October 9, 2026 RICK POLETE

How to Replace the Rate Switch on a Schumacher Battery Charger

After decades of use, even a workhorse like the Schumacher SE-4020 can develop problems that affect its performance. One of the most common problems with these chargers is a broken or failed rate switch. The original rate switches are no longer available from Schumacher, and after 30 or 40 years of use, the plastic stems can break, deteriorate, or simply become difficult to turn.

In this walkthrough, Rick and Scott demonstrate how to replace the original six-position rate switch with a compatible replacement switch, including how to understand the different terminal numbering systems, transfer the wiring, mount the new switch correctly, and test the finished repair.

Key Takeaways

Original switches for the Schumacher SE-4020 are no longer available and can fail because of broken or deteriorated plastic stems.
The original switch has seven terminals, while the replacement switch uses a different terminal numbering system.
The safest way to transfer the wiring is to leave the old switch wired while mounting the new switch in the charger.
The replacement switch should be mounted with the stem oriented correctly so the knob aligns with the charger's off position.
A wiring reference can help match each terminal on the old switch to the appropriate terminal on the new switch.
When testing the charger without a battery connected, the voltage will be lower than it would be under load. The important part of the test is verifying that the output voltage increases incrementally as the switch moves through its charging positions.

Understanding the Schumacher SE-4020 Rate Switch

Before removing anything, it helps to understand exactly what the switch is doing. The SE-4020 uses a six-position switch, but that does not mean there are six charging settings. One of those positions is OFF, leaving five actual charging positions.

The back of the original switch is where things can get confusing. There are seven connections, but not all seven terminals represent separate charging outputs. The red and white connections are tied together by a trace on the switch, meaning those two terminals are both part of the AC input side. The remaining five terminals correspond to the charger's five output positions. Those output terminals are numbered 1, 3, 4, 2, and 5 across the back of the original switch.

There isn't an intuitive pattern to that numbering, which is one reason wiring these switches can be intimidating if you don't have a reference.

Understanding Replacement Rate Switches

Replacement switches use a completely different numbering system. Instead of the original switch's terminal numbers, the replacement switch used in this tutorial (697013) has five output terminals numbered 8, 4, 2, 6, and 10, along with the AC input connection.

The numbers on the replacement switch actually follow a logical pattern based on the physical position of the terminals. If the switch's OFF position were at 12 o'clock, the first charging position would be at 2 o'clock and correspond to terminal 2. The next positions would then move around the clock to 4, 6, 8, and 10.

On the SE-4020, however, the switch's OFF position is oriented differently, so the physical positions are effectively rotated 180 degrees. That means the terminals appear in the sequence 8, 10, 12, 2, and 4 when viewed according to their clock positions.

Understanding this distinction is important because the numbers on the old switch do not directly correspond to the numbers on the new switch. The wires need to be transferred according to their original terminal positions, not simply matched by number.

Browse and shop our replacement switches and knobs here.

1. Mounting the New Switch

One of our most useful tips in this repair: Avoid removing the old switch from the charger before the replacement switch is mounted.

The original SE-4020 switch is a twist-and-lock style. It has two small tabs and a retaining pin that holds it in place. Press the pin inward and turn the switch to remove it from its mounting position.

We recommend leaving all the wires connected to the old switch while mounting the replacement switch. This gives you a stable reference point while transferring the wires one at a time. Trying to hold both switches in your hands while disconnecting and transferring multiple wires can quickly become confusing.

Before mounting the new switch, it is important to orient it correctly. The flat on the switch stem should be positioned upward. This establishes the correct relationship between the switch position and the knob, allowing the knob to point toward the charger's OFF position when the switch is actually off.

Once the switch is correctly oriented, it can be secured with the washer and mounting nut. Tighten the nut with a 9/16-inch wrench before installing the knob.

More versions of this replacement switch with different mounting methods are available at PLP Battery Supply. The standard version mounts through the panel using a threaded barrel and panel nut, while another version of the same switch can be mounted to the face with two screws. The electrical switch itself is the same; the difference is simply how it attaches to the charger.

2. Transferring the Wires to the New Switch

With the replacement switch securely mounted and the original switch still wired, the wires can be transferred one at a time.

The first connection we make is the brown wire, which comes from terminal 3 on the original switch. On the replacement switch, that wire moves to terminal 8.
From there, the blue wire from terminal 1 on the original switch moves to terminal 4 on the replacement.
Next comes the red wire from terminal 2, which moves to terminal 2 on the replacement switch.
The white wire associated with terminal 5 is then transferred to terminal 6. It is important to distinguish this wire from the other white wire that is part of the AC input circuit and is connected through the trace on the original switch.
The yellow wire from terminal 4 on the original switch is the next connection to transfer, and it moves to terminal 10 on the replacement switch.

At this point, the five charging-position wires have been moved over. The remaining red and white wires are the AC input connections. The red wire includes a piggyback connection, which allows the white AC input wire to be connected through that existing piggyback arrangement.

The exact wire-to-terminal relationship is important, so if you're working on one of these chargers yourself, it is a good idea to document the original connections before moving anything. PLP Battery Supply keeps a terminal reference available to help customers determine which terminal on the replacement switch corresponds to each terminal on the original. Contact our team at team@plpbattery.com for support.

3. Testing the Replacement Switch

Once all of the wires have been transferred, the charger can be powered up for testing. The first thing to verify is that the timer and fan operate as expected. From there, the output voltage can be checked while moving the rate switch through its different charging positions.

In our walkthrough, the measured open-circuit voltage increases through the switch positions, moving approximately from 8 volts to 9.2, 10.4, 12.7, and 12.8 volts before reaching approximately 14 volts at the highest setting. These readings may initially seem lower or different than expected, but there is an important detail to understand when testing a battery charger: the output voltage measured with no battery connected will be different from the voltage observed when the charger is connected to a battery.

For this particular test, the exact open-circuit voltage isn't the primary concern. The goal is to confirm that the wires are connected in the correct sequence and that the voltage increases incrementally as the rate switch moves through its positions.

Once the sheet metal cover is reinstalled, the SE-4020 is ready to return to service.

Keep Your Old Schumacher Charger in Service

The age of your SE-4020 doesn't necessarily mean the entire charger needs to be replaced when a component fails. With so many of these chargers still in use, finding practical replacement parts can help keep an otherwise functional charger working for years to come.

The rate switch is a particularly common failure point, whether the plastic stem has been broken by an impact, twisted off when the switch binds, or simply deteriorated after decades of use. A compatible replacement switch provides a way to repair the charger even though the original Schumacher switch is no longer available.

The key to a successful replacement is understanding the difference between the old and new terminal numbering systems and transferring the wires carefully. By keeping the original switch wired until the new switch is mounted, you have a clear reference throughout the process and can greatly reduce the chance of getting lost in the wiring.

Need Help Identifying a Replacement Switch?

If you're working on a Schumacher SE-4020 and need a replacement rate switch, or you're not sure which switch or terminal configuration you need, PLP Battery Supply can help. Contact our team at team@plpbattery.com or 800-642-3451 for help identifying the right replacement part or working through the wiring.