904 Reverse/1st gear issues

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scott2683

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Hey guys and gals! Long story short, the 904 on my 71 duster was rebuilt about 5 years and 3000 miles ago. I'm in the process of getting the car ready for cruising season and I have a couple of issues I need some advice on. Trans is stock except for trans go shift kit.

1. If you put the car into reverse, whether the engine is warm or cold you have to ease into the accelerator in order to get the car to move back, once you get the car moving you can hit the accelerator and it will move just fine. If you put it in reverse and press the gas like normal, the engine will rev and then it will jerk/clunk into gear.

2. Today I drove it around my neighborhood testing out my new brakes and I went to manually downshift from 2nd to 1st and I felt no engine brake. You couldn't even tell that I had downshifted. However when I came to a complete stop and accelerated, I was in 1st gear.

I have not dropped the pan yet and changed fluid. When I check the dipstick the fluid is full and it's red just as it should be, no weird smells. However I have a new filter and fluid, I plan on changing it in the next week or so.

Does this sound like a band adjustment issue? If so can someone offer guidance on adjusting the reverse/low band? I imagine this can be done with just removing the pan?

When driving the car, it shifts 1-2-3 like normal, doesn't slip, and firm shift.

Thanks in advance.
 
It could be the servo spring slipped pass the retainer. When you drop the pan, look to see if the spring is hanging down. If it is you will need a new retainer. I think they sell heavy duty ones, but Im not sure.
 
Thanks for the advice! I'll definitely check that out. I'm hoping it's something simple like that!
 
Do a pressure test. That servo should have 230 to 260 psi. It sounds to me like a very loose band adjustment, or a low fluid-level,or a circuit issue. If you don't have the pressure with a proper fluid level, your pump-circuit has problems, and you will soon have issues in other gears as well.
At the absolute minimum, it could be a maladjusted manual valve; check the sync to the shifter.
In reverse all regulation is bypassed, including the governor; the gauge is reading maximum pump pressure.
 
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You say in forward AND I'M ASSUMING YOU MEAN DRIVE, "it shifts like normal".
Well, in Drive- breakaway, the L/R band is not used. Instead the rear sprag locks up. So you wouldn't even notice if the band was not there. If you pull it down into manual low; on acceleration, you still wouldn't know. but there will be no compression braking in manual low with a faulty L/R system.
In manual-Low the band is supposed to add much needed clamping power for things like; towing, being stuck, jack-rabbit starts, and initiating burn-outs,lol.
To get reverse, the trans uses that band and the Hi-drum
To get Third gear, the trans uses the Forward clutch and the Hi-drum again.
If third gear works properly, then the Hi-drum is working.......so that just leaves the L/R system.
 
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That's correct, in Drive ... it shifts 1.2.3 just fine. If I manually shift from Drive or 2nd to first there is no engine braking. So would this isolate the problem to a faulty low / reverse band?
 
That's correct, in Drive ... it shifts 1.2.3 just fine. If I manually shift from Drive or 2nd to first there is no engine braking. So would this isolate the problem to a faulty low / reverse band?
Hyup
But run the pressure test anyway, to possibly save the rest of the trans. Like I said; there is zero pressure regulation in reverse.The gauge will be seeing max pump pressure.IIRC the spec is at 1600rpm
That servo should WHAM in.
 
Thanks, I guess I'll know more when I pull the pan off this week. Can I adjust the band without having to remove anything other than the pan?
yes
But here's the deal,if you adjust the band first, and that doesn't do it.... then you will do the pressure test. And if the pressure test shows a problem, then you get to drop the pan again, and hunt up the real problem.
 
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yes
But here's the deal,if you adjust the band first, and that doesn't do it.... then you will do the pressure test. And if the pressure test shows a problem, then you get to drop the pan again, and hunt up the real problem.

I agree with you. I can do a pressure test while I have the pan off too! I don't want to keep dropping the pan
 
I agree with you. I can do a pressure test while I have the pan off too! I don't want to keep dropping the pan
No you can't. I'm talking the oil-pressure test at the L/R pressure port; the oil still has to be in the pan, and the engine running at 1600rpm

You may be thinking air-test with the VB off. That would be the 3rd test.
First oil-pressure
then inspect servo adjustment and strut install
then air-test
 
No you can't. I'm talking the oil-pressure test at the L/R pressure port; the oil still has to be in the pan, and the engine running at 1600rpm
Ok, I got you. I'll do some more research on it before I attempt it. Transmissions are not my forte!
 
Yep, it does sound like a servo or band issue.
Could even be a cracked servo piston (because it happens) and they do make a better billet one because it does.
 
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