Hi first off to let you know the situation i'm planning to do an engine swap on my 88 HF in the spring/summer with a D16Y7 from a 98 civic. I had to get an aftermarket exhaust because it was missing after the B pipe on the CRX and the exhaust on the Civic is shot too. I haven't ordered headers yet but i will need too to match up with the 2.5 inch exhaust. The ECU i bought and want to run is a PM6 OBD0 from an SI since the HF is already OBD0. So i know OBD0's run a single wire unheated O2 sensor that isn't far from the head on the stock manifold in the collector area. The problem is if I buy an aftermarket header such as a Skunk2 Alpha the collector is way down low and will not work with an unheated O2 sensor because it will not get it hot enough i will run super rich. They do offer have another sensor hole higher up in the manifold but by pictures it appears to only be one cylinder or just barely 2. It may work but i'm unsure. I've searched many threads but can't seem to get the info i need about wiring in a heated O2 on OBD0. Some people have stated they have done it but half the threads i find are for actually OBD1 wiring. Does anyone have EXACT instructions on where to splice or run wires or how it can be done for OBD0 also any other advice?

 

The 4 wire sensor has separate grounds for the signal and heater. Connect the signal wire to the O2 input, the grounds to the thermostat, and the heater 12V to a switched power source. The heater is a higher current draw, so personally I would use a relay and tap off the ECU 12V switched for the relay signal.

Only issue I see is that the sensor heater will be always on while the car is running. The ECU turns the sensor heater off once the car is warmed up, so you may end up replacing the sensor more often. It may not be a big deal, just thinking out loud.

Poking around, there is a wiring harness that makes it a more drop-in affair, albeit with no relay: https://www.summitracing.com/parts/cei-103025

All that said, my car uses a 1 wire sensor (for simplicity sake) and its wired up further downstream (aftermarket header as well). Once the car is warm it seems to idle and run fine in closed loop. My header is jet-hot coated though so it may retain enough heat to not be an issue.

 

When you mean tapping into a switched 12V for relay is that a specific Pin on the ECU wiring? That's the one thing that's really getting me confused. I'm looking at pinouts for OBD0 ECU's there's a bunch of pins broadly called main relays. Obviously i shouldn't use the relay/grounds but is switched 12v base on connectors A,B, or C?

 

That would be IGP1/IGP2 from this diagram, which is A13/A15: https://dodo-upgrades.nl/index.php/pinout-obd0/

The +12V signal wouldn't necessarily have to be the ECU power, just something that is switched.

 

You are the ******* man! That's the the kind of information I'm looking for. Now the only other info I guess i need is does it matter which of the 2 wires for the heater you run, some say it doesn't matter which one you ground or not. What's the best way to tell the difference between the 2? Also can i just cut and splice the O2 signal right into existing line or is it better to run a new wire all the way to the ECU and repin it.

 

From what I can tell, most heater wires are the same color, so the polarity doesn't seem to matter.

You would want to use the existing O2 wire as it is shielded on our cars - it has a grounded shield on the length of the wire, like a coaxial cable. Personally I'd use the OBD0 style connector on the new sensor and connect it to the stock plug so you aren't hacking up your wiring. It also makes it much easier to revert if you decide to go back to a single wire sensor.

 

That was exactly what i was thinking, leave it as is, just wire the 1 wire connector on the sensor side onto the heated o2 sensor. That way i can leave the plug running to the ECU as is and easily just plug a 1 wire back into it if i had an issue.