O2 sensor problem

[big66440]

I replaced my ECM recently with part number 8953005428 and ever since then my Renix monitor shows the O2 voltage to be stuck at 4.98V and only goes into closed loop once in a while for about 3-4 seconds, the ECM I replaced showed normal voltage readings for a good O2 sensor circuit. I have battery voltage for the heater and 4.97V reference from the ECM and show 0 ohms when I check resistance on the third wire. I tried replacing the new ECM with the old one and it's doing the same thing ( constant 4.98V and open loop) as well as installing my old O2 which I'm sure works except for the heater element being busted no difference. I looked at the terminal pins on the harness side and ECM and seem to be fine, I've wiggled the harness with the engine running and made no difference. Can you guys please tell me how to hook up my voltmeter to check the voltage of the O2? I'd like to be 100% sure what I have and don't have for my diagnosis, thank you.

[big66440]

Never mind I figured it out, I'm seeing a voltage reading going up and down but my monitor still shows 4.98. I'm going to check the resistance of the harness when I get a chance.

[big66440]

Anyone want to take a shot at this? I'm stumped right now and could use some brain storming. I'll do a quick recap, my Renix monitor shows my O2 voltage stuck at 4.98V ( all the time)and goes into closed loop intermittently for about 3-4 seconds. I have battery voltage to the heater circuit as well as a 5V reference from ECM. I have a brand new O2 sensor as well. I have also done cruisers renix tips for the ground upgrade and splices. I have checked and double checked the connection to the ECM and it's fine.

[omega_rugal]

maybe the sensor was DOA?

[87MJTIM]

Somewhere on CC is a manual for the Renix fuel injection system.  I covers everything for it.  There a section on the O2 sensors and what the proper reading should be.  

 

I tried to search but could not find it.

 

I'll search tomorrow for it.

 

EDIT:

 

Found the link in the DIY section.  Of course, it is a link, also.

 

http://comancheclub.com/topic/36875-get-your-copy-of-the-renix-fi-fuel-injection-manual-here/

[87MJTIM]

If you have not checked out this site, it has a lot of useful Renix info.  There is a part on O2 sensors

 

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

[Ωhm]

........show 0 ohms when I check resistance on the third wire. 

I take that to mean O2 sensor connector to engine block ground = 0 ohms.

 

 

Never mind I figured it out, I'm seeing a voltage reading going up and down but my monitor still shows 4.98. I'm going to check the resistance of the harness when I get a chance.

So the vehicle is running trouble free and the monitor is in error.

 

or

 

Everything you did and see is true and your engine is running lean. Your not burning Corn-85 are you?

[HOrnbrod]

 

........show 0 ohms when I check resistance on the third wire. 

I take that to mean O2 sensor connector to engine block ground = 0 ohms.

 

 

Never mind I figured it out, I'm seeing a voltage reading going up and down but my monitor still shows 4.98. I'm going to check the resistance of the harness when I get a chance.

So the vehicle is running trouble free and the monitor is in error.

 

or

 

Everything you did and see is true and your engine is running lean. Your not burning Corn-85 are you?

 

 

I vote the digital monitor's response time is too slow..............

[big66440]

The monitor was working perfectly before I replaced my ECM with the new replacement O2 sensor, so that rules out the O2 sensor and monitor issues. I'm running 89 Unleaded, I have the Renix fuel injection manual and have studied the O2 sensor diagram and I'm a bit confused. The diagram shows the solid gray wire ( terminal C ) to be return voltage to ECU Pin D9  but I'm getting a solid 4.98V with the key on engine running and not running which leads me to believe that gray wire is actually the 5V reference TO the O2 sensor FROM the ECM?. If I am correct that means the Black wire ( terminal B )is the return voltage signal and ground for the heater element to the ECM? if that is true what pin on the ECM is the return signal and ground for the O2 sensor?

[HOrnbrod]

 

For all 3-wire O2 sensors:

 

Pin A = Ignition input (12V, 5V reference, etc.)

Pin B = Ground

Pin C = Output to computer

[Ωhm]

Pin B is a shared ground circuit for both the heater circuit and the O2 signal circuit.

[big66440]

Pin B is a shared ground circuit for both the heater circuit and the O2 signal circuit.

I'm sorry but what do you mean by " 02 signal circuit" ? I understand the heater needs a ground but I thought the "ground" for the O2 signal was the oscillating voltage the ECM needs to calculate air fuel ratio :hmm: .

 

Thank you for the diagram Hornbrod

[HOrnbrod]

Nope. The switching wideband O2 output signal is referenced to ground. On a 4-wire O2 like the HO's have, there are two wires for the internal heater resistor.

 

[big66440]

 

For all 3-wire O2 sensors:

 

Pin A = Ignition input (12V, 5V reference, etc.)

Pin B = Ground

Pin C = Output to computer

Just checked voltage again and I'm getting 12V on terminal A, and a 5V reference from terminal C. I did this test on the harness side with the engine running, is this normal ? if so does anyone know where the ground for terminal B is?. I apologize once again for the ignorance.

[HOrnbrod]

Schematic says G105, so it's at the dipstick.

 

[big66440]

Thanks,So the 5V reference at terminal C is normal?

[HOrnbrod]

It's normal if you're using a digital VM as you're looking at the 5V reference. To see the output with the engine running you need to use an o'scope or an analog voltmeter. The below explains it, particularly paragraphs 4 & 5.
 

 

This question comes up a lot in threads with O2 sensor trouble codes and threads about bad mileage. I hope to use this post (thread) as an FAQ about how to properly test mounted jeep O2 sensors and the wiring and related hardware.

For the Renix years, 87-90, the O2 sensor has 3 wires, 2 black and 1 orange. The orange wire (largest gauge of the 3) is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall, and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immedietly after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

One of the black wires is a common ground for the heater power and O2 signal to the ECU, so a poor ground will give a voltage feedback from the heater power input, to the ECU causing poor mileage even with a good O2 sensor.

The third wire, also black is a voltage feed wire, 5 volts, from the ECU to the O2 sensor. The O2 sensor is an O2 concentration sensitive variable resistor. At optimal O2 concentration the 5 volt input feed to the O2 sensor drops to 2.45 volts due to losses across the O2 sensor to ground. That same wire if disconnected from the O2 sensor will read 5 volts constant to ground.

At idle that voltage should read 1-4 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 2 and 3 volts max, and is optimally running between 2.3 and 2.6 volts at 2000 rpm (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the ground and the 12-14 volts to the heater and the 5 volt feed from the ECU with power on and engine off. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth with the engine runing.

If the O2 sensor readings are not right, say they read 4 volts or 1 volt steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 volts on the orange wire, and 5 volts on one of the two black wires), and ensure the ground wire (power off) reads less than 1 ohm to the battery negative post.

A leaky exhaust system or leaky fuel injector(s), or bad compression, bad rings or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.

[big66440]

Something is very weird then

I have verified

 

12V reference to O2

5V reference to the O2

0-1 Ohm on ground circuit

2 working O2's ( one with an open heater circuit but still showed voltage oscillation)

2 ECM's one definitely working

Good connection at ECM

 

I know somethings wrong because mileage has suffered and I can see and hear the engine struggling really bad to stay in closed loop.

 

I think I can see why you dislike Renix systems so much Hornbrod :rotf:

 

All I did to "cause" this was literally disconnect my battery and replace the ECM,,,, I re installed my old ECM to verify and still no luck.

[HOrnbrod]

Yep. you can keep them. It's an illogical system and makes little sense to me. Too may "unexplainables". When they run well they are fine. But an ohm or millivolt off here and there can cause them to run like crap. The OBD-1 systems especially to a lesser extent -2 are much more forgiving.

[87MJTIM]

Could the relay be bad?  

 

Could the relay intermittently be bad?

 

I have Nick's Renix Engine Monitor.  It read the two signals to the ECU - the O2 volt and the hearter volt.  The O2 keeps from 1 to 4.98v; the heater is getting 12v.  The ECU starts in open, after 1 to 2 min, it will switch to closed.  Then it goes back to open and remains in open.  The two voltages keep reading correctly.

 

I just put in a new NKG O2 sensor.  AFAIK, the relay is original - 87, only one owner: me.

[big66440]

That's the monitor I have as well, my readings were exactly as you described before I decided to swap ECM's. Now my O2 reading is stuck at 4.98V all the time and the ECM will go into closed loop for a few seconds then pops back into open loop. I've checked all the relays and have completely taken apart the relay sockets to thoroughly clean them ( one wire at a time). I have never been so frustrated with my Comanche, I give it the best I can and can't help but feel a bit betrayed.

[Ωhm]

 

Pin B is a shared ground circuit for both the heater circuit and the O2 signal circuit.

I'm sorry but what do you mean by " 02 signal circuit" ? I understand the heater needs a ground but I thought the "ground" for the O2 signal was the oscillating voltage the ECM needs to calculate air fuel ratio :hmm: .

 

Thank you for the diagram Hornbrod

 

O2 signal circuit consist of (ECU pin D9) 5 volts on top of a fixed resistor, connected to the O2 sensor (O2 pin C), through a variable resistor and then to ground (O2 pin B. Between these two resistors the signal line is tapped, and the ECU read this voltage. This type of circuit is known as a voltage divider. Example would be when the variable resistor is of equal value to the fixed resistor, causing the output signal to show 2.5 volts.

 

As the variable resistor value increases, (high oxygen content) voltage increases on the signal line. As the variable resistor value decreases, (low oxygen content) voltage decreases on the signal line. This happens many time per second and a DVM will not show this. But when the O2 sensor is disconnected a steady 5 volts (4.98 volts) will show on the harness side of the connector (O2 pin C).

 

All of your reading and values seem good, within the limits. So why is your vehicle running lean? There was a reason you replace the ECU and O2 sensor. MAP, clogged injectors, fuel pressure, something causing this lean condition.

[big66440]

I replaced the ECM because the one I had was for an older model Manual transmission MJ and I wanted the power increase and better "manners" of the 89-90 ECM's. My old O2 sensor had a bad heater element causing a rough idle and open loop at idle. According to the Renix monitor the MAP sensor is within specifications as well as the other sensors.What ever it is it's now common to both ECM's since the one I removed from my Jeep originally is doing the exact same thing.

[big66440]

SOLVED!!!! The ECM i got from ebay must have burned out my O2 sensors!!! I got a new NTK oxygen sensor and installed my old ECM and it did the trick!!. I must have not measured voltage correctly at the sensor, either way I'm glad it's over.

[NickInTimeFilms]

Glad you figured it out, I had a sneaking suspicion the new ECU was giving you issues. I just figure I'd add a little knowledge in for later diagnostic of future users that may browse the forums.

 

For more indepth diagnosis of the closed loop process, keep an eye on the short term fuel trim reading. This is the adaptive offset from the base fuel curve and is directly affected by the o2 sensor when In closed loop. 128 is base, lower reading mean less fuel then base and higher readings mean adding more fuel then base. When the o2 is non-responsive you can watch the short term reading either drop to 0 or rise to 255. If it sits there for more then a second then the ECU will give up and go back to open loop. It's all laid out on the diagnostic screen at the very bottom so take a look sometime.

 

As for the Renix Engine Monitor, besides the heater relay which is a direct reading, it reads all its info directly from the ECU stream so long as it is reading in data properly then what you see is exactly what the ECU sees. This can be useful for checking bad connections as a sensor may be working but the ECU is not able to read it.

 

The update speed is also a good point to raise and one that has made be very glad that I can call all the shots with this device. The ECU sends about 40 frames of data a second and if I were to display them instantly then the screen would be basically unreadable as the LCDs response time can't keep up. I have added a few different speeds, ludicrous being the upper limit of what is still readable with the screen. This is fast enough to easily watch the o2 sensor swing between high and low and it's actually really fun to watch. Here's an older video to see what that looks like: https://www.instagram.com/p/BJhNDNKhEBs/

 

Anyway sorry for the info dump but hope that sheds some insight on everything!