bench-test the overdrive function

First, check the free-wheeling and lockout

The following instructions assume a 2-terminal solenoid, and a single-terminal governor. For other types, you had best consult a wiring diagram to determine how to do these tests.

1. get a battery of the right voltage for the solenoid in question
2. run a clip lead from the ground side of the battery to the transmission case (might as well 
observe "as installed" polarity here, though it really doesn't matter for the purposes of this test.

3. connect a clip lead to the other battery terminal, and touch the other end to each terminal on the
 solenoid. One terminal should do exactly nothing; the other should show a small spark and cause the
 solenoid to click or go "thunk". The sound is pretty loud in a quiet room.

If you get a fat spark, but no sound, chances are the coil is shorted or burned out. If the solenoid 
cycles normally from one terminal, but the other gives you a fat spark, then the internal grounding 
switch may be shorted.
4. Now connect a test lamp (any 6 or 12 volt lamp, as needed) between the two solenoid terminals. Then
 apply power to the terminal which you have determined is the one that causes the solenoid to 
activate. You should hear the click or thunk, and see the lamp come on as the sound happens (not 
before). That tells you the grounding switch is working, which is needed for the full-throttle kick-
down function to work.
5. Now put small flags of masking tape on both input and output shafts as pointers for counting turns,
 and ensure that the lockout lever is all the way back against its stop. Energize the solenoid, and 
begin turning the input shaft by hand. You may hear or feel some action in the transmission early-on 
in the first turn as the internal parts shift into OD ratio. From that point on, it should turn 
smoothly, and you should see the pointer on the output shaft moving faster than the one on the input
 shaft. For every 7/10 of a turn on the input shaft, the output shaft should make one full turn. You
 should also, at this point, have no free-wheeling. Turning the output shaft either way should cause 
the input shaft to turn the same, only a little less far. So far so good? We now know it can go into 
overdrive.
6. Now disconnect power from the solenoid, and continue turning the input shaft. You should hear some
 sound, and then observe the output shaft turning in lock-step with the input shaft, provided you are
 turning it forward, which is clockwise as viewed from the front of the tranny. If all you own is a 
digital watch, I'm sorry, you are out of luck. You should be able to see that it free-wheels again.

7. Now, we'll bench-test the governor. First, remove it from the transmission. A pipe wrench works 
well, if used with care. They normally aren't in there super-tight. Chuck the tip of the governor
 shaft in a variable speed electric drill, or use a piece of rubber hose as a coupler. Connect one lead from your battery to the governor case, and connect a test lamp between the governor terminal
 and the other post of the battery. Holding the solenoid in one hand, and the drill in the other,
 slowly spin up the drill. At some point, the governor should click or bump (perhaps more easily 
felt than heard), and the lamp should come on, and it should stay on as the drill revs faster.
 Now slow the drill down, and the lamp should go out, but at a slower speed than which it came on
 at. You may want to cycle the governor through this on-off cycle several times, to be certain that
 the turn-on speed is in fact higher than the turn-off speed. This "dead band" is important, because
 if it were not present, the entire overdrive unit could cycle repeatedly if you drove at exactly the
 cut-in speed, which traffic might force you to do on occasion.

If you can feel the governor's action, but the lamp does not come on, first check your connections,
 since these ad-hoc hookups can fall apart sometimes if you aren't paying attention, ask me how I 
know. If you are now sure that the governor is not switching, the most probable cause, and very
 common indeed, is simply oil on the contacts. You can easily remove the cover, rinse the contacts
 with brake cleaner, and pass a piece of fine sandpaper, followed by clean card stock, through the
 contacts to burnish them. Replace the cover and try again. If it works, then you are away to the 
races.

At this point, you can be fairly confident that the transmission will shift into and out of overdrive 
if it is properly installed, and if the electrical accessories, like the relay and kickdown switch are
 good. There is no guarantee that it will be smooth-acting or quiet in operation, as these qualities
 are a factor of the degree of wear in the overdrive's internal mechanical parts. But if you have had
 the top cover off the transmission, and it looks good in there, and if you didn't get a whole mess
 of filings when you drained the oil, then I'd say it's worth your while to install the transmission
 and try it out, if it also passes the bench test above. There's nothing to be gained by tearing down
 a perfectly-functioning transmission. If you install it, and it proves to be noisy, or displays some
 other problem, then you can pull it out and tear it down in good conscience.

Gord Richmond (SDC Forum Feb 2009)