The transmission actually uses its very own computer, mounted in my minivan against the firewall on the passenger side. At first glance it looks like the engine's brain, but it's actually the transmission's.
It looks like from the chart that second gear requires the 2/4 and UD clutches to be selected, but if you don't have any clutch solenoids engaged then it actually naturally leaves everything to be as if it's in second. So this forces the four-speed transmission into its “limp“ mode. Engaging no solenoids can be accomplished by simply pulling the plug to the solenoid pack connector, shown highlighted in this picture:
It has one 8mm bolt holding it in, and it's pretty simple to reach with a 12“ extension.
Once I pulled this, although of course the van had no top end whatsoever, it was at least driveable again. No more grinding! It wasn't the torque converter after all. And both forward and reverse work, incidentally due to the reverse running off a fluid-based clutch, not an electronically controlled one. Acceleration was not nearly as bad as I feared, and top speed was around 60mph at 4000 RPM. Fuel consumption at this speed on the guage claimed 15mpg. Usable, but not great on the freeway. It has gone 50 miles so far in this mode.
So now I'm of the opinion that the problem is either with the OD clutch itself or with a sensor that fools the transmission controller (TCM) into thinking it should be engaged when really it shouldn't be.
Moral of the story: if you have one of those wretched trouble-prone A604 transmissions found in all the Plymouth, Dodge, and Chrysler vehicles listed below, and you start having transmission trouble, you can put it in its limp mode yourself to at least avoid towing charges, and maybe use it until you can buy another ride!
yank the plug and see if its got foward before you pull the bad used trans out again!!! bummer I know they suck to do. they make good coal bins when properly located on the property on blocks.....