TAC Tech => Mechanical => Topic started by: firebirdparts on February 09, 2021, 02:15:18 PM
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This is recreated from a sticky thread we had.
Here's something that has always puzzled me. On my '79 Trans Am there is a collar around the steering column that moves in conjunction with the console-mounted (automatic) transmission shifter. If the car is in neutral, I can actually rotate this collar with my hands, which will move the console shifter and drop the car into gear. I'm thinking this is dates back to when a column shifter was actually an option, but why in late model Firebirds and Trans Ams in particular was the collar still connected to the shifter after the column shift option went away? Anybody know the history on this design?
Its called a backdrive mechanism....It controls the steering wheel lock, the neutral start switch, and the backup lights, all of which are mounted in or on the steering column. the idea is that:
you cannot switch the key to the off position and remove it from the ignition (thereby locking the steering) unless the car is in PARK. You cannot lock the steering wheel unless the car is in Park (and therefore not in motion). Once the steering wheel is locked, the transmission is locked in park. This was an anti-theft feature mandated by the US government in 1969.
For a manual car, the gearbox has to be in reverse to lock the steering wheel and pull out the key.
These parts can get lost, and they can interfere with headers, so sometimes we get questions here about how to live with this system 50 years later.
If you look under there, you'll see linkage rods going from the transmission to a shift arm on the bottom of the steering column. The linkage looks like the photo below. When you shift the floor shift, the linkage moves the arm which moves the collar on the column. This movement inside the column makes it so you can only turn the key and start the car in park and neutral, and also operates the back-up light switch that's attached to your column under the dash. And you can only turn the key to the lock position and remove the key when the shifter is in park.
(http://img.photobucket.com/albums/v159/67SS427/IMG_3225-Copy.jpg)
Here is a diagram pictures of the automatic version. It's generic, so don't get too hung up on the shape of the cross-shaft! also, on f-body, the end of the shaft fits into a cup on the frame. On the late 70's cars, the spring is necessary to hold that cup in place. Up through 1974, the cup wouldn't just fall out, but starting in 1975, they saved 5 cents on a bolt and it'll just fall out and be lost.
(http://www.firebirdtransamparts.com/forumposts/backdrive.jpg)
The adjustment procedure is to loosen the adjustment bolt, put everything in park:
-transmission in park
-shifter in park
-Column in park (it'll stay there if you take the key out)
and then tighten it back up.
check to make sure it starts in park and neutral, and the backup lights come on in reverse.
This is the same procedure for adjusting the floor shift cable. There is a bit of slop in the linkage. if the column doesn't operate exactly as you want it to, you may need to fudge the slop in it one way or another. Also, the backup light and neutral start switch is able to be adjusted relative to the column, so you can adjust it independently from the key-lock feature.
1979-81 cars are different; the steering column has some sort of mechanical block that prevents the key from turning to "start". Earlier columns had only an electrical switch.
Here's the manual 4-speed version:
(http://www.firebirdtransamparts.com/forumposts/manualbackdrive.jpg)
This allows you to lock your steering wheel only while in reverse. I guess they thought that was the least dangerous option.
The "neutral start" provision was moved to the clutch switch, so the backdrive linkage has no effect on starting a car with a manual transmission.
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Thanks for putting this up.
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i want to say on my sons 81 z, you have to have the car in reverse for the key to turn to the start position, as well as in revers to remove the key when turning the car off.
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because it's a manual.
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i want to say on my sons 81 z, you have to have the car in reverse for the key to turn to the start position, as well as in revers to remove the key when turning the car off.
Thanks for posting this. I was not quite sure how they handled that physical blocking interlock. There are an awful lot of 1979 4-speed T/As that should be the same way. I don't know if the columns are totally identical manual vs automatic, but I guess we could figure that out.
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i want to say on my sons 81 z, you have to have the car in reverse for the key to turn to the start position, as well as in revers to remove the key when turning the car off.
Thanks for posting this. I was not quite sure how they handled that physical blocking interlock. There are an awful lot of 1979 4-speed T/As that should be the same way. I don't know if the columns are totally identical manual vs automatic, but I guess we could figure that out.
Both of my 76 4-speeds, which I had when they were only a few years old, were that way. But my '78 had the back drive removed before I got it. I've since found a replacement, but haven't installed it yet.
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Well, a 76 doesn't have anything to stop you from turning the key to start. They changed that in 1979. They just had something to stop you from turning it back to lock.
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Well, a 76 doesn't have anything to stop you from turning the key to start. They changed that in 1979. They just had something to stop you from turning it back to lock.
I was pretty sure that it had to be in reverse to start. But you have seen the '76 set up much more recently than I have, so I defer to you, Joe.
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i want to say on my sons 81 z, you have to have the car in reverse for the key to turn to the start position, as well as in revers to remove the key when turning the car off.
Thanks for posting this. I was not quite sure how they handled that physical blocking interlock. There are an awful lot of 1979 4-speed T/As that should be the same way. I don't know if the columns are totally identical manual vs automatic, but I guess we could figure that out.
I just redid the column for my 79 4 sp. It seemed like the same as the one for my 77 auto. I also have another tilt and a non tilt in the shop that are identical but dont know what they are from. I had assumed the one for my TATA was original, and it has all of the earmarks to make me think so, BUT I cannot say for sure as it was loose and the date code on the horn button says otherwise. Anyway, from what I can tell the only difference is in the neutral switch at the base of the column.
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I did solid body bushings on the frame, and now the backdrive linkage wil not reach, i have ziptied the linkage to speedometer cable for now.
i have bougth a lokar linkage that is lying in my trunk, i have not gotten around to mounting it yet, but i am not sure it will work as intended as i need to keep the shift cable bracket but add the bracket for the lokar linkage.
i hope hotlinking these images work:
(https://transmissioncenter.net/wp-content/uploads/2016/01/LOK-ACA-1800_IT_xl.jpg)
and
(https://image.pushauction.com/0/0/2e2ff0f4-bf85-4276-8d1e-6c37f38460f5/03696f1e-e6dd-46be-84d6-844cecedf6b4.jpg)
need to fit on the shifter shaft together without interference.
I will have access to the car sometime in April after i get it out of winter storage.
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That ugly lever could have enough arms to do all the jobs it needs to do, but you may not be fit both. I don't know.
The factory linkage is amazingly awkward compared to the Lokar, but I guess there's bound to be a good reason for that. I don't know what it is, though.
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Anti-theft? It's hard to climb under a factory car and move rods that run against the direction they travel, compared to direct back and forth replacement. I don't have it, RHD car, only guessing.
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Don't think the Lokar linkage would work on a bird. Arm would need to be much longer to bypass the firewall. Hope I'm wrong, as it would be a slick replacement. Took a while to track down all the original parts on mine.
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The rod in the illustration photo above is cut to length, it is a bit longer and is ment to be cut and bendt to the application. Lokar aca-1800 is made for TH400 (and a few others)
My other alternative is to use a die and cut threads on the end of my existing linkage end and use a threaded rod and coupler to lengthen it, i think that is a little to shady three approach, but could work.
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This is great info about the backdrive system, which is very poorly documented in the shop manuals.
Since the time I first got my TA some 15+ years ago, the car never had any of the linkage. I had no idea this system even existed — the only part that remained was the funny lever sticking out of the steering column way back in the engine compartment, which I figured was there just for laughs. One day my car, which was running perfectly fine the day before, wouldn’t crank even the slightest. Dead silence when you turned the key. It was baffling because I hadn’t touched anything on the car, apart from pulling and reseating one of the rear spark plugs. I chased my tail all night troubleshooting the electrical system. I knew that there was a neutral start switch, but mine was wired into a anti-theft kill switch and the neutral start function never did anything. I figured the issue must be with the kill switch wiring, but everything checked out fine.
I was finally about to tear everything apart to replace the ignition switch, when I thought more about the working of the neutral start switch and what it connected to in the steering column. After a lot of digging in the shop manuals, and much confusion over how things were supposed to work when you had a console-mounted shifter, I eventually figured out that there was supposed to have this linkage from the transmission to the funny lever on the steering column. When I reached way back in the engine compartment to mess with the plug on the no.7 cylinder, I must have bumped the lever a tiny bit. I moved the lever back and the car started like normal.
I’m sure there are a lot of TA’s out there now that no longer have the linkage installed. If you’re in that boat and your car suddenly won’t crank, check your funny lever.
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And a lot that don't have back-up lights thanks to headers. Inspections seemed fairly relaxed with minor safety features.
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And a lot that don't have back-up lights thanks to headers. Inspections seemed fairly relaxed with minor safety features.
For what it’s worth I added a micro switch to my 4speed setup to accommodate headers and working reverse lights.
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Yes, many options out there, but many still do nothing.
We've had microswitches for manuals and relocated Neutral Start Switch from the steering column and onto the shifter assembly (auto) for RHD conversions since these cars were new, and both could have been done from factory LHD. Basic but effective, and hidden.
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Yes, many options out there, but many still do nothing.
We've had microswitches for manuals and relocated Neutral Start Switch from the steering column and onto the shifter assembly (auto) for RHD conversions since these cars were new, and both could have been done from factory LHD. Basic but effective, and hidden.
Interesting but makes sense. I find your guys’ RHD conversions kind of fascinating though for a restoration hobbyist it seems like they could be a pain to deal with.
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I did solid body bushings on the frame, and now the backdrive linkage wil not reach, i have ziptied the linkage to speedometer cable for now.
i have bougth a lokar linkage that is lying in my trunk, i have not gotten around to mounting it yet, but i am not sure it will work as intended as i need to keep the shift cable bracket but add the bracket for the lokar linkage.
i hope hotlinking these images work:
(https://transmissioncenter.net/wp-content/uploads/2016/01/LOK-ACA-1800_IT_xl.jpg)
and
(https://image.pushauction.com/0/0/2e2ff0f4-bf85-4276-8d1e-6c37f38460f5/03696f1e-e6dd-46be-84d6-844cecedf6b4.jpg)
need to fit on the shifter shaft together without interference.
I will have access to the car sometime in April after i get it out of winter storage.
An update to this, I ended up using swivel end that goes to the steering colum and the rod (bendt into shape) and reusing the transmission cross shaft and factory swivel. so it worked out kinda but only used half the parts.
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I am kind of shocked that there is not a package available to replace the linkage with a rigid cable and a pair of brackets to mount to the transmission and steering column to locate it properly. That could allow for engine and sub frame motion within reason as well as clearance for headers or other exhaust related components.