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It looks like the PCP isn't grounding the low side of the glow plug relay coil. Is it suppose to do this briefly at start up even when it isn't cold outside?
I believe the pcm uses engine oil temperature.....Even when it’s cold out- if the truck had already previously been ran and is still warm (hot - stopping while out shopping) the GPR will not energize it. The Wait to Start light will still come on but is a completely separate system from the GPR. A LED indicator light put on the dash wired into the large terminal (glow plug side) will allow you to monitor when the GPR is energized and for how long.
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The PCM typically will perform a quick output monitor test when the ignition is turned to run - this may be dependent on model year. This happens quickly and if the GP relay/module is checked it will happen faster than you might be able to see. Engines with relays typically will not set a code but module equipped engine should. If your relay is burnt out which is common, it can be tested and replaced as needed. I manually ground the relay and check the voltages on the terminals and at the same time it is a good practice to individually test the glow plugs.
 
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^^^ if you go to one of klhansen’s posts and look at his signature at the bottom, there is a link for the step by step process of checking GPR and glowplugs, as ford-_doctor mentioned above.
 

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And, the correct answer to your question is - yes, the GPR gets activated every single time the key is turned to the on position and the oil temperature is below about 130°. As BigHorn pointed out, the length of time it is on is controlled by the PCM based on oil temperature. In the attachment, you will find the length of time the glow plugs are designed to be activated.
 

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Slope Font Plot Parallel Rectangle
 

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I wonder why the older 7.3s had more glow plug time than the 99-03 models. The Ford diagnostic chart in the link I posted shows glow plug time going to zero above 130°. Maybe because of the slower starters?
 

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So, here's a question... would it be a good idea to wait the projected time to start at cold temps? If the GP's are supposed to be on 120 sec, for example, turn key, count to 100, and then start? I'm thinking about those really cold days and questionable starting trucks.
 
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I don’t know...... but that’s what I always do when it’s cold, I wait until almost the end of the cycle and have never not had the engine jump to life.

And RT is right on the money on the 130 degrees. It took me awhile after getting the truck to figure out why sometimes the GPR LED indicator lamp never came on after just a short drive 👍 ....... but mine sits in an insulated pole barn so it doesn’t take long.
 

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Greg, better and really cold are relative. I live in Central Alabama and my truck will start easily as cold as it ever gets here. I use the plug engine heater when it gets low 20's F. But that's for me not the truck, it gets cab heat quicker that way. That's one relative.
I have had my truck across the US, coldest was in Minot, ND at -17F for jobs. If I couldn't find a hotel parking place with 120V outlet for the heater overnight and had to start up the next morning. I would cycle the GP's on and wait aproximatly 45 to 60 seconds and start up, kinda. It would run like a missing lawn mower and the throttle is ECM locked out until it warms a bit. So I had to freeze and wait for whatever the permissive is to be achieved. That's the other extreme of relative.
So "better" for me is to include using the engine heater in extreme low temperatures. Heater, GP's and cold engine throttle control are purposeful Engineered strategies to start quickly and safely. Drivers operating in extreme cold locations would benefit from getting "better" prepared in September.
 
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Drivers operating in extreme cold locations would benefit from getting "better" prepared in September.
Yeah, like not filling up at the start of September and having 1/2 tank at the beginning of December in single digits and having the fuel gel. LOL! Don't ask how I know this. ;) I needed to drive the truck a bit more before then.
 

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I have a real time analyzer used primarily for vibration analysis and large machine rotor balancing in-place. Additionally, it has analysis features I also use for plotting and recording 12V DC vs time in a spectral plot and 4>20 ma AC signals used for industrial instrumentation.
It's 70 F. In Alabama today, not a good day for an illustration, when it turns cold again I will set up on my truck and record 12V + - DC current vs time in spectral form and post it. Seen in that format it's easy to understand how the start, gp cycle, charging and control timing work.
 

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If you have a Scan Gauge II you can watch the voltage jump up when the glow plugs shut off.
I don't have a ScanGauge, and I can see that on the factory volt gauge.
 

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on my '02 Excursion, it varies by Temperature.

yesterday, when in town and stopped for a few minutes, I paid attention to the WTS light.

It turned ON for about 3 seconds and went off.
 

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The WTS light has nothing to do with the GPR on-time.
 
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