Weil McLain 591-391-820 Wiring Harness
The Weil McLain 591-391-820 Wiring Harness is the main 9-pin control harness on the Weil-McLain GV Series 1 and 2 gas boiler. The Weil-McLain parts list (Fig. 6) describes it as a wiring harness with a 9 pin connector from the control module to the thermostat, limit switch, pressure switch and 24VAC gas valve, and notes that it is used with a White-Rodgers or Robertshaw gas valve and includes the block limit switch. In the photo you can see the white multi-pin connector at the control module end, a bundle of colored conductors, insulated spade terminals, a ring terminal, and the snap-disc limit device riding on the harness with its tag.
This is the harness that ties the safety circuit together. The thermostat call, the limit switch, the pressure switch and the 24VAC gas valve all land on this one assembly and meet at the control module through a single multi-pin plug. That is why a damaged 591-391-820 can look like several different failures: a boiler that will not start on a call for heat, a pressure switch that never seems to prove, or a gas valve that never gets power. Because the block limit is part of the harness, a bad block limit is fixed by replacing the harness rather than hunting for a separate limit.
Before you order, compare your harness to the photo. Count the pins in the plug, confirm the same ring and spade terminal layout, and check that a snap-disc limit is built into the harness. Do not confuse it with the 591-391-819, which is only the lead from the control module to the igniter. When you swap the harness, route it away from the blower housing, the jacket edges and any hot surface, seat every spade terminal fully, and test the circuit end to end before you leave. Look at the plug housing and the pressure switch terminals for heat damage while the harness is out, since a burned terminal at either end will take out the new harness too.
Key Benefits
- One Harness, Whole Control Circuit: Carries the thermostat, limit switch, pressure switch and 24VAC gas valve circuits to the control module through one multi-pin connector.
- Block Limit Switch Included: The snap-disc limit is part of the harness, so there is no separate limit to source.
- Listed for GV Series 1 and 2: Shown in the Weil-McLain GV Series 1 and 2 parts list.
- Works With Both Gas Valves: Weil-McLain notes it is used with a White-Rodgers or Robertshaw gas valve.
Common Questions
- What does the 591-391-820 connect to? The control module to the thermostat, limit switch, pressure switch and 24VAC gas valve, through a 9-pin connector, per the Weil-McLain parts list.
- Does it include the block limit switch? Yes. The parts list notes that the harness includes the block limit switch, and the snap-disc limit device is visible on the harness in the photo.
- How is it different from the 591-391-819 and 591-391-824? The 591-391-819 is only the control module to igniter lead. The 591-391-824 serves the dual circulators on the GV Series 3. The 591-391-820 is the control and safety circuit harness for the GV Series 1 and 2.