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So i had been up in the woods for better half of the day. I had been in 4wd a few times, didnt thrash on anything and while driving home about 30mins into the return trip on the pavement i noticed my speedo only showing 10mph. I know i was easily going 50mph. I changed my SGII gauge to show MPH and sure enough i was going way faster. Eventually the needled dropped to zero and stayed that way for at least 5 miles. Then i came to a stop to and made a right turn giving it a bit of throttle and the speedo began to work correctly and did so for the remaining 5 miles.
the Odom also during that time wasnt adding mileage. could this be from the rear sensor ontop of the rear diff?
The sensor in the rear diff drives the speedo but I think there is a sensor in the trans also. That may explain why the ScanGauge could still work correctly. Or I may be wrong.. and the working ScanGauge and inop speedo means the instrument cluster is failing.
Bump to the top for help from someone who knows for sure.
My memory says no signal out of the trans. The rear diff is primary, but of course there are three sensors. If there is any consistent variation between all of the three sensors the ABS yellow light will come letting you know the fault. Since you didn't state the ABS light is on, the PCM is giving the SG a good reading, and your trans shifts fine, my on-line non-instrumented guess is you have a gauge issue.
correct the ABS light did NOT come on. I will continue to watch it. I have an extended warranty with this truck so i will hopefully be able to catch it and get it to the shop while its occuring.
My old Chevy's cluster would would act up when the weather got cold. Some days it worked, Some days it didn't, other times it would say I was going 120mph everywhere. If everything else appears to be working, you could have a stepper motor going bad. I used http://stores.ebay.com/BRIANS-SPEEDO...fsub=616645015 to fix it. Reasonable and they have a exchange service. Not sure if there is a FTE recommended shop or not for this type of work.
Last edited by Jerky's_06 F350; Oct 27, 2015 at 08:52 PM.
Reason: Fix url
The trans has it's own speed sensors but as I understand it they
are only used for trans operation.
If your looking for something to do pull the plug apart and see if it's wet
or turning green. Clean with some contact cleaner and then a good dab
of dielectric grease to keep the wet out and see if that helps.
One other option is to clean around the sensor and then pull it out and
look to see if there are any metal stuck to the end. Wipe it off rel-ube
the O-ring and install. Still use the Dielectric grease.
and the working ScanGauge and inop speedo means the instrument cluster is failing. 
