tach on 15 when car off

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Mar 6, 2015 | 01:05 PM

mwm3006

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tach on 15 when car off

Tach is off. On 10 to 15 when car is off. Can you just pull the needle off and adjust like if it was a clock or will that break if? What to do??

Mar 6, 2015 | 01:41 PM

65GGvert

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From: Kannapolis NC

Quote:

Originally Posted by mwm3006

Tach is off. On 10 to 15 when car is off. Can you just pull the needle off and adjust like if it was a clock or will that break if? What to do??

What year car is it? Electronic or mechanical? When you turn the key on, does it drop to zero?

Mar 6, 2015 | 01:52 PM

mwm3006

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Quote:

Originally Posted by 65GGvert

What year car is it? Electronic or mechanical? When you turn the key on, does it drop to zero?

It's a 79. Surely mechanical. No it doesn't drop to zero.

Mar 6, 2015 | 02:04 PM

JimLentz

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From: Downers Grove Illinois

A 79 is electronic and the most likely cause is the circuit board. Does the reading seem right other than being off by 1000 to 1500 RPM when the car is running?

Mar 6, 2015 | 02:09 PM

mwm3006

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Quote:

Originally Posted by JimLentz

A 79 is electronic and the most likely cause is the circuit board. Does the reading seem right other than being off by 1000 to 1500 RPM when the car is running?

Yes it works right. Just way higher than it should be. Runs up into yellow and red when it should not. Does work right though. Just starts off at 10 to 15.

Mar 6, 2015 | 02:17 PM

JimLentz

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From: Downers Grove Illinois

Quote:

Originally Posted by mwm3006

Yes it works right. Just way higher than it should be. Runs up into yellow and red when it should not. Does work right though. Just starts off at 10 to 15.

You can remove the needle and place it back on where it should be. I am not sure on your '79, but my '77 was a real pain to get to.

Mar 6, 2015 | 02:46 PM

hwcoop

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From: Airdrie Alberta

i seem to remember them doing this even when they were new...kind of like the fuel gauge, just kind of floats until the key is turned on

Mar 6, 2015 | 03:29 PM

65GGvert

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From: Kannapolis NC

Quote:

Originally Posted by mwm3006

It's a 79. Surely mechanical. No it doesn't drop to zero.

Yeah, it's electronic. They usually drop to zero when you turn the key on without starting the engine. I'm not sure if moving the needle is your answer or not, not knowing how the reading is at higher rpm's.

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Mar 6, 2015 | 04:02 PM

mwm3006

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Quote:

Originally Posted by 65GGvert

OK. I did just turn the key on without starting and it does go to zero. Just seems to be running higher than I would expect. Idles at like 15 but car is idling about normal. Not fast idle at all. Engine has been beefed up a bit from the original 190 HP (thank god) Probably no more than 250 tops I would guess. If I recall 60 mph is running at about 3500 to 3800 I believe.
Maybe I'm just used to my newer vehicles with 6 and 8 speed trans.

Mar 6, 2015 | 04:24 PM

#10

65GGvert

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From: Kannapolis NC

Quote:

Originally Posted by mwm3006

Sounds like your tach is fine. Hook up an external rpm gauge, like a dwell meter to double check it. It's normal to read some value when switched off as long as it drops to zero when powered on. Not knowing what transmission and what rear gear ratio you have, it's impossible to gauge what your rpm should be at 60. I have a 3:08 and it runs 2600 rpms at 70mph, so you sound like you're in the range depending on the rear axle you have.

Last edited by 65GGvert; Mar 6, 2015 at 04:27 PM.

Mar 6, 2015 | 07:49 PM

#11

mwm3006

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Quote:

Originally Posted by 65GGvert

Took the car out again. 60 mph tach says 4300. 70mph is in the yellow at 4800. Don't feel like it's wound up near that much. Could have a lower rear end installed. Oh well. no big deal I guess.

Mar 6, 2015 | 08:38 PM

#12

BKarol

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Here is a large PDF file from GM Heritage about the 79 year that you can look through at your leisure:

https://www.gmheritagecenter.com/doc...t-Corvette.pdf

Appears in 79 if you ordered a L48 4spd the std rear was a 3:36 & an auto would be 3:55. I have a 3:70 and at 60mph I am at 3200 and 70mph @3400. I can't imagine someone putting in a rear gear that you are at 4800.

Check on the Willcox site for their help on tach problems or do a "search this forum" at the top of the page & enter tach problem. You will get some hits. Good luck.

Mar 6, 2015 | 09:46 PM

#13

mwm3006

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Quote:

Originally Posted by karol

thanks for the info but I know this tach has to be screwed up. I am not wound up that much at 70. Car is fairly normal and smooth at 70. I used to have a 59 vette that was wound as heck at 70. The rear end was like a 456 or something. This rear end seems pretty normal. Just trying to ignore tach. Thanks again.

Mar 6, 2015 | 10:13 PM

#14

BLUE1972

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From: Long Island

Quote:

Originally Posted by mwm3006

as a long shot, unplug the tech from the distributor and clean the contacts, also unplug the power feed to the distributor and clean the contacts. If the tech changes when you turn on the lights or turn signals check the grounds.

Mar 6, 2015 | 10:46 PM

#15

65GGvert

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From: Kannapolis NC

Quote:

Originally Posted by mwm3006

Took the car out again. 60 mph tach says 4300. 70mph is in the yellow at 4800. Don't feel like it's wound up near that much. Could have a lower rear end installed. Oh well. no big deal I guess.

That's too high. You'd know if you were winding 4800 at 70.

Last edited by 65GGvert; Mar 6, 2015 at 10:50 PM.

Mar 7, 2015 | 09:12 AM

#16

0Willcox Corvette

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St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15

The 1978-1982 tachometer is supposed to hold the last signal sent to the board when the tach is shut off. However, in this case it sounds to me as if someone has maybe replaced the board and not done a zero set on the needle.

This info will get you back on track and show you how to do a zero set. If a new zero set works, then your set. If it fails post again.

Willcox

Tach Board installation 1978-1982

Corvette Tachometer Board Installation Instructions 1978-1982

Corvette Tachometer Testing, Removal, Board Installation, Video Link 1978-1982

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