71 Tachometer quit working today
Thread Starter
7th Gear
Joined: May 2012
Posts: 7
Likes: 0
71 Tachometer quit working today
Where do I start on troubleshooting this. It was working fine yesterday and today...nothing! What feeds the tach? Is it coming off of the distributor? Coil?
All original 350
All original 350
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If your ignition system is still "stock" (as it came from the factory), the tach 'signal' is coming to the tachometer via a flexible cable...similar to the speedometer. There is a tach drive gear built into the distributor shaft; and the cable runs from there, through the firewall, and is connected to the tach gauge.
There are two failure modes that are most likely for this:
1.) the drive cable has failed and is shredded somewhere in the cable sheath, so that no rotational signal is reaching the tachometer;
2.) the teeth on that little brass cable drive gear (in the distributor) has lost it's teeth so that the cable is no longer rotating.
There are other possibilities, but those are the most likely causes.
There are two failure modes that are most likely for this:
1.) the drive cable has failed and is shredded somewhere in the cable sheath, so that no rotational signal is reaching the tachometer;
2.) the teeth on that little brass cable drive gear (in the distributor) has lost it's teeth so that the cable is no longer rotating.
There are other possibilities, but those are the most likely causes.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
unless you want a HEI upgrade....totally another topic....
You may well need a 'tach filter' too....yet another topic....
Thread Starter
7th Gear
Joined: May 2012
Posts: 7
Likes: 0
I just pulled the cable from the distributor and spun it with a drill....tach portion works so I assume that the gear set in the distributor is stripped?
I assume I must pull the distributor for this repair?
I assume I must pull the distributor for this repair?
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Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
'Fraid so. Some gear teeth are not meshing...usually because they are no longer there. If you have enough access, you can remove the cable from the distributor, then use some straight metal piece to act like a BIG screwdriver and remove the outer gear retainer. Then remove the driven gear and have a look inside with a penlight.
Another possibility (if you don't find damage to the gear teeth) is that the driven gear has "eaten" down into the body of the distributor so that the gear teeth are no longer engaged with the drive [worm] gear on the distributor shaft. To repair this condition, you need to fashion a plug of a particular size and thickness and assemble it into the distributor body to realign those gears. If that is your problem, make another post to this thread and we can direct you to helpful advice on how to repair it. Of course, you can always go buy another distributor; but repairing yours for little expense seems a better idea.
Another possibility (if you don't find damage to the gear teeth) is that the driven gear has "eaten" down into the body of the distributor so that the gear teeth are no longer engaged with the drive [worm] gear on the distributor shaft. To repair this condition, you need to fashion a plug of a particular size and thickness and assemble it into the distributor body to realign those gears. If that is your problem, make another post to this thread and we can direct you to helpful advice on how to repair it. Of course, you can always go buy another distributor; but repairing yours for little expense seems a better idea.
Burning Brakes
Joined: Jun 2010
Posts: 1,165
Likes: 93
From: Bellmore NY
Check to make sure that the end of the cable is square and that if there is any sort of plastic sheath that caps it is in good shape. If this sheath is broken or rounded at the point that the cable enters, or where it enters the distributor, this can also be a source of the problem.
Also, with the car running, you can stick your finger over the recepticle on the distributor and feel if it is moving. Better to cover all bases then to fix one trouble spot but not eliminate the problem
Also, with the car running, you can stick your finger over the recepticle on the distributor and feel if it is moving. Better to cover all bases then to fix one trouble spot but not eliminate the problem