'99 3500 van 5.9L Code P1494

[bignoisey]

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?

[autojoe]

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.

[bignoisey]

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.