Parallel Battery Switch

This topic will cover the parallel battery switch. This parallel switch allows trucks to maintain a 12V electrical system and still use a 24V starter for increased cranking power.

What the Parallel Switch Does

The parallel switch does two things. When the starter is not engaged, the parallel switch places the two battery banks in parallel so that the truck charging system can recharge both battery banks at the same time. This connectivity is shown in the 12 Volt drawing shown below:

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When the starter is engaged, the parallel switch places the two battery banks in series for 24 volts cranking power to the starter. This configuration is shown in the 24 Volt diagram below:

The parallel switch accomplishes this feat using 6 relay contacts as shown in the diagram below. These 6 relay contacts are controlled by the relay coil. When the parallel switch is not active, contacts S1-A and S1-C connects battery B2 post to the trucks 12 volt system. Contacts S2-A and S2-C connects battery A2 post to the vehicle ground. This places the upper battery in parallel with the lower battery and allows the alternator to recharge both batteries.

When the parallel switch is energized, contacts S1-C and S1-B connect B2 to terminal B of the starter solenoid. Contacts S2-C and S2-B connect A2 to B1 which places both batteries in series. Now when the starter solenoid engages, 24 volts is applied to the starter motor.

To troubleshoot this switch, simply check for proper voltages at the six contacts, using the above diagram. You will probably find loose connections or burned and pitted contacts on the relay. Some times you can repair the contacts by dressing them lightly with a small file. The contact material is made of a very soft material so file gently. The contact should be shaped with a slight and rounded bulge in the center of the contact surface.

If the contact is burned to badly, then you will have to replace the individual contacts or replace the entire relay if individual contacts are not available.

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