tach operation thoughts??

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i recently had charging issues of which have all been resolved thanks to everyones input. i just noticed something a little strange going on with my tach. i have my tach wired to a switched 12v at the fuse block and no light wired up.the tach is grounded at the dash frame , and the green at the negative side of the coil. heres the thing, when i start the car, the tach reads around 750-800 rpms, which is normal. when i shut the car off the tach drops to around 500. when i turn the key back over into to the "run" position , the tach needle finally drops all the way to 0 rpms. would this be a direct result of a bad ignition switch allowing a draw of voltage through the switch? or could this be another ignition component going bad, or is the tach itself no good? is there an easy way to check? thanks guys for your help. i have pics of the tach when all this is happening, but i didnt think they would be necessary.
 
I have a small autometer which "hangs" wherever the RPM was when you shut off power

IF it's idling at 1000, that's where it stays when the power goes off

IF you blip the throttle because you are tryin' ta' git that blonde ta turn around, and it's up 2500 or whatever, THAT is where it will be.

And if you turn the power back on, without the engine running, it will move to zero from wherever it was.
 
I have 2 of them (AutoGage brand) that do the same thing. I think the needle stays wherever it was when you turned the key off. They have been that way for years. One in my 59 Suburban and one in my wife's Mustang so it does not seem to be vehicle brand specific. I would not worry about it.
 
67Dart273 said:
I have a small autometer which "hangs" wherever the RPM was when you shut off power

IF it's idling at 1000, that's where it stays when the power goes off

IF you blip the throttle because you are tryin' ta' git that blonde ta turn around, and it's up 2500 or whatever, THAT is where it will be.

And if you turn the power back on, without the engine running, it will move to zero from wherever it was.
Click to expand...
ok great, thanks again 67 and everyone, much appreciated. i guess i just never noticed it before. the tach is a Sunpro supertach 2 ,almost brand new.
 
They might be using a stepper motor to drive the needle, many newer automotive gauges have them. They are a type of brushless motor. They are also used in CD and DVD drives to control the read head. The magnets hold the last position when powered off. A nice feature for fuel gauge, not so good for others.
 
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