How to Fix Shurflo 4008 Pressure Switch Failure: Complete DIY Repair Guide

Short Answer

You're absolutely right about the Shurflo 4008's high pressure switch being a common failure point with regular use. The microswitch developing high resistance or failing contacts that causes continuous pump operation is a classic symptom of this model. Your approach of replacing the pressure switch assembly is the most practical repair, and having backup parts is smart given how common this failure is. The repair typically takes 15-20 minutes with basic tools and costs around $15-25 for the pressure switch housing versus $73 for a complete pump replacement.

Checking for Leaks First

Before assuming switch failure, check your entire water system for leaks, as this is actually the most common cause of continuous pump running. Look for dripping faucets, toilet valve leaks, or water line connections. Even small leaks can cause the pump to cycle continuously.

Understanding the Problem

The Shurflo 4008 uses a pressure-actuated microswitch in the high pressure switch assembly that cycles the pump on and off based on water demand. With regular use for showers and sinks, this switch experiences thousands of cycles, and the internal contacts gradually wear out or develop corrosion. When the switch develops high resistance or fails to properly open the circuit as you experienced, it can't stop the pump, resulting in continuous operation that will quickly burn out the motor if not addressed.

This is particularly problematic in remote locations like Alaska where replacement parts aren't readily available. The switch failure often starts intermittently - you might notice the pump running a few extra seconds after turning off a faucet - before progressing to the continuous operation you experienced. The good news is that this is typically an electrical failure rather than a mechanical pump problem, so replacing just the pressure switch assembly usually solves the issue completely.

What You'll Need

Parts Options (as you mentioned):

• Shurflo pressure switch assembly (housing with microswitch) - $15-25
• Individual microswitch for DIY repair of old housing - $5-10
• Complete Shurflo 4008 pump as backup - $73
• Thread sealant or Teflon tape for reassembly

Tools Required:

• Phillips head screwdriver
• Small flat blade screwdriver (if replacing just the microswitch)
• Needle-nose pliers
• Multimeter for testing (optional but helpful)

How to Fix It

Replacing the Pressure Switch Assembly:

1. Turn off the water pump switch/breaker, then turn off the 12V power to the pump and open a hot water faucet to ensure complete pressure relief
2. Remove the three screws securing the pressure switch housing to the pump head
3. Carefully disconnect the two wire terminals from the microswitch, noting their positions
4. Unscrew the old pressure switch assembly from the pump head
5. Apply thread sealant to the new assembly and thread it into the pump head, tightening to 15-20 ft-lbs per Shurflo specifications
6. Reconnect the wires to the new microswitch in the same configuration
7. Secure the housing with the three screws and test the system

Testing the Repair: Turn the power back on and open a faucet. The pump should start immediately and stop within 1 second of closing the faucet. If it continues running, double-check your wire connections and ensure the new switch isn't defective.

Leave It to the Experts

While this is definitely a DIY-friendly repair that most RV owners can handle, consider professional help if you're uncomfortable working with 12V electrical connections or if you discover additional issues during the repair. If the pump housing shows signs of cracking or if replacing the pressure switch doesn't solve the continuous running problem, there may be internal pump damage requiring professional diagnosis.

Your strategy of keeping multiple repair options on hand is excellent for full-time RVers, especially in remote areas. Many RV service centers stock these parts now due to the common failure pattern, but having them with you eliminates the downtime and potential for helping fellow RVers as you mentioned.