• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Testing Jeep O2, Oxygen sensors

Well congratulations!!! Well done.

How about we move the remaining cold start question to a new thread? And as you mentioned the O2 sensor is not used in starting or running while cold. ECU ignores it till its warmed up.

I greatly appreciate your help Ecomike. I’ve posted a new thread “extremely cold natured Yj”.
 
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.

My next post will be about testing the HO years O2 sensors, as they are a different animal.

Where do you get Orange and two black wires?
The 1990 FSM shows the engine side harness wires as Orange at the (A) on the plug, Black (B) on the plug and Grey (C) on the plug.
Orange going to the oxygen sensor relay, Black going to ground and grey going to D9 computer pin.
 
Good to know, thanks for sharing. My 87 is black on both. Perhaps AMC was using what wire it in 87, Black, LOL. Or the gray one is black with age, LOL. I used an ohm meter
to sleuth out which was the 5 Volt supply and which was ground.
 
Randy, 99% sure the O2 sensor relay that powers the electric heater element in the sensor is stuck ON, and draining the battery. Change that replay and see if the power draw does not vanish. My compliments on your post, great detail of the story, made it easy to know what is wrong! Not sure if that image below is right, so check some others online or the FSM for the right location.
picture_php_pictureid_18907_jpg_a6419108be3f1555d840dfeed843fcb82f14aab8.gif

Does the 90' renix o2 sensor supposed to have a normally closed relay or normally open relay?

Normally closed relay pin 30 to pin 87. Normally open pin 30 pin to pin 87a
 
I and a few other NAXJA/ECU tuning guys are working on reverse engineering the entire RENIX ECU (both hardware and software) but we haven't touched the 2.5 one yet. It is very different from the 4.0L one, TBI vs MPFI, only one MCU instead of two on the PCB, etc.

I do have a spare 1990 2.5 manual ECU but I need it for our project and it sounds like the ECU is probably not the problem here.

How is the reverse engineering the ECU going?
 
Does the 90' renix o2 sensor supposed to have a normally closed relay or normally open relay?

Normally closed relay pin 30 to pin 87. Normally open pin 30 pin to pin 87a

A 5 pin relay has the option for both modes, and like you mentioned:
- 87 is Normally Open (Regular)
- 87a is Normally Closed (Inverted)

The o2 heater circuit for the 4.0L uses pin 87a.
 
The Renix O2 sensor heater relay is only closed only when power is applied from the ignition switch powering the ignition relay near the battery that then powers the heater relay.

That would be a normally open relay IIRC.

Nice to see you posting here Nick!

A 5 pin relay has the option for both modes, and like you mentioned:
- 87 is Normally Open (Regular)
- 87a is Normally Closed (Inverted)

The o2 heater circuit for the 4.0L uses pin 87a.
 
The Renix O2 sensor heater relay is only closed only when power is applied from the ignition switch powering the ignition relay near the battery that then powers the heater relay.

That would be a normally open relay IIRC.

Nice to see you posting here Nick!

Check those diagrams just to make sure, the o2 heater circuit is rather convoluted and backwards seeming.

The Relay is actually controlled from 2 different locations making diagnosing fun since it doesn't work with KOEF.



It's Normally Closed, and the ECU grounds the coil to open it during WOT and other things.

The main switch is actually the Load Supply on 30 which is switched on and off by getting power from the Fuel Pump relay.



So it will be active when the fuel pump is on, and the ECU is not commanding it to open.
 
How is the reverse engineering the ECU going?

Haven't had a chance to do anything in a while sadly. I'm swamped at work and it's cutting into my spare time in the evenings.
 
I think I have that backwards.
Correction.
Normally closed relay pin 30 to pin 87A. Normally open pin 30 to pin 87.

The FSM says that o2 power wire goes to 87A.

If the O2 power wire goes to 87A which is normally closed then the o2 would be powered when not energized (Key Off). Thats what I have going on right now. Which is why I'm thinking that the o2 relay should be a relay for the o2.

I'm thinking that if the o2 power wire was going to 87A on a normally opened NO relay then there would be no power going to the o2 with key off. And then be powered when key is on.

I get 12v to o2 power wire, 5v reference wire and zero ohms to ground wire.
 
The FSM says that o2 power wire goes to 87A.

If the O2 power wire goes to 87A which is normally closed then the o2 would be powered when not energized (Key Off). Thats what I have going on right now. Which is why I'm thinking that the o2 relay should be a relay for the o2.

I'm thinking that if the o2 power wire was going to 87A on a normally opened NO relay then there would be no power going to the o2 with key off. And then be powered when key is on.

I get 12v to o2 power wire, 5v reference wire and zero ohms to ground wire.

Right, a 5 pin relay is a 5 pin relay, you can't get a normally open or normally closed version. Use 87 for open and 87a for closed.

Follow the wiring diagram further up and you'll see the rest of the circuit. The o2 relay gets its input power (Pin 30) from the fuel pump relay.

So when the Fuel pump is off, the o2 is off.
Fuel pump on, o2 on, UNLESS the ECU commands it off by grounding the coil and opening the latch.

Probe pin 30 of the o2 relay and see what it's doing, and check out the fuel pump relay while you're at it.
 
Back
Top