B2559

Code

Other B — Body

Aux A/C Blower Motor Relay Circuit Short to Battery

Brand: Other

Type: B — Body

Views: UK: 28 EN: 40 RU: 23

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Damaged wiring (chafing, pinched, melted) causing relay control wire to contact battery voltage source
Shorted relay socket or terminals with battery feed
Failed blower relay with internal short between coil and battery feed
Faulty control module (HVAC control module / Body Control Module) sourcing battery voltage incorrectly
Corroded or loose connector, aftermarket accessory wiring tied into circuit

Symptoms

Auxiliary A/C blower runs continuously or at full speed
Loss of blower speed control or unable to turn blower off
Blown fuse for blower/relay or repeated fuse failures
Battery drain with ignition off (if relay stuck closed)
DTC stored, HVAC system may show limited function

What to check

Visual inspection of relay, relay socket and immediate harness for melted insulation, pinched wires, or signs of arcing
Check related fuses for blown condition and inspect fuse holder and wiring
Identify wiring colors and trace relay control wire back toward HVAC module or BCM using wiring diagram
Swap known-good identical relay (if available) to see if behavior changes
Check for recent aftermarket installations (alarms, audio, lighting) spliced into blower or relay circuit

Signal parameters

Battery voltage on relay supply terminal: ~12–14.5 V (key on)
Expected relay control voltage when OFF: near 0 V (pulled low or open, depending on design)
Expected relay control voltage when ON: near battery voltage (~12 V)
Blower motor current draw (when running): varies by vehicle; typically 2–15 A depending on motor and speed
Short to battery condition: control wire measured at ~12 V regardless of command state

Diagnostic algorithm

Obtain vehicle wiring diagram and identify relay terminals: coil + (battery feed), coil - (module/control), and blower power circuit.
With ignition off, disconnect battery if you will be disconnecting connectors or replacing parts. For voltage checks leave battery connected and take care with live circuits.
Visually inspect relay, socket and harness for heat damage, melted insulation, pinched or chafed wiring. Repair obvious damage.
With connector reconnected and ignition ON, backprobe the relay control terminal. Command blower OFF/ON from HVAC controls and observe control terminal voltage. If control terminal is ~12 V when it should be low or floating, suspect short to battery or faulty module.
Remove relay and measure continuity between control terminal and battery feed terminal at the socket. There should be no direct continuity. Continuity indicates shorted socket or melted terminal bridging contacts.
If relay/socket appear okay, disconnect the control module connector and measure voltage on the control wire. If battery voltage disappears with module disconnected, the short is downstream (wiring/connector). If voltage persists, the module may be sourcing battery voltage — consider module diagnostics.
If wiring/connector suspect, perform wiggle tests while monitoring voltage to locate intermittent shorts. Repair or replace damaged sections and protect with conduit/loom.
After repairs, clear DTCs and verify normal operation across multiple blower settings and ignition cycles. Verify no parasitic draw with ignition off if relay was sticking closed.
If module is suspected after wiring and relay checks, follow manufacturer procedures for module diagnosis or replacement; avoid replacing module without confirming wiring/relay integrity.

Likely causes

Shorted or repaired wiring near relay or harness (most common)
Faulty relay with internal short to battery
Connector contamination/corrosion at relay socket
Aftermarket accessory or prior repair improperly spliced into circuit
Failing HVAC/BCM output stage (less common)

Fault status

⚠️ Status

Stored DTC indicates the auxiliary A/C blower motor relay control circuit is detecting a short to battery voltage. The control circuit is being held at or near battery potential when it should be switching low/open; relay may be stuck energized. Further isolation of wiring, relay and control module required.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours