Speedo Failure

S

sico24

Member
Hi

Today, the P6 had its first outing of the year. As I was driving, I heard a high-pitched 'zzzip' and the speedo stopped working. On getting home, I have disconnected the speedo angle drive and cable from the strip speedo. The angle drive seems free enough and truns when you use a screwdriver. Driving the car and watching the inner cable end, it is not turning (either in or out of the angle drive). So I undid the inspection hole and undid the cable (the car is a 1967 2000TC Manual). Turning the gearbox end of the inner cable manually rotates the speedo end of the inner cable OK. The cable seems well-lubricated.

So I assume its a drive gear issue? How easy is it to do and how easy are the parts to get? I did this on my Lotus Europa years ago, but the part had to come from America!!

Thanks

Simon
 
It's unusual to have problems with the drive or driven gears in the gearbox, but if you're sure that's where the problem lies it should be the driven gear as that's plastic and the drive gear is steel. If you take the inspection plate in the tunnel off, you should be able to fish the driven gear out. If it's the same as the 3500S one I've got one, but although I'd imagine it would be it would be wise just to confirm the part numbers. The only other thing I can think of is the rear flange on the gearbox is loose, or the mainshaft stud is snapped, but that's normally accompanied by problems with the synchro on a 3-2 downshift.
 
Harvey

Thanks, that's really helpful. Do you think I could test the speedo by driving the geardox end of the inner cable with a cordless drill? (the old 'clockers' trick!)

It is great if you think that I can fish the driven gear out through the inspection port - I had to take the rear assembly off when I did the Lotus.

Simon
 
sico24 said:
Thanks, that's really helpful. Do you think I could test the speedo by driving the geardox end of the inner cable with a cordless drill? (the old 'clockers' trick!)
Click to expand...

You can, as long as you've got "Reverse" on the drill, but I normally just put a bit of old cable into the back of the head, or if the cable is there, spin it with my fingers and that normally give me an idea whether it's all working OK.

sico24 said:
It is great if you think that I can fish the driven gear out through the inspection port - I had to take the rear assembly off when I did the Lotus.
Click to expand...

I've got a 3500S rear housing here, I'll see how easily (or not) the gear comes out of that to give you an idea if I get time.
 
There's nothing that holds the speedo driven gear and sleeve into the tailshaft housing, so in theory it will just lift out, but in practice I don't think that's going to happen if the one I've just done is anything to go by, it would only come out by being drifted from inside the housing, so I'd try through the access hole, but I wouldn't waste too much time on it. As I said though I can't ever remember one failing.
 
Hi Harvey

Seems like there's a first time for everything! Took off the cover plate from the gearbox again again and the nylon gear actually came out with the cable.

See pictures - cog teeth completely mashed - all the high parts knocked off.




Twisting the gearbox end of the inner cable sharply by hand made the speedo strip go up, so I guess that is OK (although my cordless drill seems too slow to show a speed?).

Any ideas please? (and also what made all the teeth get knocked off!). I will try the usual (Wins and JRW) tomorrow to see if they have one

Thanks

Simon
 

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If you count the teeth I can tell you if I've got one. Does the gear turn in the sleeve? If not that's the problem, otherwise I think I'd check the rear flange bolt. The only other possible cause might be the rear bearing, as if the cage has broken up and all the balls are on one side that could pull the shaft away from that gear, but I would have thought that would be very noisy.

I'll go and count the teeth on the one I have here.
 
I have to say it looks like your one has got mighty hot for some reason.

The one I have has 22 teeth and looks to be identical to yours. (Or what yours should look like.....)
 
Hi Harvey

Mine is 22 teeth. I've ordered a new one from Wins, thanks, so will try that. But as you say, I need to do a little investigating to see why these might have been stripped.

The box is not noisy, so I will check if the gear is spinning in the sleeve. I will look and see what I can see inside where the gear came from. Otherwise it will a trip under the car and see if I can see anything.

Thanks

Simon
 
Hi Harvey

The gear does turn in the sleeve, although it was quite stiff (although it seemed lubricated OK). As you mention, it looks as if it got stuck or very hot (or both).

I can't see anything amiss down the hole where the sleeve was - the drive gear looks OK.

Simon
 
I think I'd have a poke about down the hole and see if I could make the drive gear move on the shaft to give an idea whether the rear flange is loose, and also see if the mainshaft moves about at all. All very carefully obviously.

I can't remember ever seeing one that's done that.
 
Hi Harvey

Had a look inside - the drive didn't seem to be loose, nor the shaft. The new speedo drive gear I got from Wins rotated much easier in the sleeve (especially after I cleaned the sleeve), so I am hoping it generated all the heat from the friction with the sleeve. Also my speed cable seems very long (?)to get from gearbox to speedo, so I tried to re-route it carefully.

After reassembly, went for a drive round the block and seems OK up to 30 mph. However it was about 50mph when it went before. So I will keep my fingers crossed........

Thanks for the help

Simon
 
Hi

OK - it happened again! On the way to the Scottish All Rover Rally (lovely day with some great cars), after travelling about 15 miles, I was on the M8 doing 50mph, when THUNK and no indicated speed! The speedometer was perfectly fine until then.

Once home, I discovered that the inner cable had sheared about 1 inch from the gearbox end - just above where it comes out of the gearbox drive (thank goodness, so I could get it out). I don't know the condition of the drive gear (that I replaced last time) as I can't get that out.

So....given that the problem both times was at the gearbox end, if there was an intermittent problem with the angle unit at the speedo head (or actually the head itself) could this have caused the inner cable to jam and then shear first the drivegear and then break the cable?

What do people think? I will obviously need a repair for the MOT, although I can drive on the rev counter at the moment (17 mph/1000 revs in top roughly?)

thanks again

Simon
 
Strangely the speedo is not an MOT fail, but it is an offence to drive without a working one I believe..
The speedo seizing can definitely break the top angle drive, so it seems plausible the gearbox drive could break under similar circumstances.
Harvey has mentioned to me before that it's seldom the gearbox drive that initiates problems, he may mention it again :wink:
Jim
 
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