'72 1150 merc thunderbolt tach wiring

[monk-monk]

i just read a Post in another boating forum where a guy was having probs with his thunderbolt tach...the answer to his Post was submitted by Clams Canino and went something like this: don't use the thunderbolt tach (get a newer tach) and don't wire it to the integral tach circuit in the controller, but instead wire it to the stator lead(s)...can anyone elaborate on this?

 

[achris]

Re: '72 1150 merc thunderbolt tach wiring

Correct...

This applies ONLY to the thunderbolt ignition system that has a distributor (like yours)

The early thunderbolt tachos (designed to run on the tacho feed connection on the side of the switchbox behind the front engine support member) had problems... Use a normal marine tacho that can take a '12 pole Alt' input... Connect the tacho feed wire to one of the 2 yellow wires on the rectifier that come from the stator.

Should look like this when you finished....

Chris.......

 

[Moody Blue]

Re: '72 1150 merc thunderbolt tach wiring

Installed a 12 pole tach this past summer on my 1971 Merc 800. You can take the tach signal from either side of the rectifier (either yellow wire).

If you have an older tach already installed, chances are the tach signal is taken off the brown lead on the starboard side of the switchbox. You will need to disconnect the brown wire from the switchbox (not distributor side) and connect it as Chris has shown in the illustration to the rectifier.

 

[monk-monk]

Re: '72 1150 merc thunderbolt tach wiring

thanks guys, i understand! the reason behind my research of these tachs is because i can only get 4000RPM's (so the gauge says) out of this 1150...one person suggested the prop was to big, but it is the original prop. the prop people said it could simply be the motor was getting weak. i have never checked the compression on this engine but it "purrs like a kitten" so i became suspect of this gauge...there is a ground wire jumper from one of the posts to the frame of the gauge and it is completely deteriated (like all old merc wiring)! i love the gauge as it matches all the other gauges in my dash panel and would like to keep it...i will probably get a gauge as y'all have described and hook it up as you've told me and see how close the two are!