I recently went through this with my old beast. Thought the Speedometer might be bad or the cable is toast; turns out I just needed to replace the cable (Bonus!) but I had to test the Speedometer to make sure it was functioning and hadn't gone to the parts bin in the sky somewhere so here's how I did it in the event someone else has the same issue.

First - Remove your dash and the Speedometer cable. I found it easier to remove the cable from the transmission (mine is a manual so it's located at the back of the transmission close to the driveshaft) and push some of the excess cable up through the dash. It's also a good time to replace the cable since they do apparently "wear out"
disconnect the cable from the back of the Speedometer and then take apart your instrument panel

Second - Unbolt the Speedometer from the instrument cluster and set it aside for a minute; make sure you don't lose the screws! (Found out I had a hole in the floor pan which is being replaced next ha!)

Here's where it gets fun - Since I replaced the Speedometer cable anyway, I cut about a 6" piece off the old cable and chucked it into my drill so it was tight and would spin easily (forgot who mentioned doing that in one of my previous threads but thanks for that idea!!!) if you're not replacing your cable then your best bet would be to find a piece of wire or something else you can stick in the end of the Speedometer to make it turn; again you'll need to chuck it into your drill

Finally!! You want your drill to go the same way the needle goes; Counter clockwise so your drill should be set to reverse. DO NOT instantly push your drill to full throttle!!! That would be bad juju! Start your drill off slow and SLOWLY amp it up, make sure the needle on your Speedometer is actually moving. If it's not then you're Speedometer is toast!

Ill post the video of this later on this evening so it can be referenced - hope this helps someone else out!
Nate

 

good deal Nate.
glad to hear it was just a bad cable. speedometer heads for these trucks are getting harder to find as they got older.
the newest one you are going to find will still be 26 years old now.

 

Great write up. A lot of people don`t know about testing a spedo head with a drill.
I have done a few in the PNP`s mainly Mercedes to see if the low mileage indicated was correct, or if it was slipping which is common on the VDO heads.
Those are easy to take apart and repair.
Wonder how hard it is to take apart one of these Ford Spedo heads?


Now 26 yrs ago would be 91. aren`t those Digital/LCD or electronic?
If so, when did Ford go from cable driven to Digital?


I have a 86, so pretty sure a 91 would not work.
What about the gassers, they the same?


Charlie

 

charlie, 92 was the switch to the digital speedo head.
86 and older, 87 to 91, then 92 to 97 were digital.
gas or diesel did not make any difference for speedo head, that difference was for the tach.

 

Thanks Tom for the info


Charlie