Electrical fuse problem
 

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?

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?

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

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

Kevin,

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

Ed

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

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

Pete
 

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

Schematic
 

Let's see if I can attach the JPEG.

Jim,

I'll PM you

Thank you, Ed

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

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

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

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

Pete

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

Ed,

Did the diode fix the problem?

Jim,

After replacing the diode the problem remains. Pesky little ioinic greminlin! I'll be looking at the RUN/StandBY switch next.

Ed

Ed
Did you replace both diodes?
You can only get battery voltage direct thru these diodes if they go short circuit. If you remove the fuse marked 11/12 on the diagram and the power goes off then the problem is a diode.
With the run/stby switch in the run position there the wiring goes nowhere.
Pull out the master switch and using a DVM check wire no KPB18 to KPB10 if there is 28v on this point the only place it can come from is the diodes.

Pete

Hi Pete,

I replaced the diode that was on the Fuse rack, above the battery box. You maybe right, but I don't see where the second diode is located on this 67' 337B. Any idea where to look?

Ed

Ed,

This problem is perplexing.

I do not see any way a bad diode in that circuit could cause the master solenoid to engage. I can see a shorted diode causing the fuse to blow. Those diodes should be allowing current from the alternator field to flow back to the bus via either the alt field circuit breaker or back to the battery via the other diode, through the GEN fuse to the positive battery lead attached to the master solenoid.

The only way I can see the master relay engaging by inserting the GEN fuse is by having two faults.

1. the circuit between the two diodes is grounded, and

2. the wire from the fuse to the master solenoid is connected to the wrong terminal on the solenoid, i.e. the same terminal which the master switch grounds.

It might also be interesting to disconnect the master switch lead from the master solenoid to see if the symptom changes. This would be rare but could be caused by corrosion inside the master switch. Corrosion in electrical circuits does strange things when energized.

Try verifying the wiring connections and if that is OK, try disconnecting the master switch lead from the solenoid.

Also, be sure there is insulation between the diodes and the airframe. I think the metal case of the diodes is connected to their cathodes and if it is not insulated from the mount, it will be grounded. That coupled with the miswiring described above would cause the problem you have described.

I have not looked for the location of the other diode, but I expect it would be behind the panel somewhere between the master switch and the Run/Stby switch. There is also a resistor / diode circuit board behind the interior left paneling just forward of the circuit breaker panel. I don't think there is a third diode on this board though. This board disables the avionics bus when either starter is engaged.

Maybe I should ask an intelligent question. When you insert the GEN fuse, do you hear the master solenoid engage? If you don't, probably all I've said above is inapplicable for your problem.

Jim

Jim,

I'll verify that it's the battery soleniod that is picking up and pull the master switch out for a check up. I'm becoming a contoursionist to look under the panel. Not easy for us non lite weight types. The location of the second diode is the key hunt

Ed

Ed
Nothing heard from you, did you fix the problem with the electrics?

Pete

Hi Pete,

Well, I finally located the 2 diodes in question. They are located on the left side, just behind the fire wall, forward of the panel and perfectly situated where I can't reach them. Has Anyone ever replaced these with any easy?

Eddie

Hi Ed

Now i remember were they are! I have been trying to think where they put them.
I seem to think i had to bring to instrument panel forward to get at them, with the power pack up there it is one of the most awful jobs.
You need a very slim mechanic with long slim fingers and hands fitted with universal joins.
There is no easy way to do this, apart from removing the power pack, i would not go that far. I would try and run another wire from the fuse to the new diode then to the master switch etc.

Let me know how you get on.

Pete

Pete,

Thanks for confirming the degree of difficulty to replace these diodes. I can reach the wires using a mirror from the top avionics port. I will relocate the diodes well within reach.


Eddie

It is done!

Replacing these diodes is meant for a very small person (12 inch height max) that can wiggle around and get inside the well. After serveral pints of blood, sweat and tears I've managed to replace these little buggers back in place and all is back to normal. Wow!

Pete Sommers and Jim Stack, thank you for your assistance, you dudes are great.

Now I got to let my arms heal from the cuts, scrapes and bruises. I still love this Bird and this web site is the best.

thanks guys

Eddie

Hey Ed
Great news that the problem is solved.
I remember doing that job and ended up with shredded arms and losing your temper, getting stuck under the panel, not and easy job. Taking out the instrument panel help a lot. The joys of aviation!!

All the best
Pete

Ed,

Congratulations on the fix. I know you are glad to have that behind you.