Oxygen sensor voltage?

leakyfaucet · Dec 11, 2004

I've been spending some time lately trying to figure out why my oxygen sensors won't turn on. I borrowed an analog voltmeter to do these tests.

I tested the main harness plug that hooks into the oxygen sensor harness. One of the prongs was putting out 5V. So I assume that's the heater. I unplugged an oxygen sensor from the harness and tested the plug directly from the sensor. I grounded the voltmeter on the body of the oxygen sensor itself. At idle after running for a few minutes, I tested the prongs and none of them put out any voltage. Or at least I couldn't measure it. Is it possible I was grounding the voltmeter poorly by using the sensor body?

I've been told that the sensors generate their own electricity, so I should be getting some kind of reading from one of the prongs. However, I got none.

Suggestions?

leakyfaucet · Dec 11, 2004

Also, an unrelated question. While I was running my car at idle while it surged, I noticed a hissing sound just like a vacuum leak, although it could be almost anything whooshing. I would start hissing and then stop all by itself, then start again. It sounded like it was coming from somewhere around the fuel injector lines under the manifold. Is this normal?

HISSIN50 · Dec 11, 2004

what kind of meter were you using (analog or digital)? what setting were you on?

one should use an analog meter (digital meter avg's readings). dont do this w/o confirming it, but IIRC, you should use one with a high input impedance (cant remember how high).

my recollections are from something that Jrichker said - search his posts on the subject for more info.

good luck.

leakyfaucet · Dec 11, 2004

I was using an analog meter. I had the setting on 2.5DC volts. The needle never moved when I touched the prongs. I thought it was supposed to if the sensors are working correctly?

HISSIN50 · Dec 11, 2004

leakyfaucet said:
I noticed a hissing sound

was i standing behind you?

a stockish motor should have ~15-20" of vac at idle.

use whatever method you like to track the leak down. if the idle hangs when coming down from higher revs (while the car is stationary), that suggests a leak. a wandering idle also suggests a leak.

fuel rails can make noise, though i think your ear would have to be pretty good to hear it. injectors are like doors opening and slamming quickly. the fuel in the rail sloshes a little as the door is opened and closed.

good luck.

leakyfaucet · Dec 11, 2004

Yeah, The vacuum didn't seem to change during this. It was almost like the car controlled it. Very strange indeed. Any likelyhood of one of the intake manifold gaskets leaking? They've never been off the car. I don't know how I could test for those, spraying fluid on it won't work because of the fan blowing.

But my main concern is those damn oxygen sensors. Any insight?

Edit: When the hissing noise started and stopped, it didn't happen gradually. It was instantaneous. I really don't think it's a vacuum leak.

Michael Yount · Dec 11, 2004

The O2 sensor is a very misunderstood beast. They only measure within a VERY small band around stoichiometric. As a practical matter, they basically act like an on-off switch telling the computer whether the mixture is rich or lean on either side of a range between about 14.5:1 and 14.8:1 a/f ratios. Their voltage signal is very small as well - a used sensor will put out a 700 milli-volt signal around 14.5:1 ratio, dropping down to about a 200 milli-volt signal around 14.8:1. So the signal's very small.

Use a good digital volt meter, and get the car good and warm - and you should be able to see what's going on with yours. New O2's will have a bit of a broader voltage range for the same a/f ratios - say 900 mV down to 100mV.

HISSIN50 · Dec 11, 2004

Michael Yount said:
The O2 sensor is a very misunderstood beast. They only measure within a VERY small band around stoichiometric. As a practical matter, they basically act like an on-off switch telling the computer whether the mixture is rich or lean on either side of a range between about 14.5:1 and 14.8:1 a/f ratios. Their voltage signal is very small as well - a used sensor will put out a 700 milli-volt signal around 14.5:1 ratio, dropping down to about a 200 milli-volt signal around 14.8:1. So the signal's very small.

Use a good digital volt meter, and get the car good and warm - and you should be able to see what's going on with yours. New O2's will have a bit of a broader voltage range for the same a/f ratios - say 900 mV down to 100mV.

Michael, i need to get some Yount-ucation (i think the fruit cake killed some brain cells. or the sheer gravity of it is causing all blood in my brain to head toward my gut). i thought the lambas' voltage switched quickly (like multiple times a second)..?? if so, would a digital meter not just try to average the readings out? i like the analog gauge idea because one can see the needle sweep and get an idea. i was just curious what you thought.

351LX · Dec 11, 2004

I have a regular single wire oxygen sensor installed in my exhaust that I use for tuning purposes.At idle I find there is not enough exhaust volume to get a meaningful reading off it, maybe a heated one would be different.I use a digital voltmeter set on a 2v scale and that seems to work fine.I would try to find the output wire and hook a voltmeter to it in the car and drive around and see if you get a reading.Since I have a carb I just try to get readings below about 750mv and higher than 400 and I am happy.

giddyup306 · Dec 11, 2004

Michael Yount said:
Use a good digital volt meter, and get the car good and warm - and you should be able to see what's going on with yours. New O2's will have a bit of a broader voltage range for the same a/f ratios - say 900 mV down to 100mV.

Exactlly. If you can try and borrow a scantool. It's much easier. Your voltage should sweep frequently between about 150-850mv. Also make sure that your car is in CLOSED LOOP. Other wise the tests are void. On OBD 1 cars this may take several minutes.

Mike

Michael Yount · Dec 11, 2004

JT - I've never hooked one up to mine - don't know how often it can alter the voltage it's 'sending'. So it seems your education is right up there with mine. If you don't have Probst book on the Ford EEC it's a good buy. Great sections on the O2's and the entire emissions sections.

Na5.0HO · Dec 11, 2004

Just thought I'd throw my 2 cents in.... the O2's work like batteries. They're energized by the heat from your exhaust, which is why the car needs to be in closed loop to get an accurate reading (they need to be hott) So, I would reccommend checking them for a large amount of carbon buildup. See if you can clean them off a little. As stated above it's only a (.1 - .9v) reading so every little thing matters.

As for the possible vaccum leak... if you think that's the problem you can always let your car cool... temorarily disconnect the fan or unbolt (not necessary, your choice)... and carb n' choke around your intake. Look for bubbles and listen for any change in idle. Worst case you get a shiny intake when u clean if all off.

Have fun and good luck,
Jason

jrichker · Dec 11, 2004

Been there & did that...

My trusty 28 year old Radio Shack Analog voltmeter says between .1 volt and .7 volt on a 1 volt scale while the engine idles. It is a 50K Ohm per volt input impedance. Not as good as a Simpson 560, but much cheaper.

Yes, it did switch back and forth between .1 & .7 volts many times within the 30 second widow I observed.

The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. Measuring it at the computer saves you from having to jack the car up, put the jackstands in place & crawl under with with the engine running. OUCH! those cat converters are HOT!

Michael Yount · Dec 11, 2004

As far as cleaning them - the ceramic tip is shielded by a metal housing with slits in it. There's really not much you can do to clean the business part of the O2; and the ceramic is quite sensitive to contamination so be careful if you put anything on them at all. If they've got more than 50K on them, they simply need to be replaced.

Oh - and most of the Stangs have the later heated sensors. The sensor is preheated by the system to speed movement into closed loop. Instead of it taking 90-120 seconds for the exhaust to get the sensor up into the 500-600F range, it can get the heat it needs to operate as quickly 15-20 seconds.

HISSIN50 · Dec 11, 2004

thanks Joe and Michael for all that.

to add to Michael's last post - while the HEGO's are pricey, they are generally able to last longer than EGO's. i say this only for general info for future searchers - people have had ideas of trying to retrofit cheap EGO's. not a good idea, IMHO.

leakyfaucet · Dec 12, 2004

Okay, I'm trying to test at the ECU for o2 sensor voltages. I don't see where to put the positive prong. There is a large gray female connector that goes to the ECU. I'm assuming it needs that plugged in to run, so how would I test for voltages?

Pierce the insulation on a wire?

giddyup306 · Dec 12, 2004

leakyfaucet said:
Pierce the insulation on a wire?

Use a gorilla snot connector if need be.

leakyfaucet · Dec 12, 2004

What's that?

giddyup306 · Dec 12, 2004

It's a crimp on deal that has a hole for your lead filled with silicone. This will prevent oxidation. GM sells them. I don't know what they're officially called though.

leakyfaucet · Dec 12, 2004

Hmm. So I shouldn't even try this without that? I guess you can get this at a Chevy dealer?

Also, I was looking at this test:
http://www.veryuseful.com/mustang/tech/engine/images/o2SensorTests.gif
And it appears to be for a newer mustang, I think. My connectors only have 3 prongs. What should I do?

Oxygen sensor voltage?

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