Speedometer needle is erratic

[67D100Pickup]

Hello again. I have a 67 D100. The speedometer needle doesn't read correctly.

Years ago the needle would jump up and down erratically. I put a few drops of oil in it and it started working perfectly.

Then I switched out the rear end from 353 to 272. Of course the speedometer showed the wrong speed but was otherwise unaffected.

Then... I changed the speedo gear in the tranny to adjust the speed higher. It worked perfectly and showed the correct speed for about 50 miles.

Now the needle is I'll say "sticky". It is slow to rise to the current speed, if at all, and doesn't fall back down immediately on deceleration. In fact, during steady travel it will often climb higher and higher. When I stop it usually eventually falls back to zero.

The only clue I have as to why it started misbehaving is the new "slave gear" in the tranny has a more shallow hole for the drive cable. I had to cut the cable shorter to screw it back into the tranny but I'm not sure I cut it short enough. I theorize it is conceivable that the inner drive cable was still too long and worked it's way up the cable housing during that first 50 miles which put pressure on the needle?

I have since loosened the tranny connection, easing any potential pressure the drive cable could put on the needle and reoiled behind the gauge.

No help.

I guess I have two questions...

1. Since I haven't seen inside the speedo I don't know if the drive cable being too long could have caused this failure. Could it?

2. Can I fix the instrument or must I replace it?

Jeff

[712edf]

First thing I would try would be to remove that inner cable, inspect it for burrs then completely lubricate the entire length of it.
Also check the cable housing to see if it is in a bind somewhere along its layout.

I would think that the gear you installed would have the same depth of hole as the one you removed & no adjustment to the cable length would have been needed.

Bucky

[67D100Pickup]

Hello again. I have a 67 D100. The speedometer needle doesn't read correctly.

Years ago the needle would jump up and down erratically. I put a few drops of oil in it and it started working perfectly.

Then I switched out the rear end from 353 to 272. Of course the speedometer showed the wrong speed but was otherwise unaffected.

Then... I changed the speedo gear in the tranny to adjust the speed higher. It worked perfectly and showed the correct speed for about 50 miles.

Now the needle is I'll say "sticky". It is slow to rise to the current speed, if at all, and doesn't fall back down immediately on deceleration. In fact, during steady travel it will often climb higher and higher. When I stop it usually eventually falls back to zero.

The only clue I have as to why it started misbehaving is the new "slave gear" in the tranny has a more shallow hole for the drive cable. I had to cut the cable shorter to screw it back into the tranny but I'm not sure I cut it short enough. I theorize it is conceivable that the inner drive cable was still too long and worked it's way up the cable housing during that first 50 miles which put pressure on the needle?

I have since loosened the tranny connection, easing any potential pressure the drive cable could put on the needle and reoiled behind the gauge.

No help.

I guess I have two questions...

1. Since I haven't seen inside the speedo I don't know if the drive cable being too long could have caused this failure. Could it?

2. Can I fix the instrument or must I replace it?

Jeff

UPDATE: For the record... I loosened the speedo cable from behind the instrument and the needle stopped binding! So yes, the cable CAN apply pressure to the needle housing causing it to bind.

Now it is smoothly reading speed! Whoopie

However, now it reads too high by about 10% so I'm dumbfounded again. ‍

I had a 35 tooth cable gear with the 353 rear end. I calculated that I needed a 27tooth gear with the 272 rear end. If I recall correctly this forum confirmed that and a chart I found online agreed. And it read correctly for the first 50 miles or so.

How the hell could it have read correctly for 50 miles then start reading higher once I got the needle housing to operate freely?

Maybe it was binding "a little bit" before, causing it to read the correct speed? That sounds bizarre.

It is a newish cable that clearly spins smoothly.

Thoughts?

Jeff

[67D100Pickup]

First thing I would try would be to remove that inner cable, inspect it for burrs then completely lubricate the entire length of it.
Also check the cable housing to see if it is in a bind somewhere along its layout.

I would think that the gear you installed would have the same depth of hole as the one you removed & no adjustment to the cable length would have been needed.

Bucky

I confirmed the cable spins freely and the housing isn't binding.

Yes, I thought the new gear would have the same depth hole too but it doesn't. The original is about a 1/2" deeper.

Jeff

[712edf]

In order for those speedometer gears to be exact in EVERY vehicle (car or truck) for a given rear axle ratio it would require each vehicle to have the exact same circumference of tire. I doubt every Mopar passenger car with say a 3.23 rear ratio had the exact same size tire, nor would they likely be the same as a truck with a 3.23 rear. Maybe the factory varied the speedo gear by a tooth one way or the other based on tire application, but I suspect each transmission got the same speedometer gear based upon the rear axle & therefore some vehicle speedometers were inaccurate from day one. Just my guess.

Lord knows the other gauges weren't precise.

Bucky

[Wildergarten]

Lord knows the other gauges weren't precise.

Interestingly, I was told by somebody in the business that OEM Dodge truck speedometers of that era were made by Stewart Warner. One wonders if they made the cables and gears too.

[712edf]

Could be. The "sweep" speedometer panels used in Dodge trucks/cars looked identical to what some other manufacturers had back then.

I didn't mean the other gauges (oil, fuel, temp) were always wrong or inaccurate, but given their markings they just were not precise. Exactly what does Cold, Normal, Hot mean?

Bucky

[Wildergarten]

Exactly what does Cold, Normal, Hot mean?

Not exactly a calibrated scale, is it? I think my truck will be hot, even when it's cold.

I guess it depends upon whether the truck is in heat. I know Sweptlines were used for deliveries, so who knows?

I'll go hide now.

[67D100Pickup]

Hello again. I have a 67 D100. The speedometer needle doesn't read correctly.

Years ago the needle would jump up and down erratically. I put a few drops of oil in it and it started working perfectly.

Then I switched out the rear end from 353 to 272. Of course the speedometer showed the wrong speed but was otherwise unaffected.

Then... I changed the speedo gear in the tranny to adjust the speed higher. It worked perfectly and showed the correct speed for about 50 miles.

Now the needle is I'll say "sticky". It is slow to rise to the current speed, if at all, and doesn't fall back down immediately on deceleration. In fact, during steady travel it will often climb higher and higher. When I stop it usually eventually falls back to zero.

The only clue I have as to why it started misbehaving is the new "slave gear" in the tranny has a more shallow hole for the drive cable. I had to cut the cable shorter to screw it back into the tranny but I'm not sure I cut it short enough. I theorize it is conceivable that the inner drive cable was still too long and worked it's way up the cable housing during that first 50 miles which put pressure on the needle?

I have since loosened the tranny connection, easing any potential pressure the drive cable could put on the needle and reoiled behind the gauge.

No help.

I guess I have two questions...

1. Since I haven't seen inside the speedo I don't know if the drive cable being too long could have caused this failure. Could it?

2. Can I fix the instrument or must I replace it?

Jeff

Ok. In case it helps anyone else, here is thenroot cause and solution.

The new 27tooth speedo gear is right and now reads within MPH of my GPS app.

The root cause was the new gear hole where the cable inserts was not as deep as the old one. When I screwed the cable into the gear it pushed the "drive" cable through the sheathing putting pressure/friction on the needle casing, the metal part that the magnet spins.

Instead of just the magnet spinning the needle, the friction also influenced it, making it spin too fast.

I had to remove the cable from behind the speedo and push the cable back through toward the tranny, with a decent amount of force, until the little collar near the end snapped back into position.

Not being a mechanic this little stuff can eat my lunch ‍

Jeff

[PwrWgnDrvr]

Figuring that all out and fixing it makes you a decent mechanic!

[IHWillys]

Figuring that all out and fixing it makes you a decent mechanic!

I wholeheartedly agree.