Common power ground for o2 sensor.

[Lugnuts]

My E30 cigarette lighter provides a 12v switched power source. DMM indicates it's dead unless the ignition is "on". Sounds like a perfect source to power my o2 controller.

The o2 sensor doco says it's mandatory that the ECU and the o2 controller share a common POWER ground.

Linking the o2 controller to the ECU for lambda signals is done via 2 wires - one for signal (out) and one for a "compensating" voltage (in, from the ECU) to avoid offset voltages.

See the bottom of this page for how you're supposed to wire up the ECU and o2 controller: http://wbo2.com/sw/wblinout.htm

Can I use the ground wire for the cigarette lighter as power ground for the o2 controller? Is this considered "common" ?

Edit: ECU is Megasquirt DIYPNP. o2 controller will be linked to ECU for signal via "O2" and "SG".

[varg]

There may be enough voltage drop between the lighter and the ms that it affects the reading. Have you considered wiring it in to the stock O2 sensor connector by using a cut off stock O2 sensor pigtail?

[ak-]

I wired my 12v switched to the yellow defroster switch on the HVAC panel with a fuse in-line as instructed.

All of my grounds, 02, MS, controller, went to the ECU ground (passenger, or driver for you?) shock tower bolt/nut.

My AFR gauge matches what tuner studio's AFR reading measures.

[Lugnuts]

Have you considered wiring it in to the stock O2 sensor connector by using a cut off stock O2 sensor pigtail?

Hey that's not a bad idea. Wish I'd kept my old O2 sensor now!

I think for the interim until I get everything working smoothly, I'll power + ground the O2 controller using the cigarette lighter wires as planned (NOT using the CL socket itself). Easy to use - just means crimping on a couple of spade connectors. I'm not expecting to get this all working well in a single weekend, so being able to undo everything and drive the car come Monday morning is important.

I contacted TechEdge and they said using the CL socket is not a good idea (I never planned to, they misunderstood my first email) - "they often have an impedance of an ohm or so - this is way too high (at a starup current for the heater of 5 Amps that's 5 Volts LOST), so the wideband unit will see 12.0-5.0 = 7 Volts under load.". They then went on to say using the CL wires is fine.

Once I'm happy the MS is tuned and working correctly, I may well butcher my existing NB sensor and wire the controller as you suggest...or via an "add a circuit"...or via the auxillary port in the glovebox...or just leave it using the CL wires..not decided yet. I unfortunately don't have an auxillary fuse box like I've seen with other E30s.

All of my grounds, 02, MS, controller, went to the ECU ground (passenger, or driver for you?) shock tower bolt/nut.

Driver :)

From what I've worked out so far, my DIYPNP MS will be grounded using 3 of the 4 ground lines available in the factory wiring loom - pins 2, 14 and 24. Not sure if I should also ground via pin 19 since that's for the always-live 12v feed (vs pin 37 that I'm going to use, which is switched). I'm basically following this:

https://www.diyautotune.com/diypnp/apps/b55/usdm-bmw-325-8792-m20b25-mt.html

IAT sensor will be grounded via one of the two SG points on the MS. The other SG will be used by the TPS. O2 sensor will be ground via the CL wires, but using their differential signal wiring which I'm assuming will compensate for any voltage offsets (if present...)

I'll make a point to compare what the O2 controller thinks the AFR is and what the MS is reporting.

So to answer my OP, using my CL wires as a switched 12v source + ground for the O2 controller should be fine.