P138B – Glow Plug Control Module System Voltage


By Reinier (Contact Me)
Last Updated 2019-03-28
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P138B Glow Plug Control Module System Voltage Wiring, Defective glow plug control module, Glow plug(s), PCM

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What Does Code P138B Mean?

OBD II fault code P138B is a manufacturer specific code that is defined by carmaker Ford as “Glow Plug Control Module- System Voltage”, and is set when the PCM (Powertrain Control Module) detects an abnormally high or low voltage feeding into the glow plug control module.

NOTE: It should be noted that code P138B refers to an abnormal voltage in the glow plug control modules’ control system, and NOT to a failure of the either the glow plug control module, or the glow plugs.

Since the combustion of diesel fuel is very difficult to initiate in cold engines, all diesel engines are fitted with glow plugs to provide an initial source of heat to assist in the ignition of the diesel fuel/air mixture in low ambient temperatures, or when the engine is cold- typically after not having run for a period of several hours.

In practice, glow plugs screw into the cylinder head much like spark plugs on gasoline engines, and consist of a steel tube containing a heating element that is embedded in a ceramic-based material to improve convection. On older systems, the glow plugs were supplied with current from the battery via a relay and timing device when the ignition was turned on.

On the latest iterations of glow plug control systems, the glow plugs are controlled individually by a dedicated control module, both to improve glow plug control, and to reduce glow plug warm-up times. This is made possible by the fact that glow plug technology has now made it possible to produce glow plugs that reach their optimum temperatures in about 2-3 seconds, as opposed to older glow plugs that required as many as 10 -12 seconds (and sometimes more) to warm up.

Improved control of the glow plugs is made possible by the fact that modern glow plug control modules have the ability to sense the electrical resistance of each individual glow plug’s heating element, which is important, since no two glow plugs on any diesel engine are identical in all respects. While modern mass production methods have high repeatability rates, it is impossible to eliminate all differences in all glow plugs that could potentially affect the operation of a given glow plug relative to other, seemingly identical glow plugs.

Nonetheless, by being able to sense or measure the electrical resistance of each glow plug, a glow plug control module can modulate the current that is fed to each individual glow plug via a dedicated plug lead. The practical advantage of this is that all the glow plugs are heated to the same temperature in the same amount of time, which has the benefits of reducing engine-starting times, while reducing harmful exhaust emissions, at the same time.

However, in terms of overall control of the glow plugs, the PCM uses primary input data from one, or sometimes two dedicated engine coolant temperature sensors. When the ignition is turned on, the PCM will calculate the actual engine coolant temperature, and if the coolant temperature is below a specified threshold, the PCM will enable the glow plug control module to feed an appropriate current to each glow plug.

Conversely, if the engine coolant temperature is at, or above a specified threshold, the PCM will disable the glow plug control module, since the residual heat in the cylinders is sufficient to ignite the air/fuel mixture. Note though that in extremely low ambient temperatures, the glow plug control module may continue to feed current to the glow plugs for several seconds after the engine has started both to improve combustion, and to reduce exhaust emissions.

As a practical matter, the neither the PCM nor the glow plug control module can control the glow plugs when the specified current is not available to the glow plug control module. Therefore, when the correct current is not available, the glow plug control module will sometimes be disabled altogether, and code P138B will be set and a warning light illuminated.

Where is the P138B sensor located?

The image shows the location of the glow plug control module (circled) on a 6.0L Powerstroke engine. On other applications, the glow plug control module may be mounted on the firewall, or on a shock absorber tower. In almost all cases though, the glow plug control module will be located in the engine compartment. If however, the module is not immediately visible, simply trace the leads (resembling spark plug leads) that are attached to the glow plugs back to the control module.

Note the location of the electrical connectors; loose, damaged, corroded, or broken terminals in these connectors are the most common cause(s) of this code on almost all Ford applications.

What are the common causes of code P138B?

The most common causes of code P138B could include one or more of the following-

  • Damaged, burnt, corroded, disconnected and/or corroded wiring in the glow plug control module’s wiring harness
  • Damaged, corroded, or loose terminals in the glow plug control module’s electrical connector(s). Note that given the glow plug control module’s location in the engine compartment, this is among the leading causes of issues with glow plug control modules on many Ford applications
  • Defective glow plug control module
  • One or more defective glow plugs
  • Failed or failing PCM, but note that since this is a rare event, the fault must be sought elsewhere before any control module is replaced.

What are the symptoms of code P138B?

The symptoms of code P138B are much the same across all Ford applications, and could include the following-

  • Stored trouble code and one or more illuminated warning lights
  • While the engine may crank, it may not start when it is cold, or extended cranking times may be required before the engine starts
  • If the engine does start, it may not idle, or the idling may be rough and/or erratic during the time the engine warms up sufficiently to sustain efficient combustion
  • If this code is not resolved in a timely manner, extended cranking times could damage both the battery and the starter motor

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