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MG MGB Technical - Speedo died?

While cruising along at around 60 mph, there was a sudden loud and fast ticking noise. Scared teh crap out of me as i was 100 miles from home, then notice that the speedo had stopped working. I reached under the dash and disconnected the cable from the speedo, and the noise was gone. Obviously i had to drive home w/out the gauge, which is no big deal as it was bouncing so much i use the tach for speed. The end of the cable looks ok, a little rounded but not much.

Any ideas fro the experts?? is it the cable or the speedo that is shot?

Ken
Ken Harris

Ken. The answer is a definite yes, it is the speedo or the cable that is shot. Unless it is the angle drive unit. That is all that can be diagnosed over the internet--you need the speedo in hand to do any more diagnosis.

Pull the speedo (pain in the neck on the pre-77 cars) and decide whether you want to make a tool to fit into the rear (to simulate the cable), or whether you want to sacrifice a speedo cable to turn the magnet inside. Yes, the speedometer works off a magnet affecting the needle. The odometer, on the other hand, is direct drive. If, with the cable hooked up to the speedo, the speedometer does not register, but the odometer keeps working, it is a speedo head problem. If neither is working, it is most commonly a cable problem.

The second possible cause of the speedometer function and odometer function not working is a bad angle drive unit.

First thing to do is to put the rear of the car up on jack stands, start the engine, put the car in gear and see if the cable is turning. If not, you have a cable/angle drive problem. Make sure the cable is fully seated in the angle drive/transmission output before testing. If the cable is turning, it is either a bad cable end or a bad speedometer.

Chuck an old piece of cable end into an electric drill, insert the square end of the cable into the speedo and turn on the drill. If you have proper functioning, it is a binding cable and the cable and outer sheath need to be replaced.

Les
Les Bengtson

I had the same thing happen lat last season. In my case, the loud ticking noise was a broken cable inside the housing. If you remove the cable end from the tranny, you can chuck a cordless drill to it and see if the guage will register. If you find that the cable is binding, you'll know the cause immediately!

If you replace the cable, buy a replacement for an overdrive. The extra length allows a larger radius bend at each end which places less stress on the cable itself.
Jeff Schlemmer

Ken: While testing the cable with the drill, I believe you use it in "reverse" to turn the speedo. (experience is a wonderful teacher):)
jim

Ken,

If you have a jumping speedo needle (but it seems accurate otherwise), it could have been caused by a fraying (breaking) cable. Since the speedometer in the '70 B I care for was jumping and off by about 20 mph, I took that instrument over to APT in Bloomington Minnesota (www.gaugeguys.com) and had them go thru and clean and calibrate the instrument.

You can check for the cable operation simply by putting a piece of tape on the end of the cable that you've disconnected from the instrument, and watch the tape rotate as you push the car. It happens that with a push of 52.8 feet, you should get 100th of a mile of cable turns, or on a speedometer marked 1280 on the face, about 12.8 turns if you are running something close to standard tires. Note, you don't have to be in gear for this, nor do you have to jack up the car if you've already disconnected the cable from the instrument and can place the end somewhere where you can see it. The cable should turn very smoothly. If it doesn't, you can trouble shoot from there, like remove the cable and see if it has separated inside.

I hope this helps.

dave
Dave Braun

OK, with the car moving the cable does turn, so I guess that eliminates the angle drive unit. i hooked the cable back up to the speedo and nothing happened. Neither the odo. or speedo moved. Now I'm moving onto the cable . Yes, I will be purchasing an o/d cable for the added length.

Question though. Should there be an angle drive on the speedo? Mine has one. I would have thought the cable would attach directly to the back of the speedo.

Ken
Ken Harris

I routed my cable a little differently than stock to attach it directly to the speedo without the angle drive. That's one of the bonus features of using a longer cable, although you may be able to do it with a short one?
Jeff Schlemmer

I'm with Les in that maybe the speedo itself should
be checked by attaching a drill motor up to the speedo via a short piece of old speedo cable
(or, I use a finishing nail that is filed square on
one end).

Many times the input gearshaft (inside of the
speedo) will seize.

When this happens, the cable will continue to turn
but the gears in the cable angle drives will pop out
of mesh during each revolution and I'm betting
that's where your ticking noise is most probably
coming from.

Daniel Wong

You'll need to take the speedo out. The speedo comes out forward. It is held in place by two little struts that attach to two longish bolts sticking out of the back of the speedo and (the struts) push against the back of the dash. Indo the two knurled nuts on the longish bolts, with the extream edge of the fingernail of your littlest finger while standing on your head. Actually if you managed to get the speedo cable on and off you won't have a problem.
Don't forget to accidentaly disconect the high beam headlight wire and not discover this until weeks later.

Once out, you turn the chrome bezel on the front and it and the glass come away. The mechanism is held in the casing by a couple of screws on the back.
Peter

This thread was discussed between 03/05/2006 and 05/05/2006

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