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« on: November 07, 2014, 05:02:32 PM »
1949 and the F-series trucks got OD for 1953. (Before 1941 & 1949 Ford, Lincoln & Mercury used a two speed axle to lower engine revolutions – The Columbia axle - not discussed here).
To understand the B-W OD, the features it has and how to properly drive it, one needs to understand the driving conditions that existed during the 1930s & 40s. At that time all vehicles were operated by a low horsepower (80-100HP) low compression engine, with a very long stroke (4+ inches would not be unusual). This engine design is very appropriate for in town driving conditions. (start & stop, no hills and speeds that do not exceed 40 mph).
To make it possible for a low power 1930s-40s engine to climb hills and haul loads manufactures installed very low rear axle ratios. (High numerically) 4:10+ was common and some vehicles approached and sometimes exceeded a 5:1 ratio. These ratios made it possible to climb a hill (road building technology was equally behind 21st century techniques, grades were steeper back then) but would also spin an engine very fast at highway speeds. (60 mph was exceptionally fast in 1940!) Overdrive transmissions made modern freeway speeds possible while saving gas, oil and engines. (Thrown rods were common in the 1930s & 40s).
A Borg-Warner OD has sometimes been called a ‘cable-operated’ overdrive...IT IS NOT! A B-W OD is electrically operated. The only control visible to the driver is a cable located somewhere on the dashboard. However all this cable does is ‘lock out’ the entire system. When the knob/cable is pulled out the transmission operates entirely as a standard three-speed transmission. With a properly-operating system and knowledgeable driver, the cable will be pushed in 90% of the time. When the OD cable IS pushed in, the first operating feature of the B-W OD transmission becomes apparent it “freewheels”. This means that while the engine can drive the wheels, when the vehicle coasts, no power is fed back to the engine. No engine braking, just coasting.
When the electrical components of a B-W OD are not functioning this is all that the transmission will do - freewheel. Freewheeling at speed is dangerous and can lead to loss of control due to increased brake use. However a properly functioning B-W OD does NOT freewheel at speeds above 28 mph. In a properly functioning OD engine braking DOES happen. The thought that engine braking is not possible is a major misconception for a B-W OD transmission.
The other operating control for a B-W OD transmission is not readily visible to the driver (and not visible at all from the driver’s seat of a 67-72 Ford truck) it is the kick down switch. The kick down switch is a push button type switch that is operated only when the accelerator is fully depressed (mashed to the floor) Before 1967, Ford trucks used a switch design that goes all the way back to the 1930s. It protruded through a hole in the floor and was activated by the back of the gas pedal. Starting in 1967 (1963 for cars) the kick down switch migrated to the firewall and was operated by the throttle linkage.
Driving with overdrive
Driving a B-W OD vehicle starts off with the cable pushed in and the truck pulling away in first gear as normal. The truck accelerates and the driver shifts into second gear. At approximately 28 mph (BTW - nobody knows why 28 mph and not 25 or 30 mph) a small click can sometimes be heard from under the hood (activation of the relay on the firewall by the governor on the transmission) the driver then backs off on the gas and the transmission automatically shifts into overdrive. The shift feels exactly like a shift from an automatic transmission.
The driver must release the accelerator completely such that the power comes back from the wheels toward the engine to complete the shift to OD. In an automatic transmission a shift will occur (eventually) even if the throttle is held open. For mechanical design reasons in a B-W OD set up, the driver MUST let off the throttle completely to allow the shift to happen.
With the vehicle in second gear overdrive, the overall ratio is not quite as high as third, but higher than second. This is a perfect ratio for in town use. Between stoplights you don’t need to shift into third. However, if you continue accelerating and shift into third (from 2nd OD) and you will then be in third gear overdrive, the perfect ratio for the highway.
An often asked question is “Can you overdrive first gear?” The answer is yes, however you would have to exceed 28 mph for the system to activate. The real question then becomes ‘Can you exceed 28 mph in first?’ 28 mph is pretty fast for first gear so 1st gear OD is rarely achieved. This means that while a B-W OD transmission has sometimes been called a ‘6 speed’ it is really provides only 5 practical forward ratios.
While you are in overdrive if you back off the gas you will feel engine braking. (second or third) Because it is an overdrive ratio, the braking is not as evident as if the transmission were in 2nd or 3rd direct drive but the transmission does NOT coast. If the transmission does coast, the OD set up is not operating properly.
Third gear OD is great for gliding along at highway speeds but it can lack power for passing or hill climbing. To get the transmission out of overdrive, mash the gas pedal to the floor. The engine will rev up and when the pedal compresses the kick down switch, it will suddenly and quickly shift back into direct drive. This shift feels exactly like the kick down of an automatic transmission. Complete your pass or top the hill in direct gear, then let off the gas completely for a moment and the transmission will shift back into overdrive.
If you begin slowing down and shift from third OD to second OD, then slow down further as if approaching a stop light, as soon as the speed falls below 28 mph the power to the OD will be cut and the transmission will then be back in direct drive.
This automatic loss of OD is a designed in safety feature because you must start out from a complete stop only in direct drive. To try to start out from a standing start in overdrive would create an incredible strain on the driveline is certain to damage something or at least result in increased clutch wear.
Owners have sometimes rewired their systems and by-passed the governor by adding a manual switch to turn the system on & off. If this has been done it is very easy to forget and attempt to take off from a stop in first gear-overdrive. Doing this is as fool hardy as by-passing the neutral safety switch on an automatic transmission.
So as you slow down below 28 mph you are automatically put back into second gear direct drive. HOWEVER, because the sun gear is no longer being held (no OD ratio) the transmission will freewheel. This event can be very surprising to someone who is not familiar with the operation of an OD transmission as suddenly there is NO engine braking! Since this occurs only below 28 mph there should not be much need for engine braking and using the foot brake to stop the truck should be just fine.
But this is also where another novel aspect driving with a B-W OD transmission becomes apparent. All B-W OD transmission set ups (from all manufactures in all years) used a non-synchronized first gear. They never built a B-W OD transmission with a synchronized first gear.
By the 1960s full syncro transmissions were fully available, so why didn’t they make a fully synchronized OD? Turns out a B-W OD trans doesn’t need to be synchronized!
Drive in second gear OD, slow down below 28 mph and the trans falls out of OD and into freewheel mode. Step on the clutch and pull the lever into first and you will find it slips into first gear as easily as if it were synchronized! Even if you are rolling.
This ‘synchro effect’ happens because the freewheeling clutch prevents power from being transmitted from the driveshaft into the transmission. Push in the clutch and there are NO forces on the gears. When you pull the lever into first gear it slips in easily. The most clashing you get when shifting into first gear at a rolling speed, is a slight “ratcheting” of the gear teeth that you would expect if you were shifting from neutral into the low granny gear of a truck 4 speed or the reverse gear in any transmission.
The freewheeling feature also makes clutchless shifting possible. Start in first, pull away and then WITHOUT depressing the clutch pedal, back off the gas and shift into second as easily as if you had pushed the clutch! If you shift into third gear before 28 mph (and engaging the OD) again there is no need to depress the clutch pedal to make the shift. If however you have allowed the transmission to engage the OD in second (backing of the gas) then depressing the clutch is necessary to shift from 2nd to 3rd.
These operational features are what made the B-W OD transmission very desirable in the days before fully automatic transmissions. (Especially with the ladies). No clashing shifts into first; No clutch necessary to shift into second; Automatic shift into second OD around town.
One other aspect of the freewheeling clutch needs to be discussed; parking and pushing.
Pulling out the OD cable on the dash operates a lever on the side of the transmission and mechanically locks the sun gear to the planetary gears. (Ideally the cable should only be pulled out while the vehicle is stopped).
When the OD cable is pushed in then the whole system just freewheels. This happens as long as the vehicle is below 28 MPH (or the system has no electrical power) the OD will not engage. This means that to push start an OD equipped truck, the cable needs to be pulled out for the wheels to send power to the engine when the clutch is released.
Parking is the other situation that pulling out the OD cable is necessary. If you park pointing downhill and put the shifter into first, second or third without pulling out the cable, the forward motion will freewheel over the engine and the truck will roll away. There is no compression lock. To overcome this you can either pull the OD cable out or place the shifter into reverse.
In order for any OD transmission to back up, the freewheeling clutch MUST be locked out. There is a shaft/rod built into the transmission that automatically accomplishes this whenever the transmission is put into reverse. So to park safely (set the brake) and then either pull out the OD cable OR place the shifter into reverse. This locks up the driveline and prevents all rolling.
Couple the OD transmission with the very low rear end gears that existed in the 30s & 40s and you effectively had an automatic transmission. Owners manuals from that time suggested that around town you could (should?) start out in second gear slipping the clutch only slightly, (possible with a very low rear end gearing) then letting the trans shift itself into second overdrive. No need to touch the lever and limited use of the clutch, what more could you ask for?
Speaking of rear end gears how are they affected by the overdrive? All B-W ODs overdrive function at a 0.7 overdrive ratio. Since all transmissions use a 1:1 ratio in high gear, to find out your final drive ratio in OD simply multiply the rear end ratio by 0.7.
4.11 = 2.87 3.70 = 2.59
Even a very low 4.56 ratio can be tamed by an OD into a very functional 3.19.
This is what makes the B-W OD so appropriate for use in a truck. In a truck a low geared rear end could help it haul a heavy load but you don’t want to spin the engine so fast at highway speeds or when the truck is empty. With a functional OD you get the best of both worlds, strong low end pulling AND practical highway and unloaded use.
An OD can also compensate for small diameter wheels & tires. Smaller wheels & tires spin an engine faster. 16” wheels turn the driveline slower than 15” which spin the driveline slower than 14” (as the wheels/tires get bigger they carry more weight and lower the engine speed but they also reduce the available power. All things are a compromise) All these factors need to be considered when outfitting a vehicle with a B-W OD transmission. If you use a rear axle ratio that is too high (numerically low) and/or couple it with wheels/tires that are too big in diameter a vehicle can actually slow down or use more fuel when it is operating in OD.
Troubleshooting
The Borg-Warner overdrive transmission combines both mechanical and electrical components. The mechanical components are very stout and as long as the transmission AND THE OVERDRIVE unit is filled with gear oil (see maintenance below) the only problems should be electrical, which are very easy to trouble shoot.
Take an OD truck out for a test drive with the OD cable pushed in, if it freewheels above 28 mph (and you don’t feel the automatic shift) the trans is OK but there is an electrical problem.
The OD electrical system is protected by just one fuse clipped to the relay on the firewall. This fuse gets power whenever the key is turned on. Begin your diagnosis by checking for power at both sides of this fuse. Because the relay is under the hood, the fuse and its mounting gets very corroded, very easily. Remove the fuse, and clean all the contacts thoroughly. If there is no power here, trace the wire back to the key switch to find the break.
Testing the system
If you have good power on both sides of the fuse on the OD relay, the next check is made under the vehicle. At the back of the transmission is the governor. This is a cylinder shaped device that is driven by the speedometer gear with ONE wire coming out of it. Inside the governor are weights that spin with the driveshaft. When they reach the magic speed of 28 mph, the wire going into the governor is grounded. The full circuit is that simple!
For some reason the insulation on wire at the governor is always made from the same cloth used since the 1930s. This wire always seems to have a frayed spot. With the ignition key turned on, jump the wire to ground.
(There could also be a wire connector between the governor and the OD harness. Pull it apart and ground the wire that was going to the governor). However you ground the governor wire, you should then hear a click from the relay on the firewall.
Power comes from the fuse through the relay, which is then activated whenever it is grounded.
Power from the relay to the governor does pass through the kick down switch. If there is no relay click when grounding the governor, check for 12 volts at the wire and trace it back to the kick down switch and then back to the relay to find the open circuit.
If you have 12 volts at the governor and you hear a relay click when you ground the governor, then the relay should be sending power down to the solenoid. (the steel can on the side of the trans) As soon as the solenoid gets power, it too should click. (the solenoid is trying to push its plunger shaft into the trans) If the solenoid does not click, check to see that it is receiving 12 volts of power directly from the relay.
If you have determined that 12 volt power is being sent to the solenoid from the relay, the solenoid can be tested by providing it with 12 volts directly. There are two wires on the solenoid. One wire activates the plunger and if it is given power, it should engage. The other wire goes directly to ground (part of the kick down circuit). Clip your 12 volt power source to one of the wires, it should either activate the solenoid or be completely grounded. (You have a 50% chance of picking the correct wire)
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