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A magnet that magnetizes "teeth" of an iron gear located on the motor. In general... The magnet magnetizes the "teeth" as the approach then de-magnetizes as they move away.. which creates an electric field.. The wire attached to the magnet connects to the tachometer which displays the rpm,
There are multiple types out there. The modern, most exact, is to have a laser or infra red light with a receiver on the opposite side. When the wheel ( or shaft rotates) it has a mirror or a tab that obstructs the path of the light every time it revovles ... then there is simply a chip to count the number of obstructions per minute. This one is extremely accurate and can handle some of the highest speeds.
But there are a host of older versions ... many still in use.
One is quite the same as above except there is a magnet on the shaft and a sensor ( Google 'reed switch' for example ) nearby that detects the magnet passing.
Another way is a chip attached to a switch that is physically pushed or contacted by a cog on the shaft.
Still another is to have a generator attached to the shaft and the Tach is actually a voltage meter telling the reading off that generator.
Finalley ... saved this one for last ... chances are you looked up this question because you wanted to know how the oldest Tachs worked back when electrical gear wasn't commonly used in cars and such. How did they pull it off back then? Well here goes: The end of the shaft had 2+ arms attached to the side that are weighted and would swing outward further and further, the faster the shaft rotated. The weights had a collar attached below them that would rise upward, the faster the shaft ran. Still with me? Check out this graphic if you need http:/www.railway-technical.comgovernor.gif the wieghts are the round black objects and the rising collar is green. So now all that is left is to have a hook ( with wheel to reduce friction ) that is hooked in the path of the collar so that it will get pulled up with the collar. The hook is attached to a cable ... on the other end of that cable is ( drum roll ) the sneedle of the Tach ... tugging it against a spring, over more and more as the shaft spins faster and faster. No electricity and no chips involved! Obviously this is somthing of a percision device and only as accurate as its most recent callibration ... but it worked enough. There were even hand held versions, very precise instruments that machinist would press against the axels of gears and shafts to get RPM readings. Now they have little hand held laser devices that are pocket size and much more accurate.
What else can I help you with?
Mazda b2500 tach works speedometer does not?
The Mazda tachometer and the speedometer have separate cables. The tachometer cable probably does not work properly. Change the tachometer cable.
If you had a speedometer with a tachometer and you replaced it with a speedometer without a tachometer will it work?
it sure would work.. behind the dashboard are plugs for each function. one for tachometer, one for speedometer, fuel, water temp. ect.. if your new dash doesn't have a tachometer it will not interfere with the speedomet aspect of it. but you will not have a tach obviously. that sucks kid
Your tachometer on your 1996 ford explorer does not work why?
One possibility is that the tachometer connections to the circuit board of the cluster have become corroded. It is also possible that the tachometer has just become worn out and needs to be replaced.
Why will the tachometer odometer and speedometer not work on a 1995 Silverado?
because they are broken.
Why does your tachometer not work on my 2005 Kia Sedona?
rev counter not working
What would make a 92 sc400 miss fire and tachometer not work properly?
There are several things that can cause your engine to misfire and the tachometer to not work properly. The most common cause is a malfunctioning vacuum hose.
My tachometer only works about once per year. How do you check for loose connections on the tachometer in a 99 Honda Civic Si?
If the tachometer only works a few times a year the VSS is malfunctioning. This can occur when oil is spilled on the unit and messes with the sensor. Cleaning the sensor will allow the tachometer to work properly.
Why won't my 87 acura tachometer work and could it be affected by a bad ignition coil?
A bad ignition coil can cause your 1987 Acura tachometer to quit working. A bad tachometer cable or ground wire can also cause it to quit working.
My speedometer and tachometer in my 97 Jetta GT stopped working I replaced the Speedo sensor in the transaxle and it still doesn't work. What can I do?
Replace the dash cluster that's what it sounds like to me since your tachometer and speedometer both don't work
Does the tachometer work off the coil?
Usually. It can also pull the signal from a spark wire.
How do you install a tachometer on a 1991 mercury cougar?
Remove the instrument panel cover. Remove the tachometer retaining screws. Remove the tachometer cable from the back of the tachometer. Reverse the process to install the new tachometer.
On a 1986 El Camino does the tachometer work off of vacuum?
The tach works off of the distributor.
