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S-L-O-W CRANKING STARTER - SS-101

Question:
".... 57 MGA 1500. I recently replaced the battery .... and it was slow cranking then fast. I checked terminals including terminal going to starter. The inner bolt seems to turn but it was tight and clean."


I presume you are talking about the cable terminal stud on the end of the starter motor. This has an insulating nylon bushing and fiber washer to allow the stud to pass through the end plate without grounding out. The first (inner) nut has to be fairly tight to secure the stud in the end plate, prevent rotation of the stud, and assure a good contact for the heavy wire on the inside. If you tighten this nut and end up with a short to ground, then you need to replace the insulating grommet around the stud, which requires removal of the end plate from the starter motor. After the stud is secure and tight, then you install the cable and the outer nut. If this cable joint is loose, the bad connection can result in high resistance and a significant voltage drop (and lots of heat in the joint when cranking), which will make it slow cranking (or no cranking).

When you think you may have a starter problem, the first thing to do is to check the battery. For that go to Battery Maintenance and Testing. Don't waste time messing around with the starter motor or cables or switches until you know the battery is in good condition AND FULLY CHARGED.

To diagnose starter problems you need to use a volt meter to measure the voltage at different points in the circuit when cranking (high current conditions). There are about a dozen electrical connections in the high current circuit outside of the starter motor, so you need to have good clean and tight terminals with less than 1/4 volt loss in any one of these connections. The acid test (pardon the pun) is the net voltage at the starter motor during cranking. Put one lead of the volt meter on the tip of the stud on the end of the starter motor (not on the nut or cable end). Put the other lead of the meter on the case of the starter motor. Read the voltage while cranking the engine. 10 volts or more should make it spin over well. 8-9 volts will be noticeably sluggish, and less than 7 volts may not turn it at all. If you get less than 10 volts here you need to figure out where the voltage loss is occurring. You can also measure the voltage at the battery (on the posts, not on the terminals) while cranking (somewhat higher that across the starter). The difference between these two readings is the total voltage loss in the cables and the cable connections, and if you're losing 2 volts or more the car might not start.

The battery should have an open circuit voltage of about 12.6 volts when fully charged (measure twin six volt batteries in series). To measure the voltage drop in any one connection of a high current cable, switch your meter to a low voltage scale. Put one lead on the meter directly on the terminal post or threaded stud, and the other meter lead on the cable end. Then crank the engine and read the meter. This is where you should see less than 1/4 volt on the meter. If you see anything like 1/4 volt or more drop across any one terminal point, this is cause to disassemble the terminal, clean all of the mating parts, reassemble it and retest it. Most solidly attached and clean connections will have a volt drop more like 1/10 volt (100 millivolt) or less. Consider both ends of the cables between the (original) two 6-volt batteries, the hot cable from the battery to the starter switch, the internal voltage loss of the starter switch itself when cranking, the cable from the starter switch to the starter, the grounding connection of the starter motor to the engine rear plate, the grounding cable between the engine and the chassis of the car, and the final return ground cable from the chassis to the battery. If you have an accessory battery cut-off switch, this might be installed on one of the battery posts, or it might be installed in series with one of the cables connected to the battery.

You can test multiple connections at one time. For instance, measuring the voltage between the engine block and the chassis of the car while cranking is actually checking the two end connections of the engine grounding strap together.

".... While doing this I by accident removed the plus coil wire. Not knowing this I tried to start the car and it cranked real fast. It didn't start because that wire had come loose. After seeing the wire and replacing it I figured since it cranked fast it would again, but with the wire in place it cranked slow."

This is caused by the kickback of the engine when you have too much spark advance. When ignition of the fuel in the cylinder occurs before the piston reaches top dead center, this wants to make the engine turn backwards, which is fighting the starter motor and causing hard cranking. You need to retard the spark timing by rotating the body of the distributor counter clockwise (the direction of rotation of the rotor).

As a good beginning point, go by the book. Rotate the engine until the timing mark on the flange of the crankshaft pulley is near the zero pointer at the bottom of the pulley. The longest pointer is zero (TDC). The other two points are 5 degrees apart. Rotate the crankshaft to line the mark on the pulley up at about 7 degrees BTDC. Then put a test light or a volt meter on the wire on the side of the distributor. Rotate the body of the dizzy anti-clockwise until this terminal is grounded through the ignition points to the engine block. Then rotate the dizzy body slowly clockwise until the points break open and drop the ground connection, which will yield an abrupt change of state of the volt meter or test light. That's the 7 degrees BTDC static timing setting. Secure the dizzy body in that location and try starting it again, and it should crank over much easier and quicker.

Regards,
Barney Gaylord

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