Forgive me if this is noobish, but I'm new to GM cars. I've noticed that the tachometer in my daughter's 2.5L powered 1986 2m4 reads about 1000rpm when the engine is off and key is in the off position (or even if the key is removed). Is this normal? Or is it a symptom of an electrical problem in the making. The needle drops to 0 as soon as the key is turned to the ignition position and reads appropriately when the engine is cranking and running.
84-87 Fieros used gauges that float around a bit when you turn them off. Something to do with magnetic fields.
I didn't think that 88s were any different with respect to gauges.
Anyway, the full-sweep gauge needles work like a compass. A permanent magnet (attached to the needle) follows the direction of the magnetic field lines surrounding said magnet. Two perpendicular fixed electromagnets generate a magnetic field around the permanent magnet. The direction of the resulting magnetic field depends on the magnitude and polarity of the electrical current supplied to each electromagnet. The integrated circuit that controls the gauge injects the current into each electromagnet such that the needle will point the right way.
When there is no magnetic field present (or only a weak field, such as Earth's magnetic field), then the permanent magnet (and hence needle) has no tendency to move if not disturbed. It will stay where it was left at.
When the gauges are not powered, there is no magnetic field created by the electromagnets.
The full-sweep gauges don't have return springs; that would be counterproductive, as the electromagnet + permanent magnet combo would have to be strong enough to fight the spring's returning force... for no good reason at all.
The tach needle movement is two coils wound around a magnet. When current is fed to the two coils, the needle moves to the resulting position (there is a sin relationship relating position to current). When the power is removed, there is no force being applied to the needle at all.. it is floating. But the needle weight is balanced, so that it does not have a preferred weight.. so it should just sit there. If it is weighted slightly off, it may drift to a certain position.
All Fiero and many other GM lines... None of the Gauges are "Zeroed" when car is shut down for any reason. And yes, changes in magnetic fields can make them move w/o power. How much depend on strength and orientation of magnet(s) etc.
Gauges Should read Zero or whatever when car is turned on. IOW Fuel gauge and others are only accurate when key is on.
GM might have covered this in the owners manual... get @ http://www.fieronews.net GM did publish a TSB covering this "problem"/complaint by owners but most don't have access to read them.
Even then don't trust most or all of them because iffy wiring etc can make them lie. Worse on any 15+ year old car not just 30+ year old Fiero. Sender, gauge or anything else can make the gauge lie. Even just One iffy ground in the car can make make any or all gauges read off. Example: Low or High Oil Pressure on the dash gauge often can't be trusted until you check w/ a real gauge on the engine. If real gauges = same as dash gauge then you have problems that can wreck the engine. V6, VIN-9, 30-45 PSIG L4, VIN-R, 36-41 PSIG Low pressure is "common" problem for V6 but Is Not Normal regardless of posted here and other places and engine can die in minute to years depending on many factors. Many dash gauges read high even after replacing sender and even the gauge because of iffy wiring adds to resistance from the sender. (Fuel/OP senders operates only 0-90 Ω.) But Real high pressure when tested can be bad for the engine too. Oil Pump for most engines are driven by the cam or timing set and high pressure puts a huge load on cam and timing set.
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