7.3L IDI Manual Glow Plug Control Switch

How to Wire & Install a Manual Glow Plug Switch on a 7.3L IDI Diesel

Applicable Models:

1988 - 1994 Ford F-250, F-350, F-Super Duty

Applicable Engine(s):

International 7.3L IDI V-8 diesel (naturally aspirated & turbocharged variants)

A manual glow plug activation switch is a somewhat common modification to the factory 7.3L IDI glow plug system. Rather than allowing the factory controller to control glow plug heating cycles, a momentary push button style switch is mounted in the dash that controls the system in an "on demand" fashion. With this setup, the glow plugs are only energized with the key in the "run" position and the momentary switch depressed. The glow plugs will only received power while the switch is depressed, typically for 7 to 10 seconds (depending on ambient conditions) before starting.

The benefits of manual glow plug control is that it allows for the glow plugs to be cycled only when they need to be. Ordinarily, the glow plugs are cycled prior to every start. However, the glow plugs are not always necessary for starting when the engine is still warm (i.e. was recently turned off). This modification allows the driver to cycle the glow plugs only as needed. A 10 second cycle may be necessary on a cold Winter morning, while a quick 5 second cycle is all that's needed on a hot summer day. In addition to giving the driver full control, this can increase the life of the glow plugs since they are likely to be used much less in most cases.

The danger of installing a manual glow plug switch is that a driver may cycle the glow plugs for too long. Anything more than ~15 seconds of heating may damage the glow plugs and, in this setup, there is no fail safe - the driver has 100% control of the glow plug cycle time, and the controller no longer has the ability to switch off the glow plug relay.

Manual Glow Plug Switch Installation

Disclaimer - the installation of a manual glow plug activation switch overrides the factory system and removes all fail safes. It is therefore possible to over cycle the glow plugs, possibly swelling the glow plug tip and/or increasing the risk of a tip breaking off inside the combustion chamber.

7.3L IDI Manual Glow Plug Controller wiring diagram

• Remove the white wire from the glow plug relay and cut off the terminal. Install a crimp or wire nut on the end of the bare wire, tape it up, and tuck it away so that it is secure - this wire is no longer being used.

• Cut the blue wire for the wait-to-start light halfway between the electrical connector and controller. On the end of the wire that goes into the controller, install a crimp or wire nut at the end, tape it up, and tuck it away - this wire is no longer being used.

• Crimp roughly 12 inches of wire to remaining blue wire and install a ring terminal on the other end. Plug the wire connector into the matching connector in the wiring harness, then install the other end to the terminal on the glow plug relay that you removed the white wire from - this will allow for your wait-to-start light to illuminate while the glow plugs are being manually cycled.

• Run a wire from your momentary push button switch (do not use a toggle switch or switch that remains activated after your let go) to the terminal on the relay that you removed the white wire from (where the blue wire should also be installed).

• Run the other leg of the momentary push button switch to a good ground (preferably to the frame or engine block).

How the Manual Glow Plug Switch Works

To cycle the glow plugs, turn the key to the "run" position and then hold down the manual glow plug switch for 7-10 seconds, then release and start the engine. Do not cycle the glow plugs for more than 15 seconds at a time . If the glow plugs need to be cycled again, wait 5 seconds and repeat as necessary.

The modification works because it controls the ground for the glow plug relay (which receives power anytime the key is in the on position, but the glow plug controller normally switches the ground). When the button is held down, it completes the circuit and activates the glow plug relay, bypassing the controller and supplying power to the glow plugs for however long the button is depressed.