Low volt reference to o2 sensor renix

I have a 1987 jeep renix. I have been getting poor fuel mileage, weird cold idle and it seems to be running rich. I have done a lot of cruisers tips and to be honest the harness in my keep is in amazing shape. I'm the 2nd owner and the first was an old man. Has low Niles as well. Now, my problem is this. I started checking sensors and ground circuits for an issues before I start just replacing parts. Here is what I found. With the o2 sensor unplugged and checking the vehicles harness I'm getting my 12 volts for the heated circuit. I checked the ground and it is solid and giving the same 12 volt reading as it would with my meter hooked to the negative side of the battery or hooked up to the ground circuit. No reisitance found. Now... My 5 volt reference is only reading 2.5 volts. Unplugged from sensor... I chased and cleaned every connector including the ecm connector. At the ecm, I'm still getting 2.5 volts. What gives?

OK. I am under the impression that 87-90 ecus interchange. Is that true? I picked up an 88 computer today and I want to install it and test voltage but am a little hesitant to do it.

So. I found info from cruiser about swapping in an 88-later ecm for a better tune. Installed, and I'm still getting 2.5 volts right out of the ecm for the 5 volt reference to the o2 sensor. If I had a short to ground somewhere under the hood, would it bring the voltage right at the ecm connector down as well?

I'm far from a *Renix* O2 sensor expert, EcoMike, Cruiser or 5-90 are. Three wires going to your O2 sensor, orange battery voltage in, black ground (shield), and grey voltage *out* to the ECU.

If I were you I'd check the intake exhaust bolt torque, careful with the end studs. Personally I don't touch the end studs unless I absolutely have to.

Check your MAP senor vacuum line. The vacuum line can rub through on the bottom and leak vacuum.

Check your TPS ground, the TPS function and adjustment.

When I've had a bad O2 sensor on a Renix it always ran lean, especially after it was at operating temperature and running around 2200 RPM.

Vacuum leaks are something to look at closely. Many of the vacuum lines hook into the intake manifold off center, so some cylinders get a touch more air than others if a vacuum line is leaking. Makes for a poor idle and throws the true extra oxygen readings at the O2 sensor off.

A whole lot of stuff can cause a poor idle or miss at idle. Renix are kind of notorious for this.

I've had a Renix for around 27-28 years now and never needed an ECU, they do occasionally go bad, but this would be my last choice when troubleshooting. So many other things can fail and do fail, before the ECU does in my experience.

I have read that at the o2 connector on the vehicles side unplugged, I should have battery voltage at the orange wire for the heated element, ground at the black, and 5v at the grey wire when not plugged into the oxygen sensor as voltage drops when its plugged in due to how the o2 sensor works. My jeep is running rich. With half of a volt reference I'm assuming that that it's causing the ecm to see a lean condition. Very lean. So its making it run rich. Trying to figure out why I have half of my 5v reference. I kept checking closer and closer to the ecm and eventually pulled it out from under the dash. Check the connectors which where spotless. Cleaned them anyways and plugged it back in. Backprobbed the o2 wire at the connector at the ecm and I'm still getting 2.5 volts. Shaking the harness did not change the readout. I opened sections of the arness up and its in very good shape.

Two ways I test a ground circuit.

One is an ohm meter from the connector to the battery negative.

The other is with the sensor plugged up and powered, I use the low volt scale and check from the ground (back pinning) to the battery negative. This will give you a standing voltage or the voltage that isn't making it through the ground circuit.

Where are you testing the O2 sensor reference voltage to ground? Are you testing back into the O2 harness or directly to battery negative?

Just a shot in the dark as I have mentioned O2 sensors aren't really my thing. I might try unplugging the TPS and maybe the MAP to see if there was any voltage change at the O2 sensor. Maybe a waste of time, may indicate an issue. I know in the later models sensors sometimes share reference voltage, a bad (shorted) sensor on a parallel circuit can affect the reference voltage to other sensors.

The only other thing I can think of is maybe low battery voltage/amperage into the ECU. A damaged wire or corroded connector can give you an acceptable voltage, but is unlikely to give you sufficient amperage. One way you might pick this up is to measure the voltage at the battery and then again as close to the ECU as practical, with the ignition on and then maybe again with the motor running and see how much line loss you have.

It also isn't a bad idea to use a different meters and compare the two. Sometimes the base readings can be off. Make sure you clean all the oil off of your meter leads, big difference testing 12 volts and anything less than around 6 volts.

I initially wanted to just test the o2 and its circuit to verify it was functioning. First I tested at the connector down by the o2 sensor. I tested using the battery as a ground and then the plugs ground to verify a good ground. So I started working my way up the harness and back probed the connector near the firewall. Still low voltage. Then I pulled the ecm out from under the dash and was able to back probe the connector right there. Same voltage. Connector is super clean. Contacts are good. Most of my sensors on the engine are getting a 5 volt reference. This wire seems to be a direct patch to the ecm. It passes through one pigtail near the firewall and goes from Greg to orange and then a foot and a half later is at the ecm.

Is that orange O2 sensor wire plugged into the D-9 slot, D-10 slot? 87 is kind of a one off year for some of the wiring and it can be hard (impossible) to find a completely accurate wiring diagram.

Been awhile since I've looked at the plugs for an 87 ECU, seems unlikely they'd have two orange wires next to each other D-9 and D-10 with two different functions?

You may have better luck following the wiring and double checking the pin number the wiring plugs into. Double checking may be prudent. An 88 wiring diagram is likely to be close enough.

They do screw up those harnesses on occasion (lowest bidder). I've found crimps, uncrimped, with the wires just sitting in the crimp. I found the ground for the TPS sensor transmission half swapped with the TPS motor (ECU) half of the TPS. Corroded connectors is common. The wiring where it makes the bend at the firewall from the injector rail harness is prone to copper fatigue under the insulation on the individual wires.

The voltages you mention seem unlikely unless you are rounding them off. Having 5 volts to the TPS is rare, it is most often around 4.6-4.8 volts sometimes higher, sometimes lower. Battery voltage is rarely (never) 12 volts exactly.

Seems unlikely you'd be getting low voltage to or from the O2 sensor with two different ECU's, unless the in voltage is off or you have a circuit flaw that is affecting both ECU's the same way. I've never tested this voltage myself, never had the need. I pop a new Bosch O2 sensor in there every two years (smog inspection).

The only real trouble I've ever seen with this circuit is the (O2, knock sensor, engine temp) harness is prone to cook onto the exhaust where it passes through or behind the power steering pump bracket. Many mechanics route this harness wrong when they change out a water pump and it cooks on the exhaust manifold. Where it typically cooks is almost impossible to see from the top or the bottom, I've had to disconnect the sensors and actually pull the harness up to see the burn spot. Seriously the only real issue I've ever had with this circuit is bad wiring. Mileage typically averages around 15-16 miles per gallon, some people say they do much better than this, I never have.

Running rich is most times a worn out O2 sensor (or wiring issue), vacuum leak, cracked exhaust manifold or something with the MAP sensor. Maybe a leaky diaphragm in the fuel pressure regulator dumping raw fuel into the intake. A cracked exhaust manifold won't always make it run rich, but it may make it run rich. Depends on the nature of the crack and whether it venturi (sucks) extra air into the exhaust manifold or not.

I can usually feel when the motor begins to switch over from open loop to closed loop, usually about a quarter dial on the temp. gauge, maybe a little higher. When the O2 starts to crap out and sends faulty info, the motor may run lean at operating temperature, it may even buck and shake at certain RPM's and/or vacuum scenarios and may never reach the upper RPM range (above 3500 under load). When the O2 sensor is completely out of range, it seems doubtful the ECU will make the open loop, closed loop transition. Open loop is typically a slightly higher idle and runs a little richer for one thing. Just info, closed loop is incremental, it doesn't have to be all or nothing. I do a lot of troubleshooting with my butt dyno, it isn't perfect but it works.

Cruiser may be around sooner or later and have some better ideas. EcoMike pops onto the board periodically and has done some extensive O2 tests. 5-90 is pretty much an expert and may have an answer, but Kelly isn't around much anymore. I do pretty well with the hard wiring and motor basics.

I had to follow the wire into the cab to make certain I was chasing the correct wire. I have read that there are two black and one orange which is not the case on mine. My 5v reference is gray. It changes to orange at a pigtail near the firewall. I was rounding up. Most voltage had been in the high 4.8 area checking sensors. There are indeed two identical colored orange wires at the ecm connector. One gives me that same 2.5 volts and the other nothing while not running. So. I chased it to the ecm and its still low. I'm wondering if I have a short to ground or a couple of wires melted together causing my drop on that circuit. I assumed that if I found the other end of that circuit with higher voltage. I'd get into the harness and find an issue. There areas I did open where in great shape so I didn't further investigate. Even the factory grounds looked amazing. I've always been super happy with how well it ran and how clean and unmolested it was under the hood. But I do wheel it. And I have taken it apart. i know its running rich. The new cat I put on yesterday is showing signs of discoloration due to excessive heat it the cat. Which means I'm burning too much unburnt fuel in the convertor. I pulled plugs and they looked great. Replaced them anyways. Egr is clean. All Mani bolts are tight. No exhaust leaks anywhere. Map sensor is good. Tps is good. Clean mat sensor. Wasn't that dirty. New air filter because it needed it anyways. Ccv system is clean and functioning properly. Fuel pressure regulator is not leaking fuel. It runs great on the highway. There is a flat spot in the 1500-3000 rpm range. Its not huge and most people wouldn't notice it. But I drive my jeep everyday. And I notice it. Once I hit an rpm that the fuel can be burnt properly in, it had more power. I have been going over the jeep and started with a ton of cruisers tips. And my own list of to dos. I have not just thrown parts at it. Looking for a problem and I found one in this circuit. Plus the way it runs at idle when cold and warm is another reason I'm hunting in the o2 area.

I had to follow the wire into the cab to make certain I was chasing the correct wire. I have read that there are two black and one orange which is not the case on mine. My 5v reference is gray. It changes to orange at a pigtail near the firewall. I was rounding up. Most voltage had been in the high 4.8 area checking sensors. There are indeed two identical colored orange wires at the ecm connector. One gives me that same 2.5 volts and the other nothing while not running. So. I chased it to the ecm and its still low. I'm wondering if I have a short to ground or a couple of wires melted together causing my drop on that circuit. I assumed that if I found the other end of that circuit with higher voltage. I'd get into the harness and find an issue. There areas I did open where in great shape so I didn't further investigate. Even the factory grounds looked amazing. I've always been super happy with how well it ran and how clean and unmolested it was under the hood. But I do wheel it. And I have taken not apart. I know its running rich. The new cat I put on yesterday is showing signs of discoloration due to excessive heat it the cat. Which means I'm burning too much unburnt fuel in the convertor. I pulled plugs and they looked great. Replaced them anyways. Egg is clean. All Mani bolts are tight. Now exhaust leaks anywhere. MP sensor is good. Tps is good. Clean mat sensor. Wasn't that dirty. New air filter because it needed it anyways. Ccv system is clean and functioning properly. Fuel pressure regulator is not leaking fuel. It runs great on the highway. There is a flat slot in the 1500-3000 rpm range. Its not huge and most people wouldn't notice it. But I drive my keep everyday. And I notice it. Once I hit an rpm that the fuel can be burnt properly in, it had more power. I have been going over the jeep and started with a ton of cruisers tips. And my own list of to dos. I have not just thrown parts at it. Looking for a problem and I found one in this circuit. Plus the way it runs at idle when cold and warm is another reason I'm hunting in the o2 area.

Does it have the c101 connector on the firewall? Have you checked the Intake air temp?

It does have that connector. I pulled it apart yesterday. Cleaned it very thoroughly. Used die electric grease to seal it but not on the direct contacts. But. My o2 5v reference wire does not seem to pass through that connector. I chased the gray wire to a pigtail at the end of the valve cover before the harness passes through the firewall. Checked before and after there as well. The wire goes from solid gray to solid orange at that connector. It then stays that color to the ecm. I should have checked what oinout number I was testing. But its the only orange wire that passes through the firewall with 2.5 volts like the whole circuit.

The O2 sensor heater wire goes from 87A on the relay, through the c101!!

O2 sensor itself grounds at G105 on the driver's side inner fender

But I'm getting battery voltage from the heated o2 circuit just fine. Good ground as well. Just a low 5v from the ecm. Only getting 2.5 with the o2 unplugged.

Just a piece of info. With a MAP supply voltage of 2.5V, you should be running real lean, NOT rich.

Not map circuit. The o2 circuit. And if the o2 circuit is getting less voltage than its supposed. Once plugged into the o2 sensor that 2.5 v reference drops off close to 0vs.causing the ecm to see too lean. Then it corrects by adding a ton of fuel and the o2 sensor will never tell the ecm that its even passed the lean threshold.

Ditch the C101. Tip 27.

I'm going to pull the harness apart from the o2 plug to the ecm. I think I have a short to gekundcausing my low 5v reference. Its almost gotta be that.

You may be using a low impedance meter that is giving you a faulty reading.

I did have a similar problem in my 98 sport, about .8 volts reference to all sensors, turned out my alternator had almost died, my jeep seemed like it was missing w cylinders, misfired, died easily, replaced although and all cleared up