FAQ
TL;DR: E39 blower-speed faults usually come from the FSR “hedgehog” control: verify 12 V on the thin control wire; “On one thin cable you must have 12 volts.” Check fuses and signal before blaming the climatronic. [Elektroda, melas, post #16901619]
Quick Facts
- Fuse No. 8 behind the glovebox is part of the blower/FSR feed; verify it first. [Elektroda, malina156, post #16892377]
- Forcing the hedgehog’s heatsink to ground drives the fan at full speed, proving the motor and power feed. [Elektroda, malina156, post #16903067]
- Matching FSR part numbers avoids compatibility issues; users recommend buying the same number. [Elektroda, sebap, post #16896173]
- The thin control wire should show ignition 12 V and vary with climatronic commands. [Elektroda, melas, post #16901619]
- BMW WDS wiring diagrams help trace the IHKA-to-FSR signal path. [Elektroda, melas, post #16901619]
What is the E39 “hedgehog” and what does it control?
It’s the Final Stage Resistor (FSR) with a finned heatsink that modulates blower speed. The climatronic (IHKA) sends a control signal to the FSR, which then regulates power to the cabin fan. A grounded heatsink makes the blower run full speed. [Elektroda, melas, post #16894610]
How do I diagnose no airflow control when the blower still spins?
Do three checks. 1) Verify blower fuses (including No. 8). 2) Momentarily ground the hedgehog’s heatsink; full-speed means motor and power are good. 3) With ignition on, measure the thin wire for 12 V and variation while changing fan speed. [Elektroda, melas, post #16901619]
Does the FSR need identical part numbers on a BMW E39?
Yes. Owners advise buying a hedgehog with the same number to avoid incompatibilities. This ensures the control circuitry matches the IHKA signal and avoids erratic speeds or no response. “Buy a hedgehog with the same number and don’t fret.” [Elektroda, sebap, post #16896173]
Where is the hedgehog located on an E39 sedan?
It sits inside the cabin HVAC housing, not under the hood. Access is typically behind the glovebox/center area, depending on trim. Users in the thread clarify it’s inside for the E39 sedan. [Elektroda, malina156, post #16894474]
Could the climatronic (IHKA) be the cause if speed control doesn’t work?
Yes. If the thin control wire shows no 12 V or no change when adjusting the panel, the IHKA signal path or controller may be at fault. “Check this thin cable… if it gets a signal from the climatronics.” [Elektroda, melas, post #16901619]
What voltage should I see on the thin control wire?
Expect about 12 V with ignition on, then a changing signal as you adjust fan speed. Lack of 12 V indicates a control-side issue upstream of the FSR, such as IHKA output or wiring. [Elektroda, melas, post #16901619]
Which tool is best: oscilloscope or multimeter for this test?
Use a basic multimeter to verify voltage and continuity at the FSR connector and blower. A budget unit is sufficient for these checks, and safer than probing with an oscilloscope in tight spaces. [Elektroda, Dawidws442, post #16895575]
How can I confirm the blower motor itself isn’t dead?
Momentarily ground the hedgehog’s heatsink. If the fan runs at maximum, the motor and power feed are fine, and the issue is control-side. This is a quick, decisive test. [Elektroda, malina156, post #16903067]
What wiring issues should I watch for at the hedgehog connector?
Inspect for tarnished or heat-damaged wires and loose plugs. Poor contacts drop voltage and prevent proper fan control, even when the blower and FSR are good. Clean and tighten as needed. [Elektroda, Dawidws442, post #16893172]
I replaced the FSR twice and still no control. What next?
Stop swapping parts. Measure the thin wire at the FSR for ignition 12 V and signal change from IHKA. If absent, trace IHKA output and related fuses using WDS diagrams to find the break. [Elektroda, melas, post #16901619]
Is using an OEM brand like Valeo a guarantee of success?
No. One user tried an original Valeo with matching numbers and the issue persisted, pointing to wiring or IHKA signal. Part quality matters, but correct diagnosis matters more. [Elektroda, malina156, post #16901396]
What is BMW WDS, and why is it useful here?
BMW WDS is the official wiring diagram system. It shows the IHKA-to-FSR-to-blower path, connector pinouts, and fuse links, helping you find breaks quickly and avoid guesswork. [Elektroda, melas, post #16901619]
What do brown wires mean in this context?
Brown usually denotes ground in BMW wiring. If the brown ground paths show no continuity or no return, the blower will fail or behave erratically even with a new FSR. One case reported no current on brown wires. [Elektroda, malina156, post #16903067]
Can a failed fuse still let the fan run at full speed when grounded?
If grounding the heatsink triggers full speed, the high-current fuse and feed are intact. The remaining fault is the control signal or connector integrity at the FSR. Check the thin wire next. [Elektroda, malina156, post #16903067]
Quick How-To: How do I test the E39 blower control path fast?
- Check blower fuses, including Fuse 8 behind the glovebox.
- Ground the hedgehog heatsink; confirm full-speed blower.
- With ignition on, meter the thin wire for 12 V and response to fan changes. [Elektroda, melas, post #16901619]
The fuses are ok all. I have no idea what devices are you writing about, so is it possible that the climatronic has fallen ???? I have not checked the cubes yet, but 99% is ok. 