G951

After hooking up my Autometer A/F gauge, I decided to do some voltage readings from the O2 sensor to see if I was getting any deviant outputs from it (O2 sensor is old and hence the test).

Anyway, here are my results. This is on a stock 951 (mods are not on the car yet). The car was warmed up.

Disconnect O2 sensor at firewall and put multimeter (+) lead to the O2's black wire (signal wire) and (-) lead to battery ground. I got readings from .810 to .850. This appears to be normal as the disconnected O2 sensor will cause the engine to run rich. I will call this the baseline reading.

Funny thing is that if I throttle up sleadily, the O2 sensor drops to about .780. Isn't it supposed to go higher than the baseline?


I also wanted to check for proper grounding to the A/F gauge. I inserted the (+) lead into the DME harness side of the O2 connector. The lead fits in perfectly. Then, I took the (-) to the battery ground. Car was idling and I got a reading of .435. Now, isn't the DME side to just read nothing at all?

I went to the A/F signal line and hooked up my multimeter there. Same exact reading as the O2 sensor connector (.435). At least I'm getting the same voltage reading on the O2 connector and at A/F gauge end.

How come I'm getting a voltage reading at the DME when there's no O2 sensor hooked up?

Is the DME doing something I don't know about?

951Tom

If the O2 sensor is disconnected, the DME will use a default value of 0.45 volts which is roughly 14.7 to 1 A/F ratio. Consider it a sign your DME is working properly.

G951

[quote]Originally posted by Tom L.:
<strong>If the O2 sensor is disconnected, the DME will use a default value of 0.45 volts which is roughly 14.7 to 1 A/F ratio. Consider it a sign your DME is working properly.</strong><hr></blockquote>

Hey thanks!

I thought about that but it just seemed odd that there would actually be a voltage reading (output) on the DME/O2 connector. I assumed the voltage would be read inside the DME, processed and then spit out to the injection system.

Then again, I guess that's why some 944/951 owners who have faulty O2s seem to have a car that runs better when the O2 sensor is disconnected - the DME automatically goes to STOICH. Guess this is one way of troubleshooting an O2 or DME problem.

I'm gonna change my O2 sensor anyway since it seems as though it hasn't been changed.

Outlaw952

[quote]Originally posted by G951:
[QB]

Then again, I guess that's why some 944/951 owners who have faulty O2s seem to have a car that runs better when the O2 sensor is disconnected - the DME automatically goes to STOICH. Guess this is one way of troubleshooting an O2 or DME problem.QB]
<hr></blockquote>

Can anyone verify this? My car runs better without the O2 plugged in. Is this an indication that the O2 sensor is bad?

G951

[quote]Originally posted by Outlaw952:
<strong>

Can anyone verify this? My car runs better without the O2 plugged in. Is this an indication that the O2 sensor is bad?</strong><hr></blockquote>

Our cars run in closed loop mode during idle which means that the A/F mixture is being either richened or leaned by the signal from the O2. It's a constant adjustment process to achieve a 14.7:1 air/fuel ratio.

If at idle, you disconnect the O2, the DME attempts to go directly to STOICH (14.7:1). In effect, it is actually just reading a preset voltage (as it appears).

I'm not saying that a better running car w/ the disconnected O2 is a sign of a bad O2 sensor. Case in point would be that you have a vacuum leak. The car sucks up more air, the O2 sensor reads a lean condition, DME picks up a low voltage (lean), and adds more fuel. What happens is that adding more fuel causes a rich condition, the O2 sensor picks up less oxygen in the combustion= rich, DME picks that up and goes to a lean condition.

As you can see, this can be a pretty bad cycle of lean to rich, lean to rich and so on. This may cause the erratic idling that many people get.

So, the answer is that NO, it is not an indication that it is bad. You could have a vacuum leak or some other problem. But, it CAN mean that you have a bad O2 if it's no longer reading the oxygen content properly. I believe these Bosch O2s are good up to 60K miles. Changing it is better than not changing it - helps to rule it out of the equation if you do have a vacuum leak somewhere.

83na944

I think I read that the voltage from an o2 sensor is actually negative and should be about -0.5 volts. This would explain why the readings wen the wrong way when you open the throttle.

The sensors are actually reasonably cheap and easy to change.

My car runs better with a good sensor.

zucker

The sensor voltage is dropping because of the increase in exhaust gas temperature (it is a common trait of the sensor).
Your sensor is functioning normally.