Testing old Lionel transformers - voltages and circuit breakers.

The test procedures for the 5D tester have the transformer test specs. in them.  The 5D procedures are in the back of the Greenberg's Postwar Repair Manual.   Using a tender to test the whistle controller will require additional load to get whistle relay to work properly. Maybe put three passenger cars with two incandescent lamps in each one on the same track as the tender. 

The outpiput voltage of a transformer is determined by the ratio of the high voltage turns to the low voltage turns. If the output voltage, without a load, appear low, check that the input line voltage is the same as what is stated on the transformer label. 

The workings of traditional Lionel Train transformers and whistle controls has been explained here many times. The Lionel service manual has a section on the operation of their transformers. The whole manual is available online. Here is a link to the overview section on transformers.
And here is a link to the instructions on using the 5F Test bench, it also has the information David Johnson mentions above.

The circuit breaker tests use a fixed resistance as a load, and are based to how long it takes for the breaker to trip. Check the values for larger Lionel transformers and it should become very apparent why many of us recommend using external protection.

sometimes on some  transformers the moving part that rides on the transformer winding slips and so u get a lower voltage then the full out put as specified. and yes the zw transformer you'll hear a clicking sound and the  red short will come on where is a short and the breaker will trip and reset by itself that's the way there designed to due! as far as the 1041 transformer the whistle button sometimes they get out of adjustment due to the contact gets bent that's what make you have to press hard down. if you replace the push button  switch or straighten the contacts the problem will go away. mot of the older transformer you can adjust the linkages or swath contacts and make them work correctly again. you have to remember how old they are and have had a lot of use for many  many  years. most parts are available at tca train show if you need them!

For whistle testing, I have a test tender, a 6466W, that is fully serviced and works well.  I notice that on some transformers, e.g., the 1041, you have to hold down the button part way in just the right position for the whistle to blow.  What's going on there ?

A whistling tender by itself is not a valid test of a transformer whistle control circuit.
When you push the button partway, you are keeping the switch in the "pickup" position, which adds about 6 volts and runs all power through a rectifier, creating 1/2 wave DC.
So the relay activates and runs the whistle motor.

Continue to push the button in all the way, and the switch moves to the "holding" position, which puts a resistor in parallel with the rectifier. This allows some A.C. through, but keeps a little DC bias.
The DC bias keeps the relay closed.

The amount of DC bias generated is proportional to the load on the transformer. The whistle tender alone isn't enough load for a valid test.

The Lionel test benches with real whistles built-in for testing transformers and external whistle controls had more than a whistle in the circuit. They had an extra coil to properly load the device under test.
The last two test benches, the "D" and the "F" had a fixed load and used meters to check the condition of the whistle control circuitry.

Thanks for all of that advice.  I'm going to try the whistle test with another load on the track as suggested.  I've got plenty of passenger cars with lights.

I'll alos take a look at those references.  I read them at an earlier time but didn't understand the significance of what I was reading.  Now that I have four years experience, I'm finding that as I re-read Dr. Tinker's maintenance books, they are much more meaningful.  Always seems to be something new to learn, and that's one of the reasons I being in this game.