PACCAR · Engine Coolant Level Sensor CircuitMedium

PACCAR SPN 111 FMI 3: Coolant Level Voltage Above Normal

SPN 111 FMI 3 (FMI 3) — Engine Coolant Level - Voltage Above Normal, or Shorted to High Source

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Quick Answer

SPN 111 FMI 3 on PACCAR MX engines means the coolant level sensor circuit voltage is above normal, usually a short to 5V or power. The surge tank may be full while the ECM reads a fault. Verify physical coolant level and test signal voltage with the sensor unplugged.

Overview: PACCAR SPN 111 FMI 3

Coolant level warnings on PACCAR trucks should always start with a visual check of the surge tank. ProfessionalDieselRepair.com sees SPN 111 FMI 3 caused by scaled probes and rubbed harnesses more often than actual coolant loss.

Repair Snapshot

Time

1 - 2 Hours

Difficulty

Intermediate

Parts Cost

$45 - $220

Dealer Labor

$320+

Tools Required

  • PACCAR Davie4
  • Jaltest
  • Multimeter
  • Coolant Test Strips

What SPN 111 FMI 3 Means

SPN 111 FMI 3 on PACCAR MX-11 and MX-13 engines indicates coolant level sensor voltage above the expected range, typically a short to 5V or battery positive. The ECM may ignore low-coolant warnings or set false alerts, and some calibrations apply mild derate when level data is implausible. Verify physical surge-tank level before replacing the sensor or radiator cap.

Symptoms Drivers Notice

  • Low coolant level lamp illuminated with full surge tank
  • SPN 111 FMI 3 active in PACCAR Davie4 or Jaltest
  • Coolant level gauge pegged high or showing erratic readings
  • Possible inhibition of idle shutdown or fan strategies with bad level data
  • Fault present with engine off and key on

Most Likely Root Causes

Level sensor signal shorted to 5V or battery positive

Common

Chafed harness between the surge tank and battery box pulls signal voltage high.

Failed coolant level sensor or scaled float probe

Common

Mineral deposits or a shorted internal element send high voltage even with adequate coolant.

Connector moisture bridging supply and signal pins

Occasional

Glycol seepage into the Deutsch connector creates a conductive path between pins.

Common Misdiagnoses (Don't Waste Parts)

Adding coolant repeatedly for a voltage FMI 3

Why it happens: The dash lamp says low coolant.

Do this instead: Parts-cannon fluid top-offs do not fix electrical shorts. Verify surge tank level and unplugged signal voltage first.

Replacing the radiator cap instead of testing the sensor

Why it happens: Cap failures cause real low-level symptoms.

Do this instead: FMI 3 is voltage high, not valid low level. Test the circuit before cooling system parts.

Master Tech Commentary

Mechanic's Notes

ProfessionalDieselRepair.com reminds PACCAR operators that SPN 111 FMI 3 is not a low-coolant emergency if the surge tank is full. Float sensors fail from scale buildup; clean the probe before ordering parts.

Step-by-Step Diagnostic Procedure

  1. Step 1: Check the surge tank sight glass or dipstick with engine cold; level should be at the COLD FULL mark.
  2. Step 2: Connect PACCAR Davie4 and monitor SPN 111 voltage; above 4.8V with a full tank confirms FMI 3.
  3. Step 3: Unplug the level sensor at the surge tank. Key on, measure signal voltage; high reading unplugged means harness short.
  4. Step 4: Inspect the sensor float and probe for corrosion; MX engines use a resistive or voltage-divider style sensor in the tank neck.
  5. Step 5: Repair harness or replace sensor; typical resistance at full level is 80-300 ohms depending on sensor type. Clear code and verify level reading matches physical fill.

Live Data & Freeze-Frame Tips

In PACCAR Davie4, compare SPN 111 voltage to manual tank level during a controlled drain-and-fill. Voltage should change smoothly; a pegged-high reading with changing physical level confirms a sensor or harness fault.

Safety Warnings

  • Open the surge tank only when the system is cool to avoid scalding.
  • PACCAR MX systems hold pressure; use caution when removing tank caps.
  • Dispose of contaminated coolant per regulations.

When to Limp In vs Tow

Tow only if a separate SPN 110 overtemp or visible coolant loss accompanies the fault. FMI 3 alone with a full surge tank does not require towing.

Prevention Tips for Fleets & Owner-Operators

  • Use distilled water mix to reduce scale on level probes.
  • Inspect tank harness clips at every coolant service.
  • Clean the level probe during scheduled PM if readings drift.

Related Fault Codes

SPN 111 FMI 4 — Coolant Level

Voltage below normal; open signal or grounded level circuit.

SPN 111 FMI 1 — Engine Coolant Level Sensor

Valid data but low coolant level; real fluid loss rather than voltage high.

SPN 110 FMI 3 — Engine Coolant Temperature Sensor Circuit

ECT voltage high; may share harness routing on PACCAR chassis.

Frequently Asked Questions

What does PACCAR SPN 111 FMI 3 mean?

Coolant level sensor voltage is above the ECM expected range, typically an electrical short.

Is my coolant actually low?

Not necessarily. Verify the surge tank visually; FMI 3 is a voltage fault, not valid low level.

Will this trigger derate?

Some PACCAR calibrations apply mild derate when level data is implausible until repaired.

How do I test the level sensor?

Unplug and measure signal voltage key-on; high voltage unplugged means harness short. Check resistance at known fill levels.

Can scale on the probe cause this?

Yes, buildup can cause erratic or high readings; clean the probe before replacement.

What scan tool works for PACCAR?

PACCAR Davie4 is OEM; Jaltest also reads SPN 111 on MX engines.

More PACCAR codes on this system

All PACCAR fault codes

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