Wiring o2 Sensor to be Powered Only with Engine On
 

Anyone have tips on wiring a wideband o2 sensor to only be powered on when the engine is running (e.g., in tandem with the fuel pump), instead of being powered with ignition on? The generic recommendation is to “power it from the fuel pump relay,” but with the wiring of the relay being unlike most vehicles and the safety cutoff from the air metering plate brown/black wire, I’m unsure how exactly to wire the o2 sensor to come on with the fuel pump. Is it as simple as running the o2 power wire with an inline fuse to the black wire coming from terminal 30 of the fuel pump relay (the wire that goes to the fuel pump) or directly to terminal 30 of the relay? Or do I need to use a separate relay for the o2 sensor and use the fuel pump wire to switch the relay on? Is there anywhere in the engine bay to tap into for power only when the engine is running?... possibly a way to use the brown/black wire from the safety cutoff?

Surely someone has their o2 powered to be on only with the engine running, as that seems to be the widely accepted install method to ensure a long sensor life (i.e., sensor heats up as the engine heats up).

I'm likely overthinking this... any help would be greatly appreciated!

Specs:
1976 911s 2.7
EFI conversion (all CIS parts removed, wiring still present)
Megasquirt 2 ecu running MS2extra
14point7 Spartan Lite v2 wideband controller w/Bosch LSU 4.9 sensor

Silly question, but is this on your 911 with EFI? I don't know much about MS2, but can't you designate an output with this function and have it activate a relay to power the O2 sensor?

I have a wideband O2 sensor in my CIS car; I just flip a switch to power it...

On all of my race engine setups (Haltech and MoTec) we always powered on the O2 sensor when the ignition was turned on. Common practice was to turn on the ignition and wait at least 30 seconds before starting the car.

I decided a 15 second delay was the simplest method. I’ve got my afr gauge in my ‘72 911, the car has no modern electronics besides the afr gauge. And with MfI, it sometimes takes a few tries before it’s up and running. 15 seconds means I reset the clock if I need to restart. So I’m not cycling the heater circuit in the O2 sensor nearly as much.

I used this one. https://a.co/d/a7dAduB

Wiring o2 Sensor to be Powered Only with Engine On
 

Why do you want it only powered when the fuel pump is running? The sensor needs to be heated in order to operate properly and powering it with the car on allows for that.

FWIW, Here’s the wiring I had with ms2, the o2 sensor is powered from a fuse panel that powered all the efi when the car is on.

https://www.dropbox.com/scl/fi/vx8hdsgr86ypmp3z6vrhi/EFI-Wiring.pdf?rlkey=j3yhdebhvwm387jron5zdfoxc&st=rjfk hvnc&dl=0

Yes, you're correct. I can control it from a currently unused output from the ECU and program it to come on with RPM. I just don't know enough about MS to do so. I suppose I need to research that some more.

That was my plan on wiring as well, as I have a fuse panel powering the ECU and injectors, until I noticed the install instructions indicate the o2 sensor should only be powered with the engine running. So I dove in and did some research and found it is indeed the suggested installation method to power it only with the engine running. And powering it with ignition on is said to be the leading cause of premature sensor failure (cold engine emitting cold moisture on an already hot sensor and causing thermal shock), as detailed in the following article. It's also specifically stated in the install instructions with the wideband controller to install so it's only powered on with the engine running.

https://www.nzefi.com/bosch-lsu-wide-band-airfuel-ratio-lambda-sensors-fail-often-aftermarket-performance-applications/

I believe that Megasquirt ignores the sensor for 30 seconds after powering to avoid corrupt readings but yeah, if the mfr says to have it wired only when the car is running, follow that.

Too bad the x-factory kit is wired that way /facepalm

Utilizing the outputs in MS isn’t terribly hard- you set it up in TunerStudio. The manual is pretty clear and if you need help the msextra forum is robust. (I’m away from my computer otherwise I’d help more) you’ll need a relay as the outputs are signal level voltage, not 12v.


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Thanks, Julian. I will head over to the msextra forums today and post over there if I can't find what I'm looking for.

I just figured it might be easier to power it from the fuel pump relay. I reached out to DIY AutoTune and they first suggested using an unused ECU output, but then followed up with the suggestion to power it in tandem with the fuel pump, which led me to find that is what most people recommend.

This is how the O2 sensor heater is powered on the stock 3.2. In tandem with the FP from the FP relay (called the DME relay). FP power is then fused downstream, but not the O2 sensor.

That is what I suspected, as well as with the SCs that have o2 sensors. I believe this is how most/all factory o2 sensors are powered, which is why they last so long.

So yeah, I would like to mimic that. Anyone know how to actually wire it like that?

If you still have the stock FP relay wired with safety cutoff through the sensor plate switch, you can use terminal 30 of this relay as a source of power for the O2 sensor heater.

Terminal 30 is where the FP gets its power from. Of course there are already two wires connected to this terminal, one for the FP and one for the WUR. But you can probably piggy back one of them, preferably the black one that goes to the FP.

I am referring to the diagram for 1982 SC.

Great, thank you! I still have the stock FP relay in place with the FP wire and WUR wire going to it. Although, I no longer have the WUR installed, so I suppose I could remove that wire from the relay… wait, could I tap into the WUR wire in the engine bay to achieve this?! That would be amazing!

I made sure the red/white WUR power wire went to pin 10 on the 14 pin connector in the left rear then checked continuity from pin 10 to the fuel pump relay terminal 30 and all is well. So now I just need to figure out the best way to tap into the red/white wire in the engine bay, preferably at the 14 pin connector.

Anyone see anything wrong with this?

The red/white wire in the engine harness turns ON when you turn the ignition switch to the Ignition position. Thus, it's on when the engine is not running. Your first idea, to wire it to the fuel pump, or the output of the fuel pump relay (pin 30), is the way to go. In the SC models, the pump only runs when the engine is cranking or running. IIRC, your 1976 works the same way. No need to power the O2 sensor through another relay, as the current draw of the o2 sensor is small, relative to the full pump. The fuel pump fuse is around 25 amps, but the pump draws about 10 amps or less.

Pete, are you sure the red/white turns on when the ignition is switched on (but engine is not running)?
See the diagram below for MY76. It shows that the red/white is tied together with the black at FP relay pin 30. And at the other end, it is at the 14-pin connector in the engine bay.
Hence it should follow the FP on/off cycles and could indeed be used to power the O2 sensor heater. Its gauge is only 0.5mm2, but that should be enough, since the WUR is no longer there.
http://forums.pelicanparts.com/uploa...1754731580.jpg

Wazzz, this is my conclusion as well, using the factory diagrams and my multimeter. I haven’t tested it with the battery connected and the key in the various positions, but I will be doing so later today. I’ll post my findings. Thank you!

If that's the correct wiring diagram for the 1976, then you're right about it. I wasn't sure because my engine and harness are from an SC, and I know that turning the ignition on powers the CIS components. However, my car is a 1973 car, so there is probably something different about adapting the SC wiring harness to a 1973 car.

So, ignore my earlier comment.

Just wanted to post a follow up to wrap this up. The above method of picking up power from the red/white WUR wire in the engine bay at the 14-pin connector works just fine to power a wideband o2 sensor to come on only when the engine is running. As the power is coming from the fuel pump relay, it operates the same. Thanks, everyone!