Finally asking for help... both ho2 circuits out

[Kamel]

I posted this same post on another forum, before realizing that forum looks totally dead. I apologize for posting in 2 places, not something I normally do so just wanted to mention that up front.

Alright, I have been working on this for quite some time now. Finally, I've decided to ask for help. Not like me, because I enjoy figuring things out on my own or without having to ask and bother others with my troubles. Anyway, here's what's going on.

1995 Ford Ranger XLT 2.3L 5speed

Throwing codes: p0135 & p0141 (heated o2 sensor heater circuit, both banks)

Only had it for about a year, not sure what prior owner has done to it (had codes when I got it)

I went ahead and replaced both o2 sensors, and they looked in pretty good shape -- they also broke free pretty easy, so I'm pretty sure they were new (ugh, wasted money sucks). Just to be sure that didn't fix it, I cleared my codes and they came back after a quick drive around the block.

I also checked fuse #20 in the cabin fuse box and it's fine. Also checked for voltage to both sides of the fuse with the key on, both good (I also used a test light to confirm... I once had an electrical problem that was reading good volts but not powering a test light so I always do this lol). I decided to try and test amperage draw on the fuse, and it ended up being 0 amps, so whatever it is doesn't draw much current or it's got a broke wire somewhere.

I couldn't determine any other fuses that the o2 sensors could be on, but I'm not sure if I'm looking at the right fuse. I've failed at finding a decent wiring diagram for my truck. I saw a few things on the net talking about checking a "light blue/orange cable" to the o2 sensor, but all I'm seeing is 2 white cables, a grey one, and a black one. I couldn't see the color of the wires coming from the harness of the vehicle itself all covered, but I'm going to try again soon.

Having said that, I have a couple of questions.

1) Even if it doesn't help me fix this issue, it'd be cool to find a decent wiring diagram if anyone knows where I can get one
2) Is fuse #20 (15A) in the cabin fuse box the correct fuse?
3) Should I be able to verify the o2 sensor heaters are getting power by turning the key in the on position then testing for voltage between the frame of the vehicle and each pin of the o2 sensor plug? How many of the pins are powered, just one?
4) How is the wiring routed for the o2 sensors? I see it going up and around the transmission, then I lose it

and finally...
5) Am I on the right track? Maybe you have a totally different idea, I'm open for suggestions!

Thanks a lot guys

 

[koda2000]

free wiring diagrams: http://search.ebscohost.com/. Use rrcc and rebsco when signing in

 

[tgibbonsfordluv]

A four wire 02 sensor supplies both hot and ground for the heater. It also supplies ground to the sensor itself (voltage generator type) and the last is a signal wire back to the PCM.

After figuring out which is which, make sure the sensor heater is receiving power. Also, check the sensor output when hot and engine is running to make sure it is creating a signal. It's going to be a rapidly changing voltage between .5v and 5v. That is normal function.

From my own experience, the PCM throws O2 codes, not because of sensor malfunction but because the sensor output is outside preferred range. This basically means there is the wrong amount of Oxygen in the exhaust. Typically an air/fuel/ignition problem, but can also be caused by exhaust leaks.

 

[tgibbonsfordluv]

Thinking a bit more about it though, if you're not getting any power at the fuse you're likely to find the same at the sensor end. Seems very odd the fuse block is showing voltage but not amps. How did you test voltage? If it was any other way than pulling the fuse out and probing both sides of the fuse port, try it again. Do the same for amps. DO NOT use a test light on this. Use a digital multimeter only. A test light can force an amp draw that can short the pcm.

At any rate, if you're coming up no voltage and no amps, I'd check the fusable links powering the fuse block. There's usually a main ground for the fuse block too. I'd also check that.

 

[Kamel]

koda2000: Thanks man!

tgibbons:
Thanks for the warning on the test light... hadn't thought of that

Since the code failures literally read "heated oxygen sensor heater circuit malfunction", I thought it was specifically meaning the heater circuit was malfunctioning, not the entire o2 sensor. Having said this, is it still possible it could just be the output of the o2 sensors are outside of the range the PCM expecting?

I tested voltage by putting a lead from my dmm to the frame, then the other to the outside test on the fuse (fuses have little tiny metal contacts on the outside that you can test while they are in place, not sure if you're familiar -- I didn't find out myself until recently).

I got voltage on both sides of the fuse when testing to verify the fuse definitely has continuity (I also checked the fuse by itself for the heck of it, definitely not blown -- I wanted it to be blown very badly though). I did not pull the fuse out and try to test for voltage in the fuse slot itself.

I tested amperage by removing the fuse, and placing my test leads in place of the fuse. Since my meter can handle 10+ amps I was pretty sure I'd be good since the fuse hadn't blown (also I recall reading that a totally different code is thrown when the o2 sensor heaters try to pull more than 2.5amps). This is when I determined I was getting 0 amps drawn at the fuse after starting the vehicle with my meter in place.

I'm going to go ahead and test for voltage at the actual o2 sensors, and check the wiring diagram to make sure I'm at the right fuse. Hopefully this will tell me where the problem is, but I'm thinking there's got to be a break between the fuse box and the actual plugs for the o2 sensors. I'll find out soon and report back

Thanks again for all of your help!

 

[Kamel]

Alright, I've finished my diagnosing for the night...

First, a bit of a scare, I learned something, and thankfully all is well now. I went to test voltage and still had my test leads in the amperage slots... really bad idea! on amperage, basically the volt meter is like a jumper wire, and i ran it from the fuse to the frame. I got some impressive spark, and scared the crap out of me (especially given what you just mentioned about putting any amps on the PCM). Fortunately, I used the wiring diagram to trace the circuit back, and it is a relayed circuit that ends up going through fuse #13, PCM power in the engine relay box which is a 30 amp fuse. I definitely didn't damage the PCM, and didn't even blow the fuse. Whether it damaged my meter is still to be determined, but so far so good.

The wiring diagram really helped, I was at the right fuse for sure. I got 12v on both sides of the fuse like before, then I tested with the fuse out. The right side must be the unfused side, as it was reading voltage and the left side was not reading voltage.

Clearly, there is a break in the light blue/orange line somewhere, and probably up in the engine bay somewhere. This is because the same wire splits off and powers both o2 sensors. I tested, and both o2 sensors aren't getting voltage.

The o2 sensors both connect to a harness that goes above the transmission and I lose it after that. I tried looking today, and only found a few harnesses in the engine bay that didn't look related. Is it possible that the harness enters the firewall somewhere around the top of the transmission and then up through the dash to the fuse box? I am having a terrible time following this thing

 

[Kamel]

i poked around on that website you gave for the wiring diagrams and found a wiring harness section that i think is gonna help a lot! thanks again, this is greatly appreciated

edit: sorry to keep posting here guys, i have read over the wiring harness document like 10 times and i can't find the o2 sensors or transmission part anywhere. this simple problem is driving me crazy if i could just find the wire i think i can fix it! lol

 

[Dono]

Don't both o2 wires go straight to the engine bay harness the plugs in to the pcm?

I don't know for sure, but I do remember unplugging them as part of the wiring harness pulling the motor out.

Could you just run a fresh hot wire from where you know the wire is still live down to the o2 wires? Its not finding the broken wire, but would prove your troubleshooting. It could even be a permanent fix.

 

[Runnin'OnEmpty]

There's a good chance the problem's in the PCM
connector. I'd try disconnecting it and spraying with
a contact cleaner or WD-40, and re-connecting.
Might save some grief trying to chase down the
specific wire.

If that dosen't work, you can find the pin and then
check for voltage and use Dono's trick with the
jumper wire.

 

[Kamel]

Don't both o2 wires go straight to the engine bay harness the plugs in to the pcm?

I think the wires you're thinking of are the data side of the o2 sensors, the heater circuit goes from battery hot, to the fuse box under the hood, to the PCM power relay, to fuse #20 in the fuse box in the passenger compartment.

Could you just run a fresh hot wire from where you know the wire is still live down to the o2 wires? Its not finding the broken wire, but would prove your troubleshooting. It could even be a permanent fix.

I'm not the least bit opposed to bypassing the wiring with a wire of my own, but I don't know where the wire comes out at the fuse box. The only thing I can think of is to get behind the instrument cluster and try to follow it. I've been behind the instrument cluster before and it required a great deal of disassembly to get there. I'd prefer stay away if at all possible but I'd gladly take my whole dash apart to fix the issue if that's where I'll find my fix lol. I get the feeling I wont find what I'm looking for though =\

I'm about to go on a trip, I'll be back Sunday and probably give this another go Monday.

 

[Kamel]

There's a good chance the problem's in the PCM
connector. I'd try disconnecting it and spraying with
a contact cleaner or WD-40, and re-connecting.
Might save some grief trying to chase down the
specific wire.

If that dosen't work, you can find the pin and then
check for voltage and use Dono's trick with the
jumper wire.

Since I'm getting power at the fuse, that tells me that the relay is working properly (and therefore being activated by the PCM), so I don't think that would be it... would it?

You probably know better than me, so if I'm mistaken please spell it out for me sometimes I need that lol

 

[tgibbonsfordluv]

There's a good chance the problem's in the PCM
connector. I'd try disconnecting it and spraying with
a contact cleaner or WD-40, and re-connecting.
Might save some grief trying to chase down the
specific wire.

If that dosen't work, you can find the pin and then
check for voltage and use Dono's trick with the
jumper wire.

That's a good point, the master PCM connection is known to cause problems. I havn't looked at the wiring diagram myself but I think the short is AFTER this connection.

This is a real head scratcher, to be honest. I'm gonna look at the wiring diagram myself and come back to this.

 

[tgibbonsfordluv]

http://repairguide.autozone.com/zne...96b43f/80/21/b9/87/large/0996b43f8021b987.gif

 

[tgibbonsfordluv]

Looking at it, probe the light blue/orange wires with a t-pin. Attach your positive lead from your voltmeter to the pin. Put the negative to a known good ground. If there's no voltage, it's definitely a short in there somewhere. Check that one by itself to a known good ground first to isolate the circuit.

If that one passes, keep the t-pin in there and also probe the white/black. Put your negative there and see if you've got voltage. DO the same with the red/white. These are ground supplies from the PCM. If the fault is found to be in these curcuits I'd look at the master harness between the PCM and the Main Engine Harness. All of these should be checked with key on/ engine off.

 

[Kamel]

Back from my trip, thanks a ton for your response, I really appreciate it. I didn't get a chance to look at it today because I was just so worn out from traveling (28 hours of driving total, sheesh).

I checked all of the wires in the o2 sensor plug before I left though, and got no voltage. It could be a bad ground like you said though... I'll check it out when I get a chance and report back.