The 7.3 PSD is equipped with a Power Train Control Module (PCM) controlled Glow Plug System to preheat the cylinders. The system has one glow plug per cylinder installed in the head next to the Injector and is powered by a Glow Plug Relay via the Under Valve Cover Harness (UVCH). According to Ford the Glow Plug relay does not come on if the Engine Oil Temperature (EOT) is above 131°F in the F series and 86°F in the E series vehicles. The Glow Plug System runs for a maximum of 120 Seconds the runtime dependent on the Altitude and EOT.

The 7.3 is tried and tested to work in climates below freezing when all systems are functioning as they should. If something stops functioning properly issues develop when starting in extreme cold. Ultimately the 7.3 cannot function when it is cold without a properly functioning Glow Plug System as well as following the recommended use of the Engine coolant/oil heater when temperatures are below 30°F. Without the Glow Plug System cold starting is near impossible. Proper functionality starts with strong batteries. Without solid performing batteries when it is cold the glow plug system will not function as effectively as it should causing cold starting issues. If  your 7.3 PSD is having cold starting issues the first step to ensure your batteries are properly charged and test good.

Glow Plugs

We recommend only using Motorcraft/OEM Glow Plugs because there have been issues of third party brand glow plugs snapping off in the head while trying to remove them.

94.5 – 2003 Motorcraft Glow Plug Model # ZD-11 part # F4TZ-12A342-BA

Glow plugs should be tested every 100,000 miles to ensure they test within spec. It may also be a good idea to replace them around that time if you are in a colder climate where the Glow Plug System is absolutely necessary.

Glow Plugs should be tested by using a multi meter to check each glow plug’s resistance to ground to check resistance. A resistance of 0.1 – 2 Ohms indicates a good glow plug, however, some recommend a tighter resistance window between .8 and 1.2 Ohms. The lower the resistance the less effective the glow plug and a plug should never exceed 2 Ohm resistance. The first way to test a glow plug is to perform the resistance test with the glow plugs out of the vehicle measuring the resistance between the side and the top electrical connector.

An easier test is to use a multi meter to test the glow plugs while they are in the vehicle by disconnecting the main harness plug or plugs from the UVCH depending on the year of your 7.3 PSD (pre or post ’97). Looking at the plug(s) that is attached to the valve cover the outer pins on each side are for the glow plugs and the inner pins are for the Injectors. The inner pins are connected to the injectors which operates at a much higher voltage and different current than the glow plug system. Newer 7.3s have 2 wide pins on the outer edge of the plug and older 7.3s that have a pair of plugs on each valve cover gasket have one pin per outer edge for glow plugs. Using a multi meter set to resistance touch the (-) Black lead to a solid ground and then the (+) Red lead to each of the outer pins on each UVCH plug taking note of the resistance to see if they all fall within spec mentioned above. Replace any glow plugs that are out of tolerance. To be safe if  one is going out others may be soon to follow so all 8 should be replaced.

The Glow Plug Relay

While older 7.3 vehicles only have a Glow Plug Relay newer vehicles have 2 relays, one for the Air Intake Heater (AIH) and the other for the Glow Plugs.  The relay towards the front of the engine is the AIH relay and the one closest to the turbo is the Glow Plug relay. You can verify the AIH relay by following the power cable from the AIH in the intake manifold “spider” back to the relay. Knowing this is important if you ever need to test the relay to ensure you are working with the correct relay.

To test the Glow Plug relay, with the vehicle off, connect a multi meter set to DC to read 12 volts with the (-) black lead going to a ground on the vehicle (negative battery terminal or a proper ground source such as the engine block) and the (+) or red lead on the right/rear side of the Glow Plug relay and verify you are getting a reading of 0 volts. The other terminal should read 12V because it is directly connected to the battery so ensure you are on the correct terminal of the relay. Next, have someone turn the ignition key on (no need to start the engine) and the voltage for the relay terminal that was at 0v should now show 12v and should stay powered on for 30 to 120 seconds then shut off. If this fails the relay may need to be replaced.

If you need to replace your Glow Plug relay there are aftermarket options that are heavier duty than OEM such as the White Rodgers 586-902 or the Western Snow Plow Relay 56131K-2.

Glow Plug System Wiring

As noted above the glow plug wiring from the main harness connects to the  outside pins on the UVCH plug. This doesn’t often go bad but sometimes the ends of the main harness connector that goes into the UVCH plug get damaged or wear out. Inspect and ensuring that these wires are in good condition to ensure proper operation of your Glow Plug System. Replacement ends can be had from most auto parts stores or online.