Speedo reading half

[HQentity]

Heya everyone!

Today I changed the worm drive in the gearbox. The old one was pretty beaten up so it clearly needed it!

The speedo now works, but reads half the correct value which I find a bit baffling to say the least?

The previous owner said to me that the gearbox has been reconditioned during the time he owned her, and I believe the differential and wheels are all standard. The speedo cable was replaced as well.

Gearbox is 3 rail non overdrive.

I couldn't get the screw in that holds the worm drive in place, I'm pretty confident its in far enough, but just not rotated correctly. I simply put the screw up tight against the worm drive instead of into it.

[sprint95m]

Kyle, I think you have answered your question?

When I removed the drive gear, the lock screw's head was flush with the gearbox casing.
So, I am thinking you should undo said lock screw and then wiggle the drive gear back and fore until it drops into position fully
and then you should be able to correctly tighten the lock screw.

What I think is happening is that the drive gear is only part engaging....hence the under reading?




Ian.

[Howard81]

Most new cables aren't greased adequately enough which can cause irrational readings. You can remove the centre spindle from one of the ends, re-grease with low-friction light grease and then reassemble. I think I used bicycle cable grease.

[HQentity]

Hiya,

Have done both those things, speed has improved greatly but still about 20% out. Maybe it will improve over time? I know i hasn't been working on at least a year so it might just need some miles to loosen up?

Thanks again

[MIG Wielder]

It is unlikely to improve over time. Can you check the colour of the nylon gear wheel on the old speedo drive gear and compare it with the new one. ( They are colour coded according to colour ) . Alternatively can you count the number of teeth on the old and new gears as a comparison. What part number does the new gear have ?
Do you have the early type of speedometer installed ( perhaps a photo ? ) and what are the small numbers on the speedo; front face near the 0mph marker ?
Tony.

[sprint95m]

The drive gear came out of a very early 1850 three rail 'box (sub WF10,000) and the speedo was reading correctly.
If I can find the intermediate 1850 parts book I'll check the part numbers for the drive gears,
but I thought intermediate 1850s had single rail 'boxes?




Ian.

[AlanH]

Has any grease got into your speedo ?

If you grease the whole cable it can work it's way up into the speedo's guts. Usualy this results in the needle wildly flapping about but it may be worth a look.

[Edin Dundee]

The drive gear came out of a very early 1850 three rail 'box (sub WF10,000) and the speedo was reading correctly.
If I can find the intermediate 1850 parts book I'll check the part numbers for the drive gears,
but I thought intermediate 1850s had single rail 'boxes?




Ian.

My Sep 75 intermediate 1850 has single rail.

[sprint95m]

The July 1974 parts catalogue shows all 1850 three rail gearboxes used the same speedo drive gear.


Thanks for confirming what I thought Bill. I guess that Kyle's 1850 must be amongst the last with a 3 rail 'box.





Ian.

[HQentity]

I took the worm drive out to count the number of teeth (both the new and the old worm gear have 23 teeth) and have found that the gear is starting to get damaged in exactly the same way as the old gear:



For now I have replaced the worm gear with the old one to save the life on the new one.

I believe I have a 3 rail because of this post here. My reverse is next to 1st?

I've put a small amount of grease at the base of the cable near the gearbox, and a small amount of wd40 in the speedo end. It hasn't got any worse from it thankfully.

[Jon Tilson]

Looks like the internal cog is damaged or there is a foreign body lurking within.

Is this a non overdrive box?

If so you need the angled drive on a dolomite....not on a Sprint bizarrely.

Jonners

[HQentity]

Yeah this is a non overdrive box. But I spoke to Tony a while back and we concluded I didn't need an angle drive, is this what you mean?

Maybe I should try and get inside my gearbox and see whats going on...

[sprint95m]

I believe I have a 3 rail because of this post here. My reverse is next to 1st?

Indeed Kyle, you have a three rail 'box.

Like Jon, I suspect your 'box is the problem.
It may be best to change the 'box for a s/h one?
I have a working 'box from a '72 1850 (56,000 miles) which you are welcome to (if you can arrange collection from KW1).

By the way, you don't need an angle drive. These are required for o/d 'boxes though.




Ian.

[matt of the vivas]

It could be the speedo head at fault, partly seized. Fairly common on older cars. It would explain both faults - the repeated chewing up of the drive cogs and the low speed reading.
Worth seeing if you can find a good used replacement to try.

[HQentity]

Thanks Ian, will definitely consider it if I can't fix the speedo! I'm a bit hesitant to change the gearbox though as this one is great. Do you know if Parcelforce etc have weight limits?

Is it possible to dismantle speedo heads? There has been a small amount of noise coming out of the speedo head so I suspect you might be right!

Update: dismantled the speedo, everything seems to be in good order, there was one part that was a bit stiff, a sort of plastic 'latch' that connected to the odometer cog. I cleaned it up. Otherwise all very smooth though, no reason to think it would struggle that badly.