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Old 22nd Mar 2005, 20:04   #1 (permalink)
Join Date: Dec 2000
Location: North West, UK
Age: 48
Posts: 714
Cessna 150 master switch

We have a Cessna 150 and have been experiencing problems with a low voltage light coming on during flight which we have now rectified.

In the process of checking the work that had been done to correct this problem, we discovered that whilst running the engine at a high RPM and then turning the master switch off to isolate the electrics, the radio and cockpit intercom systems remained live and would not switch off with the master switch.
When it was retried later with the engine stopped, isolating the master switch would cut power to all equipment as I think it should do but did not do this when the engine was running at 2300 RPM.

Is this normal and has anybody else experienced this.???
EGCC4284 is offline  
Old 22nd Mar 2005, 23:47   #2 (permalink)
Join Date: Mar 2005
Location: Townsville Australia
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ive never really come across this problem in the 150 i fly but all i could really say is that it might be caused by vibration and a loose wire some where.
but i would suggest you get some one to double check that wiring for that voltage light they might of made a mistake some where.
hercboy is offline  
Old 23rd Mar 2005, 00:45   #3 (permalink)
Join Date: Mar 1999
Location: Vancouver, Canada
Posts: 37
master should shut off all electrics

I'm a pilot, not a mechanic, but I'm certain the master switch in a C150 should shut off the alternator and remove power from all electrics except the clock and the hobbs meter. I have 500 to 1000 hours in the C150.

It is common to see the red voltage light illuminate immediately after start. In later models, which have a separate switch for the alternator, this problem is eliminated by not selecting the alternator on until after start. In some airplanes it's labelled the "overvoltage" light, because the alternator shuts down automatically in the case of too much charge, and then the light illuminates. In either case, it tells you the alternator is offline.

I have a pilot's manual for the version with the split alt/bat master, but not for the one with the single switch. It has only a very simple circuit diagram. I'll bet if you first remove the fuse for the alternator, the electrics don't keep running when you switch the master off. (I'm assuming you have the old kind with fuses and the single master, and not the split master plus CBs, because you said "master" not "battery side of the master.")

I would not consider it safe to fly a C150 with the master switch malfunctioning as you describe. In-flight electrical emergencies require that the pilot be able to select electrics off with the master switch.

I can't imagine that's any help, because the big book from Cessna must give you more info than that, but whatever it takes to keep the magic smoke in the radios and out of the cockpit.
Luftwaffle is offline  
Old 23rd Mar 2005, 03:27   #4 (permalink)
Join Date: Mar 2005
Location: Townsville Australia
Posts: 22
I have a copy of the single switch master electrical system schematic the one supplied in the pilots handbook i can scan it then sent it to you if you want???
hercboy is offline  
Old 23rd Mar 2005, 06:34   #5 (permalink)
Join Date: Jan 2001
Location: Dunstable, Beds UK
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Early 150's had generators then later ones have alternators

With Generators you have a voltage regulator and reverse current relay ( A la early cars ) on the bulkhead.

Alternators just have the regulator.
GotTheTshirt is offline  
Old 23rd Mar 2005, 08:22   #6 (permalink)
Join Date: Jul 2001
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Have you tried removing the low voltage light bulb?
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Old 23rd Mar 2005, 17:59   #7 (permalink)
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The low voltage light problem was a wire that had vibrated off a connecter spade and has now been put back on correctly, we think.

The switch is a double rocker type. One half battery and the other the generator/alternator?

I agree that the power to the radio and headset intercom should of gone dead when the battery and alternator switch is turned off but whilst the engine was still running at 2100 rpm, did not do this and was allowing me to use the radio for ATC and I was able to still talk to the guy sat next to me through my headset.

It was only when I stopped the engine and tried switching off of the double master switch for a second time did it cut all power to all electric systems ???
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Old 23rd Mar 2005, 23:00   #8 (permalink)
Join Date: Mar 2005
Location: Townsville Australia
Posts: 22
I'm a student pilot, not a mechanic, but i recon that a wire hasn't been put in the right spot or accedently not put back in, its been bypassed somehow. but im sertian on the conditions that have been happening that its accedently been left out.
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