47 replies to this topic

[maieth]

Okay, speedometer theory question:

Last week my wife's speedo needle started to bounce back and forwards erratically, gradually dipping lower and lower until it cut out completely. The speedo now works right sometimes, badly sometimes (the needle doesn't pick up enough speed), but mostly not at all.

So the speedo cable has been replaced, but the problem persisted. We've removed the speedo to inspect both the speedometer itself and the cable separately, and when moving the car, the end of the cable turned as it should. Equally, when turning the speedo manually, the needle moves as it should. So both things independently seem fine.

 

 

We put everything back together again and the speedo worked. Yay. Until about 2 miles down the road when the original problem happened all over again.

 

Anyone have any ideas what on earth is going on?

I've thought about replacing both the cable and speedo in case there is a problem with the way they are connecting (maybe the cable is slipping, socket is rounded, etc?), but I don't know if there could be any problem the other end. Could there be an issue with the final drive or speedo connection that end that would cause the speedo to gradually slip like this?

 

Car is a 1988 Mini City, with an MG Metro engine.

 

[sledgehammer]

is the speedo cable connecting fully at both ends -

 

if it rubs on one end , it will still work intermittently then may stop all together

 

had a similar thing on a customers car years ago - turned out a spider had made a web in the return spring / pickup dish area

 

after a second read may be the nylon / plastic gear in the gearbox

Edited by sledgehammer, 22 February 2015 - 04:08 PM.

[dklawson]

As above, it could be gearing in the gearbox, rounding out of the cable drive holes in either the gearbox or the back of the speedometer, or a poorly made cable which is not fully engaging the square drive holes.  If the cable is a bit "out of spec" and the speedometer is starting to have drag from thickening grease due to age, the cable may be engaged for a while then slip out of engagement while you drive.  In short, you have to methodically check the cable and both drives where it attaches.

[maieth]

is the speedo cable connecting fully at both ends -

 

if it rubs on one end , it will still work intermittently then may stop all together

 

had a similar thing on a customers car years ago - turned out a spider had made a web in the return spring / pickup dish area

 

after a second read may be the nylon / plastic gear in the gearbox

 

As above, it could be gearing in the gearbox, rounding out of the cable drive holes in either the gearbox or the back of the speedometer, or a poorly made cable which is not fully engaging the square drive holes.  If the cable is a bit "out of spec" and the speedometer is starting to have drag from thickening grease due to age, the cable may be engaged for a while then slip out of engagement while you drive.  In short, you have to methodically check the cable and both drives where it attaches.

 

 

Thanks so far guys.  Both of those suggestions sound like sensible things to start looking at.

I will start inspecting the cable and drive holes asap.  Should be easy enough at the speedo end, but what is the best way to inspect the drive at the gearbox end? Is there any way I can get a clear line of sight without it being engine out?

 

In terms of things blocking or clogging the mechanisms and drive at the speedo end, is it worth trying to clean up or re-grease the speedometer at all? This isn't a mechanism I've really had to deal with till now, so I don't want to go into dissasembly and refurbishing if it's not necessary or helpful.

[dklawson]

If you are able to place a square tipped nail into the speedometer drive hole and turn it easily by hand... that is all that should be required for "freeness".  If you find tight and looses spots as you turn the nail, that suggests professional cleaning and possibly odometer worm drive component replacement.   Avoid greasing the speedo.  Only a tiny drop or two of 20W or 30W oil on the drive hole is all that should be required from the "outside".  Too much lube of the drive on the speedometer can cause problems.

 

I forget how the cable is constructed.  There are collars on one end that prevent the inner cable from moving far out of position.  Disconnect the speedometer cable at the top end and try to pull the central core out.  If it moves up, push it back down and try twisting the cable.  If you can spin it, that says the drive gear in the gearbox or its square drive hole are damaged.  If you cannot twist it, focus your attention on the top of the cable and where it fits the speedometer.  

 

Over the years there have been reports of problem replacement cables that use cable cores swaged into an oval profile instead of square and the oval profiles slipping in the square drive holes.  Along with those complaints there have also been issues involving the length of the drive ends of the cable being "too short" and not engaging the square holes fully.

[Guess-Works.com]

Also consider if the path the cable takes has any tight turns or kinks... The cable is essentially a really long tightly wound spring, so if there's a tight turn it can slow down and then suddenly spring round... which gives the effect of a bouncing needle.

 

Once a cable starts doing this then it can also cause resultant damage to the speedo head and also the plastic drive pinion in the gearbox.

Edited by Guess-Works.com, 22 February 2015 - 06:14 PM.

[Spider]

I've had 2 so far where the Plastic Drive Gear on the Speedo Drive in the Gearbox started spinning on their shafts. The first one was fine when the car was stone cold, then at pretty much bang on 7km's it would stop. Took us a few days to work out what it was. The only way I diagnosed it was after eliminating everything else (as you have done), removing the gear checking it.

 

If it is this, it can be replaced with the engine in, I remove the radiator to get at it.

Edited by Moke Spider, 22 February 2015 - 09:48 PM.

[maieth]

If it is this, it can be replaced with the engine in, I remove the radiator to get at it.

 

Best thing I've heard so far!  Was bricking it that such a small part might need the engine out.

 

I'm hoping I can get home before the light runs out tonight and start inspecting things.

[l_jonez]

I've had 2 so far where the Plastic Drive Gear on the Speedo Drive in the Gearbox started spinning on their shafts. The first one was fine when the car was stone cold, then at pretty much bang on 7km's it would stop. Took us a few days to work out what it was. The only way I diagnosed it was after eliminating everything else (as you have done), removing the gear checking it.
 
If it is this, it can be replaced with the engine in, I remove the radiator to get at it.

I was just about to say this exact thing, I've also had the nylon gear spin on its shaft once its warm and expanded a little

[Ethel]

The odometer should tell if it's a fault with the speedo or its drive - false mileage reading = the drive.

[maieth]

The odometer should tell if it's a fault with the speedo or its drive - false mileage reading = the drive.

 

  Good thinking that man.  A quick run out in the daily driver to get a measurement and a followup run in the mini should help there.

Thanks

[adampat84]

Have exacltly the same issue with mine at the moment. Have replaced the cable with no change.

 

Was working initially when driving the car but after a period of time it would start bouncing then bounce its way down to Zero.

 

In the last few days it has ceased to operate all together.

 

Will keep an eye on this!

[dklawson]

 

The odometer should tell if it's a fault with the speedo or its drive - false mileage reading = the drive.

 

  Good thinking that man.  A quick run out in the daily driver to get a measurement and a followup run in the mini should help there.

Thanks

 

 

Just keep in mind that failure to drive could be caused by either end of the cable not being right, the square drive holes in the speedo or gearbox gear being worn, the gearbox drive gear teeth being damaged, or the gearbox drive gear spinning on its shaft.  You will still need to look at all these parts to determine which needs attention.

[Spider]

I'll add, though doubtful that it will be this, I've had one only where the end of the Mainshaft in the Gearbox got broken off, the end with the slot to drive the Spindle for the Speedo Drive. Not in my own can, but some PO made a bearing support for that end of the shaft, similar to but not a KAD one, only a needle roller bearing, only it was about 0.020" off centre. The guy siad the Speedo stopped working about 2 km after having the gearbox overhauled. That was an engine out job, but I'd be fairly confident that all other repairs would be engine in.

[maieth]

Well the missus and I did a few drives the other day checking mileages and our odometers match up fine, so the speedo itself is functioning okay.  What we found is that the speedo functions as normal for around 4-5 miles. once this kind of distance/running time is reached the needle starts to do its little dance and within a mile will be out of action.

This seems to fall in line with experiences by a few others and would seem to suggest the problem is related to something warming up with the engine and slipping, so at the moment I'm expecting to find the nylon gear is knackered.  However, I'll do this properly. No plans for tomorrow so I'll start taking things apart and examining them properly (any checks so far have been done with weak torches in low evening light :s ).

Intending to check:

 

- Cable end at speedo

- Drive hole in speedo

- Cable end at gearbox

- Drive hole at gearbox

- Condition of nylon gear

 

Fingers crossed one of these will give a solution.

 

Last question for now, does anyone have any tips on removing/accessing  a) the speedo drive on the gearbox  b) the upper dash rail to access the bracket for instrument panel (quite ashamed of this one, but I've never had to do much work behind the dash)