'99 3500 van 5.9L Code P1494

I got a check engine light with a code P1494 - Leak Detection Pump Sw or Mechanical Fault.
It happened after replacing the engine heads, but not immediately. I drove the vehicle around for a couple of days then replaced the thermostat, and then the light was there. I only had to remove the alternator to change it thermostat, but I don't see how this cold cause the above fault code. Has anyone seen this before?

this code relates to the evaporative emmissions leak detection pump(LDP)SYMPTOM
P1494-LEAK DETECTION PUMP SW OR MECHANICAL FAULT

WHEN MONITORED
Immediately after a cold start, with battery/ambient temperature between 40 deg. F and 90 deg. F and coolant temperature within 10 deg. F of battery/ambient.

SET CONDITION
The state of the switch does not change when the solenoid is energized.

POSSIBLE CAUSES




LDP Switch Sense Circuit Shorted to Ground
Wiring Harness Intermittent
LDP Vacuum Supply
Faulty Leak Detection Pump
LDP Switch Sense Circuit Open
Powertrain Control Module
most of the time it is a faulty (LDP)
make sure have at least 13 inches of vacuum to pump.check electrical wiring for opens and shorts....check electrical connector.
there are 3 wires....one from generator provides power to pump......a sense wire that pcm monitors to make sure pump is working when it grounds pump.....and a pcm control wire that the pcm controls the pump.
I use a scan tool with bi-directional controls to test circuit and a vacuum guage to test for vacuum to pump.

Good information, autojoe. In my case I found I had accidentally pulled the vacuum hose off at the base of the throttle body when I was removing the generator to get access to the thermostat I was changing. I put it back and presto! problem solved.