flyboy0681 Posted January 19, 2013 Report Share Posted January 19, 2013 A couple of months back I reported here a situation where the fuse was blowing on the post lights. After hours of trial and error, checking each individual bulb and the wiring harnesses, we came up with nothing. We recently discovered that when the fuse is replaced the lights will work just fine on battery and ground power. After engine start the lights continue to stay on until power is brought up to about 1700rpm - then poof. It sound like excessive voltage, but the two digital voltage readouts on-board show normal charging taking place. Has anyone experienced a similar situation? Quote Link to comment Share on other sites More sharing options...
DonMuncy Posted January 19, 2013 Report Share Posted January 19, 2013 I have the same problem. But no solution. And those $15 fuses add up. Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted January 19, 2013 Report Share Posted January 19, 2013 On my 77J there are lights on the glare shield, and they were randomly blowing the fuse. Someone had put aluminum tape behind the bulbs to act as a reflector and the tape was shorting out the lights. I would look for something that is shorting out because of vibration, chafed wires and such. You can eliminate the voltage situation by pulling the alternator breaker and reving the engine. If the fuse still blows then it is vibration. I have found another part number for those fuses and you can get them from Digi Key for about $4.50, I will look it up if you like. They are exactly the same fuse with an industrial rating instead of a military rating. Either way they are a much more robust fuse then the 3AG fuses used in so many places. Quote Link to comment Share on other sites More sharing options...
Skywarrior Posted January 19, 2013 Report Share Posted January 19, 2013 The problem is most likely not excessive voltage, but excessive current flowing through. Quote Link to comment Share on other sites More sharing options...
flyboy0681 Posted January 19, 2013 Author Report Share Posted January 19, 2013 The problem is most likely not excessive voltage, but excessive current flowing through. Which is most likely a symptom of what? Quote Link to comment Share on other sites More sharing options...
tony Posted January 19, 2013 Report Share Posted January 19, 2013 I had a pinched wire between the intrument panel and some structure, causing a high resistance short. Happens sometimes when the guy reinstalling the instrucment panel. is not careful. MIne wasnt enough to take out the breaker but it was enough to bring the voltage down so the lights wouldnt light. I think this is a great excuse to install some nulites and get rid of those post lights. Quote Link to comment Share on other sites More sharing options...
Skywarrior Posted January 19, 2013 Report Share Posted January 19, 2013 Which is most likely a symptom of what? I would guess a short somewhere. Try to find out what changes, in the electrical system, when you hit 1700 RPM. Perhaps some electrical component allows too much current through when that RPM is reached. I'm not a double-E. Just remembering my Basic Electrical Circuits class. The clue is the fuse - a fuse blows from excessive current, not excessive voltage. Quote Link to comment Share on other sites More sharing options...
takair Posted January 20, 2013 Report Share Posted January 20, 2013 Does it happen at all dim settings? have you tried with the dimmer turned down? Could be a couple of things. As others have suggested, it could be a short in one of the post light wires. It could be shorting under vibration. Rarely, a post light can develope an internal short. The post is a tube with the conductor running up the middle. If the insulation chafes it can short. A more likely failure point are the transistors. These are near the fuse, below the breaker panel, as I recall. These can fail in various ways; open, partial internal short, full internal short. I couldn't find a 201 schematic to get into more detail, so I'm going from memory. Might be worth replacing these to see if it helps. I don't remember if they are soldered or plug in. I do remember they fail on occasion and blow the fuse. Quote Link to comment Share on other sites More sharing options...
DrBill Posted January 20, 2013 Report Share Posted January 20, 2013 Put an OHM meter, preferrably an older one with a needle (shows intermittants better than digital) but a digital will also work ok, on the lamp side of the fuse to ground with the fuse removed. Measure the normal resistance to ground (if your lights normally draw 3 amps then you should see about 4 OHMS.). Now you can start up the engine and rev up all you want. By watching the OHM meter you can watch the resistance change when the intermittant SHORT to ground (very low resistance, e.g. 0.1 to 0.5 OHMS) which would cause the fuse to blow if it was in place. Once this is verified, the challange is to find the short but you can isolate it by disconnecting 1/2 the post lights. If you still see the short, then it's between the fuse and the point you disconnected. If you don't see the short, it's between the point of disconnect and the end. Good luck BILL Quote Link to comment Share on other sites More sharing options...
DonMuncy Posted January 20, 2013 Report Share Posted January 20, 2013 N201MKTurbo, Yes please. I would like to utilize this source for the fuses. Quote Link to comment Share on other sites More sharing options...
takair Posted January 20, 2013 Report Share Posted January 20, 2013 Bill Not sure that will work due to the transistors in the dimming circuit. However, by pulling the fuses to remove power, you could do something similar by ohming out from one post lights with the bulb removed. That would not find a failing transistor though. Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted January 20, 2013 Report Share Posted January 20, 2013 OK the fuses are FM01A125V5A The industrial number is GMW-5 here is the best price I could find: http://nationalfuse.com/north-american-electrical/micro-fuse/gmw-5/prod_1183.html They are $4.64 Here is the datasheet you will note at the bottom that it says they are compatable with FM01-5A: http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/Data%20Sheets/Bus_Ele_DS_2058_GMW_HWA.pdf Quote Link to comment Share on other sites More sharing options...
flyboy0681 Posted January 24, 2013 Author Report Share Posted January 24, 2013 Well folks, the mystery has been solved. Thanks to the suggestions here (especially N201MKTurbo), the process of elimination led our avionics shop to a loose, dangling wire behind the circuit breaker panel. Many thanks to you and the techs at Sarasota Avionics. Quote Link to comment Share on other sites More sharing options...
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