What it is

Electrical "gremlins" are the faults that vanish the moment you put a meter on them at rest. The unit reads a perfect 244V sitting idle, every wire looks tight, and yet the compressor won't start sometimes, or trips on overload, or the system dies for no reason and comes back. These are almost always high-resistance connection problems and weak contactors — and the reason they hide is that resistance only causes trouble when current flows through it. A static voltage check at no load can't see them. You have to measure under load.

This is a root-cause discipline more than a single fix. A loose lug, a corroded spade, a backed-out wire nut, or a pitted contactor doesn't just cause its own symptom — the heat it generates can cook nearby components (that's why the capacitor or board "next to it" keeps failing). Finding resistance where there shouldn't be any is one of the highest-value skills on the truck because it kills callbacks and prevents fires.

How it works

A good connection has essentially zero resistance — current flows through it without losing voltage or making heat. A bad connection (loose, corroded, oxidized, pitted) has resistance. By Ohm's law, when current flows through that resistance, you get a voltage drop across it and heat at it (power = current × voltage drop). The bigger the current, the worse both get.

That's why these faults are load-dependent and intermittent:

  • At rest / no load: no current, no drop, no heat. Your meter reads full voltage everywhere. The fault is invisible.
  • Under load (compressor running): current flows, the bad connection drops voltage and heats up. The motor downstream now sees less voltage than it needs, so it draws more current (a motor is a constant-power-ish load), which makes the connection drop even more and heat even more — a runaway that can open the connection entirely, trip the overload, or weld/burn the contact.

A weak contactor is the same physics at the contact faces: pitting and oxidation from years of arcing add resistance across the closed contacts. The contactor "closes" but the contacts don't make good metal-to-metal contact, so there's a voltage drop across a closed contactor — and heat that pits it further. Eventually it won't pass enough current to start the compressor, intermittently at first.

In the field

The whole trick: measure under load, and look for voltage where there should be none.

  1. Take voltage readings with the unit running, not idle. Idle readings lie about these faults. Get the compressor pulling current, then measure.
  1. Measure voltage drop across each connection and the contactor — under load. Put your meter across a closed connection (both sides of a lug, both sides of the closed contactor, across a wire nut, across the disconnect): a good connection reads ~0V. Any meaningful voltage across a connection that's supposed to be solid means resistance there — that's your gremlin. More than a volt or two across a closed contactor or a lug under load is a problem.
  1. Check incoming voltage under load vs at rest. If voltage is fine idle (244V) but sags badly under load (into the 220s or below), there's resistance upstream — a loose lug at the disconnect, the panel, or the utility connection. Voltage that drops when the compressor pulls in is a connection telling on itself.
  1. Use heat as a finder. High-resistance connections run hot. Carefully feel (or use an IR thermometer) along the lugs, the contactor, the disconnect, and the terminals after a run. A connection noticeably hotter than its neighbors is the bad one. Discoloration, melted insulation, or a scorched lug is the smoking gun.
  1. Inspect and load-test the contactor. Pull the cover and look at the contacts — pitting, blackening, and erosion. Then measure the drop across the closed contacts under load. A contactor that drops significant voltage across closed contacts is weak even if it "works." Don't just look; measure.
  1. Wiggle-test under power (carefully). With the system running and respecting live-panel safety, gently flex the wiring at lugs, spades, and the contactor. If the unit hiccups or amps jump when you move a connection, you've localized a loose/corroded one.
  1. Tighten, clean, or replace — and find the collateral damage. Re-torque lugs to spec, clean corroded terminals, replace pitted contactors and burnt connectors. Then check whether the heat from the bad connection damaged a neighbor (the capacitor, the board, adjacent wiring) — the gremlin often takes hostages.

Normal values & targets

Orientation only:

  • Voltage drop across a closed connection (lug, wire nut, disconnect) under load: essentially ~0V. More than ~1–2V indicates resistance to fix.
  • Voltage drop across a closed contactor (line to load, same pole) under load: should be negligible. A drop of more than a volt or two means pitted/weak contacts.
  • Incoming voltage under load: should hold near nominal (~240V single-phase). A sag into the low-220s or below under load = upstream resistance (or a genuine utility problem).
  • Voltage imbalance (3-phase): legs should be within a small percentage of each other; a high-resistance connection on one leg shows as imbalance and overheats the motor.
  • Connection temperature: all similar connections should run at similar temps. One running hot = the bad one.
  • Compressor current: rises when supply voltage sags (motor pulls more amps at lower voltage) — high amps plus low voltage under load points at a connection, not the compressor.

Common faults & what they mean

  • Voltage drop across a closed contactor under load: pitted/weak contactor. Replace; look for what caused the arcing (short-cycling, voltage issues).
  • Voltage fine idle, sags under load: high-resistance connection upstream (loose lug at disconnect/panel/utility). Find it with drop measurements and heat.
  • A connection or lug running hot / discolored / scorched: high resistance there. Re-torque or replace; this is also a fire risk.
  • Compressor won't start intermittently, hums and trips: could be the contactor not passing enough current, low voltage from a connection, or a weak run capacitor — measure under load to tell which.
  • Repeated capacitor or board failures in one spot: heat from an adjacent bad connection cooking the neighbor. Find the resistance, not just the dead part.
  • Low voltage + high amps under load: the connection-resistance runaway. The motor's starved for voltage and overdrawing. Fix the connection, not the compressor.

Tech tips & gotchas

  • Measure under load — idle readings hide every one of these faults. No current, no voltage drop, no heat; the whole class of problem only appears when the compressor pulls amps. Single most important habit here.
  • Voltage across a closed connection IS the fault. A good lug or closed contactor drops ~0V. Real voltage there means resistance — probe across connections under load, not just to ground, and you've found it without guessing.
  • The bad connection takes hostages. Heat from a loose lug or pitted contactor cooks the capacitor, the board, or the adjacent wire. When a part keeps failing in one spot, suspect a resistance problem heating it — fix the cause, not the repeat victim.
  • A contactor that "works" can still be the problem. Pitted contacts pass some current — enough to look fine, not enough to reliably start a compressor under load. Measure the drop; don't just confirm it clicks in. And don't overlook the disconnect and whip — outdoor terminations corrode.
  • Re-torque to spec; don't just snug it. Over- and under-tightened lugs both fail. Aluminum especially loosens over time (cold flow) and needs proper torque and sometimes anti-oxidant.

Safety / code notes

  • All of this is live-panel work under load — use a meter and leads rated for the circuit, keep one hand clear, wear appropriate PPE, and be deliberate. Measuring drop under load means probing energized conductors.
  • A high-resistance connection generating heat is a genuine fire hazard, not just a performance issue — scorched lugs, melted insulation, and discolored terminals warrant immediate correction.
  • Re-torque connections to manufacturer/code-specified values; improper terminations (especially on aluminum) are a code and safety matter — see the applicable electrical code section for conductor terminations and torque requirements.
  • Bleed capacitors before handling motor terminals; a weak-contactor diagnosis still involves the run circuit.