Hello and welcome to JustAnswer!
Let's see if any fault codes are stored. Cycle the key from off to on three times ending with it on and watch the digital odometer display. Don't crank the starter, just off to on. The codes start with a P and have four numbers, there may be more than one stored. Write down any that you see and post them here.
That one wouldn't be related I'm afraid. Is that the only one you saw before it said "done"?
Great! That one is related.
Do you have a voltmeter or test light to do some electrical testing, or are you more looking for some possible causes?
I have one, but I won't be able to do any testing until the morning. The "boss" just took the van to work.
If you can explain any possible causes in the meantime, that would be OK as well.
The cruise control is controlled by the cruise servo. This is mounted just to the right of the battery tray. It has an electrical connector, one vacuum line, and a cable that runs up to the throttle body to open the throttle.
The P0594 sets when the powertrain control module (PCM) detects that there is no power at the cruise servo when there should be. Power is supplied to the cruise servo through the brake lamp switch which is mounted under the dash on the pedal bracket. The servo is powered when the brake isn't pressed, but then power is lost when the brake is pressed.
This could either be a problem in the wiring to the brake switch, from the brake switch to the servo, or it could be the brake switch itself. It's pretty likely to be the brake switch, which is a cheap and easy repair.
What kind of electrical testing should I do in the morning?
You'll need to check for power at the cruise servo first. With the engine running press and hold the cruise On button and check for power on the violet/white wire at the cruise servo connector.
If there is nothing there (there shouldn't be) then you would need to go to the brake switch under the dash. Again press and hold the button and check for power on the violet/white wire. If there is nothing, press and hold the cruise button and check for power on the violet/yellow wire. Most likely you will find this is ok, but there is nothing leaving the switch on the violet/white wire, and the brake switch will need to be replaced.
I have the switch out, and I ohm-checked it. It has six blade-style contacts in a row: 1-2, 3-4, 5-6
Well, with the plunger pushed in, it reads:
With the plunger out, it reads:
My guess is 1-2 controls the brake lights because pushing the brake pushes the plunger out (just checked it out; I jumpered the pins at the connector and the brake lights lit). So it seems the switch is working. Maybe by taking it out and jiggling it, something has broken loose. So I'm going to road test and see what happens.
Sounds great! Let me know what you find and we'll take it from there!