E-150 1990: HOW IS O2 SENSOR SUPPOSED TO READ???
<hr style="color: rgb(209, 209, 225); background-color: rgb(209, 209, 225);" SIZE="1"><!-- / icon and title --><!-- message --> I have been battling a code 31 and 41 on the "off test OBD1" procedure and that indicates the O2 sensor is always low. I tied a volt meter to the output oi the sensor and I read after warmup bertween 0.7 to 0.9 VDC and it stays at that voltage until I decelerate, at which point it drops to 0.03 to 0.4 VDC. It nevers "sweeps" between low and high voltages (for that I mean below and above 0.5VDC which theoretically would be the stoichiometric optimal value).

Not being an expert on Ford injection systems, does the computer sweep the sensor with a voltage to read its value, just like the Bosch systems L-jetronic, Motronic and others do?

If I had a healty running engine, what should I read on the O2 sensor tap I made? a steady voltage or a sweeping voltage?

Thanks for any input!

P.S.: Oh yeah, car drives like heck, stumbles, jiggers and it is not the fuel pressure as I have a calibrated gauge on the rail and it is spot on.

Namarena

 

The computer supplies a reference voltage to the sensor and it delivers a voltage back based on the O2 level in the exhaust compared to what it is in the atmosphere. When an engine is running correctly and the EFI system has it under control the exhaust mixture will hover around stoic(14.7) and the O2 sensor output will oscillate between about 0.2v and 0.8v which mirrors the fueling changes the EFI system is making in an attempt to hit the target A/F ratio.

And you can't really test an O2 sensor with a multimeter, the act of attaching the meter will corrupt it's output.. you need an oscilloscope with a high impedance probe or hardware that interfaces with the PCM.

 

Paul: thanks for the reply. My multimeter is a high impedance meter/oscilloscope and have measured the voltage coming out of the O2 sensor for years.

My question was not about how to measure an O2 sensor, but what is the EEC doing with the signal. As stated I get a constant 0.8VDC while cruising and that is not right, as this voltage would mean the mixture is rich. I already changed the sensor for a Ford original part number and it behaves the same.

I guess what I need to know is : how do I make sure the computer is scanning the sensor properly and determine if there is something else wrong?

Thanks,

 

OK. You need to get into the PCM and see what's going on and the obvious first thing to do is pull the codes. If there are sensor problems there should be codes, if there is a serious problem the PCM will stay in open loop and that richer mixture will mean the O2 will almost never switch. What you have to realize is the O2 doesn't control anything it's just a feedback device for the PCM, so that is where you need to look for the cause of the problem.

 

Paul: that makes sense, I thought the computer was always in open loop since the reading was always rich. What is the PCM you mentioned about?

Thanks,

Namarena

 

PCM = Powertrain Control Module it's the technical term for the computer that controls both the engine and automatic transmissions in these trucks.