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hey all, i'v always been curious about how speedometers and tachometers work/read the engine speed and speed of the car. i know theres a speedo gear or sumthing on the transmission, but how does it all work really? Is it the same mechanism on older cars? (i got a 2000 civic ex) My friends 87 celica's speedo like fluctuates and jumps up and down roughly, and i always wondered how they worked and what causes that.
anyone have a link or short description of how that stuff works? and if i were to ,say just randomly, swap in a b18c5 and just hook everything up to my tach now, would it be an accurate rpm reading? or would i have to get an acura integra gauge cluster to match? im justt very curious. i appreciate any info on this topic.
 

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i dont know much bout the "truth" behind the technology but jumping tachs and speedos are pretty commin in older cars. . . its basicly all electical, this is a good post i'm just not sure if it belongs in the tech forum
 

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older cars speedometer function (basics):

the speedo driven via a cable coming off the transmission. The speedo cable would have a "drive" gear with a certain amount of teeth and inside the tranny on the output shaft there would be "driven" gear, also with a certain amount of teeth. The teeth on the driven gear would correspond to the first/final gears of the tranny and the teeth on the drive gear would correspond to the axle ratio. As the tranny turns and in turn, turns the speedo gear and cable, the speedometer would turn and it itself has a set of worm gears inside it which turn in relation to the amount or pressure being exerted on it by the speedo cable. Please note that even though you are shifting through the gears the output shaft on the transmission does NOT slow down as it is STILL in contact with the drive shaft/axles/wheels which are ALL continually gaining momentum...thus as the speed of the cable
increases, the speedo needle will go up in speed as the amount of force on it increases to push it farther along the face....it actually takes QUITE a bit of spinning force to move that damn needle as I found out on an extra 71-73 mustang speedo face I had...I could get it to spin quickly to about 20mph myself as fast as I could spin it like a top on the back...that's it...

as they wear out the cable frays or breaks and thus will slip or stop all together which ends up in either erratic function or cease of function.

newer cars speedometer function (basics):

same principal except the cable drive is replaced with an electronic V.S.S. (vehicle speed sensor) which is attached to the drive gear. as speed increases, it increases voltage along the wire sent to the speedometer and the needle travels (functions almost exactly like a reostat). And just like a reostat, as it wears out the voltage will fluctuate and you'll get goofy readings...

tachometer function

the tach is just like the electronic speedometer only MUCH more simple. the tach feeds off of the negative side of the ignition coil and reads the duration of the phase off of the negative side of the coil. The "duration" of the coil is read as the time between discharges of the coil as it sends a signal to the spark plug that is firing...the coil acts like a capacitor...remember that. It sends this signal to the tach face which is also like a reostat of sorts. the longer the duration between discharges, the lower it reads. the shorter the duration between discharges, the higher it reads. thus, as you increase rpms you get a higher rpm reading on the tachometer.

well, that's both in a nutshell though there's actually a LOT more involved in both than this such as gear ratios and drive gear ratios for speedo gears and the differences of stock and aftermarket tachs (which there is NOT much difference...)
 

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so based on this, if my speedometer sometimes works and sometimes does not, do I need to replace the cable? or the sensor? its a 1992 Civic DX
 

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its on the transmission. Just go to the parts store and get them to bring you one so you can look at it then go outside to your car and find it on the tranny.
 
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