Originally Posted by Sir Octobus
Can low trans fluid have anything to do with this?
I believe on the 3000 series the reverse lights are activated by the TCM. there is a switch internal to the transmission.. since this is a fully electronic trans, the engine will need to be running and the transmission will need to fully engage the gear for the reverse signal to be sent.. so yes low fluid can affect it..
for reference the MT643 or the AT545 uses a pressure switch in the side of the trans.. . the switch activates when reverse gear engages and pressurizes the switch.. reverse lights wont work if the engine isnt running or trans fluid is very low.
the allison 1000/2000 use a physical switch based on the position of the shifter lever.. on those the switch can be internal or can be an external switch on the side of the trans at the shift lever.
ive only seen a couple HT7xx series transmissions and they are as John describes, an external switch on the side of the trans.