Handbook 652 Body Electrical Diagnosis Junction Blocks and The wiring for most vehicle circuits passes through one or more junction blocks (J/Bs) or relay blocks (R/Bs) . J/Bs and R/Bs are often convenient test points because they contain connections to so many circuits. Understanding how to find those test points for a particular circuit is a critical skill in diagnosing electrical problems. Relay Blocks Junction Blocks Junction blocks distribute power and ground to various circuits. Inside the junction block, layers of insulated, solid metal plates create internal connections between the pins of different connectors.
This eliminates the need for many additional splices, and improves reliability. On the wiring diagram, junction blocks are color-coded gray. Note the diagram does not show the entire junction block. It shows only the part of the junction block that affects the circuit. Connections to the J/B are indicated with an oval . Inside the oval is a number or letter followed by a letter which together are the J/B ID . The number part of the ID represents the J/B location. For example: 1 = Junction Block #1 2 = Junction Block #2 I = Instrument Panel (LH) NOTE Don ’ t confuse the letter I with the number 1. J/B ID IF and 1F are in different junction blocks. Technical Training 77