Speedo acting strange
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Tuned
Joined: Apr 2019
Posts: 310
Likes: 50
Speedo acting strange
Hey guys, just wondering if anyone would know why my speedometer (1977 F250, auto C6, 2wd) would decide all of a sudden to only register my speed when I get to 15 mph. It seems to be working properly from there on and when I decelerate, it reads speeds below 15 mph. I just noticed this recently and I know it was working fine up until then. It doesn't really bother me, I'm just curious as to what could be happening here.
Grumpy goat
Joined: May 2017
Posts: 3,058
Likes: 594
I did not know that. That could explain why my needle bounces at low speeds. I was assuming the drive gear in the transfer case was causing it.
Logistics Pro
Joined: Jul 2005
Posts: 4,217
Likes: 154
Probably flying magnets at the end of the cable, which spin around a metal disk which is attached to the needle.
There is also a light copper spring which returns the needle to zero, and provides the proper resistance so that the needle points at the correct speed, as the flying magnets attempt to make the needle spin with them.
There is also a light copper spring which returns the needle to zero, and provides the proper resistance so that the needle points at the correct speed, as the flying magnets attempt to make the needle spin with them.
Laughing Gas
Joined: Nov 2013
Posts: 1,207
Likes: 60
From: S.E. Louisiana;so far USA
It may be a case of the needle sticking a bit on the upswing and it requires a little more of the spinning magnets to "break it loose" and get it going. You could check it for accuracy once it gets going. If you're good at delicate things, you possibly could disassemble the head and resolve the problem.
Last edited by Ozzie H.; Sep 3, 2020 at 01:48 PM. Reason: spelling correction
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Laughing Gas
Joined: Jan 2018
Posts: 898
Likes: 70
From: Coos Bay
I think most of us have! Just like Victoria, she doesn't have a lot of secrets left......
Back on topic though, are there any adjustments that can be made to the speedometer head itself?
Back on topic though, are there any adjustments that can be made to the speedometer head itself?
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Logistics Pro
Joined: Jul 2005
Posts: 4,217
Likes: 154
It may be a case of the needle sticking a bit on the upswing and it requires a little more of the spinning magnets to "break it loose" and get it going. You could check it for accuracy once it gets going. If you're good at delicate things, you possibly could disassemble the head and resolve the problem.
When the bushings wear, the 'drive' magnets will deviate from their orbit and start to contact the 'driven' metal disk, making a subtle scraping sound and causing the needle to jump up higher than your speed.
But in the OP's case, it definitely sounds more like a problem on the needle/disk side.
I believe powdered graphite is what you want for a lube here?
Fleet Mechanic
Joined: Dec 2007
Posts: 1,506
Likes: 339
From: Wildwood Pa
Some of the speedo heads do that. No obvious cause for it that I have been able to find, or repair. I've had probably 5 speedo heads out of 100 that have done this. One took 30 seconds of spinning at 1000 RPM (I use a drill motor) before it would react. That one got tossed.
If I had to guess, I would say it is something to do with the magnets
I use a synthetic clock oil to lube the speedo's.
Your jumping needle at low speeds ifs probably due to the cable. Pull it out & apart, clean the old grease out, re lube & reinstall..
If I had to guess, I would say it is something to do with the magnets
I use a synthetic clock oil to lube the speedo's.
Your jumping needle at low speeds ifs probably due to the cable. Pull it out & apart, clean the old grease out, re lube & reinstall..
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