Printed Circuit Board Relays

Did this ever happen to you? You're working on an electrical concern and you think a faulty relay maybe the culprit and while you may or may not hear the little critter clicking in the bussed electrical center (BEC), you can't quite put your finger on where it is. The diagram in the cover of the BEC may indicate that it is there and it is. Let's look a little closer.

Certain relays are soldered right to the circuit board of the  BEC and are not replaceable or serviced separately. In the wiring schematics, if the letters "PCB" are in the description of the relay, then the relay is part of the BEC printed circuit board. If an issue exists where one of the PCB relays is inoperative, then the BEC must be replaced as a unit. The photo below shows the circuit board with the relays attached. 

The picture below is an example of how the relay would be called out in a schematic. If your diagnosis leads you to a faulty relay and the BEC is to be replaced, make sure to follow the latest procedures in SI and use care to install the mating electrical connectors at the bottom of the BEC to avoid terminal damage.

Diagnosis;
 In most cases the BEC manufacturer will place the fuse in the circuit a PCB Relay controls after the switched contacts of that relay. This fuse becomes the test point for that relay. If a technician were to use the test lamp and touch the metal at the top of the fuse, and then operate the relay, the test lamp should either go out or come on depending on circuit operation. The illustration above is an example of the "downstream" fuse.

In some cases there is no fuse to use as a test point or there are two relays used and the circuit is protected in some other way. This can happen in circuits that use two relays to do a series or parallel type circuit, door locks, wiper speed or DRL lamps. Since there may not be a fuse, a "Test Point" may be placed in the floor of the BEC so the technician can probe through the floor of the BEC and watch the test lamp change state when the relay is operated. Test points are identified on the BEC label as "TP" (See the illustration above). As a technician working on a problem that may be related to a PCB relay, you may want to reference the Bussed Electrical Center views in SI . Navigate to them by going to "Power and Signal Distribution">"Wiring Systems and Power Management">"Component Locater">"Electrical Center Identification Views".  Once there, scroll down to the related BEC and look at its "Device Usage" to see if any PCB relays are involved.  In the illustration below you see that the Back Up Lamp and DRLs have fuses listed that a tech can be used as a test point. In the Fan Control PCB relay, no fuse is listed, the tech would then be referred to check at the TP provided in the BEC as part of the Global Diagnostic Chart. 

So that's the story on the missing relays - which of course are not really missing - they can be accessed and diagnosed if you know where...and how...

Mik Stubing - Lead Instructor GMTCNY