HT 740D Transmission lockup issues

[joshb817]

Hey everyone. I am actually a fire engine owner but hopefully this will be valuable to all of you. Almost all of the 700 series maintenance questions I have seen while searching the internet have been on this group and it has been more helpful than the fire service groups.

Im having a hard time finding good info on an old HT740d. I have one in the 1985 fire engine I purchased. This is for training so I know how to run it but not repair it. Anyone here able to help diagnose? Allison advised they sold off all of the HT series and had no info.
It runs fine until it gets warm, then once the converter locks up, I can't get it to unlock and it dies when slowing to a stop. After sitting for a while it will unlock and I can up and downshift fine until it gets warm again. Changed fluid to transynd and external filter but not internal. Could be nothing but looks like a very small leak on/around the modulator assembly on the side of the pan. What direction should I be looking? I'm hoping this is as simple as a solenoid or something like that, but the repair manual I bought doesn't give much detail on that, and I am having a hard time finding parts to reference. I've included a couple photos for reference as there is a pto driven fire pump that could also be a potential cause. I believe it pumps in lockup.
There is usually a torque converter lockup solenoid on the bottom of the bell housing on these, but on mine, its just a plug I believe for checking pressure. Any ideas where the solenoid may be then?
Ive attached some photos. On a photo from the top. On the right side, is that a temp sensor inline with the coolant line? All the HT manuals show the temp sensor being a separate plug next to the coolant line, not in it directly. Im at a loss here, as nothing seems to be lined up with the manuals.
On the bottom side, can anyone explain the diaphram type device that all the hydraulic lines run into on the back of the transmission behind the pan? Thanks all.

[luvrbus]

The 740's in fire trucks use a external valved electric control to lock the converter up, you are not getting voltage to the valves or you have a bad valve in the lock up circuit start with the dash switch, my fire truck has 2 valves normally closed from the governor to one of the 4 plugs on the transmission between the pan and the bell housing   

[joshb817]

Are you referring to these plugs? I was told the top one should have the solenoid but it obviously does not. Is the governor the same thing as the modulator that is bolted to the side of the pan? Thanks for the help. I'm obviously clueless.

[luvrbus]

The governor is on the tail end of transmission it has plugs above the 4 bolt plate that covers the governor, I have never saw a converter lock up without the electric solenoid valve on the converter housing for lock up, who knows how a fire truck can be valved, mine would not disengage if the pump wasn't disengaged   

[luvrbus]

If the engine has Jakes that could be your problem leaking solenoids, broken o-rings taking time to build  oil pressure then releasing after the engine is stopped