Suburban SH-42 Heater Wiring Diagram: Complete Thermostat Connection Guide

Quick Take

After replacing your Suburban SH-42 thermostat, proper wiring connections are critical for reliable operation. The SH-42 typically uses a two-wire thermostat connection - a red wire (R terminal) for 12V power and a white wire (W terminal) for the heat call signal. These connect to the corresponding terminals on your new thermostat. I'll walk you through the specific connection points and verification steps to ensure your replacement thermostat is wired correctly.

Common Causes

Suburban furnaces have several typical failure points that match your symptoms. Common issues include loose connections at the limit switch terminals, which are located near the heat exchanger and prone to corrosion from temperature cycling. These connections can work intermittently when disturbed, exactly like you experienced.

Another frequent problem is the sail switch connection becoming loose or corroded. This switch confirms proper airflow and is essential for startup - if it's making poor contact, the furnace will fail to ignite consistently. The control board connections are also suspect, particularly the thermostat wires and the 12V power feed.

Since you mentioned the unit started randomly while troubleshooting, thermal expansion or vibration likely restored a marginal connection temporarily. The wiring harness where it connects to the gas valve is another common failure point on these units, especially if the RV has experienced road vibration over the years.

Preparation

WARNING: Before starting any electrical work, turn off the propane at the tank and disconnect the 12V power to the furnace at the main panel or by pulling the appropriate fuse. Never work on gas appliances with the propane supply connected.

The furnace wiring consists of: 12V positive (usually red) from the fuse panel, ground (black or white), thermostat control wires (typically white and red), limit switch wires, sail switch wires, and the gas valve control wire. Download the furnace manual from Suburban's website for the complete wiring diagram for your specific model.

Have your phone's flashlight ready and consider using a small mirror to inspect connections in tight spaces. Take photos of all connections before disconnecting anything so you can reference the original positions. The control board is usually accessible from the front after removing the lower access panel.

Repair Steps

Start by removing the front lower access panel to expose the control board and wiring connections. With power still off, carefully inspect each wire connection for corrosion, looseness, or heat damage. Pay special attention to the limit switch wires - these are usually connected with spade terminals and are prone to working loose.

Check the thermostat connections at the control board. For the SH-42, the red wire from the thermostat connects to the 'R' terminal on the control board (12V supply), and the white wire connects to the 'W' terminal (heat call). Since you just replaced the thermostat, ensure these specific wires are properly seated in their designated terminals and making good contact. Use your multimeter to verify continuity through the thermostat when calling for heat - you should see continuity (closed circuit) between the thermostat terminals when the temperature is below the setpoint and calling for heat.

Locate the sail switch (the SH-42 uses a sail switch - a small paddle-shaped device in the blower housing) and check its wiring connections. This switch must close when the blower creates airflow, and loose connections here will prevent ignition. Clean any corrosion with non-flammable electrical contact cleaner in a well-ventilated area and ensure tight connections.

For the gas valve wiring, trace the wire from the control board to the valve itself. This is typically a single wire that provides 12V to open the valve during the ignition sequence. Check for voltage at this wire when the unit attempts to start - you should see 12V during the ignition cycle (typically appears after the blower starts and airflow is proven, and may only be present for 15-30 seconds during the ignition attempt). If connections look good but you're still having issues, the control board itself may be failing, which is common on older units.

When to Get Help

WARNING: Call a qualified RV technician immediately if you find gas leaks during your inspection, if the gas valve isn't getting proper voltage signals, or if you're uncomfortable working with propane appliances. Gas-related repairs require special tools and expertise to ensure safety.

If your troubleshooting reveals that the control board needs replacement, use the correct part for your model (verify with dealer) that typically costs $150-250 and requires careful wire-by-wire transfer. While it's possible to do yourself, the complexity of ensuring all connections are correct makes this a good candidate for professional service.

Also seek professional help if you discover damaged wiring in areas you can't easily access, or if the heat exchanger or limit switches need replacement. These components involve both gas and electrical connections in tight spaces, and improper installation can create serious safety hazards.