Other Articles - December - 2017

A Look at Wiring Diagram Changes

2A lot of technicians just aren’t comfortable with the newer format that many manufacturers use to display wiring information on their a schematics. It isn’t surprising: Schematics and their formats have changed significantly over the past few years.

For example, let’s look at the GM schematics and some common aftermarket applications, such as Mitchell and Alldata. Alldata and Mitchell receive their information from the manufacturer, which means the format and the schematic will be the same as the OEM’s, for the most part.

The schematic format has slowly transitioned over the past eight years to what you see today. Like most manufacturers, GM currently develops their schematics for use on a global scale. They write the service information once and then translate it into the language needed for each individual market.

Since a lot of information is lost in translation, the OEMs have moved to simplify and standardize their formats for worldwide use.

During development of the service information, modules and components receive a unique name and identification code. For example, GM now provides component codes as part of the schematic. They then use that code as part of the component identification and diagnostic process in the rest of the manual.

Understanding the component codes makes it much easier to use the diagnostic information and your scan tool.


The module/computer component code is a 3-digit descriptor. This 3-digit component code is used for everything from the schematic to reprogramming a module. Most of the module/computer codes start with the letter K.

Here’s a list of some of the common module/computer component codes:

K17 — Electronic brake control module
K19 — Suspension control module
K20 — Engine control module
K33 — HVAC control module
K34 — Glow plug control module
K38 — Chassis control module
K44 — Power takeoff control module
K67 — Trailer brake control module
K69 — Transfer case control module
K71 — Transmission control module
K111 — Fuel pump control module

…and there are many, many others. These designations appear throughout the service information and, in many cases, the service information lists the 3-digit code rather than the name of the module.


The components also have codes to help identify their functions and locations. Each type of component has its own general and specific component designator to identify the type of component.

In the schematics, each component has a base code followed by an extension to identify the position and quantity. For example, the schematic may display a B5LF to identify a left front wheel speed sensor, where:

B5 — wheel speed sensor
LF — its location

When running a diagnostic, the service information may not list the complete name B5LF; it may just list its name, such as B5. This is because the diagnostic process is common among all wheel speed sensors. The diagnostic procedure will typically specify something such as “appropriate part,” which means it’s up to you to figure out which wheel speed sensor to diagnose.

Remember, each component will have a different designation, so a wheel speed sensor may be B5 while a vehicle speed sensor may be designated as B14A or B115.

Inputs, such as sensors and switches, typically use a base ID of B followed by the component number and sometimes a position code. For example:

B12A — Pressure switch manifold
B13 — Transmission temperature sensor
B14A — Output speed sensor
B14B — Turbine speed sensor
B14C — Input speed sensor
B14D — Intermediate speed sensor
B15 — Internal mode switch IMS
B34 — Engine coolant temperature sensor
B74 — MAP sensor
B107 — Accelerator pedal position sensor

Outputs typically use a base ID of Q (Transmission and engine), T (Ignition coils), E (Lights), F (Air bag systems), G (Fuel/coolant pumps and fans) followed by the component and sometimes a position code. Some examples include:

Q17A — Injector 1
Q17B — Injector 2
Q23 — Line pressure control solenoid
Q27 — Most 6-speed applications
Q27A — Pressure control solenoid 1
Q27B — Pressure control solenoid 2
Q27C — Pressure control solenoid 3
Q77 — Most 8-, 9-, and 10-speed applications
Q77A — Pressure control solenoid 1
Q77B — Pressure control solenoid 2
Q38 — Throttle body motor
E1L — Accent lights
F107 — Steering wheel airbag
G10L — Cooling fan left
Example Q17A: Q — Output, 17 — injectors, A — injector 1


Many fuse blocks no longer label the fuse functions within the block. When looking at a fuse, you may see a schematic designation such as F12UA:

F — Fuse or circuit breaker
1 — Position/location within the fuse block
2 — Fuse block where the fuse is located.
U — Area
U — Engine compartment (underhood)
D — IP (Instrument cluster)
P — Passenger compartment; not in the IP. Could be in the glovebox or center counsel, as an example.
R — Rear body, such as the rear quarter panels or truck deck lid.
A — Number of fuse blocks in the area: A=1, B=2, etc.

So, in example F12UA: F — Fuse, 12 — within the fuse block, U — Fuse block located in the engine compartment, A — only one fuse block in this area.

Relays are very similar in designation. A relay will typically start with a module ID followed by an R and a relay number. For example, this may appear as KR75:

K — Module control
R — Relay
75 — Relay number

Fuse boxes or electrical centers are typically labeled using a base ID of X, such as X50A:

X — Fuse box
50A — Underhood fuse block electrical center

Looking at a schematic can be confusing. Hopefully, the next time you look at a GM schematic, you’ll have a clearer understanding of what the information is trying to present. In the future, we’ll tackle some other schematic formats, including your biggest challenge: VW/Audi.

Until next time, and remember, Quality isn’t an act; it’s a habit.