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GCU exitation problem


MacFiePict
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Having a problem with an H-model GCU.

Number 4 generator was showing no volts/freqs. It was determined that the generator was fried and was replaced along with a GCU. Second gen fried. Upon troubleshooting we found that the GCU had an output of 50 vdc to the exitation field.

The GCU is getting 106 vac from the PMG. All wires have been ohmed for shorts to ground (none found), breaks, etc. None found.

All grounds have been checked and found good.

Slaved #3 GCU into #4 harness and had the same exitation output of 50vdc.

Put #3 GCU back on it's proper harness and it checks good.

#3 gen PMG is 96vac.

Slaved #3 gen PMG into #4 (isolating #4 pmg wire). Still same output of 50vdc on #4 exitation.

Has anyone encountered this or a similar problem? Any possible suggestions?

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Make sure your Gen and GCU are (matched) same manufacturer.

Depending on what version you have installed.

(I know it should not matter but I have seen issues from mixing...)

When you say:

"Slaved #3 GCU into #4 harness and had the same excitation output of 50vdc.

Put #3 GCU back on it's proper harness and it checks good.

#3 gen PMG is 96vac.

Slaved #3 gen PMG into #4 (isolating #4 pmg wire). Still same output of 50vdc on #4 excitation."

How did you accomplish this?

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Alright, first there are some things that really helps to know before T/S these problems. What year a/C and has the A/C gone under any heavy maintenance such as depot, ISO, or center wing box replacement. I'm taking it you are checking the excitation (F) coming from the GCU at the terminal board. I'm taking it is a typo saying that on number three is 96VAC and four is 50VDC. something is drawing the voltage almost in half. have you check the horse collar cannon plugs for any bent pins and pushed back pins. When you turn on the generator and you get the out light what is the fault message on the GCU? This sometimes leaeds you in a different direction. this could be a fault protection of the GCU to limit the excitation under a faulty condition.

I guess all I need is what fault message and the year of aircraft..

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"have you check the horse collar cannon plugs for any bent pins and pushed back pins."

Ditto on that! Got skunked pretty hard once trusting another on that. Also, doesn't the new GCU eliminate the need for separate voltage regulators by automatically sensing the PMG output and adjusting accordingly?

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Yes the GCU does determine what type of generator it has online according to the PMG that it receives from the generator. Each GCU monitors (PMG) input from the main generator to determine the type of regulation to provide. If the PMG voltage is 30 VDC, the generator is a Bendix unit and the regulator for the Bendix generator PMG is selected. If the PMG voltage is 108 VAC, the generator is a Leland unit and the regulator for this type PMG is selected. One thing to keep in mind is that If the GCU selects the Bendix generator when in fact a Leland generator is installed, high PMG voltage will be established on the GCU’s PMG line. However, an internal protective (fail-safe) circuit limits the PMG DC voltage to 70 volts, When the high voltage is detected, the field relay and line contactor control relay open, which isolates the GCU from the generator system. This fail-safe system is independent of the microprocessor, and will automatically reset if the internal logic reverts to a correct generator selection. …….That’s why with the GCU’s it is usually better to troubleshoot this system by the GCU fault tree. This problem might be related to another system fault causing it to revert to a lower system excitation voltage.

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  • 1 year later...

I've been on the road before and had a generator that wouldn't come on line so I had them shut down the engine then I hit the reset button a few times... they fired back up and were good to go. Well this recently happened again and I said that they needed to shut down the engine completely (not just pull breakers and cannon plugs) for it to completely reset. They didn't do the shutdown, they pulled the breakers and cannon plug and pressing the reset button did not work - still had codes. I agree that pulling the plug would take away power but I think you have to start from a shutdown engine for the reset to work properly.

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I've been on the road before and had a generator that wouldn't come on line so I had them shut down the engine then I hit the reset button a few times... they fired back up and were good to go. Well this recently happened again and I said that they needed to shut down the engine completely (not just pull breakers and cannon plugs) for it to completely reset. They didn't do the shutdown, they pulled the breakers and cannon plug and pressing the reset button did not work - still had codes. I agree that pulling the plug would take away power but I think you have to start from a shutdown engine for the reset to work properly.

Never a great idea to disconnect an electrical plug with it's circuit(s) energized!

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  • 4 weeks later...

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