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!
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!