Interesting Manual Hubs Phenomenon

[B94Sport]

I was driving down the freeway yesterday, going (what I estimated to be) about 65 MPH. The entire time, my speedo would fluctuate back and forth rapidly between 50 MPH and 80 MPH! I just re-greased the cable about 3 months ago when I replaced my gauges, so I start thinking "What has changed since the last time I drove?" and I eventually remembered that I had locked my manual hubs the day before, but had forgotten to unlock them. 4x4 was not engaged, but the hubs were still locked. So I pulled over and unlocked them, started back on my way, and the speedo did not fluctuate at all. Does anyone know why this happens? How would having the hubs locked affect the accuracy of the speedo? I really don't know a lot about the workings of the drivetrain, but it seems to me that even if the hubs are locked and the front axle is spinning with the wheels, it still does not influence the transmission/rear axle unless the transfer case is engaged.

 

[B94Sport]

bump

 

[410Fortune]

Hmm this is interesting.

Okay first off the speedo cable gets it's lubrication from the ATF in the transfer case, so when you say your greased it I am assuming at the back of the speedometer correct? First thing to check is that it is seated properly since you recently had it disconnected.

The other end of your speedo cable is attached to the speed sensor which is mounted in the transfer case. You are right in thinking that the front hubs do not affect the speed sensor, they should not, even with the 4 wheel drive engadged.

If the cable connection on the back of the speedometer checks out, you can then pull the speed sensor (1-10mm bolt, pull it out firm) and check onthe condition/existance of the speedometer gear (plastic gear on the end of the speed sensor, this is what you change on a Ford to calibrate the speedo after a tire/gear change)

If the gear checks out I would replace the cable, they are about $20 and easy to install. A fluctuation in the speedo like that is most likely a bad gear or a cable on it's way out...........when I first installed my headers my speedo would jump all over, I couldnt figure it out, until I saw that my headers melted my speedo cable.....