Thread: 350 trouble

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Welcome to CHR!

Let’s see if we can turn this into a teaching moment. An internal combustion engine must be rotated at a great enough speed to suck both fuel and air into the combustion chamber(s), while the intake valve(s) are open, and continue turning as the valve(s) close to compress the mixture which will be ignited by a timed spark. The starter motor is a compact device that packs a lot of punch and is geared very low to turn an engine over (via the flywheel or flexplate) to provide said rotation. The challenge is engaging the starter motor only when necessary. Bendix versus pre-engagement (using a solenoid) – two very different animals.

Vincent Hugo Bendix (really interesting guy – look him up on the web) developed the Bendix drive in 1910. As per Wikipedia,
“The Bendix drive places the starter drive pinion on a helical drive spring. When the starter motor begins turning, the inertia of the drive pinion assembly causes it to wind the spring forcing the length of the spring to change, and allowing the pinion to engage with the ring gear. When the engine starts, backdrive from the ring gear causes the drive pinion to exceed the rotative speed of the starter, at which point the drive pinion is forced back and out of mesh with the ring gear.
The main drawback to the Bendix drive is that it relies on a certain amount of ‘clash’ between the teeth of the pinion and the ring gears before they slip into place and mate completely; the teeth of the pinion are already spinning when they come into contact with the static ring gear, and unless they happen to align perfectly at the moment they engage, the pinion teeth will strike the teeth of the ring gear side-to-side rather than face-to-face, and continue to rotate until both align. This increases wear on both sets of teeth. For this reason the Bendix drive has been largely superseded in starter motor design by the pre-engagement system using a solenoid.”

The pre-engagement system uses a solenoid (using an electromagnetic engagement) to move an actuating arm, forcing the pinion (drive gear) onto the flywheel, typically when the ignition key is in the “start” position. When the key is released, the power is dropped from the electromagnetic circuit and the solenoid returns to its original position. As such, when functioning properly, the starter motor is only engaged during the starting operation.

In your case, it appears that the solenoid is not functioning correctly and therefore failing to move the actuating arm and engage the pinion gear with the flywheel. Pretty straight forward fix and in most cases, a rebuilt GM starter motor comes with a new solenoid.