The MGA With An Attitude
Speedometer LOSS of DRIVE - ST-104

At 08:34 PM 5/3/06, Barrie Graham wrote:
>"Although my speedo worked with an electric drill attached to the cable, it doesn't work when driving the car..... I read your article on the speedo pinion and I assume I can change it with the trans in the car????"

Yes, you can R&R the pinion gear (below oil level) with the gearbox in the car. But do a little more investigation first. It may not be that part.

>"but the article says that it may be a loose nut on the tail end of the gearbox. When I changed the tail end bush, could I have caused a malfunction of the drive gear for the speedo???"

The fact that you have a tail bushing means you have a 1500 type gearbox with sliding spline rear coupling. Your car will not have the big nut on the tail end. The 1500 gearboxes have the retainer nut on the mainshaft just aft of the speedo drive gear. You have to remove the tail housing to get at that one, so if that nut is loose you have to remove the engine to get the gearbox out of the car for service. For the 1600 type gearbox with flanged output you can check or tighten the rear nut by disconnecting the propshaft flange from the gearbox.

>"When I changed the tail end bush, could I have caused a malfunction of the drive gear for the speedo???"

The tail bush has nothing to do with the speedo drive gear. If the rear most nut on the mainshaft is tight the speedo gear is fine. For the 1600 type boxes you can check or tighten the big nut at the output end just by disconnecting the flanged propshaft. For the 1500 type with sliding spline output the big nut is inside.

This is more likely a problem with the cable core being a little too short, or not properly engaged at one end or the other, or a problem with the pinion gear or pinion shaft, which is easily removable.

Start by disconnecting the cable from the back of the speedometer. Hold the cable in one hand and put your thumb on the end of the core cable to apply a little thrust load. Drive the car (or jack up one rear wheel (or both) and run it with transmission in low gear). If the cable turns under your thumb the drive gear and pinion are most likely okay. The cable center core may be a little too short for complete engagement at one end.

If the cable does not turn when driving, the problem is at the bottom end. Leave cable disconnected from the speedometer. From under the car disconnect the cable from the gearbox. You can pull out the cable core and insert the bottom end of the core into the drive spindle in the gearbox to check for secure engagement of the square drive end coupling. Try turning the core to see if the pinion shaft will turn (it should not).

If the pinion shaft does not turn your problem is with the cable (center core too short) or with the coupling end (square drive not engaged). Sometimes the square hole in the pinion shaft gets rounded out and fails to grip the cable core. Sometimes the cable end gets rounded off. Sometimes it's a combination of both, or the cable may not be engaged far enough to have a good drive connection.

If the pinion shaft turns you need to remove the driven gear for inspection. Be prepared for drain oil, as this assembly is below oil level. If you jack the rear end of the car high enough you may not have to drain the oil.

When you have the pinion gear out, check to see if the nylon gear is tight on the pinion shaft. If the pinion gear turns on the shaft you need to replace the part. If not, then check again the fit of the cable end in the pinion shaft. Also reach through the speedo drive port with a large screwdriver to see if you can manually turn the drive gear on the mainshaft.

If the driving gear turns on the shaft you have to tighten the rear nut on the mainshaft. For the 1600 type this is at the rear flanged coupling. For the 1500 type this is inside the tail housing.

Good hunting.

When installing a new speedometer cable it may help to connect the top end first. The top end contains a collar on the inner cable for axial location. If you connect the bottom end first the tip of the cable may not properly engage the square drive spindle. If you then connect the top end last you might jam the bottom end of the cable along side of the drive spindle without proper engagement. The cable may not drive, the speedometer may not work, and you could damage the cable in the process.

The same thing applies if you are only changing the cable core. If you only disconnect the top end to withdraw one cable core and insert another, the new core may not properly engage the square drive spindle at the bottom end. It is best to get under the car to disconnect the cable jacket at the bottom end, and reconnect it only after the top end has been reconnected.

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