Still working through my '55 TBird.  Upgraded the engine using a '57 top end and totally rebuilt the brakes.  Car runs great, does not overheat, stops and I'm getting used to driving with my knees bent  (Ha Ha).  Now on to the Overdrive tranny.   This car has a Borg Warner T86 3 speed O/D tranny.   When I bought the car it had already been converted to 12 Volts, but the positive ground remained.  I switched the cables and reflashed the voltage regulator.  It's now 12 volt negative ground and all seems OK.  The generator light works and the battery charges.

I have been driving the car with the under dash O/D cable pulled out.   I tested the car for the first time today with the cable in.  I now have a tach in the car, so I can see if the O/D is working.  Here are my questions.

Are any of the O/D electricals voltage or polarity sensitive?  To get the car to shift into O/D my understanding is you have to totally let off the gas.  Should I be able to tell when the shift into OD occurs?  Is it a big drop in RPM or just a little?  I am just getting into the O/D issue at this point.  I can tell the engine is freewheeling, but I'm not sure the O/D is working as it's hard to tell driving around my neighborhood.  I have never driven an old car with O/D before so I don't know what to expect.

When the car was converted to 12V did they change the 6V solenoid? I'm not sure how changing to 12V negative will effect the relay and governor either?
When it shits to OD you should feel about a 30% drop in rpm.  It should only freewheel under 28 mph

Check this site, it covers the BW overdrive pretty well.  http://www.fordification.com/tech/overdrive.htm

Ben HessTulsa,OK

Welcome to the wonderful world of B-W O/D. To answer your questions, none of the O/D electrics care about polarity. The solenoid does, however, care about voltage. It's an electromagnet and its operation depends more on amps than volts. Since a 12v system has 1/2 the amps of a 6v system the solenoid, though it will operate on 12v, won't like it much and will get hot. If it's in good shape it can last a good long while on 12v but it would be best to replace it with a 12v unit or place a voltage reducer in the line between the relay and the solenoid. There are a couple of sellers on eBay that have come up with reducers that actually work for the solenoid; the generic ones you get at the auto parts store or the usual suspect old car parts suppliers won't work. Been there, done that.
You do have to lift off the gas to shift into O/D. Depending on load you may have to lift completely. On light acceleration just a slight lift will do the trick and sometimes the shift is so seamless you may not even notice it. Usually you will hear the solenoid click and feel a slight lurch when it shifts up. Most of Ford's O/Ds were 30% so you should see RPMs drop by about 1,000. If you have the O/D enabled (handle pushed in) and it freewheels when you lift off above the governor cut-in speed then the O/D is not engaged and you'll need to diagnose it. Most O/D problems are electrical.

Go here for the official B-W operation/maintenance/repair manual:
http://www.oldcarmanualproject.com/manuals/trans/Borg-Warner%20Overdrive/index.htm

BTW, if you change the oil DO NOT use anything marked "hypoid". Many modern manual transmissions use it but the additives that make it hypoid will attack the brass synchronizers in vintage transmissions. The proper oil can be had at NAPA (if they don't have it they can get it), Tractor Supply (they call it Ford tractor transmission oil), and probably CO-OP and other agricultural suppliers.

Thanks for the replies.  As soon as I have a chance, I'll drive the car in third gear with the OD lever pushed in.  With the handle out, I run about 3,000 rpm at 60 mph.  We'll see what happens.  Something is happening when I drive it. I can feel it freewheel when I decelerate in gear.   Be back soon.

I drove an overdrive for several years. It was normal for the car to free wheel anytime the o/d handle was pushed in, regardless of the overdrive solenoid being engaged or not. That is why it is important to have it in reverse when parked so the car doesn't roll away. When shifted into reverse an internal mechanical interlock is actuated that locks out the free wheeling so the car won't roll away when parked with the o/d handle pushed in.

And apply the parking brake. Belt & suspenders.

If the O/D is working properly it shouldn't freewheel above the governor cut-out speed, about 21 mph. If it freewheels at any speed above that there's something awry.

The thing went in out of o/d ok, but I did have a bad problem of trying to throw a good shift to second gear. It had a good Hurst synchro loc  shifter with the gate adjusted right. I eventually put a regular 3 speed manual t-85 in which would shift ok. Maybe something was wacky.

Letting off the gas in OD at low speeds can feel a bit like freewheeling since there isn't much engine braking in OD.

I took the car out on a straight road and the OD is definitely working at least in high gear.  With the OD handle out my RPM is about 3,000 at 60 mph..  With the handle in, it's 2,000 RPM.  The OD shifts so smoothly and quiet I can't tell when the shift occurs.  This works for me as I will never use OD unless I'm on the highway.   A couple of questions please.  Is it OK to push the handle in and out when the car is moving?   If the car is in OD high and I pull out the handle, what will happen?   Is there any chance the car will start out in 1st gear OD with the handle in?