speedometer question

[bevans6]

1980 MCI MC-5C, speedo seems accurate up to about 40 - 50 mph then jumps to 65 mph and continutes up to 80 mphsomewhat normally.  In other words, over 50 mph or so it just adds 15 mph to the speed.  Now, I know I'd like to be able to do 70 in third gear, and cruise up hills at 90, but if there is an easy fix I'd live to hear it.  Is there anything to usefully clean in the sensing arrangement on the left front hub?

thanks, Brian

[gus]

Brian,

I know nothing about MCIs but since you got no response I'll give it a whirl.

If it is mechanical disconnect the cable at the hub and turn it with a variable speed electric drill.

If it still does the same thing it is the cable or speedo head.

Then remove the head and do the same thing, if all is still the same you know it is the head.

With all your vehicles you probably know a good instrument shop. On the other hand, a mechanical speedo is as simple as they get and may just be dirty or bent. A small magnetic disc rotates parallel to another disc attached to the needle, if these two drag they cause problems in accuracy. Sometimes it is just dirt in the small gears involved.

If it is electric forget all the above!!

[bevans6]

Thanks for the reply, Gus, I  appreciate it.  I've doing the google thing for a few days now and found some stuff that leads me to think that the issue is the alignment and spacing of the four trigger bolts that are supposed to be in the rim of the brake drum that the magnetic sensor is supposed to sense as the wheel turns.  I've found a site that sells a speedo kit that actually has my symptom (speed suddenly increasing at higher speeds) in their instructions, and the cause is the sensor too close to the bolts.  My speed goes up by 25% suddenly, I bet that cleaning things out and adjusting things I'll find that one or two of my trigger bolts are closer than they should be to the  sensor.  if it works, I'll post it so the next busnut can do a search and find out what happened.

Brian

[JackConrad]

Brian,
   Also check for a build up of rust paticles on the bolts or on the magnetic sensor.  Jack

[pvcces]

The voltage produced by the sensor depends on the strength of the magnetic field and the speed with which the field passes the sensor. At high speed, the voltage may be too high for the speedometer head.

For what it's worth.

Tom Caffrey

[gus]

I assume from this discussion that you have an electric speedo?

It is still a mystery to me how my mechanical speedo manages to work with about a 45' cable making a bunch of bends??