RV Lights Dim When Water Pump Runs - Battery vs Wiring Fix

Tools & Parts Needed

Find these on Amazon: Digital Multimeter , Battery Load Tester , Battery Terminal Cleaner

What's Going On

This is a common issue, and here's the scoop. When your RV's 12V lights dim during water pump operation, it's usually caused by voltage drop in your electrical system. This can stem from weak batteries, poor electrical connections, or undersized wiring. Water pumps like SHURflo and Flojet models typically draw 4-8 amps, which can expose underlying electrical issues.

Why This Happens

Weak or Aging Batteries

RV batteries lose capacity over time, especially flooded lead-acid types. Batteries over 3-4 years old may not hold sufficient charge. Load test batteries or check specific gravity with a hydrometer. Replace if capacity has dropped below 80% of rating.

Poor Electrical Connections

Corrosion at battery terminals, loose connections, or oxidized wire joints create resistance. Clean all connections with a wire brush, tighten to manufacturer specs, and apply dielectric grease to prevent future corrosion.

Undersized or Damaged Wiring

Long wire runs with inadequate gauge cause voltage drop. Water pumps should use 12 AWG wire for runs up to 10 feet, 10 AWG for longer distances. Check for pinched, cut, or heat-damaged wiring.

Failing Converter/Charger

Progressive Dynamics, WFCO, and other converter units can fail, providing insufficient charging voltage. This forces the system to rely entirely on battery power, causing dimming under load.

Getting Ready

Before diving in, gather these items:

• Digital multimeter
• Battery hydrometer or load tester
• Wire brush or battery terminal cleaner
• Dielectric grease
• Basic hand tools (wrenches, screwdrivers)
• Safety glasses and gloves

Walking Through the Fix

Here's how to tackle this:

1. Test battery voltage: With the RV disconnected from shore power, measure battery voltage with a multimeter. Fully charged 12V batteries should read 12.6-12.8V. If below 12.4V, charge or replace batteries.
2. Check voltage under load: Turn on the water pump and measure voltage at the battery terminals, then at the pump itself. More than 0.2V difference indicates wiring or connection problems.
3. Inspect battery connections: Look for corrosion, loose terminals, or damaged cables at both battery terminals. Clean with wire brush and apply dielectric grease.
4. Test converter/charger: With shore power connected, voltage should read 13.2-14.4V at batteries. Lower readings indicate converter issues.
5. Check ground connections: Ensure solid ground connections from battery negative to frame and from pump to ground.
6. Examine wire gauge: Water pumps should have 12 AWG wire for runs up to 10 feet, 10 AWG for longer distances. Undersized wiring (16 AWG or smaller) will cause voltage drop.

Situations That Need a Professional

Contact a certified RV technician if:

• You're uncomfortable working with electrical systems
• Wiring needs to be run through walls or under the RV
• The converter/charger unit requires replacement
• You discover evidence of electrical burning or melting
• Multiple electrical systems are malfunctioning simultaneously