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DIY: How to Replace Your Diesel Glow Plugs

    Diesel engines do not have spark plugs or ignition system of any kind. So it is up to the glow plugs to get it going when the engine is cold or it's cold out.
» Part 1: What You Will Need
» Part 2: General Info
» Part 3: Glow Plug Design
» Part 4: Before You Start

Design Of Pencil Element Glow Plugs

Except for the heater element, the design of a quick-start pencil element glow plug is the same as that of a pencil element glow plug. The heater element consists of a heater and control coil connected in series.

When the glow system is switched on, each glow plug will be subject to a current of approximately 30 amps. The glow plug is heated very quickly by heater coil. With increasing temperature, the control coil increases its resistance and limits the current to approximately 815 amps. This will protect the glow plug against overloads.

While there is no scheduled replacement interval for glow plugs, they are often forgotten until they go bad. That is why I, personally, recommend replacing them every 60,000 miles. If the winters get as cold as they do in here in Minnesota, you will like knowing that your glow plugs are not going to crap out when it's 40 degrees below zero.

Chrysler Vehicles

Some Chrysler vehicles equipped with an optional diesel engine do not use glow plugs; they use an Intake Manifold Air Heater Grid to heat the air going into the cylinders. In the instrument cluster there is a Wait-To-Start lamp. The Wait-To-Start lamp gives an indication that the conditions for easiest starting of the diesel engine have not yet been achieved. The Powertrain Control Module (PCM) lights the Wait-to-Start lamp in the instrument cluster after the ignition switch is turned to the ON position.

One side of the Wait-To-Start lamp bulb receives battery voltage when the ignition switch is turned to the ON position. The PCM switches the ground path for the other side of the bulb based upon several inputs and its internal programming.

The Wait-To-Start lamp lets the driver know that the intake manifold air heater grid has had sufficient time to warm the intake air for a good quality start. The intake manifold air preheat cycle is controlled by an Electronic Air Heater Control Module. The lamp will be turned off by the PCM when the heater control module cycle is completed, or if the driver turns the ignition switch to the START position prior to the end of the heater control module cycle.

Testing glow plugs is easy and can be done with them still installed in the engine. Just disconnect the wire going to each glow plug. Connect a test light to the POSITIVE (+) battery terminal and touch the point of the test light to each glow plug terminal. If the light lights, it's good. If it doesn't, it's bad and needs to be replaced. Do you replace just the bad one or all of them? My opinion is that if one went bad, then the rest are not too far behind. So I recommend replacing all of them at the same time. I would replace, at the very least, all of the glow plugs on the same side.

Some diesel engines, Mercedes Benz diesels for example, have a Pre-combustion Chamber that houses the glow plugs. This Pre-combustion Chamber helps slow down the combustion process and aids in cold starting. They do have a tendency to get carboned up and thus rendering the glow plugs ineffective. So when the glow plugs on engines equipped with a Pre-combustion Chamber are replaced, the Pre-combustion Chamber should be reamed out to remove any carbon build up.

» Part 1       » Part 2       » Part 3       » Part 4
Additional Information provided courtesy of and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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