Power Centers are all about power. Not horsepower but routing electrical power through the various circuits and accessories in the vehicle's electrical system. Think of the Power Center as a fuse box on steroids. And because it is the heart of the entire electrical system, you can use it as a handy access point for checking circuit voltages, resistance and continuity.

Compared to the random locations of fuses and relays throughout the electrical system in older vehicles (those built up until the mid- to late-1980s), the Power Center is a welcome improvement. With most of the relays and fuses grouped together inside one or two plastic boxes, and labeled so you can identify the fuses and relays inside, the Power Center provides a great place to start your electrical diagnosis -- even if the vehicle still has a traditional fuse panel under the instrument panel.

WHAT FUSES DO

Fuses are circuit protection devices. Up until the 1980s, most vehicles used snap-in style fuses with a short piece of wire inside a hollow glass tube. In newer vehicles (1980s and up), blade-style transparent plastic fuses are used (both "mini" and "maxi" sizes). With both styles, the wire inside the fuse is designed to melt if the current passing through the fuse exceeds its rated amperage. When the fuse "blows," it opens the circuit and stops the flow of current to protect the circuit from dangerous overloads that might otherwise damage components or start a fire.

Mini-fuses are typically used for circuits that normally handle no more than 20 to 30 amps. Maxi-fuses are typically used for circuits that carry higher amp loads (40 to 120 amps). The number on the fuse is its maximum amp rating. Most fuses are the push-in spade type, but some larger capacity fuses may be bolt-in.

A blown fuse can be identified by visually inspecting the fuse. This may require removing the fuse from its holder. If the wire inside the fuse is broken, the fuse has failed and must be replaced with one that has the exact same amperage rating as the original.

Warning: Never substitute a fuse with a higher amp rating as doing so may result in circuit damage or a fire! Also, never remove or install a fuse when a circuit is on.

A "fusible link" is another type of circuit protection device that functions the same as a fuse (it melts when the load exceeds the rated current). The only difference is that a fusible link is a length of special wire that is permanently wired into a circuit or wiring harness. If it fails, the section must be cut out so a new fusible link can be spliced in. You can usually spot a failed fusible link by looking for blistered insulation around a wire. In most newer vehicles, high power fuses have replaced fusible links. This makes both access and repair much easier.

A "circuit breaker" is another type of circuit protection device that may be used in electrical circuits (such as the headlights or wipers) that may experience occasional overloads. Think of a circuit breaker as a fuse that can reset itself. Instead of a wire that melts if it gets too hot, a circuit breaker uses a heat-sensitive bimetal contact arm and contact points to open a circuit if it overloads. After the circuit breaker cools down, the contacts re-close and current is restored. There are also circuit breakers that must be manually reset by pressing a pin or button on the unit.

WHAT RELAYS DO

A relay is a switching device that uses a small electrical current to control a much larger current in a second circuit. In other words, a relay routes power to a circuit or component when it is turned on.

Relays are typically used for components that draw a large current such as the headlights, rear window defogger, fuel pump, A/C compressor clutch, cooling fan(s), heater & A/C blower fan, ABS system, the ignition circuit, even the power windows, seats and horn.

A relay is nothing more than a small rectangular box (usually plastic but may also be metal) with a magnetic coil, armature and set of contact points inside. There are typically four or five spade terminals on the bottom of the relay, and the cover may or may not have a simple wiring schematic or other identification printed on it.

When voltage is applied to the coil inside the relay, the coil creates a strong magnetic field and pulls the armature down to close the contact points. This allows voltage to pass through the output side of the relay to the device it controls.

There are three basic types of relays:
•Normally open are the most common type. The armature closes when the coil is energized to route power to a circuit or component.
•Normally closed. The armature is normally closed and is pulled open when the relay is energized to turn OFF a circuit or component.
•Dual relay. This type of relay routes power one way when it's off and another way when it is on.
Many vehicles use more than one type of relay in a particular circuit (such as the A/C compressor clutch circuit), and may use the same relay to control more than one device.

WHEN RELAYS FAIL

Relays that carry high loads and are constantly switching on and off have higher failure rates than relays which are seldom used or only carry low amp loads.

If a relay fails, it will prevent power from reaching the device it controls. In the case of a fuel pump relay, a failed relay will prevent the engine from starting because there will be no fuel pressure. If a cooling fan relay fails, the engine may overheat because the electric cooling fan never comes on. If an A/C compressor clutch relay fails, the compressor won't engage and there will be no cold air from the A/C.

On most Chrysler vehicles, for example, the cooling fan relay must be engaged before power can go to the A/C compressor clutch. If the fan relay has failed, it will prevent both the fan and the compressor clutch from operating.

DIAGNOSIS

If an electrical component isn't working, the first thing that should always be checked is the wiring circuit that provides power to the component. Start with the fuse(s). If a fuse has blown, chances are the problem isn't just the fuse but a short or electrical overload in the circuit or device it protects. Replacing the fuse may only fix the problem temporarily -- and if the new fuse blows immediately, it means there is a serious electrical problem that will require further diagnosis.

If all the fuses are intact and there is no power to a component, the relay(s) for that component should be the next item checked. In many instances, a suspicious relay can be bypassed with a fused jumper wire to see if rerouting power restores proper operation of the device (as when bypassing the fuel pump relay to see if the fuel pump will run). If bypassing the relay restores normal operation, the relay is at fault and must be replaced.

If bypassing the relay changes nothing, further diagnosis of the power supply and ground circuits as well as the wiring harness and circuit components will be necessary to find the fault. Study the vehicle's wiring diagram. Trace the power flow from the battery though any switches or relays to the component and ground. Then use a multi-meter to probe the circuit between the Power Center and battery ground using the fuse and/or relay terminal connections as your point of access.

MORE WAYS TO CHECK RELAYS

The status of many relays can be checked using a scan tool. Depending on what kind of data is available through the Diagnostic Link Connector (DLC), you may be able to view the "switch status" of the circuit in question (the cooling fan, for example). If the PCM is commanding the fan on but nothing is happening, you would know the command side of the circuit is working but that the voltage isn't getting through to the fan. If the scan tool and engine management system provide actuator tests, you can run the tests using the scan tool to see if the commanded function happens or not.

A relay can also be checked with an ohmmeter to see if the coil is within specifications, and the contact points provide continuity when the coil is energized. As a rule, most relay coils should measure 40 to 80 ohms -- but always look up the specifications to be sure. If resistance is higher than normal, the coil may still be working but it is failing, or it may not work when electrical loads are high. If the coil has no resistance, it is shorted and has failed. An infinite resistance reading would tell you the coil is open. Replace the relay.

Another quick check is to remove and shake the relay. If you hear anything rattling inside, it means the armature is broken and the relay needs to be replaced.

Substituting a "known good relay" for one that is in question is another technique you can use to see if a relay is at fault. In many instances, identical relays are used for different circuits. Temporarily swapping relays, therefore, will tell you if the relay is working or not.

Tapping on a relay that is "sticking" may get it to work, but if it is sticking it should be replaced.

Sometimes a relay that is supposed to open will stick in the closed position when the ignition is turned off. This will create a current drain that may run down the battery overnight. One way to find this kind of problem is to measure the parasitic current drain on the battery when the ignition and all accessories are off. This can be done at the battery or Power Center. If the "key off" load exceeds specifications (typically no more than about 50 milliamps), the vehicle may have a relay that's sticking. By pulling the fuses one by one, you should be able to isolate the circuit that is drawing the current.

SOME PATTERN FAILURES

• Cooling fan relays on Chrysler minivans (late 1980s and early 1990s). These often fail as a result of overheating.
• Chrysler LH cars. These vehicles have two fans and two fan relays (low speed and high speed). The low speed fan relay often fails, leaving only high speed fan under certain operating conditions.
• The fuel pump relay on various Saturn car models can fail causing a no-start condition. The relay is usually located on the fuse box inside the vehicle on the side of the radio consol
• Ford cooling fan controllers (late 1980s and early 1990s Mustangs and others models). The controller contains a relay for the radiator fan, a relay for the A/C condenser fan, and relay for the A/C compressor clutch. When both fans are on, they may pull so much amperage from the circuit there isn't enough left to fully engage the A/C compressor clutch. The cure here is to add a separate relay for the compressor clutch so it can receive full voltage from the battery.
• Late model Fords with an "integrated relay control module" (IRCM). This module contains two fan relays, a relay for the A/C compressor clutch and a relay for the fuel pump. Heat may cause the module to fail, resulting in no cooling fan(s), no A/C operation and or no fuel depending on which internal component has failed. The relays cannot be replaced separately on this application, so your customer will need a whole new module.
• 1996 GM FWD minivans. Two blower relays are used (low and high speed). If a vehicle has only low or high blower speed, the cause is probably a failed low or high speed blower relay, not the blower motor resistor block.

RELAY REPLACEMENT

If a relay has failed, it must be replaced with the correct part. Two different relays may look the same on the outside but be wired differently on the inside or be rated to handle different loads. Installing the wrong replacement relay may result in damage or premature failure.

When identifying a relay, you may have to refer to the OEM part number on the relay as well as the year, make and model of the vehicle application.

Never remove or install a relay in a circuit that is powered, as doing so may create a voltage spike that can damage the relay or other electrical components. It's also a good idea to apply some dielectric grease to the relay terminals to prevent corrosion.