What's this?

[Robbie693]

Sat in the car the other day running the engine and I noticed something I'd never seen before, two marks on the tach between 1000 and 2000 rpm. Anyone know what they signify?

Just curious - it'll probably seem obvious when someone tells me!

Cheers

Robbie

[trw99]

I thought it might be something to do with idle speed, but it is way too high for that, 800-950 rpm being the recommended range, depending on engine type.

Beats me!

Reminds me of the old Mini Cooper's, which had three orange marks on the speedo, indicating the maximum speed of each gear.

Tim

[leifanten]

guessing here: Calibration marks for assembly of needle to get the correct spring tension.

[stugilmour]

guessing here: Calibration marks for assembly of needle to get the correct spring tension.

Guessing here as well, but calibration marks of some kind. Very interesting, as my Plus 2 RVI tach has similar marks. Just did up a detailed description of the shady tree tach calibration method in the Europa thread below. Never noticed the calibration marks on the tach, and did not see a mention of them in all my extensive on line research of Smiths gauge calibration.

elan-f15/tachometer-reading-too-high-europa-t21763.html

I expect they had a known square wave signal input at the factory, and adjusted the trimming pot to get the needle to point exactly at the calibration marks. This would likely be done at the assembly bench before installing the bucket and bezel/glass, as you cannot access the adjustment pot when the tach is assembled. From my adjustment experience, one wants to adjust well off idle to get a reasonable reading across the engine rev range. I found adjusting at about 3000 or 4000 rpm worked OK.

If you look closely, all of the electric minor gauges for later model Elans and Plus 2's (fuel, temp in Plus 2, oil pressure, not sure on ammeter) have similar marks at three positions on the gauge face. I don't know about the Elan temp gauge, which is completely different design. Early Elans may have a different oil pressure gauge design as well; for reference, I am working with temp compensated bi-metallic minor gauge meter movements in the Plus 2.

These are definitely calibration marks as well. Looking at various 'net pages, they apparently correspond to input voltages of approximately 2.0, 4.8 and 7.6 volts to position the needle at the calibration marks. Again, I assume this would be done prior to the bucket being attached, as the adjustment lugs are generally covered when the bucket is installed. The two adjustor's move the needle position with a known voltage input to get the meter dial exactly positioned so it will respond to the sender resistance correctly.

The voltage inputs quoted are from a Sunbeam Tiger page referencing the temp gauge, and I have not checked them yet in practice, but will be doing this over the winter for my Plus 2 fuel gauge. As the minor gauge meter movements apparently have a resistance of approx. 60 ohms, the quoted voltages match reasonably well with resistances of 20, 70 and 222 ohms from the fuel sender, which from previous posts is roughly correct for the Plus 2. If you are sketching up circuit diagrams remember the installed fuel & electric temp gauges are fed with ~10 volts from the stabilizer not nominal 12 volts.

Anyway, all a bit of elaborate reverse engineering to determine the test bench resistance or voltage inputs (same thing electrically) to be able to bench calibrate the gauge prior to installing, as this is a heck of a lot easier than hanging upside down behind the dash! Was intending to report my results when I have adjusted, as a lot of Plus 2 folks have lousy readings at the top end of the fuel gauge and this appears to be easily adjustable without a gauge rebuild. With the correct input voltages known, a variable power supply (thinking my train set transformer here) could be used to dial in the gauges prior to installing the dash during a resto, which would be way easier.

Anorak off now. Merry Christmas to all!

[Robbie693]

Ah that makes sense - thanks guys

Reminds me of the old Mini Cooper's, which had three orange marks on the speedo, indicating the maximum speed of each gear.

Tim

I'd forgotten about that! Genius solution to no rev counter!

Merry Christmas all

Robbie