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Yep it's a dying relay in my engine start box. I just give the box a kick and it works for a few more weeks.

If it's a relay causing all the issues, I'll definitely open it up and take a look. Should be easy enough to swap out for a new relay.

This sounds extremely similar to my truck. Next time it does it, give the PCB a few taps and see if that does anything. Would at least narrow it down to some weird PCB issue. As soon as I smacked mine, it started working and it hasn't happened again but I'll see if I can duplicate it when it does.

I do believe you are correct. Maybe corroded relay contacts or something. Every day it's looking more and more like I'll be buying your box so I can stop screwing with this dang thing.

Just an update: The thing has happened a few more times and once right at startup in my driveway which allowed me to dig deeper. Turns out there is no voltage at the IGN input at the regulator when it's doing this. Or rather, not enough voltage? My meter showed 1-2 volts slowly climbing .1 volt...

That is EXACTLY what will end up happening. I've been there done that one too many other times with other vehicles.

Haha I think your 25 cents is probably worth more than a quarter. I'm just trying to understand how the regulator dying turns off the blower fan when battery voltage is still present. Of course I've driven several times the last few days and it won't act up now. It's waiting until the worst time...

Does the tach need IGN signal from the PCB to function though? These issues are somehow all related because they happen at the same time and recover at the same time. The PCB seems to be the only common point that I can fathom.

It happens when the A/C isn't running even or if Heat is selected instead. You can also hear the engine pitch change slightly when the alternator starts charging again (volt meter returns to green range). It's the same sound change as when the glow plugs are cycling right after a cold start so...

It's that 2006 M1152 in my signature. It has the 200 amp alternator. I suspect the PCB is cutting the output for tach, alternator field current and accessories for some reason or perhaps the PCB is losing the AC rpm signal from the alternator and shutting those things down? I'm not exactly sure...

Sure is. I need to follow that back to see where the source of this issue is. Maybe the PCB is cutting power to the tach, accessories and alternator field current but somehow not the gauges and IP fuel cutoff solenoid. I'll have to study the diagrams more closely to see if that's even possible.

You know what is also interesting is the PCB seems to somehow control the A/C fan power because the blower runs for about 2 seconds after flipping the ignition to OFF. I am thinking its that "+24VDC from control box" signal to the speed switch. I need to see if the tach signal passes through the...

Very interesting find! Now that you mention it, I am not sure that I ever specifically stared at the fuel gauge when it happens. I just know temp gauge and definately the voltmeter are still functioning. I'll try to get some video next time it happens to document the behavior for a closer look...

Good idea. I wish I could easily recreate it in my driveway so I could check a bunch of things but of course it only seems to happen when I am out driving around.

I am probably wrong but I have a vague recollection of reading somewhere the turbo trucks get the tach signal via the alternator/regulator. The dash gauges stay operating but the voltmeter does dip down due to the alternator not supplying current. You can hear the reduced engine load when this...

Reminds me when the driver got the truck started and off his trailer while I was moving my cars around the driveway to make room. After he left, I realized the truck was running and the front battery wasn't even connected. The lead was just flopping there haha. As long as the fuel cut-off...

Looking for some words of wisdom from a HMMWV expert for an extremely intermittent electrical gremlin that has only occurred maybe 5 times in probably 30 drives now. Basically what happens is, while driving or stopped, the tach drops to zero, voltage drops to battery voltage (no alternator...