SPN 1639: Fan Speed

POSTED IN pcodes

Updated on: August 23, 2024

Introduction

Fault Code SPN 1639 is related to issues with the engine fan speed on vehicles equipped with electronic engine control modules (ECM). When this code is triggered, the ECM has detected a problem with the fan speed signal, indicating the cooling fan is not operating as expected. While SPN 1639 requires prompt attention to prevent potential engine damage from overheating, in most cases it is safe to continue driving cautiously while monitoring coolant temperature until the issue can be properly diagnosed and repaired.

In our workshop, we've encountered SPN 1639 fault codes across various European car makes, with Volvo, Mercedes-Benz, and MAN trucks being the most frequent offenders. We've noticed this issue is particularly common in diesel engines equipped with SCR systems. From our experience, the fault is often triggered by a faulty NOx sensor, but we've also seen cases where wiring issues or exhaust leaks were the culprits. Interestingly, we've found that Mercedes vehicles tend to be more sensitive to this fault, often requiring sensor replacement even when readings are only slightly out of spec. In contrast, Volvo trucks seem more resilient, sometimes clearing the code after a few drive cycles without intervention. Overall, while SPN 1639 isn't exclusive to European vehicles, we've definitely seen it more often in these makes compared to their American or Asian counterparts.

Understanding SPN 1639

SPN 1639 stands for "Suspect Parameter Number – Fan Speed". It is a diagnostic trouble code indicating the ECM has detected an abnormal signal from the fan speed sensor or an issue with the fan speed control circuit. This code is commonly seen on heavy-duty trucks and buses equipped with diesel engines, such as Freightliner, Volvo, Mercedes-Benz, and MAN. The specific fault criteria and effects can vary between engine manufacturers and models.

Parts and Their Roles

The primary components involved with SPN 1639 are:

Engine Cooling Fan: A hydraulically or electrically driven fan mounted on the radiator to pull air through the radiator and cool the engine coolant. Typical fan speeds range from 1000-2500 RPM.

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Fan Speed Sensor: Monitors fan RPM and sends a frequency signal to the ECM. Common sensor types include Hall effect, variable reluctance, and AC pulse generators.
Fan Drive/Clutch: Controls fan speed by engaging/disengaging based on ECM commands and temperature parameters. Designs include viscous, pneumatic, and electric clutches.
ECM: Controls the fan drive to regulate fan speed based on coolant temp, intake air temp, A/C pressure and other inputs.

Common Causes

Potential causes for SPN 1639 include:

Faulty fan speed sensor providing an intermittent, erratic, or out-of-range signal
Wiring issues like opens, shorts, or high resistance in the fan speed sensor circuit
Malfunctioning fan clutch not engaging or slipping excessively
Low coolant level or air in cooling system causing fan to cycle excessively
Restricted airflow through radiator/charge air cooler due to debris, damage, or improper shrouding
Worn/stretched fan drive belt preventing fan from reaching commanded speed

Diagnosis and Troubleshooting
When diagnosing SPN 1639, follow these steps:

Connect a scan tool and verify SPN 1639 is active. Record freeze frame data if available.
Visually inspect fan drive belt condition and tension. Replace if worn, glazed, or cracked.
Check coolant level and test cooling system pressure to rule out leaks or air intrusion.
Using a DVOM, perform a voltage drop test on the fan speed sensor supply and signal circuits. Repair wiring if voltage drop exceeds 200mV.
With the key on, engine off, command the fan on with a scan tool and verify fan engagement. If fan doesn't engage, suspect faulty fan clutch.
Unplug the fan speed sensor and inspect for damage or contamination. Measure sensor resistance and compare to specification (typically 1-2 kΩ).
If all wiring and components test OK, connect an oscilloscope to the fan speed signal and verify signal frequency matches commanded fan speed. Replace sensor if signal is erratic or flat lines.

Common related fault codes include EGR Valve Position, Coolant Temperature, and Intake Air Temperature Sensor codes. These should be diagnosed using the manufacturer's troubleshooting procedures as they can cause or contribute to fan speed issues.

Repair Solutions

After identifying the root cause, perform the following repairs as needed:

Replace faulty fan speed sensor and route wiring away from heat/abrasion sources
Clean electrical connections and apply dielectric grease to prevent corrosion
Replace fan clutch assembly if slipping or slow to engage
Adjust fan belt tension to spec or replace stretched/damaged belts
Clean debris from radiator/charge air cooler fins and verify proper fan shrouding
Bleed air from cooling system and refill with coolant to proper level

Use OEM replacement parts where possible for best reliability. Torque fasteners to the manufacturer's specifications. After repairs, clear fault codes and verify fan speed signal and operation are within specs.

Repair Costs

Typical parts costs for SPN 1639 repairs include:

Fan Speed Sensor: $120-$250 USD / €100-€225 EUR
Fan Clutch Assembly: $400-$800 USD / €350-€700 EUR
Fan Drive Belt: $30-$60 USD / €25-€50 EUR

Labor times range from 1-3 hours depending on the specific repair and accessibility. Diagnostic fees are typically 1.0-2.0 hours in addition to repair labor. Additional costs may include coolant, shop supplies, and hazardous waste disposal.

Prevention Advice

To minimize occurrences of SPN 1639, adhere to the following maintenance practices:

Inspect fan drive belt condition and tension every 6 months/50,000 km (30,000 mi)
Replace fan drive belt every 2 years/250,000 km (150,000 mi)
Clean radiator and charge air cooler fins at every oil change
Check coolant level and test coolant condition annually or every 100,000 km (60,000 mi)
Inspect fan clutch and speed sensor wiring and connectors at every PM service

Frequently Asked Questions About SPN 1639

What does SPN 1639 mean?

SPN 1639 is a diagnostic trouble code that indicates a problem with the engine cooling fan speed. It stands for “Suspect Parameter Number – Fan Speed.”

What are the symptoms of SPN 1639?

Common symptoms include the check engine light illuminating, engine overheating, fan running constantly at high speed, or fan not running at all. You may also notice poor engine performance or reduced fuel efficiency.

Is it safe to drive with SPN 1639?

It depends on the severity. If the engine is overheating, stop driving immediately to prevent damage. In some cases, you can drive cautiously for a short distance while monitoring engine temperature, but get the issue diagnosed promptly.

What causes SPN 1639?

Potential causes include a faulty fan speed sensor, wiring issues in the fan speed sensor circuit, a malfunctioning fan clutch, low coolant level, a stuck thermostat, or a problem with the ECM (Engine Control Module).

How is SPN 1639 diagnosed?

Diagnosis typically involves scanning for trouble codes, visually inspecting wiring and connectors, testing the fan speed sensor signal with a multimeter or oscilloscope, and checking the cooling system for leaks or blockages.

Can SPN 1639 be caused by a bad ground connection?

Yes, a poor ground connection on the fan speed sensor circuit can cause intermittent or erratic sensor readings, triggering SPN 1639.

What is the location of the fan speed sensor?

The location varies, but it’s usually mounted on the fan clutch assembly or near the radiator. Consult a vehicle-specific repair manual or wiring diagram to pinpoint the exact location.

How much does it cost to fix SPN 1639?

The cost depends on the root cause. Expect to pay between $80-$150 USD for diagnosis. A replacement fan speed sensor runs about $200-$400, while a fan clutch is $400-$750, plus 2-3 hours labor. Total costs could range from $300-$1500.

Can a blown fuse cause SPN 1639?

Yes, if the cooling fan circuit fuse is blown, the fan won’t operate, potentially triggering SPN 1639. Always check fuses early in the diagnostic process.

Is the fan speed sensor the same as the coolant temperature sensor?

No, these are separate sensors. The fan speed sensor measures fan RPM, while the coolant temperature sensor measures engine coolant temperature. However, both can cause cooling system issues if faulty.

How do you test a fan speed sensor?

To test it, unplug the sensor and check for reference voltage (5V or 12V) on the power wire. Also test continuity between the ground wire and a good chassis ground. Finally, spin the fan by hand and check for a pulsing signal on the signal wire.

What is the normal resistance range for a fan speed sensor?

This varies by application, but most fan speed sensors have a resistance between 1-2 kΩ. Consult the manufacturer’s specifications for exact values.

Can a bad fan clutch cause SPN 1639?

Yes, if the fan clutch is slipping excessively or not engaging fully, the fan may not spin fast enough to control engine temperature, resulting in SPN 1639 being triggered.

How often should the engine cooling fan be inspected?

Inspect the fan, clutch, and related components at every oil change or 12 months, whichever comes first. Look for cracks, bent or missing blades, and excessive play in the clutch.

Can low coolant cause SPN 1639?

Yes, a low coolant level reduces the cooling system’s capacity to dissipate heat, causing the fan to run excessively to compensate. This can trigger overheating and SPN 1639.

How do you troubleshoot a fan speed sensor circuit?

Start by checking for blown fuses and inspecting wiring for damage. Measure reference voltage, ground signal, and sensor output with a multimeter or oscilloscope. Compare your readings to the manufacturer’s specifications.

What other components can cause cooling fan issues besides the fan speed sensor?

Other potential culprits include the coolant temperature sensor, thermostat, radiator, water pump, ECM, fan relay, and wiring harness.

Can a malfunctioning ECM cause SPN 1639?

Yes, a faulty ECM can send incorrect commands to the cooling fan or misinterpret signals from the fan speed sensor, leading to SPN 1639. This is usually diagnosed by ruling out other causes first.

Are there any TSBs (Technical Service Bulletins) related to SPN 1639?

Possibly, depending on the vehicle make and model. Check with the manufacturer or search online databases for TSBs that address cooling fan issues or SPN 1639 specifically for your application.

How can I prevent SPN 1639?

Prevention measures include:

Regularly inspecting the cooling system and fan components
Keeping the coolant at the proper level and concentration
Replacing the thermostat and coolant temperature sensor as preventive maintenance
Promptly repairing any coolant leaks
Cleaning debris from the radiator and fan shroud
Following the manufacturer’s recommended maintenance schedule