Hello,

Maybe one of you transmission gurus can explain this to me a little better. I have a 1994 Aerostar with a 4.0L v6 2wd. It has the A4LD Transmission in it. The original transmission failed at around 92k miles. I purchased a rebuilt unit, and installed it and 42k miles later it has somewhat failed - I'll explain and this is what I'm confused about.

When put into the normal OD position the car does not move from a dead stop. You can press the gas and the engine speed increases, but you hear something "rumbling" in the transmission. Yeah I know - its not good.

When you put the shifter down into D position, the car starts off fine and goes through 1, 2 and 3 gears normally.

Looking at the ATSG Book on the A4LD with the Shifter in OD you have the "Overdrive one way clutch" holding, the "forward clutch" applied, and the rear "one way clutch" holding. In the D Position, in addition to all those clutches applied, the Overdrive Clutch is applied. Now I honestly don't know much about automatic transmissions - I can change the fluid and filter, and change out the whole assembly pretty easily, but understanding the guts is not my thing.

What I want to know better - is why is there a difference in what clutches are applied for 1st gear when you choose between OD and D. I can understand in 3rd gear having some lockout for not going into OD, but to me it seems strange to have different engagements for 1st gear - can someone explain or shed some light on it so I can understand better?

thanks

 

In D3 the coast clutch is applied to help assist the input sprag, I guess there thinking was if you are pulling a trailer or have a heavy load you can choose the D3 position to strengthen the input sprag.

It sounds like your input sprag has failed.

 

The coast clutch is applied in D3 to provide engine braking in first, second, and third gears. When in D there is no engine braking in first, second, or third gears.

I agree that the input sprag (also known as the overdrive one way clutch) has failed.

 

Thank you both for your feedback - The little I drove it to get back home while in D I never noticed the difference in engine braking vs. what OD was, this failure makes some sense to me now


And off to a rebuild shop it goes - also thanks for the diagnosis!