When the unit stops, the confusion shouldn't start

When the unit stops, the confusion shouldn't start AlliedSignal GTCP 36-100 Series APU troubleshooting By Tim Coggin April 1998 he AlliedSignal GTCP 36-100 series APU can appear as though it is a relatively complex engine. The...

I'm talking ECU-listening?
A basic understanding of the ECU is very important because it controls everything on the engine, and all inputs to the engine go through it.

A review of the communication inputs and outputs related to the ECU are as follows:

The four inputs are really the pilot (through the master switch), the stop switch, the generator switch, and the air switch (and you can control that).

Then there are two inputs that control the ECU: the EGT probe and the RPM. You have to have a failure of either of these controls for more than a half a second before the ECU reacts to it.

There are also eight outputs from the ECU which include the starter relay, surge valve, fuel shutoff, FCU, ignitor, LCV, hourmeter, and generator relay.

Finally, there are five fault trips: speed, temperature, current, hot oil, and low oil.

On the Falcon 50 aircraft, the hot (high oil temperature) indicator is not used. The wire for the hot (optional) sensor simply comes out of the box and goes back in. I have seen this wire cut. The result is that the ECU will shut down, even though there is no sensor on the Falcon 50.

Start with a simplified schematic
You can really get bogged down if you try to troubleshoot the electronics inside the ECU. It's best to start troubleshooting with a basic schematic so that you don't get lost in the ECU. The schematic I recommend basically separates all airframe-related components from APU-related components. Everything on the left is airframe-related, and everything on the right is APU-related. The latches in the center are for fault indication only. One thing that's critical to understand is that the overspeed is always latched by the ECU whenever any other problem occurs. If you find the overspeed latched along with another indicator, it means you had a problem with the item that latched along with the overspeed. It doesn't mean you had a problem with the overspeed. There was an overspeed problem only if the overspeed is the only item that was latched.

When the unit stops, the confusion shouldn't start

AlliedSignal GTCP 36-100 Series APU troubleshooting

By Tim Coggin

April 1998

continued from page 1....

Other items before you start
Other good-to-know items about the engine that you should review before you start troubleshooting are airflow (refer to the airflow illustration) and items that are line replaceable. There are 17 line replaceable units (LRUs) on the -100 APU. They are shown in the following illustration. Don't forget that the ignition lead itself can be cause problems as well.

Also, before you start, get to know where the most problematic areas on the engine are. One typical example on the -100 is a recurring problem with the hourmeter which inevitably results in an overcurrent shutdown. Overcurrent problems are hard to troubleshoot because it is basically some item shorting in the system. To troubleshoot it, you have to start isolating each item. Also, overcurrents are often erratic. For some reason, it's quite common for the hourmeter to short out. You may want to just disconnect the wires to the hourmeter, and then if the APU starts, you're on your way. This is something wouldn't normally condone, yet it may be necessary in order to get you back to a maintenance base where the problem can be addressed properly. The hourmeter isn't cheap — it's roughly $5000 — and this creates another problem. People are removing these and putting in a Hobbs meter, which is around $250. The problem with this is that it draws more current that the AlliedSignal hourmeter, and it eventually burns out the circuit in the ECU. So now you're having to replace a $5000 hourmeter and a $17,000 ECU (exchange for $8,000 or $9,000). All this for an hour meter problem.

Some maintenance facilities have gotten around this by installing a Hobbs and rewiring it so that it gets its power from a separate source, instead of through the ECU. But remember, you just don't arbitrarily go putting equipment on your engine that isn't certified. You've got to go through the legal hoops for whatever modifications you make.

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