Common Problems

Part 2 Glow Plugs

Published in the July 2018 Issue July 2018 Ask The Expert

By Gary Fields of Diesel Pros

 Hopefully after reading the first installment in this series, you all decided to pull your hood release latch and take a gander at your batteries. Might have been the first time for some of you, and the hope is that 1 or 2 of you were able to catch a potential issue with your batteries before it left you stranded.

We covered the batteries in detail last month, and this month I’ll be explaining glow plugs in as much detail as possible. They’re a fairly simple widget with a big job to do so let’s get started.

Glow Plug Design

Most glow plugs by design are a self-regulating heating element that utilizes the same principle of operation as the glow plug relay (or coil). To be contrite about it, they are an intermediary component between battery voltage and ground.

If you’ve ever touched a live 12 volt wire to the grounded body or engine block you noticed immediately that there’s some magic happening there. Similar to the way a light bulb works, the glow plug takes incoming battery voltage and amperage and converts it to heat at the tip via resistance through its internal coil or heating element to ground. The tip then glows red hot, and inventor Captain Obvious aptly names this new device a glow plug. 

Why You Need Them

For the Cummins owners out there you can take a pass on this … I’m sure you’re already mumbling some sort of ‘lack of engineering’ statement under your breath about those ‘V-squats-2-pee-I-don’t-need-blow-plugs’ trucks but I digress.

We’ll maybe talk about your intake air heaters at some point, but the idea here is that to get a cold diesel engine started and started efficiently you need to introduce some heat while cranking. 

These engines produce a tremendous amount of compression, but that compression isn’t enough by itself to fire off a 7.3 under say 100 degrees oil temp. Remember in Part 1 (June Diesel Tech) how I covered the importance of cranking speed? Well once an engine is fired and zinging along at 500-plus rpm it can typically make enough repetitive compression events to easily keep the momentum required for those 4-strokes to occur without the assistance of a starter.  The fuel is cold, the cylinders are cold and by nature tend to make less compression when cold; so we use the glow plug to heat things up.

How Glow Plugs Operate

We covered their design already and you know that they work by converting amperage into heat like a light bulb. But we haven’t yet talked about the strategy that the designers of the 7.3 implemented to utilize the glow plug properly.

The cold-start-aid strategy in these trucks is really quite simple once you understand the individual components. You have glow plugs and those glow plugs are controlled by a glow plug relay … not terribly difficult to understand from those terms, but how they are employed can be a little bit confusing to some. 

You turn your key on your cold 7.3 and your wait to start light comes on. Everyone that has a 7.3 knows this, but what you might not understand is that when the light goes out that doesn’t mean your glow plugs stop running. The engineers based glow plug ‘on time’ on the actual temperature of your engine’s oil and the colder it is, the longer they run.

For instance on your 50 degree cold 7.3 your glow plugs will continue to run for 80 to 90 seconds after your wait to start light disappears.  Even after the engine starts, your glow plugs will continue to run and if working properly will help with that white smoke your neighbors love so much.

Since your GP’s get their power (amperage) from the Glow Plug Relay, 49-state trucks have a relay located in the center of the engine bay. It’s a 4-post coil assembly that receives its control from the vehicles PCM (powertrain control module) and is designed to allow the 150-plus amps of power through it that these glow plug systems require. They look like the starter solenoids that Ford has employed since Jesus was a PFC, and they are also the sole cause of the majority of hard or no starts in cold weather.

Component Testing

We will cover the control portion of the glow plug system in our next installment, but I want to cover the testing methods we employ on a daily basis to verify a base-concern isn’t present.

The first place we test in a potential no-start cold situation is at the glow plug relay. It consists of four circuits and understanding them will really help you out in diagnosing a problem. 

On the relay you’ll see two large posts (high amp load) and two small posts (control circuits). Inside the relay you’ll find two copper-based ‘pads’ that are closed by a coil assembly. The aforementioned ‘control posts’ are what take care of that duty and when the two pads touch, the relay is then providing the proper amount of available amperage to the plugs through their circuits; if it’s all happening as advertised. 

The most common failure of the relay is that the two pads lose their conductivity through scoring, or corrosion internal to the relay. Sometimes those copper pads make like my parents and split but again, I digress. 

Here’s a simple test: With a 12 volt test light (test it on the battery to make sure it’s operational) check for battery voltage at one of the larger posts. Assuming you have battery voltage there, move the test light over to the other large terminal and have a helper turn the ignition on. You should hear and feel an audible ‘thump’ inside the relay and if you do you know the control portion of the relay is most likely working properly. 

The next thing you should notice if it’s working properly is that the battery voltage has now moved through the relay and is headed to the glow plugs. If it thumps, and no/low voltage (lower volts than is showing on the other terminal) is noticed, then your relay is junk and needs to be replaced. 

The next place is to test the individual glow plugs for continuity to ground. This is done by a variety of methods but the most common method is to test using a test pigtail (pictured) or right at the valve cover connectors. 

You’ll set your digital voltage meter to the ohms setting, and you’ll be testing at the proper pins on the valve cover to ground.  Remember, the glow plug is essentially a resistor in place of voltage to ground, and there should be continuity. As a rule of thumb, your glow plug should show .7 to 1.1 ohms of resistance to ground if it is operating properly (2.0 ohms for example, or zero continuity to ground is a result of a glow plug or valve cover harness that is not operating normally and should be replaced).

Verify the latter by removing the valve cover and testing suspect glow plugs directly at the plug itself. Replace the valve cover harness if the suspect plug tests good at the connection point or replace the glow plug if the concern is verified at the connection point.

Hopefully you now have a basic understanding refresher in the cold start aid system your 7.3 employs to get it fired up when cold. 

In the next installment we’ll dive into the specifics of how the remaining control portion of the glow plug system operates in order to give you a more thorough understanding of the less common but just as important portion of how your truck works.  Till then, enjoy the drive.

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