Pulsing panel lights

[bede1@msn.com]

Hi, I have noticed recently that during night flying my panel lights pulse. They dim a little and go back to bright about every second or two. This happens no matter where I set the dimmer. My digital amp/volt gage stays steady and haven't had any other problems electrically. It is not a big problem but is very annoying.

Thanks for any help
Bob

[kevin]

Is it synchronized with your rotating beacon?

Kevin

[bede1@msn.com]

Kevin, don't know for sure on the matching to outside beacon light. I have all strobes on the aircraft, no rotating beacon. But it is possible that the strobes are causing it. I will check this week to see if shuting off all the exterior lights will stop it.

Thanks for the idea
Bob

[kevin]

I doubt the strobes are the culprit. What do you have in the way of electrical system instrumentation? Can you look at the current output of each alternator? If so, watch the current draw on one or the other, see if it is changing in synch with your pulsations. Also, turn off other electrical equipment, see if that is causing it. What year model airplane do you have? If '73 or newer, it is barely possible that your hydraulic power pack is cycling that fast, but again, unlikely. Only way to check is to pull the circuit breaker.

Others on the board know more about this stuff than me, I am sure you will get some good suggestions. The fact that it is cycling regularly (if I read your message correctly) means to me that something is turning off and on to cause this. If it is flickering irregularly, that is a whole different animal, probably a loose connection somewhere, or a power supply going bad...big fun to find.

My two cents, others will have better advice to offer...

Kevin

[Mark Hislop]

I have had this problem in my 73 P337. It is caused by voltage fluctuations in one or both alternators. This fluctuation is caused by resistance in the voltage regulator circuit. This resistance causes the alternator (or alternators) to "hunt", or increase and decrease there output by a volt or two.

First, try truning off one alternator at a time. The problem may be in just one circuit. Then, start measuring resistnaces or voltage drops in each segment of the voltage regulator circuit, including the master switch, the alternator field switches, the alternator field circuit breakers, and all of the little diodes and capacitors on the little circuit board mounted on the firewall. Clean all connections.

I had to replace the alternator field switches to get rid of my problem. They ohmed out OK, but there was enough resistance in them to cause the hunting problem.

Mark

[stackj]

Bob,

In my case, it was the alternator switch (a double rocker SPST switch). I was able to swap the wires to the unused halves of the switch, turn the old switch upside-down and eliminate the problem. Dirty contacts in any of the four switches between the field circuit breaker and the alternator can cause the problem. The switches most often "switched under load" would be the Alternator (sometimes labeled GEN) switches.

I would have replaced the switch, but was unable to locate a replacement.

The suggestions about looking through the alternator field circuit and all of its components is a good one. Most likely you have something through that area which is causing the fluctuation.

This won't pertain directly to your 73 P model, but if you look on the technical data page of this website, near the bottom, you will find a file entitled "Electrical System Schematic". This is based on the 38 amp Ford alternator system in my 67 337B, but it is laid out as a field current flow diagram and may help you understand your system a little better.

Unfortunately, I am out of town right now, so can't reference my service manual to see how similar the two systems are. I think they are pretty different, but the diagram may be helpful.

Good luck!

[bede1@msn.com]

Thanks everyone for all the help. I am going to do some of the things eveyone mentioned here this weekend, Will let you know what I find. By the way, the whole panel is all Garmin equipment installed last year. I also watched the digital volt/amp gage during the the flight and didn't see any fluxuations in the voltage or amp gage. Possible that the digital gage is not quick enough to catch the up and down changes. I will try shutting down one alt at a time when I take the bird up this weekend and also try shutting down some of the other electronics. For Kevins sake, I do notice when I am on the ground with the master on the hyd power pack does cycle about every 5 minutes. Don't know if this is a problem but it does not match the pulsing lights.

Bob

[kevin]

Every five minutes on your power pack is way too often. You probably have a leak in an actuator somewhere. Every 20 mins or so would be my expectation. I'd fix this fairly soon, or you could start cycling the pack long enough to overheat and/or wear it out, which is quite expensive.

Kevin

[bede1@msn.com]

Kevin,

Guess you were right on the beacon. Found that the strobe bulb was bad (didn't flash) and the electronic flash module was still pulsing. Replaced the strobe bulb and the pulsing panel lights went away. Thanks for the clue. Also found that the power pack was down below the add mark on oil. Filled it up to the proper level, flew plane and cycled gear a few times. After landing left master on in hanger and even after 30 minutes I did not hear the pump cycle once. Did find a small leak around the pressure switch. Had that repaired. Will watch for any further loss in fluid. Again thanks for the help.
Bob

[kevin]

Excellent! Glad we could help.

Kevin

check the ground. if it is on the sliding column move it to one of the braces so you have a good ground.

[SkyKing]

Just another thought on the pulsing lights and that is the older Aeroflash red tail beacons as installed on most of the 1967 through 1980 Cessna's were a real amperage hog.

Typically, on the earlier 12-volt systems the flasher unit draws 11.2 amps, and on the 28-volt systems as installed on the Skymasters, 7.5 amps.

Our 1977 P337 has two of these beacons, the standard one on the tail and a second unit mounted just forward of the main wheel well. Sometimes they flash together, sometimes not, but when they DO flash together ... well, it loads the system down with 15-amps! We definitely see a difference in the ammeter if these are switched off.

SOLUTION: Whelen manufactures a newly designed completely self-contained flashing beacon with a 35-watt halogen bulb that is a direct replacement for the older Aeroflash "amp-hogs." This newer design can be seen on the newer series of Cessna single engine aircraft. Both Chief Aircraft and Aircraft Spruce carry them and the price is modest, about $120.

Since they are self-contained there is no add-on flasher unit, so there is a small weight savings. But the BIG NEWS is these 28-volt units only draw 1.35 AMPs and they appear to be as bright —or brighter than the original units. The Whelen P/N is 70509.

Has anyone done the conversion? Kinda hard to believe that the newer 35-watt halogen lamp can be the same brightness as a 150-watt quartz halogen bulb. Maybe it has to do with the lens as it is twice the height of the old red dome.

SkyKing

[WebMaster]

Would you need to do a 337 to install these, or is there an STC that covers it. Does anyone know. I know when I turn on the navigation lights, and then the strobes ( I have whelan strobes), the whole system takes a huge load.

[skymstr02]

You must still fill out a FAA form 337. The STC is the approval status for the 337 (approved data). Also, if the current draw is much less, you may also have to replace your circuit breaker.

[Bob Cook]

Try Whelen.

I replace mine with a strobe which gives better visibility and far less power consumption. The PS sits in the right side tail boom.
Doesn't take any more than an hours worth of work.

fyi

bob