William I,
On the rear truck of all those geeps you owned is a cherry switch. One of the two wires on the cherry switch was broken, if you had to manually rev the RPM's up and down.
The way diesel sounds work is as follows; the serial data stream tells the sounds that a speed command has been issued and the sounds rev up. If there is no input on the "chuff in" circuit (this is pulsed AC ground through that cherry switch) the sounds immediately rev down. However, if there is an input to the chuff in circuit the sounds stay revved up. As you increase the throttle again (and pass a certain speed step, which i do remember what they are anymore) another command is sent via serial data to tell the sounds to rev up again. If there is input on the chuff in circuit the sounds stay revved up and so on. Once the input on the chuff in circuit goes away the sounds rev down.
Additionally, if there is no input on the chuff in circuit the squealing brake sounds will not play when you press and hold the brake button.
So, the litmus test is this; start the loco running, get it to rev up to at least RPM level 2 then press and hold the brake button. If the brake squeal sounds the cherry switch is connected. If no brake sounds then one or more of the wires on the cherry switch are disconnected.
On those earlier geeps it was not uncommon for the cherry switch wires to become unsoldered from their pin, as the rear truck swiveled a lot!
If you still have the geeps it will be a very easy fix!
Thanks,
Mike
