A warm engine does not require any glow so waiting isn't required. On the W140 the glow light goes out before the relay discontinues supplying power to the GP's so it seems to matter little.

Recently while fighting with my glow controller I removed the case and set the unit so I could watch it when I turned on the key and noticed that even after the dash glow light went out the glow controller was still activated and it didn't disengage for another 25 seconds or so.

When it's warm I seldom wait for the light to go off.

steve a

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Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others