We expect our cars to fire into life at the turn of a key, but sometimes the engine just cranks, and that’s it—we’re stuck and late. One of the more puzzling no-start situations is when the starter motor spins, but does not engage the flywheel.
No car brand is immune to this problem, but we can usually find the cure by careful diagnosis and close attention to detail. Here are some of the more common reasons for a starter that spins but does not engage.
How Does a Starter Motor Work?
Though not overly complex in nature, a vehicle’s starter is expected to perform several procedures at once. When the ignition of a vehicle is turned on, power is applied to the starter, as well as the starter solenoid.
Inside the starter, an electromagnet is activated. Once turned on, this electromagnet forces a specialized shaft out, which has a precision-cut pinion gear attached to it. This pinion gear fits into the geared outer portion of a vehicle’s flywheel, called the ring gear.
With teeth meshed, the power applied to the starter forces the “motor” inside to turn. The flywheel, or flex plate, bolted to the end of the crankshaft, starts it turning.
Once turning, various timing-related procedures can start, such as the charge being delivered to the spark plugs or the opening and closing of the valves. The pistons of the engine are also cycled in their cylinders because of the turning crankshaft.
Quite literally, an engine will roar to life almost immediately if each link in the chain detailed above functions properly. On the flip side, however, if something falls in the cracks of this process, one would generally be reaching for their cellphone to call for a tow.
Causes of a Starter That Spins But Doesn’t Engage
Even if your drive motor is functional, a lack of starter engagement can indicate different underlying problems. Knowing these problems may help you speed up your vehicle’s repairs and return to the road afterwards.
These are often the most common causes of a starter that spins but won’t engage.
#1. Low Battery Voltage.
It takes a good deal of power to energize and drive a starter motor’s Bendix, and in some cases, a faltering battery will only serve to spin the starter motor itself, but not provide enough power for actuator and flywheel engagement, too.
To make things worse, a parasitic draw from the starter can be the source of low battery voltage, too.
#2. Starter Bendix failure.
Another common reason that your starter might be having issues engaging is due to a starter Bendix drive that has failed. This means there’s a starter performing as it should, yet never mating with a flywheel or flexplate of an engine.
Such a failure ultimately means a new starter.
#3. Faulty starter solenoid/Stick solenoid contacts.
Other problems pertaining to your starter solenoid can also lead to this issue. The starter solenoid is very much acting as a relay, sending ample power for complete operation of the starter.
If your starter solenoid is pushin’ up daisies, or its contacts are stickin’, you may find your starter’s Bendix isn’t mate to the flywheel.
#4. Corroded or loose cabling.
Another possible reason you’re having trouble with your starter is if your cabling or wiring is loose or corroded. When battery cables are compromised, they may not be able to carry sufficient voltage and amperage for proper operation of the starter.
Such an issue could manifest as an overall lack of flywheel engagement.
How to Troubleshoot (Determine the Cause)
A couple of steps can be taken to assist in uncovering the source of your starter-related woes. Following such steps can help to expedite the diagnostic procedure, allowing you to place your vehicle back in service in short order.
Here are a few simple steps to help you determine the source of your vehicle’s starter engagement problems.
Step #1. Check Battery Terminals/Cables.
Begin the diagnostic process by popping the hood of your vehicle. Check the terminations on the battery for damage or corrosion. Also, check to be sure that your battery terminal-to-lead connections are snug.
Step #2. Test Battery Voltage.
Next, you will want to test your battery for voltage. A battery tester is very helpful in a pinch, but any multimeter will do for such testing as well.
While testing the battery while measuring between the positive and negative terminals, make sure that 12.6 volts or voltage is present. A reading less than this voltage is a strong sign of a failing battery.
Step #3. Verify Battery Cranking Potential.
With the help of an assistant, you will now test the drop on your battery during cranking. Recheck the voltage of the battery in the same manner as discussed above, while having your assistant turn the vehicle’s ignition to the “start” position.
During cranking, the voltage of the battery in question should never drop below approximately 10V. Should this occur, you may have a battery in question.
Step #4. Check Starter Connections.
Assuming your vehicle’s battery has checked out in spec, it will now be time to check all electrical connections to the vehicle’s starter. Be sure all connections are snug and free of corrosion.
Also, check to be sure you have proper distribution of power via the starter/starter solenoid using a multimeter. Have your assistant crank the vehicle’s ignition while you verify on the other wrench that the vehicle’s starter and starter solenoid are appropriately powered at each respective lug.
Step #5. Verify Power Delivery With Jump Test.
One can also double-check their prior findings simply by jumping the vehicle’s starting solenoid. This is done simply by placing the bain positive lug of the starter and the exciter lug of the starter solenoid.
If the starter/starter solenoid is in proper working order, a vehicle’s engine should start. Otherwise, starter removal and inspection will be in order.