Problems with my overdrive
I have restored a 1956 Chevrolet Sedan Delivery and have made a couple of upgrades to allow it to run comfortably at today’s highway speeds. It has the 3.70:1 rear end used with a standard three-speed, but then I put in an original-equipment rebuilt BorgWarner overdrive standard transmission updated with a set of close ratio three-speed gears out of a Corvette of the same year. My problem is the transmission freewheels when the cable is pulled, but it won’t shift into overdrive. The transmission has been rebuilt and the wire harness is new and connected up correctly. What’s wrong?
The MoToRs manual from the period lists probable causes of your transmission not going into overdrive as:
• Lockout pull cable is not working properly or is too slack.
• Relay or circuit fuse is blown.
• Relay inoperative.
• Kickdown switch defective or not properly adjusted.
• Solenoid not actuating.
• Wiring incorrect, shorted or corroded.
• Balk ring open end installed away from pawl.
• Overdrive misaligned with transmission.
I would first make doubly sure that the electrical components are all working and connected properly and that the lockout cable is functioning correctly. Also it is especially important to make sure the overdrive is connected to the plus terminal on the coil. However, with the upgrades that you have done to the transmission, plus going with the higher standard rear end ratio, it is possible that something other than the usual problems listed above may be happening.
Your problem may be that the governor is not spinning up to a high enough rpm to activate the overdrive at the specified car speed. If you look inside the governor you will see that it has small levers with weights on them that are held in place by a spring. As the drive shaft speeds up during acceleration the little weights open out due to centrifugal force until they touch the inner surface of the governor casing, at which point they complete the circuit causing the transmission to shift into overdrive when you lift off of the throttle momentarily.
This normally happens at roughly 27-30 miles per hour in your Chevrolet. However, tri-five Chevrolets that were equipped with overdrive had 4.11:1 differentials in them, which meant the driveline spun about 11 percent faster than with the higher (numerically lower) standard rear end in order to reach the correct shifting speed. In other words, the 3.70:1 standard, no overdrive rear end would require you to travel about three miles an hour faster in order to reach the overdrive shift point because the governor works off of driveshaft rpm, which would be lower with the higher rear end at the same speed.
Also, achieving that speed would take a little longer with those close-ratio gears. A 1956 Chevy standard transmission had a 2.94:1 first gear and a 1.62:1 second gear. But a Corvette with a close-ratio three-speed had a 2.20:1 low gear and a 1.32:1 second gear.
That gearing took away a little offthe-line acceleration, but meant that the Corvette lost fewer revs between shifts thanks to the closer gear ratios. But in your sedan delivery those gears would translate to slower off-the-line acceleration in first gear, and move the normal shift point up in miles per hour as well. And it would give you a higher second gear too, which would mean that you would rarely have to shift into high at all around town.
It is also possible that the governor itself is incorrect for the application, in which case you might try changing it out for one with different gearing to see if that solves the problem. You could also possibly try lightening the spring load on the centrifugal weights to allow the device to shift sooner. However, there is a good chance you would not be happy with the outcome because the transmission may shift too early and bog the engine down as a result.
Also, while it won’t affect the overdrive, you may also want to check whether the speedometer gear in your transmission is correct for that 3.70:1 rear end. The one for the 4.11:1 differential would make the speedometer read too slow with the 3.70:1 rear end.