One of these is the right part. “Emergency” clamping terminals should always be replaced, on either battery terminal when replacing a starter. Similarly, molded wires with swollen, split and blue corrosion-tinged insulation are also garbage. Every connection counts.
Spinning high-compression engines during cranking used to be a tough proposition. Carbureted engines, weak points-and-condenser ignitions, heavy weight engine oils and lots of internal friction made large batteries and fat starters a necessity. It’s a lot easier to spin modern engines, due to lower internal friction, lighter, often synthetic oils, hot spark and timing reduction during cranking, as well as MPFI, all requiring less rotation before the engine fires.
Why then, is starter service still an issue? The major reason is that starters are historically a heavy engine accessory that takes up a lot of under-hood room, making them easy targets for the weight reduction teams that major automakers use at the design stage to lighten up new vehicles. The result is smaller, lighter starters bolted into smaller under hood spaces with less cooling airflow, along with close proximity to hot exhaust manifolds. It’s a tough environment.
Starter service is usually a by-the-book procedure and after the standard code read, battery load and starter current draw tests, it should be apparent where to dig deeper. A slow cranking condition, however, takes a little brain power. Can the battery produce? It’s an old axiom that a decent battery should hold 9.6 volts or better during cranking, and that voltage is a good baseline to work with when load testing the unit. Charging is likely necessary; few owners give up on a weak or damaged starter until they’ve cranked the battery dead. Most techs charge automatically before testing, an especially good idea if there’s time to charge the unit slowly, with 10 amps (or less) current. After charging, remember to turn on the headlights or apply a light external load for 10 seconds or so to burn off the battery’s “surface charge,” then load test as normal.
If the battery is okay and there’s still weak rotation, think about what’s happening inside the motor. Starter motors are series-wound, and in general, their rotational speed is proportional to the applied voltage. The motor’s torque, however, is proportional to the current. While most specs call for around 200 motor RPM to fire the engine (remember the major gear reduction between pinion and ring gear) there has to be sufficient torque to get the engine’s heavy rotating mass moving, and the startup or “breakaway” torque is higher than the torque needed to keep it spinning.
Fortunately, the starter also generates its maximum torque at low RPM (technically when it’s stalled, i.e. zero RPM) and high current, which is why series-wound DC starter motors have been around for almost a hundred years. That phenomenon also has a dark side. Take away the load and the armature and field current reduces, torque goes to essentially zero, speeding the motor. In fact, it can very quickly speed up to damaging RPM, which is why a starter motor should never be bench-tested at no-load with jumper cables. If electric cars ever become mainstream, remember the field-weakening speed control trick, because it will likely show up as a control strategy. In the internal combustion world, however, it’s important to recognize that Ohm’s law is not only universal, it’s unbreakable. Voltage and current are related and proportional when resistance is constant, and the overall voltage drop from battery positive to battery negative is the sum of the voltage drops at every connection in the system. We usually test relative to battery positive, but since it’s tough to test during starting, especially with the vehicle in the air, point-to-point resistance will do. In fact, it’s better, because it’s easy to check the ground side from the starter housing back to the negative battery terminal. How much additional resistance can hobble a good starter? Take a simple example: If the battery can hold 10 volts while delivering 100 amps during cranking, total circuit resistance is 0.1 ohm, or 100 milliohms, including the battery’s internal resistance. Add five hundredths of an ohm, or 50 milliohms of additional resistance, and the current drops to 67 amps! That’s why every connection and terminal has to be clean and tight for quality starter service. One bad cable, connector or lug is enough to compromise a good starter and the trouble is, a new or recharged battery can mask the problem by driving current with a higher voltage. Starter service is relatively simple, but if you’re a parts changer, rather than a troubleshooter, new parts won’t perform or last.