Troubleshooting The Potential Starting Relay

November 3, 2002
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A simple ohmmeter is all that is needed to troubleshoot a potential relay. After taking all of the connecting wires off of the potential relay, measure the resistance across the 1 and 2 terminals. The resistance should read close to zero, since there are normally closed contacts between terminals 1 and 2. If the meter reads infinity, the contacts are stuck open, and the relay should be discarded and replaced. (See Figure 1.)

If the normally closed contacts between terminals 1 and 2 are dirty or arched and pitted, there is a chance the ohmmeter may read a resistance greater than zero but less than infinity. For example, if the ohmmeter reads 200 ohms, the potential relay should still be discarded because of dirty or pitted contacts.


Open contacts between terminals 1 and 2 will prevent the start capacitor from being in the circuit. The start capacitor is a high microfarad capacitor, which is in series with the start winding. (See Figure 2.) The start capacitor gives the start winding circuit more capacitance and gives the current a leading effect (phase shift) on the voltage.

The more the current leads the voltage wave, the more phase shift there will be, and the motor will have more starting power or torque. Without the start capacitor, the motor will usually lock its rotor and draw locked rotor amps (LRA). This will cause the motor’s overload to open and the motor will short-cycle on the overload. This short-cycling is detrimental to motor windings, starting relays, and capacitors because of its overheating effect.


Often, the relay contacts could be stuck or arched in the closed position. In this case, the start capacitor would never be taken out of the circuit. The motor would be stuck in its starting mode and draw high amps. The amp draw would be somewhere between running load amps (RLA) and locked rotor amps (LRA). The motor’s protective device would soon open. A short-cycling situation would occur.

If the contacts are stuck in the closed position, the relay would have to be checked with a voltmeter in the running mode, since the contacts between 1 and 2 are normally closed when not in operation. Once the motor is up and running, use a voltmeter to measure the voltage between terminals 1 and 2. A voltage reading of zero would prove that the contacts are not opening. (See Figure 3.) Also, a high amp draw from the start capacitor and start winding circuit still being energized is a telltale sign that the contacts have not opened.


After disconnecting all wires from the relay, ohm the coil between terminals 2 and 5. Since the coil should have a very high resistance, make sure you are using the proper scale on the ohmmeter. The (R x 100) scale is a good one to use. The R x 1 scale can fool a technician into believing that there is an open coil because of the coil’s extremely high resistance. It is not uncommon to have the resistance read in many thousand ohms.

If the ohm reads infinity on the (R x 100) scale, the relay coil is opened. The relay should be discarded and a new one installed. An opened relay coil will prevent the contacts between 1 and 2 from opening. This is caused from no magnetism in the iron core that the coil is wrapped around. This, again, will cause high amp draws. Always use the model number of the old relay when ordering a new one, or cross-reference it with a different manufacturer’s relay.

Tomczyk is a professor of HVAC at Ferris State University, Big Rapids, MI, and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached at (e-mail).

Publication date: 11/04/2002

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