What tells the glow plug light to go off?
Is it time or does it measure the temp of the glow plug?

Do they run in a series or are they parallel?

My light started going off really quickly it seemed
and I had some rough starting. Then no light.
I checked and #2 & #5 were bad. Replaced them
and all is well now. This is a 84 300SD.

Just curious......

Jeff

Parallel on an 84. (series on pre--?? (1980) models) Length of light determined by ambient temperature measured in the relay somehow. Relay compares current draw between plug #1 and the rest. If results are unusual, it tells the light not to come on, or flash or whatever.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

There is a variable resistor in the Relay that is used as a temp sensor. However, as said in other threads the Relay will continue to function for a little while after the light goes out.
You can open you Hood; turn to the key to pre-glow position;after the GP light goes off listen for how long it takes for the GP relay to shut off; click.
Other members watch their Dome Light. It is slightly dimmer during the pre-glow cycle and brightens when the Relay shuts off.

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84 300D, 82 Volvo 244Gl Diesel

Yes, I should have added that. Length of actual glow is independent of length glow light stays lit. Relays can vary. My 85 is around 25 seconds. My 79SD is around 40 seconds.
Early models sensed ambient temperature in the head. (of the engine, not the driver)

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13