1968 Tachometer
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From: Scottsdale Arizona
1968 Tachometer
I need to repair a faulty tachometer in my '68 C3. Is the tach a simple cable drive or is there also some electrical imput as well (other than the lights, of course)?
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From: At my Bar drinking and wrenching in Lafayette Colorado
It's strictly a mechanical, cable-driven tach. No electrics. Be aware that the primary failure mode is the distributor tach drive gears stripping out due to wear on the distributor housing crossgear thrust surface. In order to get the system working, you cannot just replace the gears - you have to repair the thrust surface damage. 
Lars
Lars
Last edited by lars; Nov 19, 2016 at 04:35 PM.
Team Owner
Joined: Sep 2006
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From: Westminster Maryland
Hi PC,
It's pretty straight forward to drive the cable from the end you disconnect at the tachometer with a variable sped drill.
This will likely tell you if the problem is in the distributor as Lars described, or in the tachometer.
Regards,
Alan
It's pretty straight forward to drive the cable from the end you disconnect at the tachometer with a variable sped drill.
This will likely tell you if the problem is in the distributor as Lars described, or in the tachometer.
Regards,
Alan
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From: Deerfield Illinois
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Parodi
Well you can take a shot and get lucky like I did. I bought the rebuild kit with gear, bushing and a new tach cable and installed it all and has been working great for 7+ years now. I think most of the problem is the cable gets all gummed up and the resistance is enough to crack the teeth off the gear. In some instances this damages the distributer but mostly just the drive gear. Also make sure the cable does not have any sharp bends or kinks. Be prepared for a fight getting the cable reconnected to the tach it is really tight behind the dash on our 68s!
I bought from Corvette Central but most suppliers have them.
Well you can take a shot and get lucky like I did. I bought the rebuild kit with gear, bushing and a new tach cable and installed it all and has been working great for 7+ years now. I think most of the problem is the cable gets all gummed up and the resistance is enough to crack the teeth off the gear. In some instances this damages the distributer but mostly just the drive gear. Also make sure the cable does not have any sharp bends or kinks. Be prepared for a fight getting the cable reconnected to the tach it is really tight behind the dash on our 68s!
I bought from Corvette Central but most suppliers have them.
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i was replacing the bulbs and "while i was in there" and had the dash partially apart, I replaced my cable. Its easy with the steering column dropped and the console pulled out a few inches. You might consider all the things you may want to do in there and do them all at once. I did my headlight switch, all the dash bulbs, new tach cable, connected all the wires Bubba hadnt, replaced the ignition switch, installed the vent tubes Bubba threw away, connected the door ajar wiring and courtesy light switches, replaced the speakers in the foot well, attached the courtesy lights and the wiper solenoid on the tach. While I had the center console out to get the dash apart I installed a new wiper relay, rebuilt the gauge console and repaired the bad gauges, replaced all those bulbs, bought a 69 console cheap which was modded for a DIN stereo and installed that and a new stereo.
If none of that is in need of replacing, well its easy to get carried away "while our in there"
If none of that is in need of replacing, well its easy to get carried away "while our in there"
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Joined: Mar 2016
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From: Scottsdale Arizona
68 Tach Repair
Thanks, Lars, for the quick reply! The distributor is new (HEI) so the drive gears are good. I put it in about 9 months ago. About 1 month ago the tach needle began to bounce up and down an inch when I was driving (this went on for a few weeks). Last week the tach suddenly went nuts, showing 6000 - 7000 RPMs when it was actually around 2500. Then the needle pegged in the red and snapped off! I heard the cable wizzing out of control at a very high RPM behind the dash so I stopped and unhooked the cable at the distributor. I and am driving it that way for now. Have you ever heard of such a thing?
Alex
Alex
It's strictly a mechanical, cable-driven tach. No electrics. Be aware that the primary failure mode is the distributor tach drive gears stripping out due to wear on the distributor housing crossgear thrust surface. In order to get the system working, you cannot just replace the gears - you have to repair the thrust surface damage.
Lars
Lars
Drifting
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From: Deerfield Illinois
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Alex
Sounds like a catastrophic tach implode! Have to pull the tach and get it rebuilt unfortunately. Send it to Lars!!!!!
Sounds like a catastrophic tach implode! Have to pull the tach and get it rebuilt unfortunately. Send it to Lars!!!!!
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Whenever you have a bouncing needle (tach or speedo) you should always investigate the issue now rather than later. In most cases you can prevent more damage being done.
The tachometer is just like the speedometer and the main cause of failure is the first worm and magnet bushing shown in the picture below. The first worm and magnet (A) rides in a bushing (5) and when it becomes loose can **** the first worm and magnet. When this happens the magnet portion will start hitting the speed-cup and you usually get a tinging noise, bouncing needle or worse. In your case the magnet actually hit the speedcup thrusting it forward due to this contact and more than likely, the speed cup end (where the needle is attached) broke off.
The tachometer is just like the speedometer and the main cause of failure is the first worm and magnet bushing shown in the picture below. The first worm and magnet (A) rides in a bushing (5) and when it becomes loose can **** the first worm and magnet. When this happens the magnet portion will start hitting the speed-cup and you usually get a tinging noise, bouncing needle or worse. In your case the magnet actually hit the speedcup thrusting it forward due to this contact and more than likely, the speed cup end (where the needle is attached) broke off.
Last edited by Willcox Corvette; Nov 21, 2016 at 12:03 AM.
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Joined: Mar 2016
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From: Scottsdale Arizona
Thanks to all at Willcox for all you do! I'll check out your suspicions.
Whenever you have a bouncing needle (tach or speedo) you should always investigate the issue now rather than later. In most cases you can prevent more damage being done.
The tachometer is just like the speedometer and the main cause of failure is the first worm and magnet bushing shown in the picture below. The first worm and magnet (A) rides in a bushing (5) and when it becomes loose can **** the first worm and magnet. When this happens the magnet portion will start hitting the speed-cup and you usually get a tinging noise, bouncing needle or worse. In your case the magnet actually hit the speedcup thrusting it forward due to this contact and more than likely, the speed cup end (where the needle is attached) broke off.
The tachometer is just like the speedometer and the main cause of failure is the first worm and magnet bushing shown in the picture below. The first worm and magnet (A) rides in a bushing (5) and when it becomes loose can **** the first worm and magnet. When this happens the magnet portion will start hitting the speed-cup and you usually get a tinging noise, bouncing needle or worse. In your case the magnet actually hit the speedcup thrusting it forward due to this contact and more than likely, the speed cup end (where the needle is attached) broke off.