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240 Negative Voltage from Oxygen Sensor

8

84GLT

New member
Joined
Feb 5, 2007
Car: 1990 240, 2.4, 235k km (145k M)

Upon idling in line at the border today or cruising the endless parking lot looking for spaces last month, twice I got a CEL. The codes:

1. Rich\lean (2-3-2) and missing O2 signal [2 weeks ago]
2. 2-3-2 and injector fault (1-1-3) [today]

I cleared the codes two weeks ago, and they only came back today after idling in line. The car is obviously running rich, but only throws codes when idling or barely inching along.

After it happenee today, I hooked the + lead of my DVOM to the back of the O2 wire, and the - lead to the bolt holding the ground strap to the firewall. The engine was warm. The voltage swung raggedly between .866 and .224. Then, it started dipping just barely negative. I have no idea what to make of that and I have not found much online.

I cleaned the battery terminals and the ground bolt, the reading did not change.

Anyone know the deal?

Mike
 
Could be the voltmeter. Talking with one of the electricians I used to work with (also a car guy), his opinion was that the cheap voltmeters don't react fast enough to properly measure an O2 sensor.

On a related note, my 89 wagon was throwing out the same codes. I would clear the codes, and they would come back, usually at start up or shortly after. I changed the 02 sensor and they went away.
 
Interesting. My DVOM is hardly new, and I know they are the imperfect tool for the job. The o2 sensor is two years old, I'll have a look at it and the wiring, I guess.
 
billkaz said:
Could be the voltmeter. Talking with one of the electricians I used to work with (also a car guy), his opinion was that the cheap voltmeters don't react fast enough to properly measure an O2 sensor.
Click to expand...
THe DMMs also get more erratic as the battery goes down, so if that's a couple years old, that is worth a look.
 
Generally if O2 sensor voltage drops below .100 mv, it means that the sensor is contaminated and needs replacing.
 
cleanflametrap said:
Yes, I've seen it go negative when the tailshaft seal in the transmission leaked ATF all over the back of the sensor where it sniffs the reference air. :nod:
Click to expand...

Hmm, I'll crawl under and look. Aside from an occasional drip from the rack, my parking spot is dry.
 
84GLT said:
Hmm, I'll crawl under and look. Aside from an occasional drip from the rack, my parking spot is dry.
Click to expand...

Whether or not you see what contaminated your sensor, my feeling is you don't need to question your meter as much as get a new sensor in there and repeat the measurement. And, a leaking tail shaft seal only makes a puddle while the fluid is being pumped, so it flings a lot of oil at freeway speeds before it collects enough to drip on your parking space. Wow, took me 6 edits to get the most of the errors out of those two sentences. :)
 
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Yeah, dvom can diagnose dead sensor, but oscillations should be observed on a scan tool with volts/time visible. An aging sensor will have fewer crosscounts/min, causing lag.
You can give a little propane thru a vacuum line to test full rich, then pull the vacuum line off to give full lean and test response.
 
Turns out thar I lost an exhaust hanger. I fixed that, addressed the O2 connection that was getting jostled around by the shaky exhaust. There is no more CEL and the DVOM reading looks like it does when the car is healthy.

I will very tentatively consider this a win. We'll see what the upcoming week brings.
 
It never ends.

Last week the car popped another CEL, and the o2 read dead as a doornail. I chalked it up to the missing exhaust hanger causing a shake that pulled the o2 wiring apart.

Today I crawled underneath with the (oddly hard to find) exhaust hanger and realised that the hangar post needs to be welded, which I cannot do. I rewired the o2 anyway and tried to leave enough slack that the exhaust pipe rattling would not cause it to pull apart. The o2 signal seemed fine and the car ran better.

20 miles later I got a lopey idle and CELs 232 and 113 with a steady .13v from the o2 wire. Sigh.

I'm off to get the exhaust firmly attached to the car before I condemn the o2 sensor, but I'm getting a sinking feeling.
 
604Brick said:
Generally if O2 sensor voltage drops below .100 mv, it means that the sensor is contaminated and needs replacing.
Click to expand...

Before replacing O2 sensors smrat people have been known to get theirangle grinder with coarse wire wheel and clean off the crud which can cover partially of fully the louvers on the COVER of the sensor..If those louvers are blocked the gas doesn't reach the sensor (which is inside)

Quick, easy, cheap...and it works
 
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