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Oxygen Sensor output voltages?

RaccoonJoe

NAXJA Forum User
Location
Fort Wayne, IN
OK. I've been dealing with some low fuel economy, as well as some sputtering/stalling issues. I think I've narrowed it down to a bad O2 sensor.

1st sensor tested would sit at 4.75 volts, and never move. Left the motor idle in Park for 20 minutes, still no movement.

I replaced this sensor with a spare I had (had originally came off my Jeep, with the old, cracked/broken exhaust manifold) Now, I've got some interesting results.

*due to time constraints, all testing with this second sensor was done on the open road. I've got "real-time" data. Using in-line splices, I ran 2 lead wires into the cab, and jumpered those to the test leads on my analog volt-meter*

Accelerating, my voltage would slowly climb to about 4.75 - 5 volts.
Rolling down the highway, foot off the gas, volts would shoot up to 5.
Coasting, (moving, but shifter in N), voltage would start at 3 or climb to 5
Cruising @ 55, voltage is rock solid @ 2.25
Cruising @ 60-65, voltage is between 2.75 and 3 volts.


Any ideas what this is telling me? Or is it just that the oxygen sensor is toast? I'd rather not throw money at this if I don't have to, as some medical bills have come due, and the paychecks have been a little short recently....(not to mention high gas prices coupled with low MPG)
 
Verrry Interesting!!!!!!! First off this a Renix, 87-90 correct???

No doubt the first sensor was bad, based on the second sensors data. The ECU would have ignored that O2 sensor and run in open loop all the time sucking GAS!

Second sensor is working well enough to confirm the ECU is working (supplying 5 volts to the O2 sensor) and the O2 sensor's heater voltage to the O2 internal heater is working, or else it would be locked in a rich mode reading.

Have you tested it at idle. It should oscilate from about 1 to 4 volts at 750 rpm, idle.

" Rolling down the highway, foot off the gas, volts would shoot up to 5.",

This is what it should do, the ECU runs rich on decel while trying to catch the rapid decel, so it keeps reducing fuel feed until it gets back to 14.7:1 ratio which should be about 2.45 Volts IIRC, but that can take 20 or more seconds to get back to 2.45 V. This also keeps it from running too lean during decel when it might burn exhaust valves from running too lean if fuel was shut down too far too fast IMHO.

" Coasting, (moving, but shifter in N), voltage would start at 3 or climb to 5"

This is just another decel, no load case, same results, runs rich by ECU command for about 20 seconds, then eventually drifts back 2.45 V target, with oscilation from 1 to 4 volts at low rpm idle.

"Cruising @ 55, voltage is rock solid @ 2.25"

Mine was (is) closer to 2.45, and that is the target, so you may have some signal wire losses, or resistance in there, but you are real close!!!!! How calibrated is your meter? Check the wire connections to see if you are loosing .2 volts due to some resistance in your test wires, or meter calibration, i.e check the meter zero!!!! too.

" Cruising @ 60-65, voltage is between 2.75 and 3 volts.", this one has me wondering, scratching my head, not what I would expect, I would look for a problem somewhere if that is a true steady state cruise at 65 mph. If it is under a little accelation, going uphill, or speeding up slowly, that would be normal.

For those of you new to this conversation, Raccoonjoe has strung wires from his O2 sensor to an analog high impedance ohm meter inside the cap so he can read O2 sensor data in real time while driving!!!!
 
RaccoonJoe said:
OK. I've been dealing with some low fuel economy, as well as some sputtering/stalling issues. I think I've narrowed it down to a bad O2 sensor.

1st sensor tested would sit at 4.75 volts, and never move. Left the motor idle in Park for 20 minutes, still no movement.

I replaced this sensor with a spare I had (had originally came off my Jeep, with the old, cracked/broken exhaust manifold) Now, I've got some interesting results.

*due to time constraints, all testing with this second sensor was done on the open road. I've got "real-time" data. Using in-line splices, I ran 2 lead wires into the cab, and jumpered those to the test leads on my analog volt-meter*

Accelerating, my voltage would slowly climb to about 4.75 - 5 volts.
Rolling down the highway, foot off the gas, volts would shoot up to 5.
Coasting, (moving, but shifter in N), voltage would start at 3 or climb to 5
Cruising @ 55, voltage is rock solid @ 2.25
Cruising @ 60-65, voltage is between 2.75 and 3 volts.


Any ideas what this is telling me? Or is it just that the oxygen sensor is toast? I'd rather not throw money at this if I don't have to, as some medical bills have come due, and the paychecks have been a little short recently....(not to mention high gas prices coupled with low MPG)

Before we try to interpret those results, it is vital that we know what year your rig is. RENIX (1987-1990) doesn't really generate voltage, the HEGO sensor is used as a variable resistance/voltage divider working on a reference signal of +5VDC. It is sometimes instructive to check the input voltage as well. Stoichiometry should get you a signal return of +2.5VDC, IIRC.

If your rig is later (1991-up,) the HEGO is an "air battery" used to generate a voltage signal - between, I believe, 0 and +1VDC, with +0.5VDC being the signal level for stoichiometry.

With the voltage signals you're giving, it sounds like you've got a RENIX rig, but I could be forgetting the range for the later signal (it may be +0-5VDC...) so it would be good to confirm that.
 
More background: In October, I replaced a cracked/broken exhaust manifold with one off my parts rig (had been re-welded). I left the O2 sensor from the parts rig in the manifold, installed and hooked it up. Since my problems have been getting worse, I thought that perhaps the O2 was bad, and I put in my original again. Up until October, I had been pulling a steady 20 MPG. Shortly before replacing the manifold, I whacked the downpipe on an obstacle and cracked the manifold wide open. MPG at that point was around 14.

I'll do my best to answer the questions here:

First: You're correct, EcoMike. This is on my Renix era XJ...1989 4.0L w/auto tranny

Borrowed the voltmeter, so I can't guarantee any accuracy. I can tell you that it agrees with my digital, when testing reference voltages (battery, wall outlet, etc). Also, it's been raining here for the past 2 days, so I'm sure that road water splashing on my connections may have a small part to deal with this.

At idle, I get a solid 4-4.5 volts. No oscillation, no movement from the sensor at all. This is after the motor has come up past 160* (closed loop?)

Also....I never once saw a quick oscillation of voltage. It is either solid at one voltage, or it *very* slowly climbs (with the exception of no load/WOT) From what I've read, the sensor should be putting out varying voltages, crossing back/forth over the 2.45V mark (14.7:1 AFR). If this doesn't happen, isn't the ECU supposed to ignore the sensor? Perhaps both are bad?

I'll grab my digital tonight, and do some additional testing with the digital and the analog side-by-side.
 
I miss understood your first post. No oscilation means no closed loop at all. Neither one is working right. The second one seems to be responding to something, maybe it is reporting closed loop A/F ratio readings.

I would take the meter and check the third wire, power on, engine off, don't start it, and check for 12 volts at the large orange wire in the O2 sensor wiring harness (not the O2 sensor, the engine harness side). IF there is not 12 to 13 volts there, you have your first problem spotted, probaly a bad O2 heater voltage relay. If you have 12 to 13 volts, then check the voltage between the other two engine harness wires (small blak wires) that attach to the O2 sensor (with O2 sensor not attached during the test!), it should read 5 volts. I suspect from your posts the 5 volts is there, but verify it anyway.

Lastly, power off, you can test the O2 sensors internal heater element. O2 sensor wiring harness disconected, power off, switch to ohms, lowest scale and test from the O2 sensors corresponding large orange wire to the black ground wire (one of the black wires is a ground, the other is not, so you may need to try both black wires), for a low ohms reading about 10 ohms. If it is 0, or less than 1 ohm, or more than 100 ohms I would suspect a bad heater in the sensor. O to 1 ohms, or over 1000 ohms it is toast. IIRC it is 8 to 12 ohms.

Then all that is left is the live tests you are running.

From what you have posted, either the 12-13 volts to the heater is missing, or the heater is bad in the sensor, or the sensor itself is bad. The heater is part of the sensor, so if it is bad the sensor must be replaced.
 
Hallo Ecomike, l just read this topic (l'm from italy) and l have a 89 4.0 automatic. Well, if l understood well frm the manual, there are 3 wires, one from the eater relais, one from the ECU and one from the ground. I just replaced my O2 becasue l though was bad, but l also checked those 3 wire to be sure that nothing was wrong. But for me there's something doesnt' work fine. I explain better: I checked with an analogic tester (with the engine off and the key off) the wires with the O2 unplugged,, well l found 2 negative (or grounds). I don't know if it is right, but l remeber that last summer with a friend with a digital tester, he didn't find those grounds in the wires. Maybe is me that l wrong something, maybe l have to check with the key on (no engine running of course) . Do l have this check with the key on?

And in this case from the ECu, do l have to find a voltage of 5volts?

Thanks for all your advices.

P.S.
I wrote this, because I think my Jeep sucks a bit of fuel then what it has. I have also some doubts that there's a bad connection between the O2 and the ECU. :)
 
Have you tested it at idle. It should oscilate from about 1 to 4 volts at 750 rpm, idle.

" Rolling down the highway, foot off the gas, volts would shoot up to 5.",

This is what it should do, the ECU runs rich on decel while trying to catch the rapid decel, so it keeps reducing fuel feed until it gets back to 14.7:1 ratio which should be about 2.45 Volts IIRC, but that can take 20 or more seconds to get back to 2.45 V. This also keeps it from running too lean during decel when it might burn exhaust valves from running too lean if fuel was shut down too far too fast IMHO

Mike, the renix manual states that with the engine running at above 1200 rpm and the ECM sensing a hard decel condition (it defines this as TPS reporting 0% throttle angle) , the fuel injectors will stop. I will have to read back to see if this operates in both closed and open, but I would think both. See the section labeled modes (its together with acceleration, idle, wide open, etc)

I would think, if thats the case, five volts is mostly oxygen and the lower numbers are richer mixtures.
 
I recently tested mine with power off, and with the O2 sesnor disconnected. I got 52.2 ohms from the first black wire to the battery negative post (ground). Second black wire to the battery negative post was infinite ohms. The O2 sensor, cold, read 1.5 ohms to ground on the first black wire of the isolated sensor itself, 4000 ohms to ground on the second black wire. The O2 sensor orange wire (the bigger gauge wire) had 43.9 ohms to ground. I used the battery negative post as the ground test point in all cases. I am guessing the O2 sensor case is well grounded via the exhaust pipe and thus is giving the good ground reading of 1.5 ohms.

With the engine off, power on, I read between .200 volts to .140 volts, or 200 to 140 mv standing voltage on the assembled O2 sensor to wiring harness black ground wire, and 5 volts on the second black wire and 12.6 volts on the third, orange wire. All the readings were taken between the wire and the battery negative post, ground.

While I have a good ground on my O2 sensor ground wire via the case, I seem to have some resistance on my ECU wiring harness ground wire. Not sure if that is resistance is in the ECU circuit, and thus possibly normal or if it's tell tale signs of a poor C-101 connection, or other poor wire connections on mine. I plan on comparing it to my other Renix jeep.

Anyway, to answer your question, power off, engine off, the two issolated black wires (O2 sensor disconnected) only one black wire in the harness should be grounded, the other should not, based on my tests here. On mine one is 52.2 ohms and the other is infinite resistance.

On the disconnected (from the wiring harness, but installed in the exhaust pipe) O2 sensor I got 1.5 ohms on black wire #1, 4000 ohms on black wire #2, and 43.9 ohms on the orange wire, all tested to the battery negative post. The 4000 ohms may vary depending on whether the sensor is hot or cold. Don't forget turing the power on turns the O2 sensor heater on, and gets it hot even with the engine off.

The orange wire is the 12 volt input heater wire. One of the black wires is the 5 volt ECU input voltage. I am think the third black wire may lead to a resistor ground circuit in the ECU, and may thus be the sourvce of the 52.2 ohms, but I am not yet sure of that.



Gasoline said:
Hallo Ecomike, l just read this topic (l'm from italy) and l have a 89 4.0 automatic. Well, if l understood well frm the manual, there are 3 wires, one from the eater relais, one from the ECU and one from the ground. I just replaced my O2 becasue l though was bad, but l also checked those 3 wire to be sure that nothing was wrong. But for me there's something doesnt' work fine. I explain better: I checked with an analogic tester (with the engine off and the key off) the wires with the O2 unplugged,, well l found 2 negative (or grounds). I don't know if it is right, but l remeber that last summer with a friend with a digital tester, he didn't find those grounds in the wires. Maybe is me that l wrong something, maybe l have to check with the key on (no engine running of course) . Do l have this check with the key on?

And in this case from the ECu, do l have to find a voltage of 5volts?

Thanks for all your advices.

P.S.
I wrote this, because I think my Jeep sucks a bit of fuel then what it has. I have also some doubts that there's a bad connection between the O2 and the ECU. :)
 
Thank you very much Ecomike, you been so clear :) I'll do again the test with a digital tester and with the power on (engine off). :)
 
That is correct. I keep getting myself confused between high O2 and high Fuel, rich fuel or rich oxygen. Add to that the Bosch site saying the Renix FSM is backwards on the 0-5 volt, and It gets confusing for me. Bosch says 5 volts is rich, Renix says 5 volts is lean. :doh:

I am beginning to think Renix has it right.
So it looks like you and the Renix FSM may have it right. But I would think initial decel would go rich first (not sure how long, maybe just less than a second perhaps) before going lean for several seconds. Definetely an area I would like to clear up for sure. I also want to find out where that 52.2 ohms is coming from in my wiring harness ground wire to the O2 sensor. If it is a poor ground, and not a normal ECU resistance on the way to the ground (which according the FSM. page 41, it is not), it may be biasing my system to the rich side, in which case it may be the last part of a puzzle I have been pondering for some time regading maximizing MPGs by finding and fixing the right gremlins!

Page 43 shows the B wire, middle wire in the harness connector, black wiring harness wire as a ground, so it must be that I have another bad Renix Ground wire eating my overpriced gasoline! GahhDDD!!!!:scared:

Once again, I think you hit it on the nail, that is what my renix FSM says too. I guess maybe with a properly working TPS, MAP and ECU system it works faster than our analog meters and eyes can detect, i.e in milliseconds, thus we miss seeing the instantaneous leaning out of an engine at first when the foot goes to WOT, and visa versa. What we see instead is the leaning out of fuel by closed throttle during decel from shut off of the fuel injectors until the rpm drops below 1200.

.
89CherokeePioneer said:
Mike, the renix manual states that with the engine running at above 1200 rpm and the ECM sensing a hard decel condition (it defines this as TPS reporting 0% throttle angle) , the fuel injectors will stop. I will have to read back to see if this operates in both closed and open, but I would think both. See the section labeled modes (its together with acceleration, idle, wide open, etc)

I would think, if thats the case, five volts is mostly oxygen and the lower numbers are richer mixtures.
 
Gasoline said:
Thank you very much Ecomike, you been so clear :) I'll do again the test with a digital tester and with the power on (engine off). :)

Be sure you do the ohms tests with the power off, and the voltage tests with the power on. Also don't forget with most meters one of the probes go into a different spot for ohms than for DC volts.
 
OK, I rechecked everthing this time, this data is correct, some of the prior data I posted may be off.

Power and engine off. Sensor disconnected. All readings were taken with the meter ground wire at the battery negative post. Cold O2 Sensor read:

Infinite resistance to ground on all three wires!!!!

O2 harness connection wires read:

#1 (black) 1.3 ohms
#2 (black) 4,000 ohms
#3 (orange) 53.5 ohms!!!!!!

Then with all three wires connected to the O2 sensor they read:

#1 (black) 1.3 ohms
#2 (black) 4,000 ohms
#3 (orange) 8.3 ohms!!!!!!!!!

Power on, engine off, all three wires connected to the O2 sensor read:

#1 (black) 150 MV ( I may need to clean and tighten my gounds again)
#2 (black) 5.00 V
#3 (orange) 11.87 V

Engine on, and hot:

Running 2500 rpm, then rapid decel to idle stop, I got under 1 volt for about 8 seconds!!!!!

So if renix is shutting off the fuel and running it lean the Renix FSM is wrong and the Bosch web site is right about the low voltage being lean. If the renix FSM is wrong (or mine is acting up) and the fuel is staying on during decel to 1200 rpm, then the Renix FSM is right and low volts is rich.

I got 1.5 to 4.0 volts at idle, about 1 second between swings across the 2.45 volt sweet spot at idle.

Got a rapid 2.5 to 3.5 volts at 1600 to 2500 rpm steady engine speed, about 2 swings side to side per second. It finally got to a tight 3 volts at 2,400 rpm steady engine speed once the engine was good and hot (about 10 minutes of run time). Last time I ran this test I was getting 2.45 volts almost dead on, +/- 0.1 volts, this time it was reading closer to 3 volts.

The voltage readings were taken with a high quality high impedence FET analog meter that was zeroed before the ohm tests were started. The ohms readings were taken with a high quality, high impedence digital ohm meter.
 
90 RENIX

Am dealing with similar issues. I get a solid 4.89 volts when heated up.

Does this mean that the o2 sensor is dead or just that the computer is in open loop mode? The heater side of the sensor seems good but I get no oscillation. I do not understand if that means I have to replace the sensor or if something else could be forcing open loop mode and that in that mode would the ECU even interract with the 02 sensor? I have checked all the other sensors and apart from the previous "mechanic" owner replacing the CTS with a mechanical temp guage all seems well. I have not disconnected the battery since replacing the cts...
 
90 RENIX

Am dealing with similar issues. I get a solid 4.89 volts when heated up.

Does this mean that the o2 sensor is dead or just that the computer is in open loop mode? The heater side of the sensor seems good but I get no oscillation. I do not understand if that means I have to replace the sensor or if something else could be forcing open loop mode and that in that mode would the ECU even interract with the 02 sensor? I have checked all the other sensors and apart from the previous "mechanic" owner replacing the CTS with a mechanical temp guage all seems well. I have not disconnected the battery since replacing the cts...

It does mean you have a problem. Sounds like a bad O2 sensor, but it does also mean it is in open loop. The question is why is it in open loop. Check the third wire (mine is orange), larger diameter wire and see of it has the proper 12-14 volts needed for the O2 sensor's internal heater to work before writing off the sensor. Be sure to ground the meter ground test wire when testing for the 12-14 volts. If the 12-14 volts is there, then I would say the O2 sensor is probably dead, especially if the other sensors are all OK.
 
Thanks very much Mike. Just as a note to others to maybe save time. The o2 sensor relay is only energized when the fuel pump is. On my 90 the fuel pump circuit shuts down after you leave the ignition on without cranking for a bit. (about 2 seconds) It took me a bit to diagnose that. So test instructions that say "turn on ignition without starting then test heater relay voltage" can be a little misleading.
If all is well you will get voltage until you hear the fuel pump relay clicking off. Next step for me is new 02 sensor. Will post results.
 
Thanks very much Mike. Just as a note to others to maybe save time. The o2 sensor relay is only energized when the fuel pump is. On my 90 the fuel pump circuit shuts down after you leave the ignition on without cranking for a bit. (about 2 seconds) It took me a bit to diagnose that. So test instructions that say "turn on ignition without starting then test heater relay voltage" can be a little misleading.
If all is well you will get voltage until you hear the fuel pump relay clicking off. Next step for me is new 02 sensor. Will post results.

Good point. For some reason I never ran into that problem. Mine is an 87 wagoneer, may be wired different? Or maybe I ran my tests with engine running.
 
OK I am stumped....

Done so far...
First emission test result... New Air Filter installed CTS after removing aftermarket temp guage occupying hole for it.

Cruise Limit HC PPM = 200 me 128 = PASS
CO % = 1.5 me 1.37 = PASS
Idle limit HC PPM = 220 me 286 = FAIL
CO % = 1.2 me 4.12 = FAIL


Check TPS MAT MAP CTS all within tolerances indicated by all by the way using ground from the body.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

2nd TEST

Cruise Limit HC PPM = 200 me 108 = PASS
CO % = 1.5 me 0.83 = PASS
Idle limit HC PPM = 220 me 193 = PASS
CO % = 1.2 me 2.11 = FAIL


All good nearly there but no cigar.

More work...

02 sensor
New Cat

Cruise Limit HC PPM = 200 me 101 = PASS
CO % = 1.5 me .36 = PASS
Idle limit HC PPM = 220 me 113 = FAIL
CO % = 1.2 me 1.45 = FAIL


Arghh.... So for the last test I disconnected my pcv hose.... Clearly I have a problem at idle..... I just checked while hot and I still seem to be in open loop The only other thing I can think to do is to do continuity testing at the ECU and possibly go pull one from a junkyard. Any ideas?












Replaced 02 sensor....

reseated computer terminal connections...

Dis cap /rotor/ disconnect pcv.

Redid the Cat...
 
Another thing I do not understand is that the Jeep runs fine. It does not lope.. The idle runs around 6 - 700 RPM maybe I just have got used to crappy running...
 
Check the 12-14 volt supply to the O2 sensor heater wire again.

You have a typo here:


Idle limit HC PPM = 220 me 113 = FAIL (SHOULD BE PASSED!)
CO % = 1.2 me 1.45 = FAIL

High CO at idle might be a leaking EGR valve (which reduces the O2 needed by the Cat to convert CO to CO2), or leaking fuel injector(s). I'd bet the EGR is stuck open a little, not entirely sealing off, thus running the engine too rich at idle. O2 sensor data at idle being rich would partly confirm that if the O2 sensor is OK, close to 2.45 volts at cruise rpms.
 
Thanks very much Mike.... That definitely gives me a direction to go in. I had not checked os sensor functionality at cruise and will double check
the EGR....
 
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