Mustang MonthlyHow To Drivetrain
Installing An Automatic Overdrive Transmission
Get into efficient overdrive cruising over a weekend
Performing an AOD swap in a classic Mustang is easy today thanks to the automotive aftermarket. These companies have done the homework already. All you have to do is tell them what you have and what you need. When the parts arrive on your doorstep, they can be installed in a weekend.
Few companies know more about automatic transmissions than B&M Racing & Performance, which has been building high-performance automatic transmissions for 60 years. Cal Pony Cars was founded by Ray Harrington, a Mustang enthusiast who took his profession and turned it into high-quality reproduction parts for classic Mustangs. Ray noticed how much fabrication had to be done to swap an Automatic Overdrive (AOD) transmission into classic Mustangs. That's when he and his people went to work conceiving an all-in-one kit.
Ray wasn't the only one with an eye on what enthusiasts needed. Inland Empire Driveline developed custom-length steel and aluminum driveshafts for the conversion, available from Mustangs Plus for 8- and 9-inch rear ends. Classic Tube has also developed transmission cooler lines for vintage Mustangs with an AOD conversion.
Follow along here as Gil Roiz at Mustangs Etc. replaces a '65 Mustang's C4 with a B&M AOD.
Throttle Valve Cable Adjustment
There has long been a lot of confusion about the AOD's TV (throttle valve) cable adjustment. Ford advises the use of a pressure gauge, which most novices don't have nor do they know how to use. We will caution you that TV cable adjustment is something you must get right or risk serious transmission damage.
First, some history about Mustang transmission operation. Your Mustang's original C4 transmission has two shift control systems—the vacuum modulator (throttle valve) and a kick-down system. The AOD has one cable-controlled system. C4 shift points are controlled by a combination of intake manifold vacuum and kick-down lever position. In normal driving, the kick-down cable is not involved. However, the throttle valve is always working via manifold vacuum, which depends on how much throttle you're using. With light throttle and high intake manifold vacuum, control pressure is low and you get smooth shifts up or down.
As you step on the gas, intake vacuum drops and control pressure increases to both delay and firm the upshift. You want a firm upshift at open throttle for a solid transfer of power and minimal clutch and band slippage. Punch the throttle wide open and the kick-down cable contributes to even higher control pressure and the appropriate downshift to Second or First gear for that burst of power. You don't want low control pressure at wide-open throttle, which will cause slippage, extreme heat, and serious damage.
The AOD does not have a vacuum modulator, but instead a throttle valve cable that does the work of both a vacuum modulator and a kick-down linkage. Instead of getting a vacuum signal as an indication of throttle position like we see with the older C4, C6, and FMX transmissions, the AOD's TV cable yields actual throttle position via a direct mechanical cable connection.
Control pressure is modulated by how far the throttle valve cable is moved. The more you step on the gas, the higher control pressure becomes to both delay upshift and increase control pressure for firmness. Transmission professionals we've spoken with suggest you adjust the cable with the throttle closed (idle) and no cable slack, but no cable tension either. Then take your Mustang for a road test at light throttle. If the AOD upshifts too soon under mild acceleration with quick 1-2-3 upshifts (overdrive doesn't come in until roughly 40 mph), you don't have enough cable tension (too little control pressure). Gradually increase cable tension until you get evenly spaced firm shifts under mild acceleration. Do not go into wide-open throttle until you are certain you're getting firm upshifts without slippage. Slippage will cost transmission life. It's a good idea to have a transmission shop double-check your adjustment with a pressure gauge.