What Does B1820 Mean?
This code indicates the Body Control Module (BCM) has detected an unintended voltage on the rear wiper motor relay coil control circuit that is consistent with battery voltage when the relay should be off. The BCM monitors the relay coil circuit and has found it shorted to a power source rather than being properly controlled by the module's internal driver. This prevents proper control of the rear wiper motor's up/park function.
Common Causes
45%
Damaged wiring harness with insulation worn through causing the relay coil control wire to contact a power wire or connector pin cross-connection
30%
Failed rear wiper relay with internal short circuit causing coil side to have constant battery voltage
15%
Water intrusion or corrosion in relay socket or BCM connector causing short circuit to adjacent powered terminal
10%
Failed Body Control Module with internal driver circuit shorted to power supply rail
Diagnostic Steps
1
Step 1: Perform visual inspection of rear wiper relay, relay socket, and all associated wiring harnesses for signs of damage, melting, corrosion, or water intrusion. Check connector terminals for bent pins or corrosion.
2
Step 2: Remove the rear wiper relay and use a multimeter to measure voltage on the relay coil control terminal at the relay socket with ignition on and wiper switch off. Reading should be 0-1V; battery voltage confirms short to power in harness or BCM.
3
Step 3: If voltage is present at relay socket, disconnect BCM connector and retest the relay coil control wire. If battery voltage persists, the short is in the wiring harness between BCM and relay; if voltage drops to zero, suspect BCM internal fault.
4
Step 4: If no voltage found at relay socket in Step 2, test the removed relay with an ohmmeter across coil terminals (should show 60-120 ohms typically) and check for continuity between coil and contact terminals (should be infinite resistance). Replace relay if shorted internally.
5
Step 5: Trace and inspect the relay coil control wire through its entire length, checking for chafing points against body panels, sharp edges, or contact with other powered circuits, paying special attention to areas near the liftgate hinge.
6
Step 6: After repairs, clear codes, cycle rear wiper through all functions including park position, and verify code does not return. Monitor BCM data stream to confirm proper relay control signal (ground pulse when activated).
Estimated Repair Cost
$100 - $450
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected an unintended voltage on the rear wiper motor relay coil control circuit that is consistent with battery voltage when the relay should be off. The BCM monitors the relay coil circuit and has found it shorted to a power source rather than...
The most common cause of B1820 (Wiper Rear Motor Up Relay Coil Circuit Short To Battery) is: Damaged wiring harness with insulation worn through causing the relay coil control wire to contact a power wire or connector pin cross-connection
Typical repair costs for B1820 range from $100 to $450, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving as this only affects rear wiper operation. Rear wiper may not function properly or may run continuously, reducing rear visibility in wet conditions. Repair at convenience but address before inclement weather.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B1820 to identify the root cause.
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Quick Info
Category
Body
System
Rear Wiper System / Body Control
Difficulty
Type
Manufacturer
Recommended Tools
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