I'm not getting 12 volts to the o2 sensor heater. Looking at the wiring diagram, it looks like the V+ wire goes directly from the sensor heater to pin 30 of the ECM. Does anyone know if there is a relay and/or fuse in this heater circuit?

No relay or fuse on that model.

Directly from HFM control unit.

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

That's what I was afraid of. Do you know if there is an operating condition or external signal the hfm unit is looking for that would tell it to shut off that voltage or am i going to be opening up the unit to look for an internal fault if i find no fault in the wiring from the unit?

Signal should happen at approx 35C engine coolant temp.

Heater runs all of the time from there.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

I assume that's determined(35C) by B11/3?..Any resistance/temperature reference charts available for B11/3 to test it for function?