Another Bouncing Tach
Thread Starter
Drifting
Joined: Feb 2001
Posts: 1,539
Likes: 63
From: new ulm mn
Another Bouncing Tach
I've searched the topic and there is a lot of info on the forum about this. So I know that there are 3 things that can cause this, those being a bad tach cable, a bad tach, and the gears at the distributer. My question is this, about 3-4 years ago I bought a new repro tach from a vendor, ( can't remember who as it was so long ago) and I finally got it installed. I changed it because it had the wrong redline for the 400HP motor and it the old one bounced a little. The new one bounces like crazy. When you hit the gas it bounces between 2-4000 RPM so it's a big guess as to how fast you are really spinning. I understand from reading the various threads about this, the new tachs use different style magnets inside that can cause this.
Here is the question. If I change out the cable and gears, will that solve the problem? Or will I still have the problem due to the new tach. Would it be better to buy a used tach?
Willcox, if you're listening, please chime in as I know you have rebuilt a lot of tachs.
Thanks for the good help.
Here is the question. If I change out the cable and gears, will that solve the problem? Or will I still have the problem due to the new tach. Would it be better to buy a used tach?
Willcox, if you're listening, please chime in as I know you have rebuilt a lot of tachs.
Thanks for the good help.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I've searched the topic and there is a lot of info on the forum about this. So I know that there are 3 things that can cause this, those being a bad tach cable, a bad tach, and the gears at the distributer. My question is this, about 3-4 years ago I bought a new repro tach from a vendor, ( can't remember who as it was so long ago) and I finally got it installed. I changed it because it had the wrong redline for the 400HP motor and it the old one bounced a little. The new one bounces like crazy. When you hit the gas it bounces between 2-4000 RPM so it's a big guess as to how fast you are really spinning. I understand from reading the various threads about this, the new tachs use different style magnets inside that can cause this.
Here is the question. If I change out the cable and gears, will that solve the problem? Or will I still have the problem due to the new tach. Would it be better to buy a used tach?
Willcox, if you're listening, please chime in as I know you have rebuilt a lot of tachs.
Thanks for the good help.
Here is the question. If I change out the cable and gears, will that solve the problem? Or will I still have the problem due to the new tach. Would it be better to buy a used tach?
Willcox, if you're listening, please chime in as I know you have rebuilt a lot of tachs.
Thanks for the good help.
You didn't say what year the car is so that's leaving me guessing that it's a 1968-1974 car. But if the tach works okay with the drill then I'd go to the distributor cross gears and check them next.
If the tach fails with the drill then I'd try replacing the cable first... then I'd pull the tach.. You can still test the tach on the bench with your drill after you pull it.
Odds are if it's an old tachometer the first worm and magnet is worn out, if it's a newer one you'll have to test to figure the issue out.
Willcox
Thread Starter
Drifting
Joined: Feb 2001
Posts: 1,539
Likes: 63
From: new ulm mn
Thanks, I'll try the drill thing first. It is a new repro tach I bought from another vendor. Found the original box. Like I said, it bounces wildly now, not like with the old tach that was in the car. The old tach was an original 1969 tach. The car is 1969. Thanks again. Just trying not to have to pull the tach again as it is a PITA!! Do you guys rebuild tachs? If so, I'll have the original rebuilt. You guys have done two of my interiors and they are great!!
Last edited by loflite; May 1, 2017 at 08:43 PM.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I can rebuild it.. .but if it's a new tach in the car I doubt this is your issue.
The dash pad can be pulled pretty easy if you pull the steering column. It makes the job way easier... I have detailed instructions on my help page if you do have to pull it.
http://repairs.willcoxcorvette.com/1...h-pad-removal/
Willcox
The dash pad can be pulled pretty easy if you pull the steering column. It makes the job way easier... I have detailed instructions on my help page if you do have to pull it.
http://repairs.willcoxcorvette.com/1...h-pad-removal/
Willcox
2nd Gear
Joined: May 2017
Posts: 2
Likes: 0
(C3 - 1979) tach bouncing
It's hard to tell... the best thing you could do at this point is to hook up a drill to the tach cable and see how it responds with the drill.
You didn't say what year the car is so that's leaving me guessing that it's a 1968-1974 car. But if the tach works okay with the drill then I'd go to the distributor cross gears and check them next.
If the tach fails with the drill then I'd try replacing the cable first... then I'd pull the tach.. You can still test the tach on the bench with your drill after you pull it.
Odds are if it's an old tachometer the first worm and magnet is worn out, if it's a newer one you'll have to test to figure the issue out.
Willcox
You didn't say what year the car is so that's leaving me guessing that it's a 1968-1974 car. But if the tach works okay with the drill then I'd go to the distributor cross gears and check them next.
If the tach fails with the drill then I'd try replacing the cable first... then I'd pull the tach.. You can still test the tach on the bench with your drill after you pull it.
Odds are if it's an old tachometer the first worm and magnet is worn out, if it's a newer one you'll have to test to figure the issue out.
Willcox
This is interesting. I'm a new corvette owner (old corvette) and this is one of the problems with this car along with some electrical problem causing the battery to not hold charge and the generator indicator is always lit. As far as I can tell it's the original tach so is the first worm and magnet in the distributor or in the tach. Trying to determine which end to begin with.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
First worm and magnet are in the tach- only thing in the distributor is the gears- mainshaft and cross.
Generator indicator always on could be a couple of things- bad diode in the alternator could cover both of your described problems.
Generator indicator always on could be a couple of things- bad diode in the alternator could cover both of your described problems.
2nd Gear
Joined: May 2017
Posts: 2
Likes: 0
Thanks Tim, good information and good suggestion on the alternator. I appreciate the response.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
This is interesting. I'm a new corvette owner (old corvette) and this is one of the problems with this car along with some electrical problem causing the battery to not hold charge and the generator indicator is always lit. As far as I can tell it's the original tach so is the first worm and magnet in the distributor or in the tach. Trying to determine which end to begin with.
To remove the tach, follow these instructions on how to remove the driver side dash pad the easy way.
1968-1977 Corvette Left Hand Dash Pad Removal
If you want to know what the parts are inside, this is what it looks like. A tach won't have the second and third or fourth gears because this is what drives the odometer.
-Willcox
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