Fin's Pulling His Hair Out: Odometer Edition

[Fingolfin]

A good evening to all you across the Atlantic and around the world.

Last time I drove Mog any significant distance, the odometer was reading accurately (within a few miles over a 130-mile drive), but the speedometer needle was bouncing and the works was making an awful noise -- so it was new cable time!

This weekend just past I put in the new cable, and it cured the bounce and noise immediately. However, though the speedometer needle no longer bounced and seemed to read accurately (perhaps reading 60 when traveling 55, so overreading), the odometer now significantly underreads. My recent drive from Bolivar to Fulton was 132 miles, says Google Maps' measurement tool, but the odometer only registered 98. Today I drove from Fulton to Columbia, which is 42 miles, and the odo registered 31.

What the frak? What possibly can have happened?! Can the cable somehow be wrong...?

[GBond]

Hi Fin.

The odometer is underreading by almost exactly the same ratio as before so I don't think it's the cable. Maybe it just wants to switch to kilometers.

[bmcecosse]

It can't be the cable - it's just a 1:1 drive. The speedo TPM is wrong for whatever final drive ratio you have. You've probably told us many times in the past - but please advise TPM on the speedo, and FD ratio... For example if the FD is 4.55 then 1504 speedo required, and if 4.22 then 1408 speedo and with a 3.7 FD I can tell you a Mini 1294 speedo matches perfectly for speed and miles.

[Fingolfin]

What? No, the odometer's never underread before, Gabe, I don't know what you mean. This is a new problem that just developed since I changed the cable (though it need not be linked to that). Roy, the speedometer's reading nearly accurately, so how can the TPM be wrong?

In any event, the speedo head numbers are: SN 4406/00 1525. Final drive is 4.55. It's very nearly right, though I'm rather surprised it's not 1504...probably that means this speedo is not original. Hmm.

[GBond]

Sorry Fin, thought you had changed the cable after the 130 mile trip.

I don't know too much about speedometers but I really doubt it's the cable. Unless it was slipping at either end but then I imagine it would not read accurately at all and it seems your speedo is underreading by a constant amount so I'd blame the gearing.

[bmcecosse]

The odometer is fixed gearing - so it will reflect the various ratios unfailingly - and any discrepancies therein.....(unless it's sticking/freeing off - pretty unusual...generally if they stick - they stay stuck...) Not heard of a 1525 before.....but the difference is insignificant. The 'speed' can be varied by pulling the needle off - and sticking it back on again in a different location.... I can only assume that in the past - the cable was 'slipping' giving the bouncing speed needle, and the odometer reading less since it wasn't going round all the time the car was moving.......

[Fingolfin]

No no no, I insist that the odometer used to read quite accurately. In fact it was more accurate than the speedometer needle. You're right in that I did move the needle to correspond better, but after that the odometer didn't change -- it's only underreading since replacing the cable.

In searching I came across Mr. Ufundu's fix (http://www.mmoc.org.uk/viewtopic.php?f=11&t=50437). I think this is a fair possibility, as removing the speedo head to put on the cable may have jarred the clip loose, so I am going to have a look inside. Can't hurt to try.

[Fingolfin]

Well, okay, that was a bust. There's nothing wrong with it! The plastic worm gear isn't rounded and is turning nicely with the spiral gear. The ratchet is held correctly against the main odo gear, which isn't rounded either. Turning it by hand (with a screwdriver up the cable slot) produces smooth motion. It took 25 complete turns of my screwdriver to cause the ratchet to turn the main gear one tooth; considering that that's mathematically correct (61 teeth on odo gear * 25 tpm = 1525), I think it's safe to say it's in good shape.

So -- somehow -- it must be a problem elsewhere. Maybe the in-gearbox speedometer drive is geared wrong? I did replace it a while ago...

[ajay]

I suspect we/you have missed one thing and BMC mentioned it earlier. The wild? fluctuations of the needle was/may have actually caused by the cable slipping. If so the odometer would also be out and it actually gave you an accurate reading through this.
Now go back and read this again as it is not going to be repeated. Well, not by me anyway

Now that you have got everything going ship shape and all the ducks are heading north you have no slip and your speedo is out just as it would have been if the original was not slipping prior to your repairs.

Clear as mud with eels in it but probably the root cause of the problem. No idea how to fix the problem though as my MM is almost 20% out through a change of tyres and diff.

Actually I suggest a felt pen and a calculator

[bmcecosse]

[Fingolfin]

"Not going to be repeated." Great. Need I repeat for the third time that the speedometer seems to be reading at least somewhat accurately? I believe it's within about five mph, which is good enough for my purposes. The problem has entirely been with the odometer, which is why I've been pulling my hair out.

BUT it may be irrelevant now. As I said, I disassembled the speedo head yesterday, and upon putting it back in the car I took a short unmeasured drive around town to run errands. During the errands, my subjective sense of distance seemed to match the odometer; that is to say, the numbers seemed to be turning more quickly than they had been before. I'm going to take a measured drive after classes today and see if I'm right.

[Fingolfin]

I was right!

My friends and I took a drive of 13.6 miles, from my house in Fulton to the Callaway Nuclear Power Station in Auxvasse, Missouri. (Never seen it up close before! Gotta go take a tour sometime.) Anyhow, the odo totalled the to-and-fro trip as 27 miles -- spot on, when 13.6 is doubled!

But don't ask me what I did to fix it... Or if it will last.

[bmcecosse]

It must have been slipping internally before.......