Starter Relay and Solenoid Primer

Starter Relay and Solenoid Primer

On automatic transmission models, there is a starter relay in the main power panel. The function of this relay is to prevent the starter from functioning except when the trans is in neutral or park. On 5-speed models, there is a jumper installed instead of this relay. As is the case with all relays, the start relay can fail.

Operation of the starter requires much more current than the ignition switch can handle, and much more current than any small relay (such as the starter relay that we have been discussing) can handle. Your car, as does virtually every car, has a very heavy-duty relay, or as it is more commonly called, a starter solenoid, to operate the starter. The solenoid often is used to engage the starter pinion gear with the
flywheel ring gear.

The usual arrangement is for there to be a small (10 - 14 awg) wire running from the ignition switch to the starter solenoid. This small wire furnishes enough current to operate the solenoid. The starter relay discussed above is in this small wire circuit on autos. There is a heavy (0 - 4 awg) wire running from the battery to the solenoid, and either a heavy wire running from the solenoid to the starter, or the solenoid is built onto the starter as is the case with the 928.

The solenoid is an electrically operated switch, and if the battery is weak, or if any connection from the battery positive pole thru the starter, thru the body, and back to the battery negative terminal is bad, there may not be enough current to operate both the solenoid and the starter. In this case, the solenoid makes the starter connection, that saps the power needed to keep the solenoid engaged, and the solenoid drops out, breaking the starter connection. There is then enough power to re-engage the solenoid, and the cycle repeats. This causes the all-to-familiar click-click-click no-start condition.

The most common causes for the click-click-click are battery discharged, battery terminals corroded, bad battery, bad connection somewhere in the battery/starter chain, bad starter, or bad solenoid, in pretty much that order of likelihood.

There have been a couple of cases of inadequate current available to pull the solenoid in, but this usually results in a simple "nothing happens".

Walley Plumley
928 Specialists

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