Wiring/O2 sensors (very long) persistent problem

[XJ4PLAY]

The ASD relay, going from the front of the PDC, is in the 3rd section far right.

 

[HoratioTheJeep]

THIS HAS BEEN UPDATED FROM ORIGINAL POST.

Thanks, the updated info matches my observations (confirmation is helpful).

You guys are great help, as always.

If you need the pinout drawings, e-mail me and I will e-mail them to you. does not allow posting them here.

I've sent you a PM on that.

 

[HoratioTheJeep]

I found the grounds, & the multimeter gave me a reading of 1-2 Ohms when contacting the sensor ground on the harness side (the Black/Blue wire). The 1994 FSM says it should read 5 Ohms or lower, so I guess it's in spec.

That sounds livable - but I'd still prefer being thorough, and pulling the ECM connectors to isolate the sensor ground (at which time, there should be no continuity of any sort - unless they've done something creative.)

OK. I pulled the ECM connectors, & disconnected the battery, & tested again. With the Grey (C3) ECM connector removed, I got a 0.00 Ohms reading from the BK/LtBlue wire (a.k.a. Sense Ground). No readings from any of the other 3 pins.

With ALL of the ECM connectors removed, there were no continuity readings from any of the pins, including the BK/LtBlue.

So I guess that's all right.

6) Find the pin (you'll have to backprobe for this one) that shows somewhere between +1 and +5VDC - this is the "Sense Out" lead.

Lemme take another voltage reading from the upstream harness connector, including a check for voltage on the "Sense out" lead, because I think I only got a voltage reading for the "heater source" (+12V "nominal") lead. Then again, I'm not absolutely confident in the sensitivity of my multimeter (it is an Actron), or my relatively nascent electrical diagnostic skills.

 

[HoratioTheJeep]

Did another voltage check, _upstream only_. I think previously I may have used the wrong sensitivity setting (previously: whole numbers only, this time: out to tenths of a volt), because I'm now getting more than 12V from the Heater Source lead, whereas before I saw only 12V. Well, anyway, here's the rundown:

with the engine running
"DG/OR - Heater Source" -- 13.6V DC (steady)
"BK/DG - Sense Signal" -- 1.5V DC (steady)
"BK/LB - Sense Ground" -- 0.0V DC
"BK/WT - Heater Ground" -- 0.0V DC

key ON only
DG/OR -- 0.7V DC
BK/DG -- 1.5V DC
BK/LB -- 0.0V DC
BK/WT -- 0.0V DC

So. It looks like all readings are in spec, nothing wrong. Unless "key ON" is supposed to put +12V supply to the heater source circuit, before the engine is started, but I guess not?

Maybe you already did this and just did not mention it.

I have a 96 and had a similar issue after replacing O2 sensors.

Turned out I had caused a pin in the wiring harness to back out and therefore not make a proper connection.

I, too had a couple of mechanics tell me the same things you have mentioned.

[Sorry I didn't answer before. Too much to follow.] Actually, I did check those. I took apart the connectors & cleaned them (twice, once putting Ox-Gard on the pins for good measure). In fact, I checked the pins "first thing," 8 months ago, because I was afraid I bent or damaged one when I first installed the upstream Denso sensor almost a year ago. But my pins hold in place rather well.

I'm stumped. I've been stumped for 8 months. Maybe I actually have an emissions problem? None of the mechanics seemed to think so. I'm going to go clear the codes. Then I'm going to go ditch my K&N filter for a plain one, reset the computer, & drive 30-40 miles. I'm also going to go to AutoZone & borrow their code reader, check the codes with their reader, again, in case some different code comes up (though, I can't imagine why). I'll check back in, in a while.

I'm frustrated, but I'm actually having fun learning about this part of my Jeep. Thanks, all, for your help.

 

[langer1]

Did you test the heater to make sure it's not burned out?

 

[HoratioTheJeep]

langer1

Well it looks like a bad heater or plug on the up-stream sensor. Don't worry about the down stream sensor for now, it don't control anything.
The heater can be tested with your ohm meter, you should read 7-9 Ohms between pins a and b.

Haven't checked it yet. I'll do it right now.

If it IS burned out--that would mean that I've burnt out the heating elements on THREE different O2 sensors. Not that I'm saying it's impossible, but the various mechanics I've been to checked the Denso sensors & found them to be OK, in the sense that they put out the right voltage. I assumed they also checked the heating element of each sensor, but I could be wrong. And I haven't had a mechanic look at it since putting in the sensors I bought from the dealership. The original sensor (the one with 100 000 miles on it) is still here (I've kept all of them), so I'll check all of them & post again.

 

[HoratioTheJeep]

Well, all the sensors (upstream & downstream, all brands) read 5-6 Ohms across the white wires. Now, the '96 FSM, according to the page I have from XJ4PLAY, says that's where they should be. You, langer1, are saying it should be more like 7-9?

 

[langer1]

5-6 is good, there not burned out.

 

[SteveT]

Horatio-

Any update. My '96 just died on me the other day. 4.0, AX-15, 145K.

Cranks fine, just won't start. Old Man and I talked it out a bit, but I haven't made any progress.

I did find that the rear O2 harness came loose and rubbed on the driveshaft. Three wires of the four are damaged. Can I just cut and splice to repair, or will that add resistance to the system?

Steve

 

[HoratioTheJeep]

Sorry, my bad. The 'update' is in this other thread: http://www.naxja.org/forum/showthread.php?t=71724. Big TY again to those who helped.

The short version: I gave up & took it to a dealership, the dealer's mechanic agreed it was a wiring problem, tracked it down for me (took three days)--the O2 wiring harness is grounded on the passenger side of the engine, on the block, with a cluster of other wires, above & behind the distributor, or just below & in front of the #5 cylinder spark plug--turned out the eyelet on the end of the wire at the harness ground was part-way broken off, & it was dirty from an old valve cover oil leak (since repaired).

There was enough continuity to make the wire seem OK when I tested it with my cheap multimeter/ohmmeter, but, when it came down to it, the O2 sensor was not getting enough voltage, thus causing my check engine light problem, & the driveability problems described above.


Unfortunately, if you ask me, your problem sounds quite different from mine. You say yours won't start at all? My Jeep would start with a long crank time, but it would run, though rough, & with hesitation, until it warmed up, then it was smooth running. Furthermore, my rig would start & run perfectly with the O2 sensor unplugged.

Regarding splicing the O2 sensor harness wires: Splicing them should work fine, should have no trouble with it.

I couldn't guess what is causing your no-start condition. It sounds like CPS failure (Crank Position Sensor), but I'll bet you and Old_Man already covered that.

One thing to consider: There's an Auto Shutdown Relay that might've gone bad. If your '96 is same as mine, a map of the relays, & fuses, is printed on the bottom of the lid to the relay box (actually called the PDC or Power Distribution Center). There's a way to check the relay, either with an ohmmeter for resistance, or by bridging the right terminals you might be able to start your rig. If you have a Haynes or Chiltons repair manual, it should tell you how to test relays, & what the specs should be, I think.

A bad fuel pump could also cause your problem. You can check fuel pressure at the injector rail. Look for the Schraeder valve, it looks like a bicycle tire valve. You can check it with a pressure tester while cranking the engine. Or get some paper towels & put them around the Schraeder valve, then take the cap off, &, with the engine off push the valve open. If you get a good spray of gasoline, then you have pressure. If you get a wimpy spray or a trickle, then the lines aren't holding pressure (meaning a bad fuel pump, or bad fuel pump relay, but probably the pump, not the relay).

If it is cranking, then it must not be the ignition coil. Mine did not crank when the coil went bad. So, yeah, I'd guess your problem is either fuel-related or sensor-related, but not the O2 sensor, so far as I know. But check the ground wires for your various sensors, & the braided ground strap connected from the back of the engine to the firewall. My virtual money is on a bad fuel pump, though. Check that fuel pressure, mister.

 

[nyjunkyarddog]

This MF reads like a Russian novel! Damn! I've got similar problems with mine. Makes me long for the good ol' days of carburetion. A rebuild every once in a while, stalling out from time to time, but still, all this just sucks.

 

[mrfreakinwhite]

Excellent, informative thread. Yeah, it was like reading a romance novel in Russian, but it was worth it.
Great follow up here:
http://www.naxja.org/forum/showthread.php?t=71724

I'm off to check ground wires and look for shorts in the upstram harness myself with test light, multi-meter and, a drum roll please, a fog light. Not quite a head light, but the halogen fog light will be easier to monkey around with under the hood.