Thought I'd post this as a sticky thread now that cold weather is on us.
How to check Glow Plug System
To check the Glow Plug Relay (GPR)
∑ Be sure the engine is cold, so that the PCM will tell the GPR to turn on. If the engine is hot, you wonít have as much time to check. (if oil temp is above 130F, it won't turn on at all)
∑ Locate the GPR Ė Its behind the fuel filter on top of the engine, a little bit toward the passenger side of the valley. There may be two relays there. If so, the rear one is the GPR. It will have two fairly large wires (yellow and brown) connected to one of the large posts. (circled on this photo) Another location photo here: TheDieselStop.Com Photo Hosting - GPR & IAHR Location
Photo credit F350_6
∑ With your multitmeter set to DC volts, and 15 V range (if not autoranging), clip the positive (red) lead to the output terminal (with yellow and brown wires connected), and the negative (black) lead to a good ground point (like the battery ground terminal or someplace metal directly on the engine block.)
∑ Turn the key to ON (do not start)
∑ If your GPR is good, it should click, and youíll see 11 volts or so on your meter, then, depending on temperature, it will click off up to 2 minutes later. You should do this a couple of times to make sure it consistently makes the connection.
∑ If you donít get voltage with this test, confirm by retesting as follows.
∑ Remove the two small wires from the smaller two of the four GPR terminals.
∑ With jumper wires, apply voltage from the battery across the two small terminals. If your voltmeter now reads voltage on the output terminal, your GPR is OK, and your problem is in the PCM circuit that tells the GPR to activate.
GPR for 99 & up 7.3L
NAPA GPR#109 (mounts 90 deg out of rotation from original)
Carquest #RY-175 (clone of GPR#109)
Autozone Sorensen MR-99 (also clone of GPR #109 I think)
International #1831646C1 (Identical to the one you'll get from Ford) ~$25-$30
To check Glow Plugs.
∑ Remove the electrical connector on the inboard side of valve cover at the gasket. Press down on the top of the connector latch and pry gently with a screwdriver.
Disconnecting VC harness connector.
Another photo of it loose.
∑ There will be 9 pins on the valve cover gasket where you removed the connector. The two pins furthest forward and the two pins furthest back are for your glow plugs.
∑ With your multimeter set to resistance (ohms) and low range (single digits) if not autoranging, clip the negative (black) lead to a good ground point.
∑ Probe each of the 4 outer pins individually with the positive (red) lead, noting the resistance. Good glow plugs will have a resistance between 0.6 and 2 ohms. If you get infinite resistance on any glow plug, that one is either bad or the connector under the valve cover has come loose.