Sort of stumped
 

Here goes... About 2 yrs ago I bought a 1990 toyota pickup for 1000$ that had a 3.0 from the factory. Someone had bolted in a 3.4l with a super charger.. Most of the wiring for the engine had not been done. I finished wiring everything up and got it running. Truck runs good I am nearing completion of the truck as a crawler... One of the last parts and pieces was a Urd 2.1 sc pulley and a rear o2 sensor simulator. Went to plug in the o2 sensor sim and realized that the wiring had already been done to a 3.0 o2 sensor. I've determined power ground and signal. What is the 4th wire supposed to be. The diagrams I've looked at are telling me it's a heater ground? Not super savvy with o2 sensors. Any ideas would be great

O2s only function above a certain temp (engine runs in open loop mode until they reach it). Rather than waiting for exhaust to heat them up, they have a resistor in them which speeds the process. I believe the heater grounds through the ECU, in which case it would be looking for a signal from the sensor, in this case the emulator.

This might be interesting as the ECM is going to be looking for the signal from the sensor it needs to have.

All the rear one does is see the difference from the front.

Do you have any idea what year the ecm is out of??

most often the 2 heater wires are the same color

ECM is a 97. The 3.4 harness has two blue and two purple wires coming out

I have 11.8v going into sensor and coming out a different wire then a constant ground wire. And the other wire I can't get anything out of...

Was the ECM out of a Tacoma??

The harness Colors are nothing like you describe for a 97 4runner

Yes. ECM out of a tacoma...

http://www.well.com/user/mosk/images...ma_ECUs_v1.pdf is the pinout connector list for all years and engines Tacoma. HT2 and OX2 are the post cat O2 sensor pins. Tip - if you hit "control+F" you'll bring up the search box, type in whatever you want and it'll highlight all matches for you to click through, makes searching much easier. My brief looksee didn't turn up any purple or blue O2 related wires though...

http://www.customtacos.com/forum/13-...cm-pinout.html may help even though it's for the 4 cyl.