My dad's 350sdl is having an odd issue with the power accessories. On occasion, the radio, wipers, power windows - basically all power accessories- just turn off and then back on after a few moments. Today, they didn't come on at all.

My dad says that when he has jiggled the key in the past, it seems to correct the issue, but this could just be a coincidence.

The other issue is that the glowplug warning light behaves oddly. It doesn't always come on when the car is glowing, and it comes on after running for a minute or two and then stays on. This might just be a bad glowplug, which is frustrating because I replaced them with Bosch ones about 2,000 miles ago.

Has anyone had this issue before? I am trying to decide if this could be an issue with the glowplug relay being bad and creating a big power draw, or if it's an ignition switch issue. I tried to swap the glowplug relay with a spare I had here, but I forgot that the 91's have afterglow and the older models don't, so I was unable to swap them out.

I would focus my attention on the ignition switch.

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+1

91 W140 should be still a close cousin of the 124 electrically. Pretty much everything that is controlled by the switch, directly runs through the key switch contacts on a 124, and I suspect yours is similar. Radio, wipers, glow plug relay, climate control blower go direct and power seats/power windows through relays controlled by the key switch contacts.

Be careful poking around back there, you have direct unfused battery power available.

The electrical switch deck can be separated from the main locking mechanism. There is a certain point in the rotation of the key that frees the linkage from the lock cylinder to the switch deck.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech