Why would a Falcon 50 Passenger temp run away from HOT in AUTO mode

Complaint:  Cabin cold in flight in auto mode:

EDIT squawk turned out to be : cabin cold in AUTO and in MANUAL.

How could the heat ducts become clogged in a Falcon 50?

I failed to realise early on that the cabin valve backing away from hot was normal operation.  Also, when crew is max hot, the cabin backs down even lower.  This also seems to be normal operation.

 

Observation: Pass Valve position needle will drive full hot, then immediately back down to half scale or lower. In the air and on the ground with APU.

 

EDIT 20/20 hindsight, this is normal operation. Also, it is exaggerated when the crew valve is toward hot. Who knew?

 

At about :40 seconds the valve runs cold on its own.

Reference:

F50 Pass temp electrical:   f50passtempcontrol

Associated:

With the flood duct open, the problem is reduced.

EDIT: with the flood duct open, the cabin will quickly overheat.

With CABIN and CREW valves selected OFF, the problem goes away.

With a SEPARATE temp sensor plugged into 163H, hanging in free air, THE PROBLEM GOES AWAY.   THIS is the ONLY way we have affected the squawk.

 

Troubleshooting so far:

We  have shorted 162H AMP pin A and B together. This takes the temp select system out of the system and commands full hot.  Squawk remains the same.

Crew Bleed or Pass bleed or both yield the same result.  Squawk remains the same.

AMP to 163H wires rung out carefully. M, H, J.

AMPs swapped cabin and crew.  Squawk remains the same.

162H Temp select controllers swapped for a loaner unit,  Squawk remains the same.

Back-pinned v to assure it had a solid ground during operation.

With amp unplugged, Ohm meter on M, H, and J  Both halves of the sensor read ~89  ohms at max cold, ~124  ohms at max hot.

Unrelated?

pulled 5 temp switches for a unrelated bleed oheat light when wing heat switched off, turned out to be a sticking PRV

Unrelated?

Removed  failed seals at upper and lower turbo cooler Best-O-Bell inlet pipes. Installed 4 each  new seals.

PARTS OFF: P/N 460-150 S/N NA.

PARTS ON: P/N 460-150 REV P  LOT# 0110336483.

Performed leak check.

Quoted text from John Taylor:

  1. Leading Particulars.

Type of Control …………………………………………………………………………………………………….. Pulse modulated

Temperature Selector Range ……………………………………………………………. 60°F to 90°F (15.4°C to 31.9°C)

Input Voltage ……………………………………………………………………………………………………………… 18 to 32 VDC

Maximum Output …………………………………………………………………………………………………. 2 amps at 28 VDC

Ambient Temperature Range ………………………………………………………………-65°F to 165°F (-54°C to 72°C)

Deadband ………………………………………………………………………………………………………………….. 3.0°F (1.7°C)

Transient Voltage Protection …………………………………………………………………………………. + 80 volt, -600 volt

Reverse Voltage Protection ………………………………………………………………………………………………………. Yes

Output Protection …………………………………………………………………………………………………………………….. Yes

Hi-Limit Protection ………………………………………………………………………………………………………. 160°F (70°C)

Cabin Sensor …………………………………. Negative Temperature Coefficient Disc Thermistor ( See Figure 1 )

Temperature Readout Sensor ……………………………….. Wire wound positive temperature coefficient 0.2°/F

Construction …………………………………………………………………………………………………………. Repairable Assy.

Connector Type …………………………………………………………………………………… Mates with MS3120A-14-195

Size

Length (max) ……………………………………………………………………………………… 8.400 inches (213.36 mm)

Width (max) ………………………………………………………………………………………….. 3.900 inches (99.06 mm)

Height (max) …………………………………………………………………………………………. 3.225 inches (81.91 mm)

Weight (approx.) ……………………………………………………………………………………….. 18 ounces (510.3 gm)

Temp Amplifier simplified block diagram:

temp-control