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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
The Tach in my 1985 IROC suddenly stopped working about a month ago. The Tach worked fine for about a 1,000 mile break in period on a rebuilt LB9. I am using a DUI HEI computer controlled distributor. With ignition off, Tach is pegged at 7000. With ignition on, also pegged at 7000.
There may well not actually be anything at all wrong with it, besides maybe its accuracy.
Try taking off the bezel and moving the pointer to 0 by hand with the engine off, then see what it does.
The way those work, there's no "return spring" on the meter movement, like most meters have. Instead, they rely on 2 coils oriented 90° from each other that drive the movement, one that moves the pointer "north-south" so to speak, and one "east-west". That particular tach moves 270° from 0 to full-scale; 0 is "due south", about 2300 is due W, 4600 is due N, and 7000 is due E; when the key is on and the engine is off, the N-S coil is trying to drive it S that being where 0 is, and the E-W coil is off; but if the pointer is already too far around past E, then to get from there to S it has to go CW, which it can't because the OP gauge physically is in its way. The circuit is as dumb as a bag of sand, and lacks the intelligence (and of course any knowledge about the pointer's current location) to drive the pointer N first, then W, and THEN S, to get it to return all the way S to 0.
Last edited by sofakingdom; 09-08-2017 at 06:19 AM.
There may well not actually be anything at all wrong with it, besides maybe its accuracy.
Try taking off the bezel and moving the pointer to 0 by hand with the engine off, then see what it does.
The way those work, there's no "return spring" on the meter movement, like most meters have. Instead, they rely on 2 coils oriented 90° from each other that drive the movement, one that moves the pointer "north-south" so to speak, and one "east-west". That particular tach moves 270° from 0 to full-scale; 0 is "due south", about 2300 is due W, 4600 is due N, and 7000 is due E; when the key is on and the engine is off, the N-S coil is trying to drive it S that being where 0 is, and the E-W coil is off; but if the pointer is already too far around past E, then to get from there to S it has to go CW, which it can't because the OP gauge physically is in its way. The circuit is as dumb as a bag of sand, and lacks the intelligence (and of course any knowledge about the pointer's current location) to drive the pointer N first, then W, and THEN S, to get it to return all the way S to 0.
Thanks for the information! I will give it a shot and see what happens.
09-08-2017, 01:05 AM
