I'm not familiar with the series controller, but I'd strongly suspect the original controller would need to be upgraded or bypassed to convert to parallel plugs. A 4 cylinder setup will pull 50-60A when glowing. If the wiring or contacts in the existing controller are not designed for that kind of current, you're looking at blown fuses, melted wires, or possibly a fire!

If you're competent with wiring, you should be able to install the newer controller without much fuss, or adapt the existing controller to operate a relay of some variety to control power to the glow plugs. The preglow light would no longer be relevant if you adapt the existing controller since the glow cycle will be significantly shorter.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

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1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]