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Tach reads 1K when engine off by Yorgle
Started on: 01-09-2019 03:12 PM
Replies: 7 (120 views)
Last post by: theogre on 01-10-2019 11:57 AM
Yorgle
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Report this Post01-09-2019 03:12 PM Click Here to See the Profile for YorgleSend a Private Message to YorgleEdit/Delete MessageReply w/QuoteDirect Link to This Post

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.

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olejoedad
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Report this Post01-09-2019 03:34 PM Click Here to See the Profile for olejoedadClick Here to Email olejoedadSend a Private Message to olejoedadEdit/Delete MessageReply w/QuoteDirect Link to This Post

Not uncommon

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Gall757
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Report this Post01-09-2019 04:14 PM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by olejoedad:

Not uncommon


84-87 Fieros used gauges that float around a bit when you turn them off. Something to do with magnetic fields.

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pmbrunelle
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Report this Post01-09-2019 07:06 PM Click Here to See the Profile for pmbrunelleClick Here to Email pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by Gall757:


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.

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Chris Eddy
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Report this Post01-09-2019 08:32 PM Click Here to See the Profile for Chris EddyClick Here to Email Chris EddySend a Private Message to Chris EddyEdit/Delete MessageReply w/QuoteDirect Link to This Post

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.

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Yorgle
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Report this Post01-10-2019 09:27 AM Click Here to See the Profile for YorgleSend a Private Message to YorgleEdit/Delete MessageReply w/QuoteDirect Link to This Post

Thanks for the education, guys. Little things like this seem to bug me, but if it's supposed to work that way, then I'm fine with it.

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Gall757
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Report this Post01-10-2019 09:39 AM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post

 
quote
Originally posted by pmbrunelle:

I didn't think that 88s were any different with respect to gauges.



I recall reading that the 88 gauges were changed and in fact did not 'float'. I can't find that reference now.....but I have an 88 and they all go to '0' when the car is off.

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theogre
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Report this Post01-10-2019 11:57 AM Click Here to See the Profile for theogreClick Here to visit theogre's HomePageSend a Private Message to theogreEdit/Delete MessageReply w/QuoteDirect Link to This Post

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.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)


The Ogre's Fiero Cave

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