Ok, this has been going on for a LONG time. CEL is on for many years now (I know). Had the codes pulled then and was told bad sensors. Changed them out with no luck. Finally, I got under there and noticed that there is no 12v voltage going to the sensor to heat it up --> No reading. I checked all the fuses and relays. The only other thing I can think of is a short somewhere.

Haynes indicates that I have 4-wire sensors (which I do), and that they run though the PCM. What does the PCM do? Otherwise, where else would I likely find a short, or other place to trouble shoot.'


1997 ranger XLT extended....2.3L 4 banger.

thanks...Eric

 

Welcome to the forum Eric

The O2s have battery voltage to them on the light blue/orange wire. That wire gets it's power from either fuse 3 or fuse 20 (the book appears to contradict itself) in the fuse box under the hood. There are no connectors in between the fuse and the O2s other than the ones at the sensors. You may have a bad fuse, fuse box or wire. I would check all the fuses again then lift up the fuse box and check for power on the light blue/orange wire and if that's ok then start tracing the wire to look for a break in it. The fuse and wire will only be hot with the ignition on.
The PCM (Powertrain Control Module) is the computer that runs the engine controls. It grounds the O2 heaters to turn them on.

 

If the PCM were bad, would I be having many more problems, or could that be the cause as well?

 

I'd be surprised if the PCM was the problem but anything is possible. Each O2 heater has it's own seperate wire going to it so I believe that they are on different circuits in the PCM but I don't know how you'd tell for sure.
Since you don't have 12V to the O2s that's where you have to start. They get their power from the fuse box so the PCM doesn't have anything to do with that side of the circuit.

 

Dave is right, start looking for the missing 12V. If you had 12V on 2 of the O2 sensor wires, then I would suspect the PCM....

 

ok...i'll triple check the fuses, although I tested them twice. Does the wire go directly to the sensors from the fuse box? In that case, I could check the continuity from the plug to the fuse box.

 

I don't know if it got through, but the ECM supplies a ground to the O2 sensor. That way, it doesn't have to flow power in one side and out the other with fusing issues, etc. It just says, 'ground pin XX' and that turns on the O2 heater or sensor in question. If the power never gets to the O2, it will be grounded by the ECM, but will not have power to turn on. Does that make sense?
tom

 

ok....will check the 12v situation. Please let me know if my continuity check, as I mentioned prior, will at least confirm good wiring.

Thanks...Eric

 

Yup. 12v is 12v, and it isn't going to get there another way. .. .. Unless, there is a relay involved. In other words, you have to get a schematic to know for sure that there is a direct wire from the fuse box to the O2, or if the volts go to a relay that is 'enabled' by the ECM grounding its apply coil. I am not a factory trained anything, so don't count on my answers being box-stock.
tom

 

Ok, I did some digging and have some more info for you. I only diagnose the rear sensor (the front one is a $$$$$ to get to) And FYI, the sensor has 4 wires. 1 Black, 1 Grey, and 2 White.

Haynes says - HO2 Fuse 15A (Hot in run/start) --> Lt Blu/Org --> each sensor (1)(2)--> Red/White(1), Wht/Blk(2) -->PCM

My Truck - HEGO fuse 15A -->Blue/org --> rear plug.......Sensor would be here.....White/blk--->PCM

So here, the wierd thing is that it was previously thought the 12V would be coming from fuse 3 or 20, neither provided a connection to the sensor plug. Another strange thing, if this is the true path, these two wires correspond the Black and the Grey, leaving the two white wires for the other circuit. This doesn't make sense to me. I didn't have time today, but I will determine if the sensor is truly NOT getting 12v, and ground to the battery to be sure I have a good ground.

Further Info....the Red/green wire on the plug is routed to the PCM. Finally, the orange wire, which turns into grey/red (according to the haynes) ends up at the PCM

Soooo....as far as I can tell (right now) my blue/org is my 12V, and white/blk is the ground. Then, the orng and red/green is the sensor wireing.

So. I will determine if there truly is no 12v (with the key on).




Oh....i found a great resource for O2 sensors.....

http://www.parttrackers.com/library/1/24/27/

 

JUst to be sure, you are noting the color codes in the vehicle harness, not the actual O2 sensor itself, right? The sensor wire colors don't matter, it is the factory supplied wires' color that matters, because they don't change. The shape of the connector at the O2 will determine the connection automatically, so they don't much care about color there.
tom

 

The colors are those from the harness, not the sensor. The concern I have about the colors on the sensor is this. When I purchased the new sensors, they actually initially sold me sensors with a different plug (these were supposed to fit the truck). I believe I ended up ordering a different brand, or going someplace else to get the "right" ones.

Thanks..Eric

 

ok....new question for ya, same topic. Would any of you be so kind as to crawl under your truck and indicate how the sensor plug wires coincide with the wires on the sensor.

I am beginning to beleive I may have been sold the incorrect sensor. My Blue/orng wire from the fuse box goes down to the fuse, and matches with a white wire on the sensor, rather than the black or grey. The strange thing is that there are two white wires on the sensor, which are continuous.

Let me know what you think...Eric

 

I just found the answer to my previous question... surprisingly from the guy at AZ. He enlightened me that Bosch makes universal sensors, and with these comes detailed instructions on how to apply them appropriately. I was more interested to find out that BOTH white wires are in the 12v circuit. The black wire is the sensor, and the grey is the sensor ground.

I also found out from him that it is probably only my FRONT sensor that is being the PITA. I had previously been told both were bad, so I have been troubleshooting from the easy one. I guess I will get my hands a little more dirty and try to figure out if the front sensor is getting 12v, and is continuous to where it needs to go.

Any other ideas, please let me know.

P.S. He also mentioned that he has seen the MAF sensor cause these problems, even without throwing a code. Has anyone seen this before?