Electrical fuse problem

[Ed Lutz]

Hello all,

I am having a problem with my 337B, with the master switch off, the system will power it self back on. When I pull the fuse marked "GEN" in the fuse rack, the system appears to be perating normal. I am trying to follow the schematic but this doesn't make sense. Any suggestions?

[stackj]

The description you provide doesn't make sense to me either. There is a simplified schematic of the Ford 38 Amp alternator system on the Technical Data page of this web site.

http://www.337skymaster.com/techdata.htm

Then click on Electrical System Schematic (PCX Format).

You will probably find that one easier to follow than the one in the Skymaster manual.

Can you provide any further description of the problem? Does the electrical system shut down when you turn off the master switch and then come back on? Does it just stay on? If it comes back on, how long does it stay off? Is the problem intermittent?

Also, what position is your 'RUN/STBY' switch in?

What is your Aircraft serial number?

[kevin]

I had a problem like this in flight in a 206 years ago. Turned the master off when I smelled something burning. Went away. Turned the master on, smell came back. Turned master off, airplane stayed powered up, smell got worse. Cycling the master (franticly) got it off again luckily. Relay controlled by master was sticking...about all I remember about it.

Kevin

[Ed Lutz]

Hi Jim,

this bird is S/N 3370543. I'm glad that there is a better schematic to trouble shoot the electrical, however I'm having trouble openning the file. During the last flight I lost radio trasmit and the Gen Field circuit breaker tripped. After waiting a few minutes the breaker could be reset and radio transmit was normal. This occured about four times. Shut down was normal after landing and a recheck of the system was fine. A few days later during pre flight check I found the battery dead. My first reaction was I had left something on, but a careful check revealed no such luck. I replaced the battery with a fresh spare, only to fine that the system would power up when I connected the ground cable to the battery. I've replaced the battery before during regular maintenance, but this was a first. rechecked all switchs off. The master has no effect on or off and the run/standby switch is in the run position. After attempting to follow the schematic I pulled the GEN fuse and this dropped the power to the system. I then flipped the master switch on and all came up normally. I switch the master on off several times, and even started the front and rear engine, all looked normal. After shut down I rocked the switch a few more times, all ok. I then re inserted the fuse marked GEN, and rats the system repowered up.

Ed

[Ed Lutz]

Kevin,

Was it the relay coil that went bad or stuck contacts?

Ed

[stackj]

Ed,

I can send it to you as a JPG. E-mail me.

Jim.Stack@comcast.net

You can also call if you'd like. (804) 320-5588. I'm in the middle of an annual, so I don't get home until about 9PM EDT (6PM for you).

Ill be available until about 11PM (8PM).

Jim

[Pete Somers]

Ed

More than likely diode gone short circuit from the master switch to the battery contactor.
If you have diagram No 1570000 out of the service manual it is labeled 10.
From the master switch wire No KP818 goes to the diode and from the diode wire No KPB20 goes to another diode then to a fuse and finally to the battery contactor.

Cheers
Pete

[stackj]

Is number 12 in that schematic the generator fuse? (It's not labeled.)

[stackj]

Let's see if I can attach the JPEG.

[Ed Lutz]

Jim,

I'll PM you

Thank you, Ed

[Ed Lutz]

Pete,

You guys are great, thank you for sharing. I checked the system last night for battery drainage with the GEN fuse pulled and the battery connected. No lost, held 25.83vdc after 48hrs. just as I had left it. I fired up front engine, alternator on and vdc jump to 27.79 but no amp meter deflection, turned all radios and lights on but no action. Is the GEN fuse part of the amp meter circuit? Is the battery being charged without the fuse?

I'll check the diode tonight for short,

Ed

[Pete Somers]

Ed
The diode is a system that provides 28 volts to the alternators from the battery in the event the master switch or battery relay goes bad.
If this diode goes short circuit it will provide 28 volts direct from the battery to the main bus bar. There is a fuse also involved which I think is on the front firewall marked GEN that should also isolate this problem.
I cannot remember where the diode is mounted, however a trace using the wire number will find it.
The wire from the battery relay to the fuseholder is KPB13 the other side of the fueholder is KPB14 up to the first diode then KPB20 up to the diode which is more than likely short circuit.
You will need the GEN fuse in for the system to charge.

Let me know how you get on!

Cheers
Pete

[Ed Lutz]

Pete,

I believe you've nailed it. I"ll be at the hangar tonight to follow thru on this. Do you happen to know the diode part number?

Ed

[Pete Somers]

Ed
It give it as 6F20-D which has proberly changed p/n by now.

Pete

[Ed Lutz]

Pete,

that is still a good part number for the Diode. I just ordered two from Preferred Airparts at $16ea. I guess I could have crossed over to Newark Cat. and saved some $$$$. I got the enhanced schematic from Jim, really is much better diagram and easy to flow. Very professional!

Ed