[goodllookin1722]

got a 74 alfa spider
got it all over hauled and put all back together, runs like a champ,only problem is the miles per hr gauge is way off in speed. the needle sit at 10 miles per hr, and before that it all worked great
any idea on what the heck is going on
thanks for your idea's

 

[Tifosi]

Fairly sure that if you look closely at the gauge you'll find a small pin stopping the needle from going below 10mph.
Common on the dual pod gauges (tach too) at least and does not effect indicated accuracy.

EG: the pin can be seen on this speedo head from a '69

Or do you mean that the needle doesn't move at all and just sits at 10mph no matter how fast you're moving?
In that case, the cable drive would be in question.
(might not even be hooked back up yet if you've just gotten things back together for example)


BTW, speedo is a mechanical not electric gauge, and IIRC so is your tach.
Tach went electric later on, by '82 at least if not a bit sooner, and speedo did when things changed to the monopod configuration in '86.

 

[goodllookin1722]

sits at 10, and moves, but it way off

 

[Tifosi]

Does the needle 'twitch' when you're moving?
(might be binding cable due to routing)

Did you happen to change the gearbox outright for another one?
(if so, perhaps the replacement had a different drive ratio than the original)

Tire size radically changed?
(can have a fairly significant effect on indicated speed)

 

[goodllookin1722]

good morning folks. no i never changed nothing, it was just put back together, so it does move but it so far off, moves normal. why does that needle sit at 10, or where that peg is? what is the reason

 

[Tifosi]

Without the pin stop, the clock spring that is in there would literally unwind.

The spring is used to (1) return the needle to the stop position when you're not moving or allow the needle to come down if you go from a higher speed to a lower speed, otherwise it would literally sit at the last place it got shoved to until it got shoved further along yet (2) damp motion of the needle to cut down on sway or float (not to be confused with twitch which is cable induced and a seperate matter) and (3) calibrate the rate at which the needle moves as the magnetic spool that is driven by the transmission drive spins.

Trans drives cable, cable drives magnetic spool, spool twists bucket via magnetic feild induction, bucket moves needle thats attached to it, spring calibrates the amount and speed of bucket rotation so it doesn't just fly around in blindingly fast circles and thus needle postion is steady and consistant to the given speed.

Ever see those time travel movies where the clock on the wall spins into a blur as the machine begins to go forward in time? Same premise with the gauge if the preloaded spring and pin to stop it unwinding weren't present.

 

[Farace]

The question might have been, why is the pin at 10 rather than 0? That's one I never figured out, but then again, I'm usually going faster than 10 anyway.

 

[aldo]

The question might have been, why is the pin at 10 rather than 0? That's one I never figured out, but then again, I'm usually going faster than 10 anyway.

It's embarrassing to go slower than 10mph.

 

[alfaroad]

needle is loaded by the inside spring. Lower than aprx 10km/h-15 km/h needle's small mouvement controlled by this spring's inertia. then this spring controlled from the round disc which finally takes the motion via the magnetic field produced of the magnet connected with the turning speedo wire. So, if there wasnt any pin at all in very low speeds needle will go up and down around its rest point, if needle started from zero with these conditions will be hit the pin or will give huge eratic index. The practical method here is to rest the needle at a higher point (10km/h) somewhere where the speed wire turning force overcome the first needle's system small inertia. keep in mind although this first inertia of the spring is "useless" at the first we need it after...to have quick response of the needle,when going up or going back.

 

[alfaroad]

Tifosi said a similar, however for some unknown reason saw only farace's wondering...