Are glow plugs supposed to work like this?

[Gubbtratt1]

I'm testing electrical shenanigans on my 1987 LJ70 and found that the glow plugs does some weird things, I'm just wondering if this is normal or if something is broken.

When I first turn on the key the glow plugs run on 12v for a millisecond and then switches to be connected in series with the big glow plug in the intake and the current sensing resistor, as toyota calls it. The first 12v mode works as expected, but when it switches to 6v the voltage across the glow plugs are just 3.5v, with 3.5v over the current sensing resistor and 5.5v over the big glow plug. The indicator light also isn't lit, it's only on with the very short 12v cycle. It takes several minutes for the plugs to turn off, at first I thought they would never turn off but I left them on while I wrote a post asking about that problem, and just as I was about to post it I heard the relay turn off.

 

[Gubbtratt1]

Sounds like it is working kinda like it should, only the high volt glow cycle should probably remain on a little longer.

But, even at -12c, mine only stayed on for 4 seconds before changing to low volt glow.

Thanks! I can't remember how cold it was when I made the video, but now it's maybe +5c and it's on for pretty much as long. The high voltage cycling on and off when turning on the ignition but not starting is normal?

 

[nuclearbeef]

Thanks! I can't remember how cold it was when I made the video, but now it's maybe +5c and it's on for pretty much as long. The high voltage cycling on and off when turning on the ignition but not starting is normal?

Yes, normal.
It should begin with higher volts, then drop to lower volts after a time determined by coolant temp.
It should do this regardless of whether you turn the ignition to "start".

The design is to push high volts to the glow plugs before starter engagement. Then when it switches to low volts; the light goes out and you turn the key to start.

 

[Gubbtratt1]

Yes, normal.
It should begin with higher volts, then drop to lower volts after a time determined by coolant temp.
It should do this regardless of whether you turn the ignition to "start".

The design is to push high volts to the glow plugs before starter engagement. Then when it switches to low volts; the light goes out and you turn the key to start.

Yup, I know that much. But if I don't start when the light goes out the high volt relay turns on again after a few seconds, then off, then on, and continues doing so until I start or turn off power. I'm wondering if this part is normal?

Edit: or is that what you meant? "it should do this regardless of whether you turn the ignition to "start"." threw me off a bit when re-reading.

 

[Eurasiaoverland]

Yup, I know that much. But if I don't start when the light goes out the high volt relay turns on again after a few seconds, then off, then on, and continues doing so until I start or turn off power. I'm wondering if this part is normal?

Edit: or is that what you meant? "it should do this regardless of whether you turn the ignition to "start"." threw me off a bit when re-reading.

No it should not keep cycling if the key is kept in the IGN position, only if you return it to ACC and then advance it to IGN again.

 

[Gubbtratt1]

No it should not keep cycling if the key is kept in the IGN position, only if you return it to ACC and then advance it to IGN again.

So it's still broken. It shouldn't be much of a problem though? Once all electrical work is finished I won't have much reason to turn on power without starting, and until then I could unplug the relay to really make sure the glow plugs won't overheat.

This is of course assuming it's on for long enough when the engine is cold. I found a chart for how long the light and the high voltage relay should be on for depending on temperature for 2H, would this be accurate for 2LT?