Tachometer

[jimoshel]

Has anybody replaced the calibration resistor in the tach circuit to use 4 cyl tachs in 6 cyl and vice versa? I'm thinking you can just replace the two fixed resistors with two of the proper resistance or swap in a trimmer pot. Searched search and couldn't find anything on the subject. Right now I have 3 instrument panels, one from a 2.5, 2.8 and a 4.0, on my desk getting ready to dismember them and see but don't want to rehash something if it's already been done. :dunno:

[Jeep Driver]

My 4.0 to 2.5 tach has not been adjusted.

 

Idle reads 450-500 when is should read 550-600.

 

Not off by as much as everyone thinks.

 

I live with it.

 

 

Supposedly mine is adjustable, when I change the faces out to white I may adjust it then if I can find someone with a handheld tach.......if not screw it.

[jimoshel]

My 4.0 to 2.5 tach has not been adjusted.

 

Idle reads 450-500 when is should read 550-600.

 

Not off by as much as everyone thinks.

 

I live with it.

 

 

Supposedly mine is adjustable, when I change the faces out to white I may adjust it then if I can find someone with a handheld tach.......if not screw it.

Or an audio generator. For a 6 cyl 100cycles per second will equal 2,000RPM. Or use a small transformer, 12VAC or less. 60CPS will translate to 1200RPM

[Eagle]

Has anybody replaced the calibration resistor in the tach circuit to use 4 cyl tachs in 6 cyl and vice versa? I'm thinking you can just replace the two fixed resistors with two of the proper resistance or swap in a trimmer pot. Searched search and couldn't find anything on the subject. Right now I have 3 instrument panels, one from a 2.5, 2.8 and a 4.0, on my desk getting ready to dismember them and see but don't want to rehash something if it's already been done. :dunno:

The 4.0L tach should already have a potentiometer. The clusters from 1988 and newer do have it. 1986 and older do not, but a 2.8 is a 6-cylinder, so it will be properly calibrated for a 4.0L. 1987 is the question mark year -- I don't know if they had a potentiometer or not, or if it showed up as a mid-year change.

[Eagle]

My 4.0 to 2.5 tach has not been adjusted.

 

Idle reads 450-500 when is should read 550-600.

 

Not off by as much as everyone thinks.

But it's not a fixed difference. What does a tachometer do? It counts revolutions. It does that by counting ignition pulses. For a 4-cylinder, 2 pulses = 1 revolution. But for a 6-cylinder, it needs 3 pulses for 1 revolution.

 

So ...

 

Take a 6-cylinder tachometer. In a 4.0L vehicle, 2100 RPM needs 2100 x 3 = 6300 ignition pulses.

 

Now put that tach on a 4-cylinder. Run it up to where it reads 2100 RPM. That's still 6300 pulses, but in a 4-banger 6300 ignition pulses is 3150 RPM. It's not off by 100 RPM, it's off by 50 percent.

[Jeep Driver]

I got a mechanic buddy with a shop down the road, I stop in next week and see if he has a way to test the rpms.

[alleydog]

Is there enough adjustment to read the correct rpm for a V8? I have an 89 comanche, and putting a 5.2 magnum in it. BTW, you could use a digital timing light to check rpm against a tach.