P261C - Coolant Pump 'B' Control Circuit Low
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the Coolant Pump 'B' Control Circuit Low (P261C). The signal is below the lower threshold, typically from a short to ground or an internally shorted sensor.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Coolant Pump 'B' Control Circuit Low – Permanent B+ Supply | Voltage (V) | 12.4 - 12.7 V (ignition OFF) / 13.5 - 14.7 V (engine running) | At module supply pins |
| Coolant Pump 'B' Control Circuit Low – Ignition-Switched Supply | Voltage (V) | 11.8 - 14.4 V | Ignition ON |
| Coolant Pump 'B' Control Circuit Low – Ground Connections Voltage Drop | Voltage (V) | < 0.1 V per ground point | Under load to battery negative |
| Coolant Pump 'B' Control Circuit Low – 5 V Reference Outputs | Voltage (V) | 4.95 - 5.05 V per circuit | Ignition ON, sensors unplugged one at a time |
At module supply pins
Ignition ON
Under load to battery negative
Ignition ON, sensors unplugged one at a time
Oscilloscope Waveform Analysis
500 ms/div, 5 V/divPossible Causes
Short to ground on the signal wire or an internal short circuit within Coolant Pump 'B' Control Circuit Low
Channel B:Verify dedicated wiring pins and signal pair for circuit B against the schematic.
Chafed or pinched wiring harness along the signal path to Coolant Pump 'B' Control Circuit Low
Poor ground connection or excessive voltage drop (> 0.1 V) at the ground point for Coolant Pump 'B' Control Circuit Low
Defective internal output driver or reference circuit in the control module for Coolant Pump 'B' Control Circuit Low
Symptoms
Recommended Solution
Compare signal voltage and resistance between circuit B and companion circuit A at module connector.
Measure signal wire resistance to chassis ground with connector unplugged (must exceed 10 kΩ; low reading confirms short to ground).
Verify loaded supply voltage and harness terminal pins at Coolant Pump 'B' Control Circuit Low.
Check circuit continuity and ground-side voltage drop (< 0.05 V) for Coolant Pump 'B' Control Circuit Low.
Inspect module enclosure and connector headers for water ingress, fretting corrosion, or PCB thermal damage.
Mechanic's Tip
When facing a 5V reference supply DTC, never replace the module first. A shorted external sensor (such as A/C pressure or EGR position) drags down the shared 5V internal rail. Unplug sensors one by one until 5V recovers.