Overvoltage Warning Light

Overvoltage Warning Light

I have a 1977 Cessna 172N.  Just bought the plane used in Aug 04.  I have a problem with the overvoltage warning light, which keeps coming on intermittently during flight, and on the ground before takeoff.  This problem has been occurring now for 6 weeks.

I took the plane to Roanoke, Virginia, and had the electronics shop down there check it out.  No luck.  I have tried several things to figure out what the problem is, and so far we have struck out. We have had the alternator checked, alternator guts replaced, bought new voltage regulator, thought it was fixed.  No luck. 

Then, the maintenance mechanic at my airport started following wires from beginning to end, and figured out that the sensor, to the light, was the problem.  He was supposedly jumpiing the sensor, and things seemed to work correctly.  So, he replaced the sensor, thought it was fixed again, and then I took it for test flight last weekend, and still the light comes on intermittently.  Now, we are at a loss.  Have checked the maintenance manuals, and we are now considering replacing the field wire as the next option.  Not sure if this will do it or not.

Does anyone have any other suggestions?  Has anyone had this sort of problem before?

Please advise,

Mickey Fuqua
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Re: Overvoltage Warning Light

Replace the master switch. I've had this happen on countless 172's over the years and it's an easy fix. Problem is, the contacts get dirty/pitted and make poor contact...excessive resistance. The field goes thru that switch as well. The switch is inexpensive and easy to change with push-on connectors.
Gregg Horrell
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Re: Overvoltage Warning Light

Gregg, You bring up a good point.  I had a friend who owned a 172H and he would have total electrical failure in flight, cycle the master switch then get everything back for a few minutes.  Had to puch master switch in like 1/8 inch to keep power on line, all the way out would kill power.  We replaced the master switch (old push pull type) and has been problem free over a year.  Cheers !

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Re: Overvoltage Warning Light

I just went through the same thing... check your voltage regulator. If it is an Electrosystems VR600A(light blue in color) it is an aftermarket replacement the combines the overvoltage sensor and regulator into one unit on the N model 172- Cessna issued a service bulletin that it needed to be replaced. I replaced mine because of a problem similar to yours and it cured it. Either way Zeftronics makes a direct replacement for approx. $129 and it only takes about 1/2 hour to install - money well spent.

Jeff

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Re: Overvoltage Warning Light

You can do a search and find this topic covered earlier.  I have a 1972 182P with the overvoltage sensor and at one time in its life, a warning light.  By the time that I purchased the plane, the light had been removed.

So I didn't get the warning as I had no light, but my charging system would go dead after startup periodically.  The system works by sensing an overcharge condition and cutting the charging system out.  The overvoltage system is reset by cylcling the master switch, thus you see reference to replacing or cycling the master.  If it senses an overvoltage condition again, it will cycle out again.  Sometimes recycling the master several times, until it holds will "fix" the system for that flight, but it will come back.

My fix was to remove the overcharge sensor from the system and buy a Zephtronics regulator with a built in overcharge circuit as suggested above.  I did that probably two years ago and have never had another problem.

BUT- I have seen a letter put out by Cessna I think, recommending for those that have removed their sensor and put an aftermarket VR in to put it back into the original configuration.  You will have to look deeper into that and determine for yourself what to do.

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Re: Overvoltage Warning Light

Ted is correct about the service Bulletin - Cessna recommends returning to the original configuration, however the Zeftronics route will save on both labor and parts - not to mention they are easy to come by. When the Electrosystems was installed quite a bit of rewiring took place that would have to be undone to go back to the original - the Zeftronics is a bolt on replacement. As an interesting note I called Electrosystems and they no longer manufacture the VR600A or any equivalent.

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Re: Overvoltage Warning Light

I had our mechanic replace the master switch, and put new voltage regulator on the plane, ICW replacing master switch.  I flew it one time for 15 minutes, and the light did not come on.  However, the ammeter needle is reading all the way to the left, and not the normal two needle widths to right.  My mechanic thinks the problem is still not solved.  Any further suggestions?

Thanks,

Mickey

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Re: Overvoltage Warning Light

We replaced the master switch.  Also put the new voltage regulator (VR600) on the plane at the same time last week.  I flew the plane on Monday, and the overvoltage light did not come on.  However, the ammeter needle stayed way over to the left, and not the normal two needle widths to the right of center line.  I only flew the plane for 10 minutes or so, and would like to fly it again and see what happens.  However, my mechanic thinks there is still a problem due to the fact that the needle is way over to the left. He thinks the alternator is still not charging during flight for some reason.  We did put the VR600 voltage regulator you referred to in the email below on the plane.  My mechanic insisted on me buying the Zeftronics one, listed in Aircraft Spruce Parts catalog.  This was the recommended voltage regulator for my plane.  Should I try something else?  Or a different voltage regulator?  As the emails elude to below?  Please let me know what you think about this, and what I should do now.

Thanks,

Mickey Fuqua
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Re: Overvoltage Warning Light

You may also want to check the condition of your battery.
Ask you mechanic to check the grounding points and other electrical connections.

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Re: Overvoltage Warning Light

roger.  thanks.  will do.  will have the mechanic check it out this weekend.  I think we have already ruled out the battery, as I was concerned about that from the start 2 months ago.  He said the battery was fine, but we will check it again.  will keep you posted.  thanks.  Mickey

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Re: Overvoltage Warning Light

ALCON,

Still looking for a resolution to this problem.  Does anyone have any other suggestions?  Or does anyone have further comments about the voltage regulator issue.  My mechanic has put a brand new voltage regulator on the plane.  Should we buy another new voltage regulator in the hopes that the other new one (2 months old now) just malfunctioned somehow?  Should we try a different kind/make of voltage regulator (not the VR600 as the book recommends) like the email replies elude to and see if that works?

Any help with this would be greatly appreciated.

Thanks,

Mickey Fuqua

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Re: Overvoltage Warning Light

I had this happening and it was a wiring problem. Some wire was grounding out on the block. Check those wires well, and see if that does it.

Rob Singles

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Re: Overvoltage Warning Light

    If your mechanic installed a Zefftronics alternator control unit with built-in overvoltage protection then some additional changes needed to be made under the instrument panel. The surge capacitor and overvoltage circuit needs to be removed as well. If it isn't removed then it can cause a constant discharge. Instructions are included with the Zeff unit.
 

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Re: Overvoltage Warning Light

Stuck starter contactor, mostly when take-off from rough airfield.
Reverse installation (turn "upsidedown").

Ammeter shows full scale deflection (left) as it is now a very inefficient GENERATOR!

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Re: Overvoltage Warning Light

Hi Folks  I have a 1977 Cessna 172N ; I have the same problem; have put in a new voltage regulator, new master switch and new alternator; no obvious loose wires or bad connections; haven't tried the new battery because the mechanic says it test good; the voiltage light and ammeter discharge doesn't come on till about 20 minutes into flight. Can't get it to do it on the ground. I am stumped as is the mechanic working on the plane; any more suggestions? Thanks.

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Re: Overvoltage Warning Light

HOW CAN I TEST THE OVERVOLTAGE SENSOR ONCE IT IS OUT OF THE AIRPLANE.I INSTALLED KELLY AEROSPACE VR600 AND NOW OVERVOLTAGE LIGHT DOES NOT WORK.

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Re: Overvoltage Warning Light

Well, I have a 2001 Skyhawk 172 SP with only 650 hours on it owned since birth and have started a similar problem. 3months ago I had the voltage warning light flicker then stay on and shut off the alternator system on take off with the voltage meter showing only 22 volts. At this time we had a 3 year old Gill battery going bad by the indicators on the trickle charger that we use so we replaced it wuth a new Concord battery. We also had some radio noise on take off and climb out. and replaced the L sided jack which resolved that. The plane went through annual and came back and flew fine for 4 hours of time then on climb out one day at 3000 feet the red volt light began to flicker and there were large swings of 10 degrees or more of the EGT indicator. Voltage remained at 28 volts and ammeter neutral. Returned to the airport and talked with A&P and did some research. Determined that we would replace the avionics and master switch and test and replace EGT probe if needed and clean contacts on the firewall ACU. The above were done and EGT sensor replaced and took the plane for a 15 minute flight and all seems good. Then took a longer flight and 1 hour into the flight on a climb out the Volt light began to flicker and the Ammeter had wild swings. This was in an approach with clock on so did not see the Volt indicator. Flight continues since it was in VFR condition and it resolved on its own. On the ground now cannot get any errors and Volts are 24.6 engine off and 28 engine on. Volt light works on test switch. Ammeter is normal at rest with master on -neutral. Turning off the alternator side of master does not caue the volt light to come on with the engine off but I do not know if this is normal or not. I have looked every where for loose or abraded wires and can find none. So I am going back to A&P suggesting we look at replacing the ACU on the firewall and if continues maybe bench test the alternator. Any ideas? I am stumped and frustrated. I am a physician but this diagnosis eludes me.

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Re: Overvoltage Warning Light

Let me uprdate this to save all the rest of you some misery. After a long discussion with my A&P we decided it was either the alternator or ACU ( aka voltage regulator). I said ok I have had enough . Time to replace both. Since Cessna has replaced the alternator twice under warranty due to reports of brush and spring issues in 10 years, I decide I had enough experience with a Hartzell( modified 1960 Ford ) alternator and bought a new Plane Power certified alternator and had my A&P get me a new ACU and installed both. The problem is resolved and the volt light does not come and the radios are clearer and the EGT probe is flutter free just like when the plane was new. Remember nothing tested bad on the ground including shop checks of alternator and ACU. Good luck to all and hope you never experience this!

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