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Thread: How do you test a voltage regulator?

  1. #1

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    How do you test a voltage regulator?

    Having a doosy of a problem, not going to mention that I probably caused it by arcing something during the annual.. but anyway.. out of annual and my field is not getting to the alternator, a Cessna 310.

    I have the problem solved I think, bad voltage regulator, have a couple test to confirm it's not the wiring or short in wiring into and out of the regulator, tested all other wiring. Power is routed through bus to one switch called Emer Field, than another switch Regulator Selector switch (only uses one and has a backup if main fails) than onto the voltage regulator, diagram below. I am almost positive the regulator is the problem, I have a new one today but want to confirm it its bad before installing new one. Here is what happens, the voltage regulator is showing battery voltage all the way to the field switch, but as soon as I close the field switch the voltage drops to 2 volts, but if I open the switch it comes back to battery voltage. Every time. It's not the alternator, I have tested it with a known good one in it now, ran new wire and new toggle field switch from the blue dot terminal on the Regulator Selector switch all the way to the alternator and still same problem, close the switch and voltage drops to 2, open it back to battery voltage. I thought the problem was forward of the field switch because the battery voltage always returns to good when I open the field switch, but after running a completely new wire and switch out to a new alternator and it still fails at the same place, I decided to back trace the bad voltage and it went all the way back to the blue terminal on the Regulator Selector switch, the red terminal is fine, never loses the signal there. That's when I decided to test the alternator unregulated by taking the green input on the left side of the switch off the switch and connect it directly to my new field toggle test switch with a 5 amp fuse between the alternator and test toggle switch, started up and hit the test toggle and bam alternator comes alive right up to 36 volts, that confirmed alternator is good. Shut down and ordered a new voltage regulator, have it today but just wanted to run this by someone with bad voltage regulator experience and see if this makes sense as to a bad regulator.. or could it be something else? I haven't tested all the terminal on the switch yet will do that today.

    I tried testing the regulator and must have did it wrong because I got 0 output, I connected a wires strait off the battery on each terminal the pos and neg, hooked the pos to the vr input where the red wire goes, and the negative to the neg input to the vr, and checked voltage at the blue output and got nothing or close to it, was for sure not battery voltage.
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    Last edited by supercub1999; 10-11-2019 at 12:54 PM.

  2. #2
    wireweinie's Avatar
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    Most likely a bad field breaker. Turn on the switches until you get the drop to 2 volts. Check voltage at breaker, e switch, battery switch, and regulator switch. One of them is probably bad.

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  3. #3

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    That's exactly what I did, backtraced the 2 volts and it ends at the blue terminal on the regulator switch, the output from the VR, the red terminal the input to the VR is good always, output bad but input good. I took the wire at the blue terminal off the switch and connected it directly to a new test toggle switch that went strait to the field tab on the alternator, just a fuse inline between the toggle and alternator, same result, with the toggle open I have batt volts on blue and red, close toggle and send field to alternator and blue goes to 2 volts, open switch and blue is back to good batt volts, the fuse never pops, and field switch is good. Alternator works if I connect bus power directly to that new test toggle where I temporarily connected the wire from the blue VR output terminal and bypass the VR. I have to test the wire from that switch to the VR, that's the only thing I haven't done yet. Can you tell me how to test the regulator?

    The reason I posted this is it seems strange to me that the VR is losing the signal when I close the field switch and send it to the alternator, I have ran all new test wire from that blue terminal to a new known good alternator, I am going out to plane now and testing the wire from the blue terminal back to the actual VR output. It's either that wire or the VR I think. I don't see how a short in the wire could cause the voltage loss when I close the field switch.. I think it's something in the VR causing the signal to degrade.. but I am not the circuit expert.

    And thank you very much for the help.
    Last edited by supercub1999; 10-11-2019 at 01:17 PM.

  4. #4
    wireweinie's Avatar
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    Is there 12 volts on wire P59 at all times?

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  5. #5

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    Quote Originally Posted by wireweinie View Post
    Is there 12 volts on wire P59 at all times?

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    Off to check along with checking the output from the VR at the regulator instead of the switch, will get back with you in about an hour. That p59 should have good voltage, its actually 24;though, 24 system, if it doesnt than there is definitely a problem in the switch because it provides the input to the voltage regulator.

  6. #6

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    I just realized my voltage regulator was not grounded. Now the field signal is acting strange, starts around 20 volts and goes down rapidly to 0 in about 10 seconds.

    Pin 59 as well as input at the regulator are both good steady 25 volts

    Running a new wire right now from pin 83 the input to the switch directly to the input of the VR, it has a good ground and I have a new wire coming out of the VR output, back shortly

    Same thing VR output starts about 18 volts when I turn master on than drains quickly to none.. trying the new VR now
    Last edited by supercub1999; 10-11-2019 at 10:49 PM.

  7. #7

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    Back in business, thank you for the help. Zeftronics VR for $75. off a parted out Baron from ebay worked. Power in was showing 24v and field was at 23v, hoping thats enough to light off the alternator.
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    Last edited by supercub1999; 10-11-2019 at 11:48 PM.
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