Diagnosing 2.5L Renix ECU

First post, but I've spent many hours on this site. Thank you!

Background: I just purchased an 87 cherokee 4dr, 5spd manual, and 2.5l engine. It sat in the previous owners yard for several years, he claimed it just had a coolant leak. In my quest to get it registered/smogged (in California), I have replaced the catalytic converter (the old one was completely empty), rebuilt the throttle body, new EGR valve, new manifold gasket. All of this is prior to even trying to smog it as I knew the exhaust smelled rich.

Issue: As I'm testing the O2 sensor using the excellent guide on this thread
I found that my heater circuit measures 12v, my ground is within 1ohm of battery negative, but my O2 signal wire is 0v. This is with engine off, key on, O2 sensor plugged in.

So I unplugged the O2 sensor expecting to see 5v from the ECU, but still 0v. Turn the engine on with sensor connected and it bounces between 0.45v to 0.9v. Engine on and sensor disconnected 0v.

Note, I'm using a digital multimeter with high impedance, >1Mohm

Another note,This diagram seems to be very accurate in regards to wire colors, pin assignments, and locations.

With O2 sensor unplugged, engine off and key on, I trace the wire back up to the ECU under the dash and probe just before the connector, still 0v. It occasionally bounces to 0.01v. Referenced ground was using either the engine block or the battery negative, and confirmed with 12v prior to testing ensuring good ground reference.

Diagnostics Steps taken so far:
-Measured resistance of the O2 sensor wire from O2 connector to the ECU pin 35, good <1ohm
-Measured resistance between Pin 35 of ECU connector to any of the other 34 pins (O2 sensor disconnected and ECU obviously disconnected), found no continuity. Suggesting the harness is not shorted anywhere
-Grounds on pin 1,2 are less than 1ohm but on pin 10 is 11ohms, will fix that but I don't this its relevant to not having 5v. Especially when the MAP sensor shows 5v no problem.



At this point I feel that I've ruled out the harness and engine compartment so I dive into the ECU. Why isn't the ECU outputting the 5v sensor signal. From my research it is in a pullup resistor configuration. I crack open the the Renix ECU to find the circuit board much simpler than I was expecting. Note, my career is test and failure diagnostic engineer on aircraft piston engines, although I am no electrical engineer I do have an understanding on how these circuits work. First impressions of the board were good, nothing seemed burnt or in any way suspicious. I theorize that there must be a voltage regulator that turns the 12v battery power into 5v sensor power. I see a few on the board that look like they could fit the job, but I can't find any cross reference part numbers. I see "SGS 9 644" next line "144-08". Theres several of these on the board.

Link for photos

Heres where I've come to you guys for any help, based on how simple this circuit board is it kills me to just replace it on the assumption that something MIGHT be bad. All the components seem so easy to desolder and replace. Is there anyone that has had something similar happen? These renix ECUs seem to have a good reputation. Is there something I'm overlooking?

Thanks for reading and for any input!

There is no 5 volt reference voltage to the oxygen sensor. The sensor generates a signal and sends it to the ECU pin 35.

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.

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Quoting from the sticky thread about testing the O2 sensor. It is my understanding that post-90 O2 sensors were the voltage generating type and pre-90 was the titania style that is simply a variable resistor. From what I'm measuring with my DMM, .45v to .95v I want to assume it is the newer style but that doesn't align with the everything I've read about pre-90 renix configurations.

well, with the factory scan tool on a rEnix, although a 4.0, the O2 sensor voltage varies from .1 to 4.9 volts....

farns_xj said:

Quoting from the sticky thread about testing the O2 sensor. It is my understanding that post-90 O2 sensors were the voltage generating type and pre-90 was the titania style that is simply a variable resistor. From what I'm measuring with my DMM, .45v to .95v I want to assume it is the newer style but that doesn't align with the everything I've read about pre-90 renix configurations.

Click to expand...

Looking up the part numbers on Rock Auto, both the 2.5 and the 4.0 say Titania. They have slightly different part numbers though, which may be just the connector. Other sites like CarID show the same part numbers fitting both 2.5 and 4.0.


The voltage you saw with it connected seems reasonable for indicating a rich condition. Is it possible the O2 sensor provides it's own 5-volt reference from the +12volts supply, and pin 35 on the ECU is just an analog input? You'd have to disconnect just the sensor signal and measure it.

The voltage you saw with it connected seems reasonable for indicating a rich condition. Is it possible the O2 sensor provides it's own 5-volt reference from the +12volts supply, and pin 35 on the ECU is just an analog input? You'd have to disconnect just the sensor signal and measure it.

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Ah I see what you're saying, perhaps the heater circuit provides voltage for the sensor portion. Since I'll likely replace the O2 sensor anyways I feel comfortable with cutting the wire on the sensor side of the connector and testing. Will report back once I get the jeep put back together, replacing the exhaust manifold now for several cracks.

My biggest concern here is that even though the engine appears to run smoothly, this could explain why the cat was empty. Maybe the previous owner ran it for a long time in a rich condition caused by suspect O2 circuit.

The heater warms up the sensor so it works sooner.

cruiser54 said:

The heater warms up the sensor so it works sooner.

Click to expand...

It also lets the manf put it further downstream and have a stable temperature, as the resistance of the titania type is also temperature fairly dependent,

I finished replacing the exhaust manifold yesterday, also new O2 sensor. Exhaust leaks seem to be gone, there was severe cracking on the old manifold. Probing the new O2 sensor shows no change from what I measured before, 0.35v-0.65v at idle with engine warm. No voltage engine off. No surprise there as I had a hunch it wasnt the sensor.

Additionally I cleaned the C101 connector as suggested in cruiser54s guide.

I'm stuck on the fact that the ECU isnt providing 5v excitation voltage to the variable resistor O2 sensor. I put an order in for a reman'd ECU from the parts store. Should arrive friday. Will report back.

Also, forgot to mention after finishing with the manifold and letting the engine idle up to temp I noticed that the catalytic converter was on the verge of glowing red. It was night time and just barely noticeable. I'm trying to make a link between the glowing cat and the odd O2 sensor reading, a low voltage from the O2 as measured by the ECU would indicate a lean condition therefore causing the ECU to demand more fuel to be injected.

ECU doesn't provide voltage to the oxygen sensor except for the heater.

I'd be suspecting a clogged catalytic converter and possibly a bad O2 sensor.