I wanted to share a recent “rediscovery” with you out of pure humility. I call it a rediscovery because it’s a basic fundamental I already knew, but it just hadn't come up recently enough to be top of mind until the other day.

I found myself troubleshooting a single-pole Mars 91311 contactor. I had voltage on the line side, but nothing on the load side. The wires were held on with crimp connectors, so I ended up pulling the crimp connectors off to do a continuity check. I started with the shunted side because I thought, obviously, there would be continuity there, but there was no beep.

After scratching my head for a minute, it finally dawned on me to check the actual terminal screws. Sure enough, the hex head screws were loose. Once I tightened them down, everything worked perfectly. That was my rediscovery: I was reminded, the hard way, just how absolutely critical tight electrical connections are.

Understanding the Mechanical Connection

To understand why this happens, it helps to look at how these mechanical connections are actually structured. The terminal assembly on a contactor isn't just one solid piece of metal; it’s an assembly of multiple components. On the fixed shunted side, you have the base terminal pad, the screw itself, and a small metal pressure plate (sometimes called a saddle clamp) that presses down on the spade terminals (the crimped ends of your wires).

Because this connection is built from separate stacked pieces, the entire electrical path relies completely on mechanical tension. If that single screw backs out even a fraction of a millimeter, the pressure plate lifts. Suddenly, your crimp connectors are just floating loosely between the plate and the base pad. If the screw is loose, the entire assembly is loose, and your solid electrical path is gone.

Below, you can see the complete terminal stack. When properly torqued, the screw, pressure plate, spade terminals, and base pad function as one solid electrical path. 

In the next picture, you can see the void that heats up. Even a microscopic gap under the pressure plate destroys mechanical tension, creating high resistance that starves the circuit of amperage and generates dangerous heat.

The Proof is in the Meter

Sometimes you just have to see it to believe it. I set up a quick demonstration to show exactly what happens when that pressure plate finally seats properly against the spade terminals.

The Torque Transformation: 

Watch the meter. With just a turn of the torque screwdriver, we go from an open line (OL) indicating a failing mechanical connection, straight to a closed, perfectly tight connection with next to no resistance (0.1 ohms). That near-zero reading is exactly what you are looking for.

The Breakdown: Why Loose Connections Trick Us

A loose terminal creates a point of high electrical resistance. This resistance acts like a bottleneck: it causes the amperage to drop, creates a significant voltage drop right at the connection, and generates a massive amount of heat as the energy tries to push through. Over time, that unchecked heat will melt wire insulation, scorch the terminal pad, and eventually cause a catastrophic failure. But initially, all of these symptoms can easily mimic a blown contactor.

Before you condemn a contactor on your next service call, go back to the basics:

• Tighten Before You Toss: Give the terminals a check. A few seconds with a nut driver can save your customer the cost of a new part and save you the hassle of a misdiagnosis.
• Torque to Spec: Don't just guess on the tightness. I checked the label right on the top of this contactor, and it specifically called for those hex head CU screws to be torqued to 25 in-lbs.

• Trust Your Meter, But Verify the Path: Always check for continuity and test the resistance across the contacts, but ensure the structural connections feeding those contacts are actually sound first.
• Perform a Voltage Drop Test: As an extra pro-level diagnostic, test for a voltage drop across the closed contacts while the system is running under load. Reading anything more than a few millivolts means you still have a bad connection.

Conclusion: Never assume a part is dead until you've verified the connections holding it together.

—JD Kelly