Tachometer Inaccurate?

[Coonass77]

I have a 2006 M1165, 6.5 turbo, 60mph speedo and a tachometer. My problem is that the tach rpm reading is wrong. The tach shows idle at 950rpm, but my handheld rpm meter shows 700rpm. And with the engine speed set and showing as 2000rpm on the tach, my handheld meter is reading 1500rpm.

I verified my handheld meter matches the gauge rpm on my Chevy truck, so I believe the meter to be accurate. I also verified the M1165 tach is wired per the installation instructions (it came from govplanet w/ the tach installed). Other than the incorrect rpm, the tach appears to be operating exactly as it should.

Has anyone experienced this before or have any ideas of what could be causing the error? Could the tach signal somehow be bad? Or the tach itself? Thanks!

 

[Milcommoguy]

What else is left? Think you answered your own question. One or the other know to go bad. My guess is sender. Could check tachometer with a signal from a pulse / function generator. That would be a bit science lab project.

Just a guess. CAMO

 

[Barrman]

What does the tachometer read with everything off? I have a surplus electric tach here on my desk that reads 300 rpm not hooked to anything. My guess is that is the reason it was surplused. Had the same thing happen with a mechanical drive airplane tach years ago as well.

 

[86humv]

Turbo Tach wires go to alternator regulator...no tach drive on turbo trucks.
Might be Tach defective.

 

[Retiredwarhorses]

You probably have a 400amp with the small fixed pulley, this pulley is smaller in diameter than the over running pulley, thus it shows a higher RPM, as Greg said, the Tach is wired to the generators signal for rpm source.
The turbo trucks also Tach higher due to the 3.08 differentials When driving….

 

[Coonass77]

You probably have a 400amp with the small fixed pulley, this pulley is smaller in diameter than the over running pulley, thus it shows a higher RPM, as Greg said, the Tach is wired to the generators signal for rpm source.
The turbo trucks also Tach higher due to the 3.08 differentials When driving….

Thanks for the replies!

The tach reads 0 with the engine off, so it isn't an offset problem.

The rpm error is proportional to engine speed and I do have a 400a alternator with a fixed pulley, so that explanation makes perfect sense. I also have an M1123 with a 200a alternator and an overrunning pulley so I compared that pulley circumference to the 400a fixed pulley circumference and that ratio (1.25:1) is almost exactly the same ratio as my tach rpm vs actual rpm. So assuming the pulleys are interchangeable between the 200 and 400a alternators, that also points to Retiredwarhorses having nailed it. If I wanted to swap the 200a overrunning pulley and the 400a fixed pulley, do you happen to know if they are fact interchangeable? If not, I'll dig through the TMs this weekend to figure it out. Thanks!

 

[Retiredwarhorses]

Thanks for the replies!

The tach reads 0 with the engine off, so it isn't an offset problem.

The rpm error is proportional to engine speed and I do have a 400a alternator with a fixed pulley, so that explanation makes perfect sense. I also have an M1123 with a 200a alternator and an overrunning pulley so I compared that pulley circumference to the 400a fixed pulley circumference and that ratio (1.25:1) is almost exactly the same ratio as my tach rpm vs actual rpm. So assuming the pulleys are interchangeable between the 200 and 400a alternators, that also points to Retiredwarhorses having nailed it. If I wanted to swap the 200a overrunning pulley and the 400a fixed pulley, do you happen to know if they are fact interchangeable? If not, I'll dig through the TMs this weekend to figure it out. Thanks!

No, you want to replace the fixed with over running type…don’t swap them.
also, the nut for the over running pulley is different than the 200amp nut.

 

[Coonass77]

No, you want to replace the fixed with over running type…don’t swap them.
also, the nut for the over running pulley is different than the 200amp nut.

10-4, thanks!