ECU compatible

[89base]

I have an 89 MJ with the 2.5  engine. Really bad gas mileage. Checked the O2 sensor plug for ECU voltage w/o the sensor connected.

Zip, zero from the ECU.   Suspect I blew something in the ECM when I wired a 4 wire O2 sensor in place of the 3 wire unit using the ground

point from the ECU as a heater ground.

Point is, no VDC on the O2 sensor plug. Yes I'll trace continuity back from the plug to the ECU plug , next step.

If that checks okay, then the ECU must be toast.  ( the truck runs well on this ECU other than the O2 / bad mileage problem ).

Said all that to say this: I have a spare ECU from an 86 MJ 2.5 engine.

Will this be compatible with my 89 ECU ?

 

Side note: this Comanche Club is fantastic for information, it has helped me so much during my restoration

Thanks,   John 89base

[cruiser54]

As long as both MJs/XJs are throttle body, I don't see why not.

 

Check the connector with the engine running. 

[Ωhm]

Suspect I blew something in the ECM when I wired a 4 wire O2 sensor in place of the 3 wire unit using the ground point from the ECU as a heater ground.

 

 

Not sure what you mean by ...ground point from the ECU as a heater ground. Anyways, O2 sensors generate voltage. I don't believe voltage is supplied by the ECU O2 sensor input signal.

[cruiser54]

I have a diagram I can look at in a bit.

[cruiser54]

Oxygen sensor heater  grounds along with the ICM at G104. 

 

Engine dipstick tube mounting stud. 

[89base]

Answered my question  last evening about the ECU compatibility.

Yes, the 86 ECU works in place of the 89 ECU. Engine runs as it did with the 89 ECU.

About the O2 sensor; I read an explanation  in the MJ tech section about the Renix systems supplying voltage to the  O2 sensor.

Meaning the  Renix system  uses the Titania type variable resistance sensor, here is an excerpt from that article:

 

"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. "

 

With that understanding,using a Zirconia type Oxygen sensor would prohibit the ECU functioning as it should as it produces a voltage with engine running ( zero to 1 VDC ).

 

Bottom line: should I see voltage on the O2 sensor plug (coming from the ECU w/ Sensor not connected) as the above article said ?

Thanks. 89base

[cruiser54]

Answered my question  last evening about the ECU compatibility.

Yes, the 86 ECU works in place of the 89 ECU. Engine runs as it did with the 89 ECU.

About the O2 sensor; I read an explanation  in the MJ tech section about the Renix systems supplying voltage to the  O2 sensor.

Meaning the  Renix system  uses the Titania type variable resistance sensor, here is an excerpt from that article:

 

"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. "

 

With that understanding,using a Zirconia type Oxygen sensor would prohibit the ECU functioning as it should as it produces a voltage with engine running ( zero to 1 VDC ).

 

Bottom line: should I see voltage on the O2 sensor plug (coming from the ECU w/ Sensor not connected) as the above article said ?

Thanks. 89base

Sorry I missed that in the first post about the O2 sensor.

 

They're not compatible with our Renix systems. 

[Ωhm]

89base great post on O2 sensors. Good read.  Thanks cruiser54 for posting that heater ground location.  Sorry about the blunt (vague) post on O2 sensors voltage. Needed to add the word Zirconium. Hope I didn't mislead anyone.

[89base]

As a follow up on this post: Ordered the correct O2 sensor Bosch 12008. Also ordered a rebuilt ECU as there was no measured O2 sensor ref voltage on ECU connector pin 35.

The ECU is a Cardone 79-4763. Installed and now O2 sensor output varies as measured by an analog volt meter.

Further fine tuning under the hood: checked the TPS Ref and Sig voltage:  Ref.= 4.96 vdc   X .17 should equal .84 vdc   actual reading .38 w/ key on engine off.

Adjusted TPS as near as possible to the .84 VDC.

 

Question: what ( if any) will be the noticeable effect of this  .46vdc change ( MJ is a manual transmission )

Note: This MJ has run well without any problems, just fine tuning to spec's.

[Eagle]

The effect of getting the O2 sensor back in play should be improved gas mileage. Without a functioning O2 sensor, the ECU runs in open loop (warm-up) mode, which uses a pre-set fuel delivery map (which is overly rich, like running a carburetor with the choke on during warm-up) that doesn't respond to sensor input.