I just measured my 300D and the VW and the voltage also dropped slowly on both like yours till it stabilized to around 0.1 V. I know my coolant is good they were both recently changed. Neither radiator is grounded to the chassis. Are radiators supposed to be grounded? I do not see a radiator ground strap on neither car. I also used 3 different DVM's (2 HF's) and the readings were close on 2 of them. On one of the HF where I changed the probe tips to alligator clips (looks like shiny tin plated) the voltage was much higher (started at 0.3 V and dropped slowly). I think the material of the probe that touches the coolant matters in what voltage you get.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked