Rick's Free Auto Repair Advice

Ford Glow Plug Problems

How to fix Ford glow plug problems

If you own a Ford Diesel and you’re having Ford Glow Plug Problems problems, first read up on how the glow plug system works. There are two versions of the system. The first system is used in all 49 U.S. states EXCEPT California. Due to stricter emissions laws, the California system is different.

Wait to Start

When you first turn the key, the PCM turns on the Wait to Start light and provides power to the glow plugs. The light stays on for only 3-5 seconds. But the PCM continues to provide power to the glow plugs for as long as 180 seconds to pre-heat the combustion chamber—even though the “wait” light is off.

Hard Starting Diagnostics

If you suspect a problem with the glow plug system, start here:

1.Make sure the battery has a full charge

2.At the glow plug connector (GPC) on the right side valve cover, check for battery voltage on the large black/yellow wire. If that’s present, check for battery voltage at the smaller red wire (key on).

3.If you see voltage at those two wires, cycle the key off and on while you check the other small wire pink/orange. It should have less than 0.5 volts. This is the wire that grounds the coil in the glow plug relay. Also listen for the relay to click when you cycle the key off and on.

4.If you don’t hear a relay sound or the voltage is higher than 0.5 volts, try checking for output voltage at the relay on the two large brown wires—one to each engine bank. (California models may have 1 brown and 1 yellow wire). If you see no voltage, then the relay is bad and not providing battery power to the glow plugs.

5.Finally, perform a voltage drop test to check for excessive resistance inside the relay. Place one lead of your multimeter on the positive battery terminal and the other lead on the output side of the relay. Cycle the key on and off. The reading should be less than 0.5 volts. Anything higher means excessive resistance and you should replace the relay or check the wiring for corrosion.

Testing the Glow Plug

You’ll need a high amp clamp on inductive meter for this test. Do NOT hook up an ordinary multimeter in amp mode. It will blow the internal fuse of the meters in microseconds. Always check the relay when it’s cold. That’s when they malfunction.

1.Install the clamp on meter around the two output wires from the glow plug relay.

2.Turn the key ON and look for an initial reading of 160 amps or more. Glow plugs draw approximately 17-26 amps each (20 amp draw is recommended minimum).

3.If you don’t see at least 160 amps, move the clamp meter to test each glow plug feed wire at the valve cover gasket connector. (1994-97 vehicles test the brown wires in terminals 1 and 5 of each of the four connectors) (1999 and newer test terminals 1 and 2, and 10 and 11 of the two 11-cavity connectors—wires my be yellow) Cycle the key on and off and check for a 20 amp draw at each glow plug. Replace any plug that doesn’t read 20 amps.

CAUTION: Allow some cooling time in-between each test.

California System differences

The California system operates in a similar fashion but uses a module to switch power to the glow plugs instead of a relay. The output from the module is different than for the 49-state version. It has one output wire for each cylinder. The purpose of the module is that it reports current draw back to the PCM so it can monitor the performance of each glow plug. If it see too low a current draw, it sets a trouble codes P0671 – P0678.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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