Been meaning to post about this for awhile....

I have noticed there is a difference in the amount of time that passes between key on and glow plug current draw.

When it is cold (this AM was 5F when I started the rig), it takes about 45 seconds before the voltage drop is indicated on a meter (and on the dash gauge). This is about 20 +/- seconds after the fuel pump shuts off. Voltage drops to a nominal 11v. This is typical during the winter.

The other day the temp was 30-ish and the dash volt-o-meter showed a near immediate drop (1-2 seconds) and the fuel pump was running.

Is the PCM that smart to adjust the GP activation time? Really the only thing I can think of is trying to cut the batteries some slack and not burden them with the Fuel Pump and GPs running at same time during cold start conditions.

 

It sounds to me like the glow plug relay has internal high resistance and took some time for the internal high-current contacts to finally burn in enough to transfer some juice.

There is no delay programming. I can say with certainty that the relay command circuit is grounded as soon as the key is cycled to the "RUN" position no matter the temperature; the duration of its duty is the only thing temlerature-dependent. That would be my first check on yours. Check for 12V across the small terminals (or listen for the audible click) and then check voltage drop across the two high current lugs.

The PCM is like a woman - it is hard wired to never "cut any slack" to preserve anything. It has one mission and that is serve itself; all others be damned.

 

^^^^ ^^^^

 

....

 

With that said, I have disconnected the ground wire from the glow plug relay and wired that post to a push button switch on the dash board. Also, if you ever need a relay in a pinch, such as a glow plug relay giving up the ghost 1,000 miles from home like what happened to me on a cold morning in the mountains, simply swap the intake air heater relay into it's place. Mine fried up when one of the glow plug wires rubbed against a push rod under the valve cover and shorted out. It's physically a bit bigger, so I'm assuming it carries more current and will more than suffice for powering the glow plugs.
You don't need glow plugs for anything else than to get the engine to start up when it's cold out. They leave them on for a short period of time to control the amount of smoke coming out of the tailpipe at first start, but you don't need them to keep the engine running once it's started. Wanting to extend the lifespan of the glow plugs because they are a pain to replace is why I decided to install that push button.

 

 

Does that apply to the cold weather PCMs?

Going with no programmed delay, then yes it could be a balky relay.

Although I will say that the time for the voltage drop is remarkably consistent regardless if it is +5 or -25F. Within 1 sec of 45 seconds. And when it is warmer (say above 25F), it is pretty quick after key on.

I have also noticed that the 45 second delay will remain after the engine (and compartment) is warm, and the outdoor temps are still cold.

The relay was changed in Feb 2024 when I noticed some GP performance issues (with a RiffRaff relay). I had to cycle the GP 2x instead of 1x - that type of thing. Turns out I had some bad GPs, which were replaced in Sept 2024. At the time I did basic checks on the system and nothing jumped off the page e.g. the timing of GP on was right away. But it was warm out.

I guess the batteries are SOL from the PCMs perspective. I'll let them know they should keep an eye out.

 

This project is in the queue, awaiting the install of the F650 dash panel/upfitter switches addition.

Also, my AIH relay is disconnected, but in place. For the very reason it could serve as a backup if I get into a jam.

 

Open the hood and test it. Speculation wastes time.

Why would the PCM command the Wait To Start to light to come on, go off after 8-15 seconds, THEN command the glow plug relay to close? Most people hate waiting for the light to go out and even more people begin cranking the INSTANT the light is extinguished.

On the other hand, if your engine starts when the light goes out when it's <50 degrees outside, your glow plugs are working......it ain't gonna start without them.

 

At those temps, it could be the intake air heater activating. Next time you see it drop, put the truck in gear and see if it rises again when you do that.

 

Mine is disconnected completely - feed and supply.

 

This engine will start without any sort of GP cycle action down to 20F. I don't make a habit of it. My wife does since she doesn't drive this thing often and forgets about letting it warm first.
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Ok, that was easy to rule out, lol .

 

 

So I did a solenoid voltage test at 8F, but I'm not near the rig to do a warmer temp. test for comparison. I checked voltages across all the usual characters. The tale of the tape is that doing this 4x (the table shows the first time) resulted in the solenoid closing 45 seconds after KeyOn (+/- 1-2 seconds). This was consistent over the 4 attempts. Each time the solenoid was closed for 110 seconds, with less than 1 second difference.

I am not ruling out some temperature related hangup prob with the solenoid. But at a minimum I would expect after the 3rd or 4th try it would have shaken itself loose if we are talking a mechanical hangup. Or a reduction in the amount of time the contacts close.

When I last drove my truck and parked at the airport, temp was at 30F and the truck was running at speed for 60+ minutes. I shut it off, waited a few moments and turned the key. The volt-o-meter dropped immediately. During the above testing, the volt-o-meter would drop at the same time the solenoid contacts closed (shown by a test light).

I do accept that the PCM doesn't control solenoid engagement time. But it is weird that whatever is going on comes across as something is controlling the time the solenoid closes. It is very consistent on timing, as well as temperature. Note that I have not picked up on a pattern on what happens between "cold" (like <10F) and "warm" (30F or so). I may not get that opportunity this spring.