Ken Zimmer Swapped a Modern Ford AOD Trans into His 1967 Mustang. Now it has a Shift-Timing Problem. We’re Gonna Fix It.
Ken Zimmer swapped a 351-Windsor into his 1967 Mustang convertible. Equipped with 1969 351W heads, the slightly over 9:1 compression motor sports a mild flat-tappet hydraulic cam (0.472/0.496-inch valve lift, 214/224 degrees duration at 0.050-inch tappet lift), a 600-cfm Holley carb, and long-tube headers. “I installed 3.91:1 gears in the rearend to get me grinning around town,” explains the West Covina, California, resident. “However, I did not want to be screaming on the freeway, so I also installed an AOD [Ford Automatic Overdrive] transmission to calm things down.
Ever since it was rebuilt, Zimmer says the AOD has experienced shift-timing problems. “It upshifts fine from a standing start. The line pressure is on spec, but when slowing down, the transmission does not like to downshift; instead, the car almost dies before it downshifts. If I have to change from slowing down to accelerating again, it will usually not want to downshift unless I give it a good squirt of gas. I had the shop look at it again; they ended up replacing the governor and valvebody with parts from a ‘hot rod’ Lincoln. But the AOD is still stubborn in dropping a gear or two when slowing down.”
We had Zimmer drive the Mustang over to Advanced Engineering West (AEW), where Mark Sanchez confirmed the problem after a testdrive, reporting, “The car was not downshifting completely until it rolled to a complete stop, still in Second gear. Upshifts were slightly delayed as well. In fact, it would drive away from a stop in Second gear, never in First gear. Always taking off in Second will eventually burn up the trans because the excess load put into the trans makes it run too hot.”
On AOD and other transmissions that use a throttle valve (TV) instead of an old-school vacuum modulator, proper TV-cable adjustment is critical for achieving proper shift quality and timing-but a different rear gear ratio, camshaft, and/or vehicle weight than the core transmission’s original stock vehicle application also affects shift timing. In Zimmer’s case, the so-called “hot rod” Lincoln LSC governor and valvebody should have been a step in the right direction: An LSC weighs about the same as Zimmer’s 1967 Mustang. In terms of OE valvebody and governor calibration, the LSC’s 3.73:1 stock gear ratio is a closer match to Zimmer’s 3.91-geared rearend. A stock 1980s’ AOD-equipped Mustang, although weighing in about the same as both the late LSC and early ‘Stang, had only 3.27:1 rear gears stock. Knowing all this, Sanchez suspected a TV-cable or linkage problem.
Sure enough, after popping the hood, Sanchez says, “I found the TV cable frayed at the carb throttle linkage. It told me the aftermarket TV cable was being stretched, a clear sign it was adjusted too tight.” High idle TV pressure at the trans’ TV test port confirmed the visual diagnosis.
Additionally, the carb’s throttle-shaft linkage lever had somehow been bent inward; the accelerator cable ball stud that also secured the top hole on the TV-cable corrector bracket was smaller than the holes in the bracket and carburetor throttle lever; and the bottom TV-cable mounting bracket retaining bolt was prone to loosening because it used only a plain hex nut and no washer, rather than a locknut and flat washer that’s preferred on any connection that’s an axis of rotation. During a testdrive, the TV cable retaining ball actually fell off the linkage!
The Fix: Carb Side
Sanchez straightened the carb’s throttle linkage and installed the proper-size shouldered ball stud. A new TCI AOD TV-cable corrector bracket replaced the existing TV-cable bracket at the carb. Sanchez explains, “You need to have the proper geometry of the throw-the amount of throttle movement versus TV-cable movement. We didn’t know if Zimmer’s old bracket was correct or not, but the TCI product is a known good piece.”
Typical aftermarket conversion TV cables like the one on Zimmer’s Mustang have a different configuration and method of attachment than stock-style TV cables; they’re actually close relatives to common hood-release cables. A wrecked late-model Ford Crown Victoria “donated” its hood-release cable to replace the Mustang’s damaged TV cable. “You can grab them up quickly and cheaply right now at any wrecking yard,” Sanchez relates. “They’re the same material and diameter as aftermarket conversion TV cables. The longer the cable, the better. You can cut off any excess after installation and adjustment, so I prefer ones from fullsize cars.
“If you have a collapsed sheath, the complete hood-release cable with outer sheath is a more than adequate replacement. In this case, because Zimmer’s old TV cable had a fancy stainless braided outer sheath, we pulled the cables out of both sheaths, threw away the frayed cable, and then inserted the Crown Vic cable through the braided sheath starting from the trans end. The end with the ferrule attaches to the trans adapter bracket and is the same size as the one on the TV cable. The cut-off end goes through the sheath, the carb attaching fitting, and cable end stop. The stop tightens with a set screw and its position can be changed as needed during adjustment.”
The Fix: Trans Side
With the throttle closed and the replacement cable and TCI corrector bracket installed, Sanchez adjusted the cable’s carb-side “stop” position to remove all cable slack at the trans when the trans’ TV valve is in the fully relaxed (forward) position as verified by zero gauge pressure at the test port. Correctly adjusting the cable at the carb uncovered a linkage design issue at the trans end. Sanchez explains, “With the carb safety-wired to simulate wide-open throttle [WOT], we could not get the correct trans linkage arc and rearward length of travel to completely compress the TV valve inside the trans.” This is likely why the old cable was adjusted too tight at closed throttle, in a MacGyvered attempt to develop adequate top-end pressure.
It turns out that, at least on this particular trans, the existing aftermarket cable-to-trans TV linkage adapter bracket was too short to generate full pull at WOT. Sanchez fabbed a bracket extension out of steel bar that increased the arc ratio to develop full rearward linkage-lever travel, allowing correct, linear TV-pressure rise from zero at idle all the way to the approximate 80 psi needed at WOT. If you have to perform a similar mod, be prepared to go back and adjust the carb-cable stop screw again; in fact, it may be necessary to go back and forth several times. After final adjustment, cut off any extra cable length at the carb, leaving about 1 inch excess for any future adjustment needs.
Mark Sanchez did a great job. He pretty much fixed what no one was able to fix.” – Ken Zimmer
With the cable fixed and the linkage at each end perfected, the trans now shifts the way Ford intended. Under standard city part-throttle driving, upshifts and downshifts now occur consecutively in all gears and at reasonable intervals. The trans runs cooler and should last a lot longer.
Don’t piece it together-buy a complete and correctly engineered kit. Take time to verify the TV linkage can achieve its full sweep, and be prepared to implement custom mods if it doesn’t. “If you do this at home, you must have patience,” Sanchez says. “Expect to spend a good four hours to verify that everything is correct.”
NEED JUNK FIXED? If your car has a gremlin that just won’t quit, you could be chosen for Hot Rod to the Rescue. Email us at pitstop@HotRod.com and put “Rescue” in the subject line. Include a description of your problem, your location, a photo of the car, and a daytime phone number.
Advanced Engineering West (AEW)
Mira Loma, CA
California Pony Cars
888.225.7669 or 909.923.2804
Crossroads Truck and Auto Salvage
Holley Performance Products
Bowling Green, KY
800.HOLLEY1 (nearest dealer), 270.781.9741 (tech), or 270.782.2900 (general)
Lowe’s Companies Inc.
800.445.6937 (help) or 877.GO.LOWES (products & sales)
Sears Holding Corp. (Craftsman)
Hoffman Estates, IL
888.776.9824 or 662.224.8972
The Hillman Group
800.800.4900 or 513.851.4900
The post HOT ROD to the Rescue: Adjusting AOD TV-Valve Shift-Timing appeared first on Hot Rod Network.