Tach Reading Low
Thread Starter
Melting Slicks
Joined: Nov 2006
Posts: 2,618
Likes: 149
From: Anaheim California
Tach Reading Low
The tach on my '61 FI is reading low. When at idle it shows 500 RPM when it actually should be 1000 RPM. At 3000 RPM it should be 4000 RPM. Any idea's or anyone had this problem. The tach does not intermittently fluctuate and keeps the same low reading curve. I don't want to tear the dash out if I don't have to!
The problem has been going on for a year or so and I am finally getting caught up to deal with it. I know aside from the tach it could be the cable or dist cross shaft gear, but not sure how to go about it. Thanks
The problem has been going on for a year or so and I am finally getting caught up to deal with it. I know aside from the tach it could be the cable or dist cross shaft gear, but not sure how to go about it. Thanks
Pro
Joined: Jan 2005
Posts: 510
Likes: 19
From: Waterford CT
tach error
Your mechanical tach works just like your speedometer.
The key components are the cable, a rotating magnet, an enclosure called a speed cup, a key part called a hair spring, and the needle/pointer.
1. the tach cable, powered by the distributor, spins at the same speed as the distributor shaft.
2.The cable spins a magnet around at the same speed inside the speed cup of the tach. The magnet rotates continually as the engine is running.
3.The spinning magnet creates eddy currents in the speed cup.
5.The eddy currents make the speed cup rotate as well in an attempt to catch up with the magnet. Remember that the magnet and the speed cup are not joined together—there's air in between them.
6.The hair spring tightens, restraining the speed cup so it can turn only a little way.
7.As the speed cup turns, it turns the pointer up the dial, indicating the engine speed
So, most likely, the hair spring is worn. The cable is either broken, or not. If not, you get no reading.
Check with one of the tach repair specialists if there is any possible adjustment. Personally, I always just swapped the tach. Not that big a job.
The key components are the cable, a rotating magnet, an enclosure called a speed cup, a key part called a hair spring, and the needle/pointer.
1. the tach cable, powered by the distributor, spins at the same speed as the distributor shaft.
2.The cable spins a magnet around at the same speed inside the speed cup of the tach. The magnet rotates continually as the engine is running.
3.The spinning magnet creates eddy currents in the speed cup.
5.The eddy currents make the speed cup rotate as well in an attempt to catch up with the magnet. Remember that the magnet and the speed cup are not joined together—there's air in between them.
6.The hair spring tightens, restraining the speed cup so it can turn only a little way.
7.As the speed cup turns, it turns the pointer up the dial, indicating the engine speed
So, most likely, the hair spring is worn. The cable is either broken, or not. If not, you get no reading.
Check with one of the tach repair specialists if there is any possible adjustment. Personally, I always just swapped the tach. Not that big a job.
Melting Slicks
Joined: Jul 2004
Posts: 2,869
Likes: 196
From: Maryland
The tach head could be one that was used with tach driven generators whose speed is twice that of a distributor driven tach. If that is the case the head will need to be recalibrated for distributor drive or perhaps a speed adapter from a speedometer shop can be rigged between the distributor and tach head.
Last edited by Donald #31176; Jan 28, 2011 at 11:15 PM.
