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 with the time it runs dependent on the Altitude and EOT.

The 7.3 is tried and tested to work in climates below freezing but has it’s potential issues when starting in extreme cold climates when all heating systems are not functioning properly. Ultimately the 7.3 cannot function when it is cold without the Glow Plug System as well as following the recommended use of the block heater when temperatures are below 30°F to improve cold starting. Without the Glow Plug System cold starting would be near impossible. The system is pretty simple but requires strong batteries. Without solid batteries in the cold the glow plug system will not function as effectively as it should causing cold starting issues. So ensure your batteries are properly charged and tested to prevent getting stranded in the cold.

Glow Plugs

We recommend only using Motorcraft Glow Plugs because there are horror stories of third party glow plugs snapping off in the head while trying to remove them. The OEM / Motorcraft 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 necessary.

When testing a glow plug’s resistance to ground with a multi meter 0.1 – 2 Ohms indicates a good glow plug. Some recommend a tighter resistance window between .8 and 1.2 Ohms. The lower the resistance the less effective the glow plug, however, a plug should not 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.

To test the glow plugs while they are in the vehicle you can disconnect the main harness plug or plugs (depending on the year) from the UVCH plug or plugs at the top of the Valve Cover. 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. Be careful not to touch the inner 4 pins as they 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 /ohms 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.

The Glow Plug Relay

While older 7.3 vehicles only have a Glow Plug Relay newer vehicles have 2 relays  with the one towards the front of the vehicle being the AIH relay and the one closest to the turbo is the Glow Plug Relay. You can verify which is which by following the power cable from the AIH (Air Intake Heater) in the intake manifold “spider” above the engine 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 12v DC with the – or black ground lead going to 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. Have someone turn the key on and the voltage for the relay terminal that was at 0v should now show 12v and should stay powered for 30 to 120 seconds.

Should there be no continuity there might be problem with the UVCH wiring or the glow plug itself.

If you need to replace your Glow Plug relay there are aftermarket options that are heavier duty than OEM.

Glow Plug System Wiring

As noted above the glow plugs wiring from the main harness connects to the 2 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.

If you are having issues with starting under cold conditions and all other parts of the Glow Plug System have tested OK then you may need to test the wiring to ensure 12v is making it to each glow plug. The wiring for the glow plugs is the outer 2 ports on the UVCH connector from the main harness, the inner 4 plugs are for the injectors and come from the PCM. It is important not to touch or short the inner 4 wires because the voltages used for that system are AC current and dangerous; you risk causing damage to the PCM. Power can verified from the relay to the connector for each glow plug on both sides by testing the 2 outer pins on the connector for the UVHC from the main harness with the key off. Connect the – Black lead to a good ground and the + Red lead to one of the outer pins. Have someone turn the key on and verify you see 12v. Do this for each of the outer pins on both sides of the engine to confirm 12v for all 8 glow plugs.