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HOW TO: Changing Carburetors and Making it Work with an AOD
I have noticed a few threads here lately where people are having trouble or questions with their AOD transmission. This usually starts when someone has an AOD and they want to replace their 2-barrel carburetor with a 4-barrel. While it can be done quite successfully, there are a few things the user needs to be aware of first:
I. - What makes an AOD different?
Unlike the earlier vacuum-controlled Ford 3-speed C4 and C6 automatic transmissions, the Ford 4-speed Automatic Overdrive (AOD) uses a throttle valverod or throttle valve cable to control ALL shift functions and characteristics. This device is a direct link between the throttle and the transmission that controls transmission line pressure and it MUST BE SET UP CORRECTLY or the AOD will not last!
If the geometry between the movement of the throttle is not exactly proportionate (1:1) to the movement of the transmission throttle valve via the throttle valve rod or cable, the transmission line pressure will be incorrect and the AOD will shift erratically. If the transmission pressure is too low for too long, the results are disastrous. When the throttle and the transmission are communicating correctly, the AOD will have positive, firm shifts that are spread out and on time with a smooth shift into overdrive for good fuel economy.
II. - AOD TV Rod and TV Cable
For 1980 - 1985 models with a carburetor, Ford used a throttle valve (TV) rod to control the AOD transmission.
For 1985 - 1986 models with fuel injection, Ford used a throttle valve (TV) cable to control the AOD transmission.
III. - The AOD TV Rod is NOT a Kickdown Rod!
The C4 and C6 transmissions are controlled by engine vacuum and a modulator, and it uses a kickdown rod to make the transmission "kick down" to a lower gear. If it is not hooked up, the transmission simply will not downshift on it's own, but nothing bad will happen.
The Ford AOD is a completely different animal. It doesn't use engine vacuum or a modulator at all; it relies on the TV rod or TV cable. The TV rod or cable controls all shift points, shift scheduling, and more importantly, line pressure. The basic idea is this: as you give the engine more throttle, the transmission line pressure rises in proportion, and that spaces your shift points further out along with firmer and quicker shifts for better acceleration and performance.
Ford designed the AOD TV rod to look almost exactly like the kickdown rod that was used on the C4 and C6 transmissions. And the AOD TV rod will physically connect just fine to the Ford kickdown linkage found on older carburetors that are intended for the kickdown rods for the C4 and C6 transmissions and aftermarket carburetors with the Ford kickdown linkage. So it's easy to see why a lot of people think they are the same. They are NOT.
The problem with the Ford kickdown linkage is that it does NOT move 1:1 with the throttle valve on an AOD transmission! If the TV rod or TV cable is not hooked up correctly or the throttle movement is not 1:1 with the transmission throttle valve, BAD things will happen. This could mean anything from early, soft shifts with a very sloppy feel and poor performance, or late, hard shifts with a very harsh overdrive engagement.
IV. - The correct Ford carburetor for an AOD The ONLY correct factory carburetor that is compatible with the AOD in an F-Series or Bronco is a Motorcraft 2-barrel carburetor from a 1980 - 1985 model, and it will have a throttle arm and linkage specifically made for the AOD TV rod to connect to. It will look like this:
If you are using the stock Ford TV rod, and the throttle arm and the transmission linkage doesn't look like the picture above, then you have the WRONG carburetor and you are KILLING your AOD!
V. - Aftermarket carburetors
You CANNOT use an aftermarket carburetor with the stock Ford AOD TV rod! The Ford kickdown linkage on aftermarket carburetors is for Ford C4 and C6 transmissions ONLY. If you hook your TV rod up to the kickdown linkage intended for the C4 or C6, it will physically fit, but the AOD will not last long. The geometry is all wrong for an AOD transmission, which means the line pressure and shift points will be incorrect, and that can burn up your clutches and destroy your transmission in just a few miles!
Aftermarket Holley and Edelbrock carburetors do not have the correct linkage necessary that will work with the AOD TV rod, and their throttle levers are not symmetrical to use a TV cable.
1. To correct the first problem, you will need a Lokar TV cablekit to replace the TV rod:
2. To correct the second problem, you will need a TV corrector bracket that bolts on to the Holley or Edelbrock carburetor throttle lever. This will give you the correct 1:1 ratio from the throttle to the transmission. In other words, transmission line pressure will happen immediately with throttle input and full TV pressure will be reached under wide open throttle.
If you are using the Lokar TV cable kit, the above link is not optimum for setting the TV pressure. Lokar also tells you to set the pressure using a TV pressure gauge, but I went through all of that and I am going to tell that you do NOT need the gauge to set it up correctly. If you set the line pressure with the pressure gauge, my experience is that the shifts are very early and soft and the AOD doesn't want to downshift unless the gas pedal is on the floor. While driving it like this might be nice and seamless in a Lincoln and with your grandfather driving it, this setting makes the AOD feel weak and the clutches will slip more, which builds up more heat and kills the transmission quicker. This is the MINIMUM setting for the AOD and is better suited for luxury automobiles.
Assuming that you are using the correct geometry corrector bracket, and you have the correct 1:1 ratio between the carburetor throttle and the transmission throttle valve, I have found that the best (and safest!) setting is make sure the TV cable is at it's maximum pull at wide open throttle. In other words, rotate the throttle linkage on the carburetor all the way until it stops, and then pull the TV cable all the way out until it stops, and that is where the TV cable end-stop should connect to the geometry corrector bracket. This is the MAXIMUM setting for the AOD and is better suited for performance automobiles and trucks.
This setting will give you positive upshifts, quicker downshifts, and an overall more responsive AOD.
VII. - Conclusions and my personal experience
I learned all of this the hard way. Everything I wrote above is what I learned through a lot of aggravation and confusion. I am currently using an Autolite 4100 4-barrel carburetor with a Holley-style TCI geometry corrector bracket and the Lokar TV cable kit (see links above) with my own AOD transmission. I have been running it like this for about 5 years now without any problems whatsoever.
Up until 2005, my 1985 F150 Lariat was my Father's truck. He bought it used and had the engine rebuilt by an "old timer" engine rebuilder who butchered the emissions equipment (and this was a computer-controlled EEC-IV engine!) and replaced the AOD-specific "feedback" carburetor with an older Motorcraft unit with the Ford C4/C6 kickdown linkage. Consequently, the AOD failed about 3 months later. So my Father had the AOD rebuilt and it lasted for about 20,000 miles, but it never shifted consistently before it too, burned up. Even that "professional" transmission shop didn't realize that the carburetor was incompatible because the TV rod physically fit the kickdown linkage perfectly.
When I acquired this truck a few years later, I rebuilt the AOD again and changed to an Autolite 4-barrel carburetor with the Lokar TV cable kit. Not knowing any better, I connected the Lokar TV cable to the bottom hole of the throttle lever on the Autolite carburetor...and the AOD wouldn't shift correctly. If I set the line pressure at idle, the TV cable ended up being too short and it would physically stop the carburetor from reaching wide-open throttle because the throttle valve was shorter than the throw of the carburetor throttle linkage. But if I set the line pressure at wide open throttle, the transmission would shift WAY too early with lazy, sloppy shifts. That's because the TV cable would then be too long, or the cable was slack at idle and the TV line pressure wouldn't start rising until I was well into the throttle. That is VERY BAD for the transmission, so I actually drove around for about 6 months with my carburetor unable to reach full throttle until I could figure out why this was happening.
When I discovered the geometry corrector bracket and bolted it on the carburetor, this problem was completely solved. This bracket relocates the TV cable stud location, which enables the TV cable connected to the carburetor throttle to move the correct 1:1 ratio with the throttle valve in the transmission. Now the TV cable is the correct length and is pulled the correct amount throughout the entire travel of the carburetor linkage because the geometry is now correct. My AOD has shifted GREAT ever since then with firm, positive shifts, quick downshifts, and smooth overdrive engagement. Driving conservatively, I have gotten almost 20 miles per gallon with this combination!
VIII. - Miscellaneous
See also this thread for proper use of the AOD:
I replaced the 2V carb on my 84 with an exact replacement rebuild. It was a pain to get it to shift properly. Did some searching on here, about adjusting the TV rod, and it now shifts beautiful.
I don't think any of us know if it'll work. But if it doesn't you are very likely to ruin your AOD. So, you will need to figure out if it'll work and, if not, decide how you want to overcome the problem. One approach would be to use a carb that will work.
Thanks guys I just got it all put together and your right it does NOT work it won't line up with the linkage and modifying it doesn't help, I made a lap around the back acre and it doesn't work well at all
I. - Kick Down Linkage
1980s Ford Motorcraft 2150 carburetor with the C4/C6 (w/o Overdrive) Kick Down linkage, located behind the throttle lever.
The Kick Down rod connects to the stud behind the Kick Down linkage:
II. - Throttle Valve Linkage
1980s Ford Motorcraft 2150 carburetor with the AOD (Automatic w/ Overdrive) specific Throttle Valve linkage, located behind the throttle lever.
The Throttle Valve (TV) rod connects to the stud in front of the TV linkage:
I just bought a 1984 F150 with a 302 and an AOD.
He told me it had a C6 tranny and needed adjustment when I bought it.
Only drive it a bit and noticed it shifts three times....
I look under and see "automatic overdrive" and "METRIC" on tranny pan, TV rod
hooked up like old C4/C6.
This thread tells me most of what I need to know except this:
The truck has this type of linkage
but the manual for the cable install shows an arm straight up or straight down.
Does that arm come with the kit?
How can I use the kit with my setup?
It is not at all clear...
