1988 iroc z conv - Tach issue - Third Generation F-Body Message Boards

terry m · 06-25-2009, 09:22 AM

my rpm gauge gose up to 1500 rpm ,when i turn of the motor and tutn the key to the of position, can any body help me with this problem thank terry

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okfoz · 06-25-2009, 10:33 AM

It honestly sounds like the position of the gauge needle has spun on the pin...

At 70 Mph what does it read? Depending on your rear axle ratio:

2.73 / 2.77 gear = 1750 RPM (about)
3.23 / 3.27 gear = 2000 RPM (about)
3.73 gear = 2250 RPM (about)

That would be in Overdrive at 70Mph, of course that would assume that you have stock tires etc...

John

chuck88iroc · 07-02-2009, 08:12 PM

Terry, your post is a little confusing. But, I think you are asking about the tach reading 1500 with the engine not running and the ignition key in OFF. If that is the case, I wouldn't worry about it. Mine has done that since I bought it new in 1988. I just went out and looked - it is indicating 1750 RPM witht the engine not running.

Chuck

okfoz · 07-03-2009, 10:20 AM

I have to ask... Does it read correct when it is running? IIRC It shouldn't because the tach is run by a motor that spins magnets that in turn pull other magnets attached to the needle as the tach motor spins faster the higher the needle goes... Speedo works the same way...

John

chuck88iroc · 07-03-2009, 02:27 PM

Mine does .... when the ignition switch is turned to the run position the tach goes to zero. When shutoff it will go to zero and then slowly rise.

The tach uses the frequency of pulses from the ignition system. It is all solid state. The rise of the pointer with the ignition off is most likely just a mechanical characteristic of the pointer mechanism once the electric output from the tachometer circuit is removed.

terry m · 07-17-2009, 02:24 PM

thank for the info terry