I have a surplus 737 apu?

Question

it has a 14 volt starter gave it 14 volts but no crank?any ideas?

Answer 1

First off, be sure that you're not misreading something; I don't think I've ever come across a turbine engine that didn't require 24vdc to spool and start.

The other thing is make sure that you've got ample battery power. Even the Cummin's diesel powered GPU's I used to use snorted and bogged down when we were starting the TPE331 engines of Mitsubishi MU-2's, which are essentially the same engines.

Being that it can be tough to read old marking placcards, I'd be willing to wager that you have an engine requiring at least a 24 volt power source and 28.5 is nominal. It may just look like it says 14vdc.

Those things spend decades in the grime and filth, getting about as little maintenance as possible, and they aren't easy to service, so it's more than likely that either Mobil Jet II or ETO 2380 slopped on the placcards have rendered them unreadable or misunderstandable.

I fall back on my comment that I've never encountered what you're describing as basically an automotive voltage electrical system.

12 volt systems tend to charge at around 14.4 volts, and the batteries tend to retain around 13.6 volts, which is going to drop as soon as a load is applied. 24 volt systems tend to charge around 28.5 volts and the batteries retain something around 27.2 volts.

Even if you've got the voltage correct, your power supply might not have sufficient amperage to start spooling the engine. The airframes using the TPE 331 typically designat a max of 28.5 vdc and 1100 amps.

P=I*E, P being wattage, I being amerage and E being voltage.

So, at 28.5vdc and 1100 amps, your talking 31350 watts of power, and if you're trying to start the engine with an automotive battery at call it 13.6 and a wca rating of 880 amps, you're only going to be able to push 11968 watts through it.

If you're positive, and I mean dead certain, of the required voltage, you might try wiring several large batteries in parellel to gain more cranking amps; think Class D diesel starting batteries.

Should it turn out to be that you do indeed need 24 vdc, you still may have to look at a combination big, Class D diesel starting batteries done in serial banks to achieve 24 volts, and then wired together in parallel to get the amperage and discharge time you need to get a successful start.

If you get it to start spooling, immediately stop and let the batteries recharge while you go get ear protection. Those things are NOISY.

Good Luck

Answer 2

I can almost assure you it's going to need at least 23 VDC. Around 26 or a little more is better. Ours are charged to 31.4 V in the shop, but in the plane usually sit around 26-27. And the battery is rated at 41 Amp/hours. It's dedicated solely to the APU with a separate main batt. Also APU's have an ECU ( electronic control unit), but not necessarily part of the APU, just mounted near it in the aircraft. The ECU is what will monitor the start/ stop sequence and disengage the starter at the proper RPM and monitor operating limits in case an auto shutdown is needed. Does this particular APU have an attached fuel pump? The fuel control does not pump, just controls the flow. But yes, with the proper battery and a switch you could motor the starter. But there are several conditions that need to be met and fuel flow is a big one. Thats why I ask if there is an ECU, it's the brain, and they can be simple or complex, depending on the APU. Is this used "surplus"? If it is it could be internally damaged. APU's get as little attention as possible with airliners. Not uncommon for them to crap out and get replaced.

Answer 3

Your 14volts may not have enough amps to crank it.

There are several apu's. Check out this link, it might help or lead you in the right direction.

http://www.b737.org.uk/apu.htm

Answer 4

dump em, get an air start and hopefully everybody stays clear using ground power cheaper than running that apu anyhow