Although it's not a complete list of troubleshooting problems, the following is a list of the most common items you will need to troubleshoot on the -100 engine.
Not enough battery power
You need a minimum of 16 volts before you can even turn on the ECU. After you hit the start button, the ECU allows the voltage to drop to 12 Volts because the starter draws quite a bit of current. It will allow this drop up to 25 percent engine speed, then after that, you've got to be back to 16 Volts.
Many people have this problem: the APU climbs to 25 percent and then dies because there isn't enough battery voltage. They think that it's a problem with the APU and not the battery. However, you need sufficient battery power in order to start.
In reality, though, the manufacturers have installed better, larger batteries in the aircraft, so this doesn't happen often. There was a time when the pilots had to remove the batteries from the aircraft and bring them inside the hangar so they stayed warm; the batteries were just too small.
Relays and switches
On the Falcon 50, a common problem occurs with the inlet air door relay. Power has to go through the door relay and the door switch on the GIII and IV before it goes to the ECU. If you don't get power through that, your ECU will not have power. Quite frankly, it's easy to tell if you've got power to the ECU because if you look at your EGT gauge, it's will read ambient temperature. On the Challenger, there's no door.
Some GIIIs and GIVs have what are called "hidden switches" or "stealth" switches. These are switches which prevent the APU from starting when the cowling is open by disarming power to the APU. This switch is a problem because if the switch fails, or if the cowl isn't closed correctly, the APU won't work. The switch is there because otherwise, the exhaust would be pointing at the cowling with the cowling down, and then the cowling would burn. Many people remove them, however, because they fail so quickly.
Starter resistance Another problem would be starter resistance. This is easy to check for — you just check for less than one ohm. It's about a 1/2 ohm between the positive and negative terminal. If you're getting more than one ohm, you want to exchange it for a new one.
You can also check the starter clutch. On the back of the starter, there is a slot for a common screwdriver behind a triangular plate. If you take a screwdriver, you should be able to turn it one way and notice that the APU doesn't rotate. If you turn it the other way and it should engage the APU. If it turns both ways, your clutch has failed, and you have to pull the APU because you have probably contaminated the gearbox with metal.
The EGT probes can be a problem at times. Older style probes were set in a potting compound that would break down, and the connection would become bad. Newer style probes are better and don't have that problem. If you have an open, or if one of the leads is off (or even loose nuts), it will sense very high temperatures and won't start due to the loss of the EGT signal.
Also, if the posts are shorted together, you would read ambient temperature; no matter what it's reading inside, the fuel control will keep dumping fuel in. In this case, you can literally burn up the engine.
Acceleration problems (engine hanging up):
Engine hangs at 28 percent
The starter can turn the engine up to around 28 percent, so if you're at 28 percent with no EGT, and fuel is draining from the APU, you can pretty much assume that you have an ignition problem. If you look at the ignitor connection, it should be nice and white. If you notice, however, that at either end, you have a brown-colored ignition lead, as if it is burnt, you can suspect that either the lead or the ignitor box is shorting out. The washer on the end of the ignitor plug is very important, it insulates the heat from the combustor and protects you from damaging the plug. It's primary job is as a heat sync.
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