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
4
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.