300 TDT cranks, voltage at glow plugs, won't fire

My '82 300 TDT was running great. Set cruise at 82 mph all the way across Arizona on I-40, 28 mpg.
I'm back home, but now it won't start. This started at the motel in S.
Utah before crossing AZ. It was 45 degrees and took a jump start to get going. I had filled up with ultra low sulfer diesel the day before, and that is all we can buy now in CA where I live.
I took out 4 of the glow plugs (hard to get to #1 without removing injector lines) and they all glow red within a few seconds of being connected to 12 V via jumper cables. I cleaned up the tips with a file and a light touch on a grinder.
I thought the battery might not be providing adequate current flow, so I replaced it with a new DieHard International. Cranks faster, but still no go as far as firing is concerned.
The fusable link on the glow plug relay box is good. An ammeter shows current flow at each of the glow plugs when I turn the ingnition to start.
Compression is not that bad, the motor provides resistance when I crank it. Starter is good, motor cranks rapidly, but does not fire. Like I say, it was running great before this started and even after the first problems appeared at the motel.
I loosened one of the injector lines at cylinder #4 and cranked the motor and fuel came out in bursts like it should.
Can those relay boxes go bad part way, that is, still provide some current but not enough to get the plugs hot? I guess my next test is remove a glow plug, put the wire back on, ground it and then see if it gets red when I turn the ignition to start.
Any ideas?
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The glow plugs are fine.
If you floor the accelerator during cranking????
The new variable is the fuel. It may have loosened all the old deposits and dumped them into the fuel filters.
Focus on the fuel side, look at the small clear filter - the clear plastic one - replace it if it's dark, OK if it still looks like wet redwood. If you need to replace the one filter replace the spin on filter as well.
Look at the clear plastic fuel lines - any air bubbles?? If so there's an air leak probably at a hose to fuel line joint or a filter or the hand primer pump's top "O" ring is shot and the hand pump needs to be replaced - $20.
If you find nothing pull the vacuum line off the shut down bellows. That connection is at the aft end of the injection pump - between the IP and oil filter housing. If that's the cause you'll need to shut down the motor using the throttle linkage STOP lever until the shut down device is replaced.
Finally, take a deep breath, remove the valve cover and check the timing. Turn the motor pulling and pushing on the drive belts until a notch in the thrust collar immediately behind the cam's chain sprocket is exactly aligned with a groove cut into the driver side front cam support (cam centerline). Rotate the motor only rightward, clockwise. When the notch is aligned with the groove the cam is at its 00 degrees, then read the crankshaft angle from the pointer above the harmonic balancer. It should not exceed 5 degrees, but probably will be at 7 degrees or so. Above 5 is the time to replace the timing chain anyway.
I suspect if there's no fuel problem that the chain may have jumped a tooth or two on the sprocket and that the cam is no longer in sync with the crank. The injection pump is driven by the same chain so the injection will also be "off". This is the dire news side of the suggestion I'd focus first on the filters, particularly if you've run Bio100 fuel in the car. Always look at the simple things first.
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Well, you tested all the bits separately but did you pull a glow plug, turn the key on and see if it heats up? That is if the whole system works together?
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