Control cabin zone temp illuminate onrecall

[Benx737]

Hello All,

Good day,
Iam facing repetitive problem of ata 21 about control cabin zone temp ill on recall,
when bite on both PZTC found fault message flight deck duct sensor.
reff to fim 21-62 TASKS 812-813 we had check the resistance of T477 and T478 and the result still in range,wiring check from PZTC to sensor T477 and T478 found normal,already replaced PZTC but some time the control cabin zone temp lt still illuminate on recall inflight.it seem the heating system (Lh TCV and control cabin TMV) doesnt work properly,then tried to resistance check both mix manifold sensor, M1718 sensor and S2 selector on P5-17 found still inrange.
could anyone here help me to solve that problem?

Regards,
Benx737

[sjap]

So there are still errors in the bite check? did you replace those parts?

[maro12]

Hi Benx737,

Could you please clarify:
-does the ZONE TEMP lt illuminate on recall only in flight?
-what happens when you press recall on ground?
-did you replace both PZTC? (if only one, which one and why this one?)
-what is the BITE result on both PZTC right now?
-what are the results of measurements (values for both sensors and both lines)?
-did you check lines insulation with megger?
-based on what symptoms you infer that cont cabin heating system does not work properly?

Based on what you wrote, both PZTCs during BITE show message FLT DECK DUCT SENSOR. T477 is connected to PZTC 1 and T478 to PZTC 2. During BITE PZTC 1 checks T477 and PZTC 2 checks T478. If both PZTC show message FLT DECK DUCT SENSOR, this would mean double failure (T477 or PZTC1 or T477-PZTC1 wiring) and (T478 or PZTC2 or T478-PZTC2 wiring). On wiring diagram which I have, both lines go through the same connector D41814 at the back of E3-3. Did you inspect this connector in detail? Inspect+clean contacts, check if it is properly engaged, etc.

Let us know.

Best Regards,
Maro12

[Benx737]

So there are still errors in the bite check? did you replace those parts?

yes,we already replaced both sensor (T477 and T478) and in that time both PZTC was no fault.
but in several day the trouble come again with same fault message 'flight deck duct sensor'

[Benx737]

Hi Benx737,

Could you please clarify:
-does the ZONE TEMP lt illuminate on recall only in flight?
-what happens when you press recall on ground?
-did you replace both PZTC? (if only one, which one and why this one?)
-what is the BITE result on both PZTC right now?
-what are the results of measurements (values for both sensors and both lines)?
-did you check lines insulation with megger?
-based on what symptoms you infer that cont cabin heating system does not work properly?

Based on what you wrote, both PZTCs during BITE show message FLT DECK DUCT SENSOR. T477 is connected to PZTC 1 and T478 to PZTC 2. During BITE PZTC 1 checks T477 and PZTC 2 checks T478. If both PZTC show message FLT DECK DUCT SENSOR, this would mean double failure (T477 or PZTC1 or T477-PZTC1 wiring) and (T478 or PZTC2 or T478-PZTC2 wiring). On wiring diagram which I have, both lines go through the same connector D41814 at the back of E3-3. Did you inspect this connector in detail? Inspect+clean contacts, check if it is properly engaged, etc.

Let us know.

Best Regards,
Maro12

Hi Maro12,

1.-does the ZONE TEMP lt illuminate on recall only in flight?
-what happens when you press recall on ground?
answer : zone temp lt onrecall was happen on grd too,but we can reset it on PZTC

2.-did you replace both PZTC? (if only one, which one and why this one?)
answer : actually this trouble exist on several aircraft,we have been replaced both PZTC to one aircraft but its doesnt help,the trouble come and come again.

3.-what is the BITE result on both PZTC right now?
answer : the fault still "flight deck duct sensor" in both PZTC

4.-what are the results of measurements (values for both sensors and both lines)?
answer : the result of resistance check of sensors still on range,but on severeal time we found the resistance was out of limit and sensor was bend,then we replace the sensor but in next several day the trouble come again.

5.-did you check lines insulation with megger?
answer : not perform yet

6.-based on what symptoms you infer that cont cabin heating system does not work properly?
answer : the temp of cockpit quite cold and we found the sensor was bend ( i think the sensor was hit by ice from mix manifold ).when we move the selector s2 p5-17 to warm,the temp was very slowly increase.

regards,
Benx737

[maro12]

Hi Benx737,

Even if cont cabin temp cont system was not functioning properly, the BITE result on the box is straight forward - temp sensor circuit (both). If you can reset it, fault is gone and it was intermittent. Question if temp sensors are getting damaged due to issue with temp cont sys.

Ref.4
-what range are you having in mind? In FIM I see only open circuit/short, did you find other reference with exact resistance values for given temperatures (could you share ref. doc nr so I can find it quickly)?
-did you measure sensors resistance from PZTC connector on the rack or sensor directly?
-both sensors were out of limit on several occasions you mention?
-after temp sensors replacement and clearing the fault, when fault came back, new sensors were bent and/or res out of limit again?

Ref.6
-the cont cabin air pickup is before mix manifold, it comes from left pack - pickup between check valve and mix manifold. I am not sure if ice fragments big enough to bend and/or damage the sensor (which would form on turbine outlet) would pass through condenser, this is good question, I would not think so, but from top of my head I do not remember that.

Additionally:
-Did you inspect D41814 in detail? (just to be sure all is fine there, as I see pinout numbers, this is flat rack connector - it happened to me that I found this connector not properly engaged - one side lockscrew was ok but second side was not engaged properly and this side contacts were intermittent).

Trouble exists on several aircrafts? The same problem?

Boeing gives good guidance for troubleshooting of temp cont system in 737-SL-21-108-A (it is in big part related to HX efficiency, but might give some clue what could be wrong there if this is the cause).

Best Regards,
Maro12

[Benx737]

Hi Maro12,
good day,

Ref.4
-what range are you having in mind? In FIM I see only open circuit/short, did you find other reference with exact resistance values for given temperatures (could you share ref. doc nr so I can find it quickly)?
yes,in FIM just only to check open or short circuit,but we got the limitation of that sensor in the AMM 21-61-04-000-805-002 Duct Temperature Limit Sensor Test.there is table that contain resistance to duct temperature.
-did you measure sensors resistance from PZTC connector on the rack or sensor directly?
We did resistance check from PZTC connector
-both sensors were out of limit on several occasions you mention?
yes,several time we found both out of limit with bend condition
-after temp sensors replacement and clearing the fault, when fault came back, new sensors were bent and/or res out of limit again?
a week after replaced sensor the fault came back without bent or res out of limit.

Ref.6
-the cont cabin air pickup is before mix manifold, it comes from left pack - pickup between check valve and mix manifold. I am not sure if ice fragments big enough to bend and/or damage the sensor (which would form on turbine outlet) would pass through condenser, this is good question, I would not think so, but from top of my head I do not remember that.
yes,sorry.Iam agree maybe the ice from turbin outlet,the cont cabin pick up is before mix manifold.but its should be no ice if lh tcv work properly,when lh pack temp sensor t471 or lh mix manifold temp sensor detect 35 F.(please correct me if Iam wrong).

Additionally:
-Did you inspect D41814 in detail? (just to be sure all is fine there, as I see pinout numbers, this is flat rack connector - it happened to me that I found this connector not properly engaged - one side lockscrew was ok but second side was not engaged properly and this side contacts were intermittent).
we have not check yet the D41814,thank for suggestion.we'll try to check that connector.

Trouble exists on several aircrafts? The same problem?
yes,the trouble came again and again with same fault message.we'll monitor that after we detail check the D41814

Boeing gives good guidance for troubleshooting of temp cont system in 737-SL-21-108-A (it is in big part related to HX efficiency, but might give some clue what could be wrong there if this is the cause).
the SL is about cabin hot on ground,the trouble flight deck duct sensor come with the good aircondition temp.we have been check the temp in and out of the both lh heat exchanger was quite normal ( from the SL,outlet the secondary heat exchanger was 40 C,its indicate clean heat exchanger)

Best Regards,
Benx737

[maro12]

Hi Benx737,

Ref.4
Thanks for the reference, good data.

a week after replaced sensor the fault came back without bent or res out of limit.

Was it hard fault then (unable to reset)?

I am guessing you were able to reset the fault on PZTC then. Based on below logic:

FLIGHT DECK DUCT SENSOR FAULT can only be true (fault active), if the sensor input HIGH LIMIT FAILURE would be true or the sensor input LOW LIMIT FAILURE would be true.

Therefore if fault is active (you can not reset it) with resistance of the respective sensor in limits, it would indicate PZTC as a source of the problem. If resistance is in limit, and you can reset the fault, it means resistance was out of limit intermittently some time during flight.

(If you have access to PZTC CMM, I suspect you could get more detailed info, as for how long res must be out of limit to trigger the fault etc. It would be worth to have a look in PZTC CMM if available.)

Ref.6
Correct, that is why the valve here named TCV, on some other aircrafts is also called anti ice valve. From turbine outlet the air still has to go through condenser, which in internal construction resembles heat exchanger. My question is if ice fragments can get through it. Maybe I will find this info when I will have spare minute.

Best Regards,
Maro12

[Benx737]

Hi Maro12,

Was it hard fault then (unable to reset)?
it was always be able to reset except we met the bent or out of resistance limit.

FLIGHT DECK DUCT SENSOR FAULT can only be true (fault active), if the sensor input HIGH LIMIT FAILURE would be true or the sensor input LOW LIMIT FAILURE would be true.
I found this on CMM about Duct Sensor Interface,"These interface circuits are designed to accommodate a working range of -71 to 200°F (-57.2 to 93.3°C)".
yes,Iam agree. The fault active can only be true when high limit (93.3 C) or low limit (-57.2 C) reached base on that logic schematic.I don't know maybe its happen inflight when low limit reached(is that possible?),so in the begining Iam assume that the heating system doesnt properly work,The LH TCV should be active as long as flight when mix manifold detected 35 F.
yes,Iam agree with the resistance was out of limit intermittently some time during flight.

Iam still progressing to check D41814

Best Regards,
Benx737

[maro12]

Hi Benx737,

The fault active can only be true when high limit (93.3 C) or low limit (-57.2 C) reached base on that logic schematic.

LOW LIMIT FAILURE / HIGH LIMIT FAILURE refer to resistance limit, not to temp limit. Resistance input to PZTC might be low/high for other reasons than air temperature in the duct where sensor is installed. Might be: sensor probe element failure, sensor receptacle failure, bad crimping on the ship side, contacts contaminated/corroded, wiring chafing to the ground etc. etc.

I do not think temperatures in this duct might reach temperatures you quoted. This is also safety aspect, as crew would not be able to carry out flight safely in such an environment, therefore it has to be proved during certification that this is highly improbable and protections are in place.

Also, make sure that you are installing proper sensors (double check part numbers and IPC).

Best Regards,
Maro12

[Benx737]

Hi Maro12,

Ok,I see.Thank you for information.
The D41814 connector plug was checked found normal and properly lock.
We use PN 622878-2 for T477 and T478.

Best Regards,
Benx737

[Dunny]

Did you say you have TCV faults - if so, I'd certainly rectify these first before moving on.

[Benx737]

Hi Dunny,

We have no TCV fault message on both PZTC,just shown flight deck duct sensor fault intermittenly.

Regards,
Benx737