Related to Ed's electrical comment above, I've encountered an issue on two different trucks: If the battery is in a low state of charge (you just now had to jump-start it to get it running), the alternator or generator may not have enough capacity to recharge the battery, AND kick in the OD solenoid at the same time. The first time this happened, I thought I had a weak solenoid...the second time, I finally had an 'ah-ha!' moment. I haven't measured the amount of current the solenoid draws when activated, but it must be considerable. The current required to hold the solenoid is much less.
-Kevin
