Under the Bonnet: Summer 1998

Our resident technical guru, Eric Wilhelm, will return to these pages in the fall issue of Moss Motoring, as he is very busy creating some exciting new catalogs for you! To tide you over until Eric’s next pearls of wisdom, we offer part one of a two-part feature by Louis C. Belby…

One of the most commonly asked questions on the online MGB Technical Bulletin Board deals with lack of starter motor function in 1968-80 MGBs. In this situation, when the ignition key is turned to the start position, the driver typically hears nothing, or perhaps a click, but the starter motor is not activated. The present article addresses this type of problem.

If your engine cranks but will not start, the cause could be in the ignition system rather than the starting system, or it could be fuel related. Another possibility is worn teeth on the flywheel and/or starter pinion gear, but this condition can be verified by manually rotating the engine or pushing the car in gear and then trying to start it again. An engine with a fault-free starting system will also fail to crank at all if the starter pinion gear is hung up on the flywheel, a condition which can also be remedied by pushing the car backwards while in fourth forward gear. This article assumes that you have already investigated and ruled out all these possibilities, and that the starting system itself is defective.

Problems in the starting system can almost always be traced to one of five sources: the battery, the starter switch, the starter relay, the starter solenoid, or the starter motor itself. Each of these components is easy to isolate, and thus diagnosis is relatively simple. However, an understanding of how the starting system works is necessary before proceeding to a diagnosis of defects.

The starting system is designed to deliver current to the starter motor, turn it while it is engaged with the ring gear of the flywheel, and to remove this current after the engine has started. Have you ever turned your key to start with the engine running? If so, you understand the necessity of having an inoperative starter after the engine comes to life. In order to achieve the above, current flows from the battery to the steering column-mounted starter switch on a brown wire. When turned to start, the ignition switch sends this current on a white/red wire to the W1 terminal on the starter relay located above the coil on the fender under the hood.

The starter relay has four terminals, labeled W1, W2, C1, and C2, and is held to the fender by two screws. If you remove the relay and turn it upside down, you will see that the four terminal designations are embossed on the bottom. The W2 terminal of this relay is permanently grounded, so by turning the ignition switch to start, positive current is sent to the starter relay, which closes its contacts. These closed contacts join two wires. One of them is a permanently positive (unswitched) brown wire from the battery terminal on the starter solenoid (the big bolt with several wires attached coming out of the solenoid end cap, away from the starter motor) to the C1 terminal on the starter relay. The other wire is a brown/white one running from the C2 terminal on the starter relay to a spade terminal on the starter solenoid.

By the way, if your car is equipped with factory electronic ignition, you will also see another spade terminal on the solenoid attached to a white/light green wire. This wire goes to the ignition coil and has nothing to do with the starting system. So, by turning the ignition switch to start, you are sending current from the battery via the starter relay to the starter solenoid. This current does not pass through the starter switch; rather, the ignition switch merely closes a relay which then connects the starter solenoid to the battery.

Now, let’s talk about the function of the starter solenoid. A solenoid is a switch which consists of two or more contacts that are closed electromagnetically. The starter isn’t the only component equipped with a solenoid; if you have overdrive, this unit also is activated by a solenoid. The purpose of the starter solenoid is to permit current flow to the starter motor when the ignition key is turned to start, but to withdraw it when the key is in the run position. When the key is turned to start, current is supplied via the starter relay to the positive side of the solenoid coil. Negative, of course, is always present since the solenoid is bolted to the metal starter motor. When activated, the solenoid coil draws in a spring loaded plunger. This plunger pushes against a shaft inside the solenoid that in turn pushes against a bar that bridges the battery/starter motor terminals. The battery terminal is the one referred to above (the big bolt with all the brown wires of varying thicknesses). The starter motor terminal is the other big bolt through the solenoid end cap attached to a metal strap that comes out of the side of the starter motor. Note that this terminal has no other wires attached to it!

Inside the solenoid, these bolts are attached to individual contacts that are isolated one from the other. The solenoid, when activated, bridges these contacts, thus connecting the starter motor directly to the battery and making the starter run. When the key is turned to start the plunger, as it is drawn into the solenoid, it simultaneously pulls a lever that makes the starter pinion gear thrust out and engage the ring gear on the flywheel, turning the engine and starting the car. When the ignition key is released, the ignition switch returns to the run position, which allows the car to run but which cuts current to the starter solenoid, resulting in the withdrawal of the pinion gear from the flywheel and deactivating the starter motor.

So, the current that actually runs the starter motor comes directly from the positive terminal on the battery to the big terminal on the solenoid end cap away from the starter. The ignition switch merely activates the starter relay that allows current to flow to the starter solenoid. The starter solenoid then bridges the two big terminals, activating the starter motor and at the same time transferring the motion of the starter motor to the engine via the flywheel. When you release the ignition key, the starting system, but not the ignition system, is deactivated.

With this basic understanding, let’s now turn to diagnosing an inoperative starting system. First of all, check your battery. If it is low, take it out, charge it if possible, and have it tested; most auto stores do this for free. If your battery is bad, don’t assume that this is your only problem; have your charging system checked before installing a new one, or you might be in the same boat in a few days.

If your battery and charging system are good but the engine won’t crank, proceed to check the starting system. If you hear a sharp click when you turn the key to start, you’re probably hearing your starter solenoid pulling in the plunger. The starter relay also makes a slight click, but not enough to confuse it with the louder sound of the starter solenoid. A loud click is no guarantee that the starter solenoid is working properly, though, so I would check it and the starter motor first. Jack up your car and support it on jack stands or pull it onto ramps. Chock your rear wheels, put the transmission into neutral, shut off the engine, and remove the ignition key (all of this so that your car doesn’t run away with you underneath it). Find the starter motor and solenoid on the right rear of the engine and locate the two big bolt terminals on the solenoid referred to above. Take a heavy wire (like a jumper cable) and bridge these two terminals.

By doing this, you are totally bypassing the solenoid, which, us we have seen, normally makes this connection. Since you bypassed the solenoid, the starter pinion gear will not engage the flywheel, so the engine will not crank. The starter motor itself, though, should give a healthy whirr, indicating that it is operative. If not, you have a bad starter motor that needs to be replaced: an easy bolt off/bolt on job if you reinstall the wires properly. Be sure to disconnect the negative battery cable before removing your starter! Also note all connections before removal and reattach them correctly! Most rebuilt starters also come with a solenoid, so you get two for the price of one. Install new starter and solenoid, and your problems should be over. But if your starter motor checks out okay, proceed to check the solenoid mounted on it.