Diodes & charging

[rmorris]

I've got a alternator charging issue.
Started with rear not charging, then while troubleshooting the front stopped as well.
1967 T337C, but the VRs upgraded via field approval years ago to the parallel VRs used in later models.
My question is about these diodes (see pic). Got 1 per engine. Looks like it's wired in such a way that 1 wire to battery post on alternator (on top of diode) and other wire going to master solenoid.
My thinking is that the diodes only allows one way traffic for voltage.

When I i just turn on master (engines not yet running), I see 24v on bottom posts for both engines. But on one top post I see negligible (like .8 ), on the other I see the full 24v.
Which reading is correct, what should I see on top and bottom 9f that diode? Also, what readings on the screw terminals on outer diode box?

[hharney]

I have had to replace diodes on these units. I don't think it's big stuff if you just bench test and buy replacements at the typical electronic catalog stores

[rmorris]

Yup, the diode replacement is pretty simple. But what I'm looking for is:
1) proper multimeter readings on either side of diode engine on and engine off
2) proper readings on reach of those terminals on the outside of the diode box... on that little circuit board
3) any p/n & sources for just that little circuit board

[B2C2]

I have a 1969 T337D which I believe is pretty similar to your airplane. For my airplane I had a shorted field winding on the front alternator. The older regulators are not protected for this, and will fail. So I lost both regulators as I switched from front to back, so no charging but the real issue was the alternator. I now have Zeftronics regulators for front and rear, as they have short circuit protection. If you don't have these I would strongly recommend them. In the course of all this I replaced the diode on the front but it wasn't actually bad. These diodes are pretty durable. For the diode, if you want to test it, with the engine off one side (cathode, not connected to the alternator) will read battery voltage. The other may read anywhere from zero to around 24V depending on the meter you are using. Sounds like this is the case. With the engine on, the voltage drop across the diode measured anode to cathode should be around 0.7V. However this will only be the case if the current is flowing, which it won't be if the charging system is out. If you remove the diode from the system and want to check it use the ohm meter function and test it both ways. A good diode will read a low resistance when the positive lead is applied to the anode and negative lead to the cathode, and a high resistance when the leads are reversed. You can do this in place by disconnecting the cable with the screw terminal so the diode is only connected on one side. I would suggest checking the field winding resistance for your alternators. I don't remember what they should be but you should be able to look this up. Or just check them both and see if one is much lower than the other. If you have a failure probably only one is bad.