New 684 tach runs at half speed

[rhomium]

After changing the tach on my '84 684, the new tach is only running at half speed. My old tach worked, except for the hour meter. The RPM needle worked fine. Any ideas on what the issue is? Is there some adapter I need to get for the new tach?

 

[Powered By IH]

After changing the tach on my '84 684, the new tach is only running at half speed. My old tach worked, except for the hour meter. The RPM needle worked fine. Any ideas on what the issue is? Is there some adapter I need to get for the new tach?

Could you please link a link of where you purchased the tach from?
Is it approximately half speed or exactly half speed?

 

[Mrfred54]

Beings the old tach worked, then you probably got the wrong tach or a bad one. Why not use the old tach and just install a 12 volt hour meter?

 

[MARK ROBKE]

Beings the old tach worked, then you probably got the wrong tach or a bad one. Why not use the old tach and just install a 12 volt hour meter?

I had to do that on my CIH485. No one could repair the tach (the hour meter had quit), but the rpm was fine. No replacement was available-there is something special/different about the face, I think. Except for cutting a hole in the dash, it was easy-peasy. But - guess what? electric hourmeters add up hours much, much quicker than mechanical ones. Mark.

 

[JimB2]

Usually it is the 90* gear drive that fails in the Tach drive mechanism that goes into the front of the engine and is driven off the timing gear. There is a spacer that keeps the gears meshing properly and eventually the spacer wares out and the gears start not meshing and Tach speed starts jumping then eventually stops working altogether especially in cold temperatures.
See photo below.

 

[MARK ROBKE]

Could you please link a link of where you purchased the tach from?
Is it approximately half speed or exactly half speed?

About 30yrs ago, I took a tach head from a Ford 2000 to a speedometer shop to have it repaired. We got it back, put it in, and it ran half speed. (I have forgotten the original problem). Anyway, I took it back to the speedometer shop, and he 'flashed' it with a magnet., and then it worked ok. Mark.

 

[T in NE]

I had to do that on my CIH485. No one could repair the tach (the hour meter had quit), but the rpm was fine. No replacement was available-there is something special/different about the face, I think. Except for cutting a hole in the dash, it was easy-peasy. But - guess what? electric hourmeters add up hours much, much quicker than mechanical ones. Mark.

Put a pressure switch in an oil galley so it only counts when the engine is running.

 

[MARK ROBKE]

Put a pressure switch in an oil galley so it only counts when the engine is running.

The reason the electric hour meter makes 'more hours', is they are actual hours, not hours based on engine revolutions on the tachometer. Most tachometers actually say hours at xxxx RPM, so the more RPM the engine turns, the quicker it 'makes an hour', whereas the electric hour meter doesn't care about engine RPM.
My Kubota, which replaced my CIH 485, has an electric hour meter, and adds close to double the hours the 485 made each year doing the very same jobs. Mark.