over-voltage

[kilr4d]

Originally Posted by edasmus

The diode looks like a cylinder maybe an inch long and having a diameter of about a half an inch. The thing was enclosed inside a plastic case. There is a small metal pole protruding from this cylinder that a wire attaches to. Now for all I know the diode is encased inside this cylinder or maybe the entire cylinder is the diode. I have no clue how all this stuff works. That's why I pay these guys. Well, the little metal pole that is protruding from the cylinder was loose. You could actually wiggle it around inside the cylinder. Comparing it to the other diode on the firewall, that one was very tight. No play whatsoever. Replacing that $8.00 diode seems to have solved the problem. Voltage across the system running on a either single or both alternators is rock solid at 28.5 volts plus/minus 0.1 volts and no over-voltage tripping has yet to occur. Granted, I haven't flown much yet since the repair but so far so good. It's been a long winter. Better Wx ahead. Stay tuned.

Interesting.

Could you grab a picture the next time you have the cowl off?

Thanks,

John

[edasmus]

Here's your photo....

[kilr4d]

Originally Posted by edasmus

Here's your photo....

Thank you.

[gunrunner99]

I have been looking at my firewall trying to see where this is and no luck. Could you tell me where this is located on the firewall. Inside outside? Thanks. I have this problem intermittently.

[SkyMac]

Hi Mike

On my 74G; this part is located mid firewall, basically above the vac pump. It is part of the dual alternator control system.

Ed or one of the other members can tell you if this part is even relevant to your plane as I am not familiar with the earlier models.

Dave

Ps: Welcome to Skymaster ownership and this forum, members in the forum are all very helpful and passionate about Skymasters. No matter how trivial you may think your request may be, ask away.

[edasmus]

Ditto SkyMac's post. Remove front cowling halves and look top dead center of the firewall. My airplane is a 73G model and would be identical to SkyMac's 74G. I have no idea what a 65 model would look like. Might be different. The diodes are enclosed in a case though so likely difficult to see anyway.

[cessnadriver]

Hello. I have a 1965 C337 and I know exactly where to look for the diodes. Open the avionics panel (the large panel outside-top of the front engine cowling) look outboard along the inside left sheet metal side of the fuselage, you see some large shunts about 1 1/2" wide and about 6" long, they have a wavy appearance. Just forward of this are two diodes screwed to the side of one of the shunts. To test these diodes your mechanic will have to cut the wire on the top of the diode and check it with an multi-meter WITH A DIODE SCALE. The meter should read flow through the diode with meter leads connected one way and NO FLOW with the leads connected the opposite way. The diodes are hard to remove but doable! Remove the two 6-32 screws to get at the nut on the bottom of the diodes and if you're smart your have your A&P mechanic re-locate these diodes for better access. The diode PART NUMBERS: original-6F20-D, superseded by 20H3P, superseded by (1N3881 this one is more common in non-aviation appliances and can purchased at www.mouser.com for about $6 each.)
I hope this helps, I've attached a charging system diagram the diodes are ITEM 10. This diagram is for the 38 AMP alternators the diodes are the same for the 30 AMP alternators!
regards, BILLS