That is a super simple and maybe the best way to do glow plugs. I have seen that method before, and it's what I will do if my factory relays ever burn out.This week, I had the HJ75 at Valley Hybrids/Cruiser Bros for a few days while I was cleaning up the HDJ81 to sell it and had Georg and his team diagnose my nonfunctional glow screen. This is an Aussie 12ht swap in a HJ75, so it is 12v and uses the single line glow screen. The guys put a Summit starter relay in, a 12v line to the battery, and a push-button on the left side of the steering wheel.
Initially, they thought the screen was missing another power cable and didn't think it would work as-is but the 12v screens only have one power line. They didn't want to try it out but I gave it a go this morning. We live at 7,000' and it gets down to 35º in the summer at night. Usually when I start it in the morning there's a massive noxious cloud of white smoke as the engine chugs. I have to give it some fuel to get it warmed up. It's an ordeal.
Their initial install had the power cable under the metal washer (which just caused sparks when ignited), but all the photos I found here on Mud showed the 12v line under the black insulator. This morning, I moved the 12v line under the insulator and ran the glow screen for 10 seconds and it cranked right up. No white smoke, no hard start. Just a small puff of black smoke from the direct injection. This was the last piece to the puzzle and I can't be more pleased with the install.
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HMMWVs in the early 90s were terrible for burning up glow plugs. They had 12 volt glow plugs on a 24 volt truck. The glow plug relay in the start switch would send only 12 to the glow plugs. But if you try to start the truck without waiting on the weight light to go out as guys do in a hurry it will send 24 volts to the glow plugs while you're cranking the truck and burn them out prematurely. Never saw a reliable group of Humvees in cold weather until I went to Montana. The national guard motor sergeant had bypassed all of the factory glow plug controllers and use the Ford starter solenoid and momentary switch with the power wire between the two batteries for 12v like your setup. Hold the switch for 5 seconds and crank. They had over a hundred HMMWVs and all of them were 100% reliable in the Montana winter and they had not replaced a glow plug in over 2 years since converting to the manual glow system.