73 Firebird Shifting problem

I have a 73 firebird, originally 350/350 now a 400/350. Anyway, my wife was driving to a friends' house and when she started the car up to back out of
the driveway, it just went forward instead. I checked the linkage and everything appears to move correctly from the outside of the trans. I asked her is she drove it hard or anything just before this, and she said no. I checked the transmission fluid level and color. It was full and red. I drove the car and this is exactly what it is doing.
I put it in park and the wheels lock. If I rev the engine in park, the car jerks forward. Any gear that I put it in is 2nd gear. It will start in 1st and shift to 2nd normally. Neutral, Reverse, and Park are all forward gears as well.
I took the shift linkage loose from the transmission, and it turns normally, and it clicks solidly into each position.
I am totally stumped.....
Thanx
Casey 98 Camaro Z28 73 Firebird
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Put the shift input shaft on the transmission in Reverse. Start the car. If it will hold still at throttle, or goes forward you have a stuck valve in the valve body.
If you have never serviced a valve body before, do not attempt to learn now. Take the car to a shop (not Ammco or IT), and have them disassemble the valve body. Clean the lower section in solvent, spray out the upper section and replace the divider plate. Have them remove the valves, and check for binding. You should also have the governor checked out, as well as your vacuum module.
I have had 4 TH350's that had valve body problems show up for no reason. All were good transmissions. 1 did have a mechanical damage in the valve body (like it had been dropped during a rebuild). I had to replace that valve body, the other 3 were fine after a good cleaning.
As for the problem of starting in *any* gear range, that is a problem in the neutral safety switch, or the wiring for it. If the switch bolts or screws are lose, it can move with the shifter. Burnt switches will at times default to start position. Charles
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The "S" link from the flange on the shaft:
To the plunger on the valve body might have worn out and broken also. You might want to have him drop the pan and check that first Charles, before some shop takes him to the cleaners?
Refinish King

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If the 'S' link is worn, broken or binding, the car would have had to be put in to the second gear shifter quadrant during operation. Which was not mentioned as the cause of the problem.
Most people have a hard time even changing transmission fluid, let alone understanding what does what, and what it should look like.
As for a shop taking him to the cleaners, any decent shop will just fix the problem for standard rates. If he goes in and tells them what he wants done, and nothing more with out consent, he can't get taken. Charles
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I have no problem taking it apart and doing work on the transmission. I changed the motor on that car myself, have done all the transmission/brake/engine work on my 73 Corvette, changed transmission in my old 75 T/A 455 HD (wish I still had that car!) and plenty of other mechanic work.
As for the starting in any gear...I disassembled the safety linkage when I took the shift linkage apart. I just have not re-assembled it because I was unsure as to what had to be done next.
Describe to me this s-linkage that you were talking about and what to look for, I will give it a try. Also, if I needed to try to clean the valve body myself, would this be an easy job, or pretty tough. I really don't want to pull the transmission if I don't have too, my landlord almost evicted me when I changed the motor!!
What would a shop charge to get that valve body cleaned?
As I stated before, the transmission fluid all looks nice and red, no black in it, no grime, no metal flakes. My wife tells me she didnt hear any clunks, and she didnt have to force the shifter.
I appreciate all of your help so far.

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Being able to swap a engine does not compare to internal transmission repairs. Especially if you do not know what your doing. One mistake with a check ball (small ball bearings), and things can get expensive. I know people who build transmissions for a living, they would all advise you NOT to attempt this your self.
Im not sure what people charge for it. I would charge you 4 hours labor (@ $80 per hour), plus parts.
Your transmission fluid being nice & red does not mean you do not have a stuck part, or a broken part. It means the transmission clutches are working, as is the front pump & torque converter. Not all transmissionpart failures result in debris floating in the fluid, or in damage to the friction clutches.
As for doing this laying under a car in a apartment complex, that is not recommended at all. Not only do land lords frown apon it, you do not want to get any dirt in these parts. You also want them to be cleaned, with chemicals you should not use with out a proper storage and disposal system.
Bottom line, take it to a pro, let him find the problem and correct it, as well as check out other items that wear with age (accumulator, vacuum module, seals). You will be miles ahead of attempting to do it your self.
Charles Internial Transmision servicing requires special skills & tools. If you want to learn buy a few books & junk yard transmissions. Never attempt to learn on a transmission you have to having working asap.
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I agree. NEVER depend on your "show" or "fun" car for personal transportation. If you do, you can't afford it. You should always have a backup vehicle.
I blew the head gasket in my Cavalier over the summer and the FIRST thing that came to my mind before I put a wrench to fix it was how I was going to get around in the meantime. I already had a spare car, as well as a neighbor that works in NYC who dropped me off at the subway stop, which turns out to be very convenient to get to work :).
In such case, I took my time doing the repairs and the results were well worth it. And that's on a commuter car :).
Joe--ASE Certified Parts Specialist & 10th Ann.Club Tech Director '80 Carousel Red Turbo T/A, 27k orig. '79 "Y89" 400/4 speed 10th Ann. T/A, 57k orig '84 Olds 88 Royale Bgm 2 dr, 307 "Rocket" (lol), 143k and still going....
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Refinish King has got this one right. It is almost certainly the linkage has come off INSIDE the transmission. The manual valve, under pressure will move to the "manual low" position. Very unlikely this would be a "stuck valve" problem as reverse fluid is directly from the manual valve.
The only other condition that would cause your symptoms would be a problem with the forward clutch (overheated and welded up). However it is likely to upshift to second and third woth this condition.
Drop the transmission pan, follow the linkage, and I bet you'll find the problem.
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