external tach on and saw my RPS's were double.

 

[waterone1@aol.com]

Re: OMC dual station tachometers

Is the needle of the tach steady, or is it pulsating or jumping ?

 

[allen.st]

Re: OMC dual station tachometers

waterone1 Both needles are steady, they both read the same. when engine is cold and running a little rough you can see it in both tach's.<br /><br />I bought a cheapie tach for a car yesterday and put it on and it read correct, while the other two are doing the RPM X 2. Guess what ever is wrong it must be in the design and due to age both tach's are bad???