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MG MGB Technical - Speedometer Cable?

My speedometer was acting normal, no bounce...then all of a sudden it started a +/- 10 mph bounce for about 20 miles...then went dead. I pulled the speedo cable, but it "looks" okay to me. Could a broken off piece of it still be in the tranny? It really doesn't look broken, the last inch (2.5cm) on the cable is somewhat flattened, but I assume that is how it works?? Any tricks on removing cable housing from tranny, it doesn't seem to want come apart.

ALSO, the lens on it is speedo badly faded, where does one find replacements for lens and bezal?

Joe Walck

Does your car have the 90 angle drive? This has failed twice on mine.
Does the odometer still work, if so then the cable is OK and the instrumnet has failed.
Stan Best

Cable should just unscrew with the round knurled 'nut' same as on the speedo. An inch of squared off end sounds about right. The right-angle drives are notorious for stripping the plastic gears, neither of mine came to me with one, as long as the cable is long enough to make a gentle curve at the gearbox end (probably under the crossmember rather than over which would be the normal case) it should be OK without.
PaulH Solihull

Stan, The speedo needle is on the bottom stop, no movement since it stopped bouncing and died. The trip meter hasn't moved since either.

Paulh, In Moss parts catolog it is hard to find the 90 at the tranny. I've ordered a new cable, but maybe that isn't the problem after all...I'll try the re-route and see if that works. What is the PN for the 90 if I need it?

BTW, when I picked up the car last month, engine/tranny mounts were bad/broken, so the engine was basically sitting on the oil pan/cross member...the lower pulley even put a nice groove in the cross member before I got it home and the mounts changed. I suspect there was more than normal pressure on the speedo cable, so the gears are more than likely stripped I'd guess.
One thing about owning MG's, you learn something new everytime you work on them. I've been learning for 35, and this is my first speedo problem.
Joe Walck

OK Joe, that indicates cable. If the ofometer was srill working that would have proved the instrument was faulty. I have found plumbers pliers good for undoing recalcitrant speedo cable knurled rings.
Stan Best

Joe
If you want the angle drive number it is.

021-511 $48.95 ADAPTOR, speedo. drive
Note: Must be used with washer 324-720

That is a moss part number but you must use the washer as on some boxes the angle is too long and ends up jamming,stripping your speedo cable and head.
The washer just spaces it out a bit to allow it not to jam on some cars.

Ste
Ste Brown

Second the plumbers pliers for the knurled rings - be gentle on the speedo end if using them there - and disconnect the battery because there are any number of likely 'short' contacts behind the dash . If I recall correctly the gearbox end involves numerous limited-sweep turns to get it home.
Roger T

Seems to me that both ends of the cable are square, and not fat.

FRM
FR Millmore

FRM X2

Methinks the tranny end has sheared off, should have a square section, like the speedo end.

Herb
Herb Adler

Fat? By 'flattened' I did rather assume Joe meant flats i.e. squared off, rather than truly flattened i.e. much wider than it is thick, which would definitely be wrong.
PaulH Solihull

Oblong oval would have been a better discription of it(note in photo how threads change where it crosses over the speedo end), the end from the speedo is square and has the swagged washer. When my cable comes in in a couple of days, I will confirm on this thread.
Joe Walck

Look at the picture, one end is squared and the other is "fat" but not square.
"Fat" is what happens when the inner layer of wires gets wound up inside the outer layer. This is from excessive torque as the cable seizes or flexes too much at a tight bend. Plain age (fatigue failure) or rust in the cable do the same. The "fat" part then increases the load as it jams in the casing, and the squared bit breaks off.
It is good practice to pull the cable through your fingers when greasing it; any lumps are places where this is beginning to happen, which will lead to a broken cable. And it is very common that such failures cause other parts (like the angle boxes) to break when the cable seizes.
Bet the square is still in the gearbox.

FRM
FR Millmore

How greasy was the top of the drive. The cable often gets over greased in an effort to stop the needle fluctuating. However the grease winds it's way into the gauge causing drag on the disc and clogging the ratchet and pawl which moves the oddometer.
It's actually quite an easy process to strip down and clean!
Allan Reeling

Definitely a broken cable end. There, indeed, should be a square section of cable at both ends. The right angle adapter isn't necessary, at the transmission, if you use the longer OD cable. I have a narrow tunnel '67, with a later 4 synchro/OD transmission, and the longer OD cable has plenty of room to operate without causing it to kink. It's been going steady for 25 years. RAY
rjm RAY

Incidentally the washer is only on the speedo end, to stop the inner dropping out if the outer should become detached from the gearbox.
PaulH Solihull

Well, the new speedo cable arrived...and it's the same as the one I removed. One square end (speedo) and one "flattened" end for the 90 degree. I may need a new 90, but will have to wait for another set of "front suspension fulcrum bolts" before I can test the speedo again...the set I got from Moss both stripped the threads off before I could tighten and put the cotter pin in. Looks like the bolt is made from softer metal than the nut.
Joe Walck

That's really odd, I could have sworn both ends should be the same i.e. square, and I'm still not convinced. But in any case, the arrangement is the same whether the cable goes in the gearbox direct or the right-angle drive first. I'd remove the right-angle and see what shape the gearbox end of that is.
PaulH Solihull

Paul, Ray, and I are eating the same mushrooms I guess!

FRM
FR Millmore

A good quality cable will have a square section formed on the gear box end of the internal, and a metal cap with a square section on the speedo end. Many new cables sold now have a plastic square section on the speedo end. In my case that plastic section was a fraction too long, so that when installed it put pressure on the internal drive of the speedo. I became aware of that only some time later when the speedo started to 'over-read' by 10 mpg or so as the drive pushed the magnet assembly inwards. If your new cable has the crappy plastic end I strongly recommend you (a) carefully measure it to make sure it does not 'bottom out' in the speedo or (b) get an instrument technician to replace the plastic end with a metal cap of the correct length.
J H Crighton

Couldn't stand this, so went digging. Found two 4 synch boxes w. speedo angle drives and partial cables. One had a square end cable, but was seized in place and the stub drive from the gbx to the angle drive was mangled. Thus appeared to be an early one, pre service interval counter, which I think was about 75-76. The second had the short cable that went to the service interval counter, and had the squashed ellipse end. The angle drive for this has still the square female drive, and either type cable fits in it. The dimensions across corners of the two types is the same, at about .150".

So, the cable end appears to have been a production change, and replacement cables could be either. If they come from Smith's or similar, they are likely to be the squished ellipse (squillipse?), but speedo shops etc probably would make up the earlier square type.

The solid metal (speedo) end types mentioned by JHC also appear sometimes, but I don't think these were OEM on any MGB; they were on other applications. Never met a plastic one, don't want to. These end fittings could not be used at the lower end, since you couldn't assemble the cable.

I think Joe will find that the angle box and/or the stub drive from the gearbox has broken.

FRM
FR Millmore

Below is a photo of the new cable, minus the housing.
The swagged washer seems a little smaller, but the squished ellipse (squillipse) is the same. I noted when I took it off, that the 90 degree had a short, square cable into the tranny, so I wonder if the squished ellipse would work without the 90. I might try that if the 90 is bad, but I'm awaiting new fulcrum bolts before I can get her on the road again.

Joe Walck

Squillipse or square it is then. Whichever, as the socket of the right-angle drive should be the same as the socket of the gearbox then either cable should fit either. Unless someone else knows better ...
PaulH Solihull

on this subject i need to replace a speedo cable on a 72 mgb gt question is how do i remove speedometer end of cable? i cannot seem to get my hand under dash. anyone have a secrect way of doing this any help would be appriciated.
RP Padilla

RP,
much earier access to the back of the speedo by taking the face level vents out and your watch off!!
Al
Allan Reeling

Even on a UK tin dash you still need pretty slim hands, the American padded dash (I assume) is different. Even though the speedo is near the centre of the car same as RHD, it's further away from the vents than UK cars. You would need experience in assisting cows to give birth, I suspect.
PaulH Solihull

Follow-up on original problem.
The cable wouldn't reach without the 90 degree, it probably will reach on RH drive MGs, but not with LH.

The speedomenter still doesn't move after installing a new cable, next step is to try lifting rear end and spinning tires to see if the cable moves at the rear of gauge, if so, then if the 90 degree at the trans spins...if so, I guess my speedomenter is bad...more later.

If I need the speedometer rebuilt, recommendations for a shop, west coast of USA? I don't see one for my 79 on ebay right now...
Joe Walck

With the cable attached to the speedo, grab the other end with an electric drill and see if the speedo works.
Very unusual for the speedo to fail in a no move, no noise mode.

Also remove the trans angle drive, and use the drill on the stub end.

Books show an angle drive on the back of the speedo, and I have seen these, but many, perhaps most, cars do not have them. If you have one, it is subject to the same sort of failure as the lower angle drive.

Finally, late MGB have no key on the speedo drive gear on the mainshaft, it is friction only, mostly maintained by the nut on the rear drive flange. Again total failure is rare, but I suppose it could happen. Usual indication of a loose nut is low reading speedo that gets lower as the car warms up. This also gives a version of the infamous rear end "clunk".

FRM
FR Millmore

FRM,
How did you know I had a rear end clunk? It starts at about 30 mph.

This may be my problem after all. Could you expand or send me in the right direction to diagnose/repair if it is the rear flange nut.

BTW, The cable at the speedo end spins when I raise the rear end and spin the tire. I'll try the drill spin trick this evening and see if I can get the speedometer moving that way...the wife couldn't spin the wheel fast enough to register, and I still get no reading when I'm driving.
Joe Walck

Joe-
This is the nut that clamps the entire 3rd motion shaft system together, locating everything off the center bearing. If you can move the front flange of the driveshaft in/out at all, it means this is very loose, but it doesn't have to be that loose to cause the speedo problem. Other symptoms may be a lot of motion of the gear lever fore and aft as you come on/off the gas. You really need to unbolt the DS and check that the big nut is very tight.

FRM
FR Millmore

This thread was discussed between 20/09/2011 and 04/10/2011

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