Electrical fuse problem - Page 2

stackj · #16 05-25-06, 10:45 PM

Ed,

Did the diode fix the problem?

Ed Lutz · #17 05-30-06, 06:27 PM

Jim,

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

Ed

Pete Somers · #18 05-31-06, 01:48 PM

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

Ed Lutz · #19 05-31-06, 07:08 PM

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

stackj · #20 05-31-06, 10:31 PM

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

Ed Lutz · #21 06-01-06, 09:57 AM

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

Pete Somers · #22 06-13-06, 03:35 AM

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

Pete

Ed Lutz · #23 06-23-06, 12:44 PM

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

Pete Somers · #24 06-23-06, 02:40 PM

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

Ed Lutz · #25 06-23-06, 03:53 PM

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

Ed Lutz · #26 07-13-06, 05:27 PM

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

Pete Somers · #27 07-13-06, 05:49 PM

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

stackj · #28 07-13-06, 05:58 PM

Ed,

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