1972 LT-1 Speedometer bouncing

[Robert D.]

[Dave S.]

Re: 1972 LT-1 Speedometer bouncing

Sounds like a cable lubrication issue to me. I just did my 65 with the same symptoms. I used a lithium speedo cable lubricant. You will need to detach the cable at the speedometer end and pull the cable through from the interior of the car.

[Paul D.]

Re: 1972 LT-1 Speedometer bouncing

I would also go under the car and make sure cable has not gotten relocated closer to exhaust or manifolds. Chip.

[Terry M.]

Re: 1972 LT-1 Speedometer bouncing

Robert
While under the car checking Paul's idea, I would suggest checking the speedometer driven gear at the transmission. Removing the gear retainer and looking at the plastic driven gear is easily done. I doubt the drive gear on the output shaft of the transmission is visible, but that too might be worth a try if you can get to it -- that might take a mirror and flashlight. The visible inspections suggested by Paul and me are far easier than removing the speedometer cable. Check the easy things before the difficult ones.

[Erich M.]

Re: 1972 LT-1 Speedometer bouncing

Robert,

I had and have the same problem. This first appeared on my car in 2013 and continues to this day since I have not fixed the problem. The bouncing only occurs when the cockpit gets hot. In the winter cold weather my needle does not bounce, no matter how long I drive the car.

There is no mechanical connection between the tach needle and the cable. The needle is moved by a rotating magnetic field which is created by a rotating permanent magnet. The magnet is rotated by the cable. A detailed description of this can be found on page 32 in the Winter 1986 edition of the Restorer in an article written by CHUCK BRIGERMANN.

I know for a fact that permanent magnet strength is a function of temperature. The magnetic field weakens as the temperature increases. The magnetism can be permanently lost if the temperature gets hot enough. It is my theory that the high cockpit temperatures generated with LT1 engines in the summer is the reason for loss of magnetism and causes the bouncing. In my case back in 2013 the cockpit got so hot that the tach stopped bouncing and would drop to "0" while the engine was running. The next day when the engine was cold the tach worked fine.

I suspect what is needed is to replace the permanent magnet in the tach. But since my tach works most of the time, I have decided to delay this project until a point is reached where the tach is totally inoperative. That has not happened yet.

Erich Meyer
Original Owner 72 LT1 Bowtie & Top Flight

[Joe L.]

Re: 1972 LT-1 Speedometer bouncing

Robert,

I had and have the same problem. This first appeared on my car in 2013 and continues to this day since I have not fixed the problem. The bouncing only occurs when the cockpit gets hot. In the winter cold weather my needle does not bounce, no matter how long I drive the car.

There is no mechanical connection between the tach needle and the cable. The needle is moved by a rotating magnetic field which is created by a rotating permanent magnet. The magnet is rotated by the cable. A detailed description of this can be found on page 32 in the Winter 1986 edition of the Restorer in an article written by CHUCK BRIGERMANN.

I know for a fact that permanent magnet strength is a function of temperature. The magnetic field weakens as the temperature increases. The magnetism can be permanently lost if the temperature gets hot enough. It is my theory that the high cockpit temperatures generated with LT1 engines in the summer is the reason for loss of magnetism and causes the bouncing. In my case back in 2013 the cockpit got so hot that the tach stopped bouncing and would drop to "0" while the engine was running. The next day when the engine was cold the tach worked fine.

I suspect what is needed is to replace the permanent magnet in the tach. But since my tach works most of the time, I have decided to delay this project until a point is reached where the tach is totally inoperative. That has not happened yet.

Erich Meyer
Original Owner 72 LT1 Bowtie & Top Flight

Erich------

I totally agree with your assessment of the problem. Even though you refer to problems with your tach, both the tach and speedometer use the same magnetic "indirect drive" system. Unfortunately, direct drive mechanical tachs or speedometers were never used in a Corvette.

[Alan D.]

Re: 1972 LT-1 Speedometer bouncing

I had this problem when the cable broke and replaced it with new cable. I found out from a GM service tech that the new cable was slightly longer and as the system heated up the cable would bind. I took the cable off at the speedometer and ground the end slightly shorter and the bouncing needle stopped. Be careful when grinding the end because on replacement cables they are not welded but swaged into a square.
Lots of luck.

[Alan D.]

Re: 1972 LT-1 Speedometer bouncing

I had this problem when the cable broke and replaced it with new cable. I found out from a GM service tech that the new cable was slightly longer and as the system heated up the cable would bind. I took the cable off at the speedometer and ground the end slightly shorter and the bouncing needle stopped. Be careful when grinding the end because on replacement cables they are not welded but swaged into a square.
Lots of luck.

[Robert D.]