1975 Merc 85 hp//850//Thunderbolt tach replacement

[Texas3141]

I have a 2 stroke, 85 hp, Merc 1975, outboard with a Thunderbolt ignition, and a pointless distributor. A year ago I replaced the original trigger and switch box to CDI aftermarket ones. The new CDI switch box directions instructed me to take the brown sending wire from the old switch box and attach it to the rectifier on the stator side. I did so and blew out my original Tachometer a few seconds after the engine started. I need to replace the tach and have been having problems locating a good source. Many of the application guides are listed for 4 stroke or for powerheads made 1977 and later. Can anyone please help me, I hate to stare at a flatlined gauge. Thnx in advance.

 

[Gomer50]

Re: 1975 Merc 85 hp//850//Thunderbolt tach replacement

http://cgi.ebay.com/ebaymotors/BOAT...8138273QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear
This might be what your looking for I use this one on my 78 90hp merc.

 

[Chris1956]

Re: 1975 Merc 85 hp//850//Thunderbolt tach replacement

Nuts, Most any aftermarket OB tach will work with that motor. Teleflex and Faria are just two of the more popular brands. Newer OB tachs work off the stator pulses.

 

[Texas3141]

Re: 1975 Merc 85 hp//850//Thunderbolt tach replacement

Hey guys thanks for the quick responses. I guess I just needed some assurance before dropping some cash on a new one. Wouldn't want to waste money on something I'll burn up right away. Probably won't get one for a few days. Might order it from the Internet. I'll post the outcome when I get one.

 

[Texas3141]

Re: 1975 Merc 85 hp//850//Thunderbolt tach replacement

I just thought about a few more questions related to this. First, I've never pulled the flywheel off to look a the stator but I did look up the replacement stator online. I counted what appeared to be 10 windings. Would that make it a 5 pole requirement or would a 6 pole be sufficient? Second, will a 4 stroke tach work on a 2 stroke? Thanks again for the help. I have looked at this forum for a long time but this has been my first post. Pretty cool website

 

[Chris1956]

Re: 1975 Merc 85 hp//850//Thunderbolt tach replacement

That Merc uses a 12P or 6C setting, whatever the Tach has.

As for a 4 stroke tach, it depends on what drives it. If it is for a 4 cycle, multi cylinder using the single ign coil on a distributor ign as a Tach signal, it likely wont work. If it is a OB tach using the stator to drive it, and it has the proper setting, it likely will work. The newer tachs are multi application now (I/O and OB), versus only one type of application, just a few years ago.

 

[Texas3141]

Re: 1975 Merc 85 hp//850//Thunderbolt tach replacement

Thanks again for all the info and your time.