Son's project [1984 F150]

Many years ago [2003?], my son's truck was converted from a 2.47 rear end to 3.55 gears. As one could expect, the speedometer was grossly incorrect. It measured way too high. The truck was taken to a speedometer shop. This shop calibrated and certified the police and Highway Patrol cruisers for the Atlanta, GA area. They installed an adapter [see attached link to a photo] and provided certification that the speedometer was accurate @ 55 mph.

https://www.bing.com/images/search?view=detailV2&ccid=EbFhkERR&id=5D1D6C169C588B569B3CC4CC04C656BAC5D12018&thid=OIP.EbFhkERR7MtKktBO7R8_6QHaE6&mediaurl=https%3a%2f%2fstevemckelvie.files.wordpress.com%2f2011%2f08%2fodometer-correction-box.jpg&exph=531&expw=800&q=cable+speedometer+calibration+box&simid=607991193303188113&selectedIndex=4&ajaxhist=0

This device has worked well for ~17 years. Recently, the speedometer jerks to a higher speed and then drops back. However, now the "drop back" speed is too high.

I am in hope that some of the "old dogs" [Gary, Bill Vose, ???] will have a idea of where to search for this piece of history. It will be this weekend before I can crawl under the truck and see if there are any identifying numbers or marks.

Thanks.

I think you need to grease the speedo cable(s).  There may be two and, if so, the split will be around the brake booster as that's where the speed sensor for the speed control went.

But you probably have only one, and it will most likely come out from the speedo end.

Will do, Gary, but how would that explain the speedometer reading higher than normal, after the "jerk"?

Speedo cables and the mechanism in the speedometer itself do strange things.  Big Blue's speedo did similar things until I greased it.

I'm not saying its a sure-fire fix, but that's where I'd start.

Thank you!

Hi David,
The short answer is what Gary told you... clean and lubricate the drive cable(s). And I would also open, clean, and lubricate the little ratio adapter since it looks like it snaps open to install the appropriate gear:






The longer answer about why it is spiking and holding a higher-than-correct speed is likely due to cable bind and jerking rotation rather than a smooth motion. Mechanical speedometers work by a rotating magnet spinning inside a cup. The polarity reversal of the spinning magnet generates rotational torque on the cup that is attached to the needle. There is a spring that limits cup travel and stabilizes its movement (as well as returning it to zero). If your cable is binding and releases suddenly, there is a magnetic field spike that will show as needle jump. If it is chattering, the spring will stabilize it some, but the sum of all the small spikes can still make the needle read high and erratic. It is less common for issues to arise with the magnet/cup/spindle and spring assembly, but if the cable lubrication does not help and you would like to try another speedometer head let me know as I should have one I can send to you.

Not the best video, but it does explain the components of a mechanical speedometer:
https://m.youtube.com/watch?v=3_vfyt6-2Ic

Thank you, Jonathan!

Saturday, 2/1/20, removed the instrument cluster, loosened the cable from the speedometer head as well as from the above described adapter, which is located approximately 6" from the transmission. The cable was lubricated and reattached. The speedometer seemed a little better, but the needle still floated at very low speeds and occasionally twitched at higher speeds.

Saturday, 2/8/20, removed the speedometer and swapped for a known good one. So far, after approximately 200 miles of driving, all seems well! There is supposed to be a cold snap in the weather, later this week. If the speedometer behaves itself, then, we will feel successful.

Success!  Glad you got it sorted, even if it wasn't just lubrication.  

1986F150Six wrote

Saturday, 2/8/20, removed the speedometer and swapped for a known good one. So far, after approximately 200 miles of driving, all seems well! There is supposed to be a cold snap in the weather, later this week. If the speedometer behaves itself, then, we will feel successful.

How did the speedometer behave in the cold snap?

Was the “known good speedometer” from your ‘86? Are you in need of another one?

The speedometer is still performing as designed!! Yes, the replacement speedometer was from my truck. No, I do not need you to send one to me, but thanks!