Interesting Tach problem

[dcraigbrown]

My tach in a 73 stock 240z wasn't working. After checking power and ground, and continuity from the b/w & g/w wires under the hood to the tach connector I finally determined I better pull it out and check the innards. This is where it gets interesting. The needle was pretty much hung wherever it happened to stop on the dial. I loosened one of the rear case screws and the needle became "unhung" and moved to the zero stop. When I tightened the screw it hung again. To make a long story short, the return spring is so delicate that having electrical posts or case screws too tight warped the movement enough to make it hang. Ultimately making sure the positive and negative posts were not torqued too tight fixed it- I was able to tighten the case screws sufficiently after that. Just thought I’d share that for anyone else with a non-working tach.

I do have one question though, since the movement is so delicate I’m worried about the accuracy of the tach after I had my big paws inside it. As a reference in 3rd gear going 40mph the tach reads 2500. Does that sound correct given a stock 73’s gear ratio?

[WarrenZ]

You can calculate the RPM for any gear using this formula:-

rpm = (mph * gear ratio * final diff ratio * 88) / circumference of rear tire in feet ie 5.25 feet

Measure the circumference of your tires with a tape.

Since your speedo will not be 100% accurate the answer may be a little off

If you want to calculate MPH for a given RPM in a particular gear then the formula rearranges to :-

mph = (rpm * cir) / (gear * final * 88)

This also gives an indication on the accuracy of your speedo.

Cheers

WarrenZ

[dcraigbrown]

Thanks for the reply. I haven't had much luck finding the variables to plug into the formula, gear ratios, etc.

[WarrenZ]

You did not say if it was a 4 or 5 speed so according to the Haynes manual for a 240Z they are:-

4 speed 5 speed

1 3.549:1 2.957:1

2 2.197:1 1.857:1

3 1.420:1 1.311:1

4 1.0:1 1.0:1

5 N/A 0.852:1

Diff ratio for 4 speed gearbox = 3.9:1 and 5 speed = 3.364:1

See how that looks

Cheers

WarrenZ

[soundmasterg]

The 4 speed and 5 speed ratios should be reversed. The 4 spd is the 3:36 and the 5 spd is 3:9.

Greg

[dcraigbrown]

Thanks for the help. Using the information provided my tach is about 1% off, I guess thats not bad.

[WarrenZ]

The 4 speed and 5 speed ratios should be reversed.

Not according to page 121 of my Haynes Manual

A 4 speed 260Z is 3.364, but 4 speed 204 is 3.9

Warren