Tach question
Thread Starter
Heel & Toe
Joined: May 2010
Posts: 19
Likes: 0
Tach question
Hi, i have a 70 corvette, after the engine warms up (5-7 min) the tach needle starts jumping and dies out. This happens everytime i drive the car. What do you think the problem is? Thank you in advance, ron
Terrorizing Orange Cones
Joined: Oct 2007
Posts: 2,572
Likes: 17
From: Northern CO
If you're addressing the factory mechanical tach, there are a couple things to look at.
You say the needle on the tach jumps then "dies out": meaning stops moving and returns to zero with no further movement through the time the motor runs?
Then after a period of time, say overnight, the same scenario repeats itself at next motor firing up? Or might the time frame be less? (attempting to id hot/cold cycle influence)
Depending on how many cycles the tach itself has run, i.e. mileage, it could be worn gears in the head unit. It could also be worn gears on the distributor side. If the needle behavior is constant, this may be the case.
If the distributor and cable are deemed stable, a common fix is then to replace the tach unit, which entails partial disassembly of the dash section containing the speedo and tach for adequate access. Some guys, like myself, remove that section for table work. Too old for contortions and this requires a lot of upside down time.
Let us know if this helps. I'd be suspicious of the head unit but I've been wrong before; those other 2 things need looking into to be sure.
If this is a DIY project, performing this replacement will usually end up discovering "other" items that may need or require addressing.
We call that "...while I'm at it...".
You say the needle on the tach jumps then "dies out": meaning stops moving and returns to zero with no further movement through the time the motor runs?
Then after a period of time, say overnight, the same scenario repeats itself at next motor firing up? Or might the time frame be less? (attempting to id hot/cold cycle influence)
Depending on how many cycles the tach itself has run, i.e. mileage, it could be worn gears in the head unit. It could also be worn gears on the distributor side. If the needle behavior is constant, this may be the case.
If the distributor and cable are deemed stable, a common fix is then to replace the tach unit, which entails partial disassembly of the dash section containing the speedo and tach for adequate access. Some guys, like myself, remove that section for table work. Too old for contortions and this requires a lot of upside down time.
Let us know if this helps. I'd be suspicious of the head unit but I've been wrong before; those other 2 things need looking into to be sure.
If this is a DIY project, performing this replacement will usually end up discovering "other" items that may need or require addressing.
We call that "...while I'm at it...".
Last edited by Dustup7T2; Apr 20, 2012 at 06:45 PM. Reason: add text
Thread Starter
Heel & Toe
Joined: May 2010
Posts: 19
Likes: 0
If you're addressing the factory mechanical tach, there are a couple things to look at.
You say the needle on the tach jumps then "dies out": meaning stops moving and returns to zero with no further movement through the time the motor runs?
Then after a period of time, say overnight, the same scenario repeats itself at next motor firing up? Or might the time frame be less? (attempting to id hot/cold cycle influence)
Depending on how many cycles the tach itself has run, i.e. mileage, it could be worn gears in the head unit. It could also be worn gears on the distributor side. If the needle behavior is constant, this may be the case.
If the distributor and cable are deemed stable, a common fix is then to replace the tach unit, which entails partial disassembly of the dash section containing the speedo and tach for adequate access. Some guys, like myself, remove that section for table work. Too old for contortions and this requires a lot of upside down time.
Let us know if this helps. I'd be suspicious of the head unit but I've been wrong before; those other 2 things need looking into to be sure.
If this is a DIY project, performing this replacement will usually end up discovering "other" items that may need or require addressing.
We call that "...while I'm at it...".
You say the needle on the tach jumps then "dies out": meaning stops moving and returns to zero with no further movement through the time the motor runs?
Then after a period of time, say overnight, the same scenario repeats itself at next motor firing up? Or might the time frame be less? (attempting to id hot/cold cycle influence)
Depending on how many cycles the tach itself has run, i.e. mileage, it could be worn gears in the head unit. It could also be worn gears on the distributor side. If the needle behavior is constant, this may be the case.
If the distributor and cable are deemed stable, a common fix is then to replace the tach unit, which entails partial disassembly of the dash section containing the speedo and tach for adequate access. Some guys, like myself, remove that section for table work. Too old for contortions and this requires a lot of upside down time.
Let us know if this helps. I'd be suspicious of the head unit but I've been wrong before; those other 2 things need looking into to be sure.
If this is a DIY project, performing this replacement will usually end up discovering "other" items that may need or require addressing.
We call that "...while I'm at it...".
Hi, Thanks for the info. I should have told you that the tach works just fine for the first 5 to 7 minutes. However, as soon as the engine is hot, that is when the needle starts jumping and eventually dies out and returns to zero. Thanks again, Ron
Team Owner
Joined: Sep 2006
Posts: 31,172
Likes: 4,249
From: Westminster Maryland
Hi Ron,
Since the tach drive is a mechanical connection from the distributor shaft gear to the tach head, I'd say the problem is in the tach head.
I beleve there's a magnet and spring in there that heat might affect.
???
Regards,
Alan
Since the tach drive is a mechanical connection from the distributor shaft gear to the tach head, I'd say the problem is in the tach head.
I beleve there's a magnet and spring in there that heat might affect.
???
Regards,
Alan
Advanced
Joined: Sep 2009
Posts: 71
Likes: 1
tach cable
Before you go pulling the dash out, just reach in by the distributor and tug on the tach cable where the crimp is and see if its loose. Mine did the same thing as yours and it turned out that the crimp came apart and the cable was slowly "walking" itself out of the distributor as I drove. I dont know if it was expansion from the heat,or just the vibration of driving it.
