Tach question

[Acollin]

1966 series V -- stock set up

I bought a used tach after my old one did not want to move much beyond 1k no matter how fast the car was moving.

The new used tach seems to measure high/fast. With my trusty old Sears meter ( rpm/dwell/volts) was used to verify( at 1k on the Sears -- it reads over 2k on the guage. It appears high beyond idle as I am close to the 6k redline cruising around 55mph.

Is there an adjustment/ calibration I can do? Is there something amiss with my new used gauge?

Please advise.

Thanks in advance for any help
Andrew Collin
Sandy, Oregon

 

[RootesRacer]

There is a potentiometer adjustment in the smiths tachs. If it is very far off of reality you will probably need to replace a capacitor or two to ensure the error doesnt continue to go south in the future.

 

[Tom H]

See my article in the Articles Archive:

http://www.sunbeamalpine.org/downloads/alpine_tach_repair_rev5.pdf

It describes several improvements, but as Rootes said the most important is to replace is the large capacitor. This also shows where to put an access hole for adjusting the tach while it is in its case. But be sure NOT to drill a hole in the case with the mechanism still inside. The metal chips will fall into the magnet in the meter movement and jam the meter movement.

Tom

 

[Acollin]

Thanks for the helpful replies.

I may have missed something by not reading the article more closely , but while I see the place to make an adjustment hole in the case, I did not see instructions for an adjustment----The how to adjust information.

I will take my oldest one apart to see if I can make sense of it, but scold me for not reading more closely or please discuss the " how to" of making the adjustment.

Thanks for the information and that amazing article. I am not sure I have the soldering or electrical understanding skills necessary, but I am usually up for a challenge.

Another question:

Is there a way of testing a tach without reinserting it in the dash?

Andrew

 

[Tom H]

Andrew, To adjust the tach, simply insert a screwdriver into the slot on the adjusting potentiometer and turn it until it reads correctly while some known signal is going thru the wire looped thru the rear.

Normally this is done on a test bench with a precision signal generator applying a known signal. In my article see a table with a sample of various frequencies and corresponding RPM (for 4 cyl engine).

Others have installed the tach and used a portable Dwell meter with RPM capability to read the car's RPM and adjusting the tach to match.

Here's an alternate method to test the accuracy on the road. You could use this method with the tach wired in but hanging out of the dash, making an adjustment with a screwdriver and then repeating the test until it's accurate enough:

How to check a tach accuracy on the road

Here's my method for getting all speed, distance and RPMs right (or close)

First measure the tire circumference this way: Put chalk marks on the tire and on the driveway, both at the point where the tire meets the driveway. Move the car exactly two (or three) rotations of the tires and make another mark on the driveway next to the mark on the tire. Measure the distance between marks on the driveway and divide by two (or three). This is the rolling circumference of your tires. This is the distance travelled for each rotation of the wheel. Convert that measurement into feet and divide 5280 feet by that measurement. This is how many tire rotations it takes to go 1 mile. Multiply that number by your axle ratio (either 3.89 or 4.21). This is how many rotations the engine makes in 1 mile (assume 4th gear, no OD). This is also the RPMs needed to maintain 60 MPH (At 60 MPH it takes 1 minute to go a mile).

In my case I measured 69.5 in tire circumference, which means 912 tire rotations per mile and with a 3.89 axle that is 3547 engine revs per mile and also 3547 RPM at 60 mph. THIS is what an accurate tach will show at an accurate 60 MPH.
The adjustment process assumes you have already put a hole in the rear of the tach case to provide access to the tach adjustment. Any calibration of the tach must be done with the case on and with the tach upright and vertical as it would be when installed in the dash.
Wire the tach into the car and take it on a highway while checking speed with a GPS. Drive the car in 4th gear, no OD, exactly 60 MPH as indicated on the GPS and observe the tach reading at that speed. It should be the value calculated above (3547 in my case). If not, adjust the tach using the adjustment potentiometer, until it reads pretty close. This may take several iterations, or you can have another person adjust while you drive.

Alternately, without a GPS, you can take it on a highway with good mileposts (or better yet a measured mile) and a stopwatch. Get the rpms up to the calculated value (3547 in my case) in 4th gear and hold the speed steady through a 1 mile stretch. (If tach is accurate you should complete 1 mile in 60 seconds) . Most likely it will take more than 60 seconds. So measure that time and calculate the % error. Multiply the expected RPM by that error % and repeat the process. Eventually you will find what RPM indication results in a 60 second mile. That is the RPM indication that produces 60 MPH. From this you can see how much you need to adjust the tach. After you adjust the tach repeat the process. If the tach is adjusted correctly it will display the correct RPMs at 60 MPH. You can also figure out the speedo and odometer errors during these same tests. But note that this process does not depend on speedo accuracy.

Any questions you can contact me directly at tahayden42@live.com

 

[Knightowl61]

The thermistor leads may have broken away (common due to age & heat).
Linear accuracy isn't the best in these old tachs. I matched the tach at 3k rpm with my trusty? old Sears meter so it should be close at working speeds.

Using test leads for ground & power with the signal loop would run the tach out of dash and keeping the tack in it's upright position.

 

[Tom H]

Actually, if you replace a few critical parts the linear accuracy is pretty good. And if you replace the thermistor with the parts described in my article the temperature stability is pretty darn good as well. Sometimes obtaining good linearity across the range of the dial requires and adjustment of the mechanical needle balance as well.

Tom