4. Now check the voltage on the starter solenoid. This acts as a relay for the compressors. Never run your compressors directly to the battery because of their amperage draw.
Excessive Resistance
A volt-ohm meter measures voltage potential and can be used to measure resistance in an operating circuit. Learning to do a voltage-drop test will help you solve the majority of your electrical problems. Some tests are easier if you have another person help. To test for a voltage drop, don't disconnect or remove any wiring or components in the circuit you're testing. This test is used to isolate excessive resistance in the wiring leading to and from your load. To check the hot side of your circuit, hook the positive lead of your voltmeter to the positive side of your battery. Hook the negative lead of your voltmeter to the hot side of your load. Operate the circuit and read the voltage displayed on your voltmeter. For high-current wires (battery cables), you should read less than 0.2 volts. Allow 0.1 volts maximum for every switch or relay contact in the circuit located between your test leads. All other wiring should read less than 0.1 volts (some fuel-injected circuits are less). To check the negative side of your circuit, hook the positive lead of your voltmeter to the negative side of your load and the negative lead of your voltmeter to the negative side of the battery or a known good ground. Operate the circuit and read the voltage on your meter. Any voltmeter readings above 0.2 volts while attempting to operate the circuit being tested indicates excessive resistance. Use your voltmeter to locate excessive resistance by isolating sections of the circuit between your test leads. If you isolate a broken wire between your test leads and attempt to operate your circuit, your Voltmeter will read battery voltage.
5. The solenoid is getting voltage on both the compressor side and the positive battery side.
Short Circuits
Short circuits show up as blown fuses, melted fusible links, and burned wiring. Never replace burned fuses with a fuse of a higher rating. Short circuits are easy to identify, but not always easy to locate. Something may be grounding out because it's been burned through or melted to another wire. Using a test light between a blown fuse can help locate the short. When the short circuit is present, the light will glow at full intensity. If the short is intermittent, the circuit should work normally and the light will not glow at full intensity until the short occurs. You can locate most intermittent shorts by wiggling the wiring in the suspected circuits. This is where a wiring diagram comes in handy, as some fuses protect more than one circuit. While the test lamp glows at full intensity, start disconnecting, one at a time, the different loads in that circuit. When you disconnect the offending load or wiring connector from a wire harness and the test light goes out, your short circuit is down the line from that point.
6. Next, we jumped the solenoid to see if the compressors were bad. The compressors turned on (ah, the sound of air). Since the solenoid was getting voltage and worked when jumped, the solenoid was our faulty part and the reason why the compressors wouldn't turn on.
Faulty Parts
By using the methods above with the appropriate repair manual and wiring diagrams, you can check all the wiring in your vehicle. These tests will help you determine whether your load or wiring is faulty. If you suspect your load device as the cause, the repair manual should be able to provide you with specific vehicle-testing methods. Some manuals will not give specific testing instructions; they might tell you to substitute a known good part. If you conduct your diagnosis properly, you can feel safe buying that expensive, nonreturnable electrical part.
7. After making a run to AutoZone to pickup a Ford starter solenoid and installing it, our compressors were purring like new.
Poor Grounds
Poor grounds cause many different problems. Dim lights, slow or no cranking, and poor battery charging are classic indications of poor grounds. Use your voltmeter to do a voltage-drop test to check for poor grounds. Hook the negative lead of your voltmeter to the negative side of the battery and the positive lead to a good ground location on your engine block and note the voltage while cranking the engine. Check the body ground by moving your positive test lead to the body and turn on all lights and accessories, noting the voltage reading. Anything above 0.2 volts indicates excessive resistance in the ground connection or ground cable.