B737-400 APU GTCP85-129 PROBLEMS

[lookilook]

Hi All,

We are stack with various problems on a 737-400 and i will start initially on APU as in subject.
First we had APU bleed surge 2 times every 30 min approx and bleed pressure fluctuation, bleed valve replaced with no luck and then speed sw replaced and was ok for a day.
since then we have APU bus #1 and #2 trip off when galley load or a hydraulic pump is selected, APU bleed pressure fluctuates and freq low<380hz.
Starting Eng#2 the Gen#2 trips with starter engaged but when we start Eng#1 first with only APU Gen#1 then start is normal.
Parts changed or robbed for t/s are ,bleed valve,low oil pressure sw,electr.speed sw,surge valve, CT T2 cleaned and inspected,APU gen Brkr cont.#2 interchanged with ext#1,eng#2 Gcu interchanged with APU Gcu .
Last night i got extra info that in flight HV and LV lights on diagnostic panel were 'on'.

Anything else you can think of and i miss? i would like to hear your opinion.

Thanks.

[sjap]

Fuel control? Looks like the speed is not kept at 400 Hz.

Once we had a similar problem and then whe replaced the combustion chamber (?) and it was fixed. The chamber was fully burned away inside...

[lookilook]

Thanks for your input sjap,

We are at a point now that the APU starts normally and both the EGT and FREQ are constant to normal numbers. When we put loads like galley ,hydraul pump and window heat switches to on still everything work normal. The problem is that when the pilot try to start the engine#2 the APU GEN#2 trips hence no start. Next step was to remove the loads mentioned before and the engine starts despite the fact that it takes a while to stabilize.
By having on only the APU GEN#1 we can start ENG#1 with or without loads so i see the problem being only on the APU GEN#2 side.

Any other thoughts?

[Boecom]

Change the 3-way solenoid valve, that sounds like the cause of the issues you have described.

Regards

Boecom

[lookilook]

Hi Boecom and Sjap,

Last night shift replaced the surge valve as the one installed was robbed from another a/c and today the pilot reported that at base he started the eng#2 with loads on and the only thing he noted is that the apu freq went from 405 to 385hz but engine started and everything went back to normal.
When at destination the apu was working fine freq=405hz, duct pressure =38 psi and galley ,hydraul pump ,window heat were all 'on' . So upon eng#2 starting sequence he noticed apu freq=382 approx surely less than the one at base and duct pressure around 22-24 psi and generator tripped.
This is what we get now and it seems to be intermittent. Is there a connection between the 3 way solenoid valve and the freq dropping so much?
Morning shift said they may check FCU and fuel filter again.
Thanks for your input.

[Boecom]

Change the 3-way solenoid. Also check the pneumatic control sense lines and t-piece fitting which often gets clogged. From what you have described and from my experience, I would say the solenoid is the source of your problems.

[lookilook]

Thanks a lot Boecom i will let you know as soon as we have this part available to install it. Might take a while due to various reasons.....

[carlito]

Hi all,

Any update regarding your problem, do you solve it ?
We have similar problem and I suspect the 3way solenoid...
Please advise.

C

[lookilook]

Hi ,No it did not change anything by replacing the 3 way solenoid but this case was messed from the beginning by bad troubleshooting and no recording of replacements. As i have left the company i have no update apart that they have contacted Honeywell for help and next step was to replace the FCU.
Let me know if you are successful.

[Jahanzeb]

Hi All,

We are stack with various problems on a 737-400 and i will start initially on APU as in subject.
First we had APU bleed surge 2 times every 30 min approx and bleed pressure fluctuation, bleed valve replaced with no luck and then speed sw replaced and was ok for a day.
since then we have APU bus #1 and #2 trip off when galley load or a hydraulic pump is selected, APU bleed pressure fluctuates and freq low<380hz.
Starting Eng#2 the Gen#2 trips with starter engaged but when we start Eng#1 first with only APU Gen#1 then start is normal.
Parts changed or robbed for t/s are ,bleed valve,low oil pressure sw,electr.speed sw,surge valve, CT T2 cleaned and inspected,APU gen Brkr cont.#2 interchanged with ext#1,eng#2 Gcu interchanged with APU Gcu .

[vortex]

could you check this site if you wanted maybe help, they repair GTCP85-129

http://www.alphaaircraft.com/

[kenga]

I am sure that problem could be in 3-way solenoid or propotional control valve. you must replase them to confirm.

[kenga]

I also advise you to perform apu borescope through compressor inlet and combustion chamber to make sure there is no damage of blades and vanes

[lookilook]

[leorizzi]

Dear people, I have problems with a B737-500 APU, after 10 or 15 minutes the APU Shutting down. I performed T/S AUTOMATIC SHUTDOWN - NO APPARENT CAUSE. No FAULT or other indications shows. Is like you put the APU Sw to OFF.
ELECTRONIC SPEED SWITCH, S634 checked resistance according T/S,
APU INTERNAL DAMAGE Checked normal in boroscope inspection (damages within tolerance)
ADJUSTMENT OF THE APU AIR INLET Checked ok, we put a jumper in the SW connector.
EXTERNAL DAMAGE TO THE APU Checked normal
APU CONTROL UNIT, M280, was replaced, first start was successful for first 5 hours, but after that, the fault start again.
THE APU FUEL SYSTEM checked, fuel arrive to APU filters (checked both normal) very good
LEAKAGE FROM THE COMPONENTS no found any lakeage
THE APU OIL SYSTEM checked filter ok.
Checked Cooling system, s/off valve operation normal. Fan and ducts checked normal. Inlet screen cleaned and checked ok.
Next step is P5-10 AIR CONDITIONING MODULE PROBLEM but I do not found the relationship.
Can you help me please!!!