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MG MGB Technical - Speedometer Woes

The speedometer needle in my 64 MGB was bouncing crazily as I drove--fluctuating 20 to 30 miles an hour. So I replaced the speedometer cable and the speedometer seemed to be working perfectly. However, after a drive of about five miles the needle became stuck at 50 mph. When I shut the engine off the needle remained stuck. The odometer seems to be working quite normally. Have I messed up the actual speedometer. Any suggestions? Thanks.
Don

Hi Don,

Smith Gauges last decades, but after 43 years We have to let man's creation die. You have a option rebuild the gauge or replace with a new one. You could also find one on eBay. But it might not last that long.

Most all MGs have mechanical gauge so when You shut off the engine the gauge needle would not return to "0" as a electrical gauge. I would check for a bend in the cable. Also You can use a LOW speed drill and a hex bit to test the operation of the speedometer.
James D

Have you checked the right angle drive on the transmission? Usually they quit and the needle stays at zero all the time, but yours might be twisted up inside and stuck at 50.
Ken
1974 mgb roadster
Kenneth Thompson

"most all MGs have mechanical guage so when you shut off the engine the gauge needle would not return to "0" as a electrical gauge" HUH? I don't think I've ever seen a proper MG speedo NOT drop to zero when the key is off, the motor is off, the clutch is in, and you are coasting down a long hill, if the speedo sticks check the head. RIC
RIC LLOYD

The speedo has no dependency on whether the engine is shut off or not, or spinning or not if it *is* shut off, it is solely controlled by the forward movement of the vehicle. If the key is off, the motor is off, and the clutch is in but you are coasting down a hill then the speedo is still registering. If a speedo is stuck at 50 mph then the problem is inside the speedo, nothing to do with the cable.
Paul Hunt 2

Don and others,
I know this may be a little late, and I don't know if this has been mentioned here before, but there is a great source on speedometer repair out there on the web published by Anthony Rhoades. The script below was taken from his repair manual.You can view and download the complete manual at his website
http://mywebpages.comcast.net/rhodes/
Just scroll down to 'smith/jaeger speedometer repair manual'.
It is an excellent pdf file with loads of techinical info as well as easy to comprehend 'fix-it' instruction and great pics too. It takes the fear out of opening up a tach or speed meter and you soon realize that you can fix a speedometer youself.


Don, Have a look at problem #2 below. I think your problem might be related.
Hope it helps (it did for me)
Dennis


"The speedometer (speed indicator, not odometer) functions in just the same way as a tachometer.
The cable spins a thin bar magnet. Just in front of the bar magnet is a disk mounted on a
spindle. Also attached to this, on the same spindle, is the pointer that is visible over the
dial face. When the bar magnet spins, it causes the disk (drag cup) just in front of it to try
to spin as well. The amount of twisting force (torque) imparted by the magnet to the disk is
proportional to the rotational speed of the magnet. If the magnet spins twice as fast, the
torque is approximately twice as great. The spindle is attached to a flat coiled return spring
to resist rotation. The amount the spring winds is proportional to the torque. In this manner,
the pointer moves progressively farther as the magnet spins faster.
There is very little that can go wrong with the speedometer. The places where a problem
can occur are the following.
1) The magnet wheel may not spin. In this case all functions cease and the drive cable
(or angle drive) will break.
2) The spindle may not move freely due to the disk binding against an obstruction. In this
case the pointer is stuck in one place or will not rise above a certain level. The spindle
pivot may lack lubrication and this will cause the needle to jump from one speed to the
next rather than move smoothly. Lack of lubrication between the spindle needle tip
and the bearing in the center of the magnet wheel may cause chatter at certain
speeds.
3) The return spring (hair spring) may be broken. In this case the pointer will wind completely
around until it hits the stop. A professional repair is required.
4) If everything moves normally, but it indicates the wrong speed, it is probably out of
calibration. This is unlikely unless the pointer has been touched directly or you are
using tires that vary substantially in rolling diameter from the originals. Also, violent
swinging of the pointer may cause it to shift if it hits the stop forcefully.
5) If there is still error at other speeds after calibration, the hair spring may have weakened.
A weakened spring will move more for a given speed than expected by the
markings on the face. While you can calibrate it for a certain speed, it will register low
below that speed and high above the speed. It is possible to unsolder the return
spring, pull it tighter, and re-solder, but it is probably best to leave this to a professional
speedometer repair shop. Alternatively, you may be able to swap the spindle/main
odometer section of another unit. I have heard that it is possible for the bar magnet to
lose its magnetism over time. If this happens, it is as if the hair spring is too strong,
and you will see the opposite problem than the weak spring described above. Professional
shops are supposedly able to re-magnetize the bar magnet."

http://www.bmcno.org/tech%20tips/speedo.pdf

Dennis Silance

This thread was discussed between 28/10/2007 and 06/11/2007

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