1985 4+3 Overdrive: very specific problem

[DaveP85C4]

I've owned my 85 4+3 since new. I know how it typically behaves. This Summer, I drove it 5,800 miles in 11 days in quest of a Sam Folz Award in NCRS. The car behaved fairly well considering it was driven further in two weeks than it had been in the previous 20 years.

On the last day, the Overdrive began acting up. Once warmed up, and having been driven for a few hours, when the button is pushed the light comes on, Direct is disengaged, but Overdrive does NOT engage. It free-wheels unless revs are brought up to Direct speed and the sprag catches. After 5-10 seconds of "coasting" along, Overdrive engages with a terrific BANG. Then it's fine until the next OD engagement.

Looking at the cut-away of the Overdrive it appears that the piston applies the Overdrive clutch, then continues to disengage the Direct clutch. There is only one piston that operates both clutches simultaneously. So I'm baffled by how Direct can be disengaged without applying Overdrive first. Unless I'm mistaken on my understanding of how it functions, which is entirely possible.

Does anyone know what the actual cause may be for this behavior? I haven't driven it since July because I don't want to break something. It really engages with a BANG once warmed up. When cold, it shifts normally.

Thanks for reading.

 

[DaveP85C4]

I asked this same question on another forum. A respondent there said that he had had two apart that had failing thrust washers. This washer is behind the direct clutch pressure plate. If it hangs up in the case because it is getting deformed, it can cause the "freewheel" I'm experiencing. He suggested I pull the pan and take a look for bronze or other flakes that could be evidence of the thrust washer breaking up. I've seen discussion over the years that thrust washer failure is common in these.