Odo works but not speedo

[JCochrane]

Hey all, I think I know the answer - "take it apart carefully and look for something that looks wrong and fix it" but wanted to ask in case someone knows before I dig in, and in case somebody else has the same issue at some point and likewise... (I searched)

Background: I took the binnacle off & speedometer apart to fix the trip reset shaft. When I put it back together, the reset would only roll to like 333.3 each time (I don't recall offhand). I could stop the press partway through to get it to stop on zero-ish, but that's obviously not good enough, so I took it back apart and checked for what I'd misaligned, figured it out and put it back together. Reset shaft works great - resets back to 000.0.

Then I go to drive it home, and find I have no speedometer. Ok, I've been here before - I must not have reinstalled the angle drive cable correctly when I reinstalled the binnacle. Take it apart, make sure the shaft is centered in the sleeve, tighten it back down. Still no speedo, but I noticed the odometer was ticking up a bit. Hmm. No time to diagnose, didn't have much option but to drive home, ~76.5mi. The odometer continued to tick up, including the trip odometer, so I kept with traffic and know pretty well my speed per gear/tach, so no issues on the way home. On the exact route I took, it was ~75.4 mi, and the trip odo clocked ~72.1mi, so... not that far off I think (~4%). (though the first three numbers are in the middle between 0/1,7/8,2/3, so maybe something's still not adjusted, or that's just how it goes).


So it seems the odometer works fine, thus the cursed angle drive (not original, but decades old) seems fine, so I clearly just mucked something up in the speedometer itself.

Any ideas exactly what I should look at? Possible the needle just isn't pressed all the way on or something silly like that?

[willisit]

I had the same thing and in my case it was the spring was in bits... but since you've had yours apart, I guess you'll know the status there. I, too, had ode, just no speedo.

[JCochrane]

I had the same thing and in my case it was the spring was in bits... but since you've had yours apart, I guess you'll know the status there. I, too, had ode, just no speedo.

If the spring is in bits (or otherwise nonfunctional), does that mean I need to replace the whole thing, or is it possible to get a replacement spring or repair it somehow? I suspect the specific spring tension is important...

[Rich]

A busted spring is fatal to the speedo as you've discovered.

You're right, you'll need to either rebuild it or replace it.

Replace: DMC offers them, including resetting the odo on the reproduction speedometer they'll sell you. At the moment they're out of stock. You can press the link on their listing to get notified when they have some in inventory. You can try to find a used one out of salvage via this forum or eBay/FBMarketplace. You'd need to take it to a shop to get the odometer set.

Rebuild: Find a local speedometer shop - yes that's really a thing - or find a speedo shop online to see what they can do for you. Broken analog speedometers are becoming a thing of the past but there are still plenty of shops. I don't know about how anybody sources or repairs the spring.

[Timeless]

Rebuild: Find a local speedometer shop - yes that's really a thing - or find a speedo shop online to see what they can do for you. Broken analog speedometers are becoming a thing of the past but there are still plenty of shops. I don't know about how anybody sources or repairs the spring.

My local tech referred me to Atlanta Speedometer for some gauge work on my 911. I'm 100% satisfied and turn around time was fast.

https://atlantaspeedometer.com/

[JCochrane]

Got it fixed! I finally took the binnacle off again last week and took the speedo out. Spring was still good, and when I spun the angle drive input, the needle did tick up *a bit*, but not as much as I thought perhaps it should (just a feeling, very little to actually go on). So I took it apart again, following the instructions for the trip-reset as I did last time, and nothing looked broken. Put it back together carefully, and pressed the needle on a bit harder, and then it bounced up quite a bit when I spun the input. Put it back together and got a test drive, and it works! It's reading about 3-4mph higher than GPS shows (shows 40 when I'm going 36), but better that way than the other way around, I think. At some point I'll take the binnacle off again and maybe adjust it slightly, but for now I'll live with it. I *think* I was just "too careful" when putting the needle back on, cause there's a strong caution in the instructions when taking it *off* not to break the needle. Or taking it apart happened to loosen whatever I'd gotten in the way.

[Gregadeth]

If the speedometer is off by a few mph, does this also affect the odometer by the same amount, or are they calibrated independently?

My speedo is always a bit off at highway speeds (no big deal - I just keep it at 2500rpm in 5th gear), but I wonder if this is throwing off the mileage on the odo as well. I can probably test this with GPS on a long drive, but wondering if someone already knows.

[DMC-Ron]

If the speedometer is off by a few mph, does this also affect the odometer by the same amount, or are they calibrated independently?

My speedo is always a bit off at highway speeds (no big deal - I just keep it at 2500rpm in 5th gear), but I wonder if this is throwing off the mileage on the odo as well. I can probably test this with GPS on a long drive, but wondering if someone already knows.

Their mechanisms are independent. Obviously they both take their movement from the rotation of the speedometer cable, but they do so differently.

The odometer is driven from a worm gear that rotates at the speed of the cable. The worm gear turns a little Tufnell gear that step advances the odometer. The odometer works off a ratio of the cable rotations per unit of distance.

The speed indicator is positioned by a rotating magnet that pulls round an aluminum drag cup. The speedometer indicating needle rotates with the drag cup. Magnetism rotates the cup working against a fixed spring. The speed indicated is a balance between the spring tension and the force imposed by the magnet's rotating speed.

In general the speedometer is more likely to loose calibration because the magnet looses magnetism over time, and the spring weakens. Luckily, they often degrade together and offset each other. But eventually either the spring gets too weak in relation to the magnet and the indicated speed is higher than actual, or vice-versa and the indicated speed is understated.

Our cars have the infamous angle drive at the wheel end of the speedometer cable, which is geared. If it is replaced and the replacement has a slightly different gear ratio, both the speed and odometer are thrown off equally. Likewise, front tire selection that increases or decreases the stock tire diameter will also affect both.

Sorry for the long winded explanation, -Ron

[David T]

Their mechanisms are independent. Obviously they both take their movement from the rotation of the speedometer cable, but they do so differently.

The odometer is driven from a worm gear that rotates at the speed of the cable. The worm gear turns a little Tufnell gear that step advances the odometer. The odometer works off a ratio of the cable rotations per unit of distance.

The speed indicator is positioned by a rotating magnet that pulls round an aluminum drag cup. The speedometer indicating needle rotates with the drag cup. Magnetism rotates the cup working against a fixed spring. The speed indicated is a balance between the spring tension and the force imposed by the magnet's rotating speed.

In general the speedometer is more likely to loose calibration because the magnet looses magnetism over time, and the spring weakens. Luckily, they often degrade together and offset each other. But eventually either the spring gets too weak in relation to the magnet and the indicated speed is higher than actual, or vice-versa and the indicated speed is understated.

Our cars have the infamous angle drive at the wheel end of the speedometer cable, which is geared. If it is replaced and the replacement has a slightly different gear ratio, both the speed and odometer are thrown off equally. Likewise, front tire selection that increases or decreases the stock tire diameter will also affect both.

Sorry for the long winded explanation, -Ron

To add some more to your good explanation:
The ratio from the tires and the angle drive sets the Odometer, the spring tension calibrates the speedometer. The odometer should be very accurate, the speedometer not as much. Typically (for most cars) things are calibrated so you get 1,000 revolutions per mile. Speedometers are calibrated at 60 MPH + _ 5% meaning they adjust the speedometer so it reads 60 MPH at 1,000 RPM's and it should read within 5% throughout it's range. Easy to check. Get on a highway and read the mile markers. You can do it over a measured mile but the longer the distance you measure it, the more precise you can be. You should also measure your MPG. It tells you the general health of the motor and if you see it decreasing you know you have a problem.

[Gregadeth]

Their mechanisms are independent. Obviously they both take their movement from the rotation of the speedometer cable, but they do so differently.

The odometer is driven from a worm gear that rotates at the speed of the cable. The worm gear turns a little Tufnell gear that step advances the odometer. The odometer works off a ratio of the cable rotations per unit of distance.

The speed indicator is positioned by a rotating magnet that pulls round an aluminum drag cup. The speedometer indicating needle rotates with the drag cup. Magnetism rotates the cup working against a fixed spring. The speed indicated is a balance between the spring tension and the force imposed by the magnet's rotating speed.

In general the speedometer is more likely to loose calibration because the magnet looses magnetism over time, and the spring weakens. Luckily, they often degrade together and offset each other. But eventually either the spring gets too weak in relation to the magnet and the indicated speed is higher than actual, or vice-versa and the indicated speed is understated.

Our cars have the infamous angle drive at the wheel end of the speedometer cable, which is geared. If it is replaced and the replacement has a slightly different gear ratio, both the speed and odometer are thrown off equally. Likewise, front tire selection that increases or decreases the stock tire diameter will also affect both.

Sorry for the long winded explanation, -Ron

To add some more to your good explanation:
The ratio from the tires and the angle drive sets the Odometer, the spring tension calibrates the speedometer. The odometer should be very accurate, the speedometer not as much. Typically (for most cars) things are calibrated so you get 1,000 revolutions per mile. Speedometers are calibrated at 60 MPH + _ 5% meaning they adjust the speedometer so it reads 60 MPH at 1,000 RPM's and it should read within 5% throughout it's range. Easy to check. Get on a highway and read the mile markers. You can do it over a measured mile but the longer the distance you measure it, the more precise you can be. You should also measure your MPG. It tells you the general health of the motor and if you see it decreasing you know you have a problem.

The answers I was looking for, thanks!