1976 Ford/ Great Eastern Fire truck trans problems

[Dsl_Dog_Treat]

Got to do the usual truck checks last night on our oldest truck in the station. Everything checked out fine fliuds on the trans good and pink.
I cycled the pump through its routine and it acted like it was struggling with an air intrusion but corrected itself.
I did notice a slight smell of what seemed to be hot brake or clutch material but couldn't locate the source and went away after a few seconds.

Let it run for awhile to get the engine and trans up to temp then went to back into the station and it would not come out of reverse.
The trans is an automatic not sure if an Allison or not but is a Cat 3208 n/a motor.
No matter what gear range selected, it will only go into reverse, but if shifted into 1 then back to neutral it will just sit there and rock back and forth abit.
It no longer has any forward gears and the fluid has that burnt smell and is somwewhat discolored now. The trans will also not release into neutral as we tried to push it forward to get it back into its' designated parking spot so we could get our new $352,000 truck back in the station, which I almost backed into because of this incident.
The selector cable was actuating fine at the trans so that rules that out.
We pulled it from service as it is now DOA!
Any thoughts?

 

[Ak HDM]

It sounds to me that one or more clutches failed. It's rebuild time!!
I'd bet it's an Allison. The trans cooler will need to be replaced or thoroughly cleaned.

 

[Dsl_Dog_Treat]

It's toast.
Well it's going for a total rebuild and in the order of $3500.00.
Gotta do it cause it's still a good truck but plenty old.