At 03:57 AM 3/19/2007 +0000, Carl ** wrote:
"1979 MGB. The radio and lights worked and still worked while trying to start, but absolutely nothing from the motor. .... I tapped on it and it turned over."

Okay, this is a pre-engaged type starter with a big solenoid on the side, and also a little relay on the fender. This may be pertinent to an MGA if someone transplants an MGB engine and 4-synchro gearbox into the MGA.

Turn the key to "Start" position, and the little relay should click gently. Relay contact sends current to solenoid on side of starter, and the solenoid should CLACK loudly as it throws the starter pinion gear into the flywheel ring gear. Upon full gear engagement, after full stroke of the solenoid, the solenoid makes connection with heavy electrical contacts putting power to the starter motor to make it turn.

Check connection of all push-on Lucar terminals first. One loose terminal and it won't go. (I hate Lucar connectors). On the end of the solenoid you will find one heavy cable that is a direct power connection from the battery. There will be a second heavy conductor going from the solenoid to inside the starter motor. This may be a flat copper bus bar or a short section of flex cable.

There will be 2 to 4 smaller terminals, probably all Lucar push-on connectors. The first will be a fat brown wire (always hot) that supplies all power to the vehicle (other than the starter), and also carries charging current from alternator to battery. There may be two heavy wire terminals for this purpose. There will be a smaller wire terminal for signal input to trigger the solenoid. This comes from the ignition switch Start terminal or from a starter relay on the fender (or from the starter switch on the MGA). There may be one additional small terminal that is a power output when the solenoid is operated. This sends power to bypass an ignition resistor to double voltage at the ignition coil during cranking. Check wiring diagrams in the MGB workshop manual to know the color codes for these wires.

You might check/test the small relay on the fender next. Find the wire (by color) that runs from the relay to the starter solenoid, and disconnect it from the relay. Put a test light or voltmeter on that output terminal. Turning switch to "Start" position, the relay should click and you should get power on the output terminal. Repeat this a number of times to be sure it is consistent and not flaky and intermittent. If it doesn't click, check relay grounding and integrity of the wire from switch to relay. You may need to replace the relay. When it works, reconnect the wire at the relay.

Disconnect the same wire at the starter end, connect test light or voltmeter at that end of wire, and check it again to be sure you have a good wire. Up to this point all the switch and relay are doing is to bring signal power to trigger the big solenoid on the starter. You can do the same thing with a simple jumper wire if the relay is not working. In fact you can take power from the big battery cable that is directly connected at the end of the solenoid, and jumper that to the small trigger terminal on the solenoid. If these terminals are close enough together you might even jump it with a screwdriver.

Be prepared for it to crank when you apply power to the signal terminal on the solenoid. Have it out of gear in neutral with parking brake applied. If you don't want it to start, disconnect the small wire(s) on side of distributor to disable spark (maybe disconnect at ignition coil). Applying power to the trigger terminal should make it CLACK and crank.

If you apply trigger power and it doesn't CLACK, then you have an open circuit for the solenoid coil connections. That is most commonly a loose connection at one end of the coil winding wire, more likely at the input end, less likely at the grounding point. Or it might be a burned our solenoid coil (very rare). Wiggle the input terminal to see if it is loose, and try again. You must get the CLACK before going on. Repair the solenoid if necessary.

If you get the CLACK but it doesn't crank, then check integrity of the connections for the battery cable and the other large cable size wire or buss bar going from the solenoid to inside the end of the starter motor. No connection here is a no-crank. Connect test light or voltmeter on the output terminal from the solenoid going to the motor and try again. Make this connection on the terminal post of the solenoid (if it is exposed), not on the output cable or cable terminal or bus bar. If you get CLACK but no power here, there is no connection on the heavy contacts inside the solenoid. If you get CLACK and power here, the fault is farther downstream.

Move the test light or voltmeter slightly from terminal post to the buss bar of cable end terminal, and try again. If you get CLACK and power on the output post but no power on the connector, you have a loose or corroded connection. If you get power on the output bus bar or cable and it still doesn't crank, you have some discontinuity inside the motor. For this you have to remove the starter and open it up for a look inside. The most common cause here is a loose connection inside where the cable or bus bar connects to the next heavy conductor going to the brushes. While you have it open check the integrity of all heavy wire and terminal connections.

You can test run a starter motor on the bench using jumper cables from the car battery, and it does not care about polarity. Connect one cable clamp on the bolt flange for a good ground, and touch the other cable on the motor input terminal to make it run. Have a very tight grip on the motor housing when you do this, as it has so much torque as to jerk the thing right out of your hand and onto the floor.

The starter motor is very robust and may last through the expected service life of two or three cars, if it is not abused. It has copper brushes that carry high current and never wear out (maybe). In normal service if you crank on it 5 seconds twice a day every day it will only see one hour running time for an whole year of service. I have never burned out a starter motor in my life, and I never expect to. I do not keep a spare in stock. The original inertia starter on my MGA has done 356,000 miles in 50 years and is still going strong. The only thing it has needed in all that time is cleaning and repainting, and once or twice replacement of insulator bushings at the power input terminal.

The trick with a starter motor is that it draws very high current and will heat up quickly. As such it is a low duty cycle motor. You may crank on it for two or three minutes before it overheats. Beyond that it has a 10% duty cycle, meaning you can only run it for one minute out of any ten minute period, having to allow the other nine minutes for cool down (or 30 seconds running out of five minutes). If the engine doesn't start and you keep cranking on it continuously it will overheat and melt some things inside, thereby destroying the motor. A normal car battery in good condition stores enough power to crank the engine long enough to destroy a start motor.

A mildly abused starter may exhibit some early signs of deterioration before complete failure. They sometimes develop a dead spot where there may be no electrical connection through one winding in the armature. In this case you might rotate the armature slightly, and then it will seem to work fine. It may repeat this problem on about every tenth start attempt, because it has one dead winding out of about ten poles on the armature. Solution for this is to have the armature re-wound.

A starter motor may suffer from stuck brushes, making bad contact to the commutator. This is more likely if the car has sat for a long time without running. This is one of those cases where if you smack the side of the motor with a wrench it might work. Then a month later it may have the same problem again. Solution is to remove the rear end cover to clean the brush holders so the brushes are free to move forward under spring force as they wear slowly.

For a car that was idle for a long time you might have a similar problem from corrosion of the heavy power contacts in the solenoid. Here again a good rap with a wrench on the solenoid might make it work. In this case once it is working it may continue to work indefinitely, as the power contacts are self-cleaning each with each cycle of operation.

Now pick up your diploma on the way out, and go fix your starter.