OK I would really like to test my oxygen senor that I adapted to my '88 190E 2.3. I bought the Bosch O2 sensor that supposedly fits 190's by just cutting the wire connectors and using the ones from the old sensor soldered.

I got my Craftsman multimeter ready but I dont know the specifications and where to hook the +/- leads. Any comments appreciated, thanks.

If its a three-wire, two are for heating, and one is for the current from the sensor to the engine computer. Hopefully you hooked those up correctly.

What you should do now is connect your meter to Pin 3 of the diagnostic socket and use the duty cycle reading on your meter to adjust the mixture to 45%. There are literally hundreds of posts on how to do this.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

If you disconnect the signal lead with the car fully warmed up it should read greater than .45V. The best test is to hook up a scope in parallel with the connected sensor so you can inspect the actual waveform. It should cycle between about .15V and .85V about one to two times per second at idle with a nice crisp jump between the two voltage levels.

A proper duty cycle also indicates a good sensor, but an anamolous duty cycle could indicate several problems.

When I do diagnostics, I usually test the O2 sensor first, and then look at the duty cycle.

Duke

What if it's a 5-wire? What is the 5th wire for?

TIA

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

I have tested by piercing the signal wire with the engine off so you do not short anything. Get past the outer shielding and into the center wire, if you have one of those little Hook type ends for you red lead of your VOM they work great. Then ground the black lead to your seat bolt and set meter to DC Voltage. Then start the car and it should be not moving and a steady .45 ish readinng until it fully warms up and the CIS-E system goes to closed loop operation. Then you will notice the voltage swinging properly, if the engine is cold you will most likely get a rich reading, over .45V. If you are talented enough go for a drive will glancing at the VOM once in a while. Once at speed you will notice it go up and down around the .45 reading. When accelerating it will go lean briefly and then to rich. When at WOT it should be above .8V until you let off the gas. This would indicate that your EHA and AFM are working properly. I actually set my EHA and IACV using the O2 reading. Got them back to where they should be after I screwed with them stupidly chasing down a rough idle and poor fuel economy. That was when I first started with the 190 before I realized there were other things wrong. Good luck.

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

Poking holes in the wire insulation and driving while concentrating on something else. Why didn't I think of that?

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1989 300 SEL that mostly works, but needs TLC

No worse than looking at the radio when driving. I had my VOM in my hand at the time and up by the steering wheel. As for probing the wires, sorry but that is all you can do and it is done by many more people than I and has been for many many years. Mechanics, Electricians, Engineers, and DIY'ers!

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

This is doing something whilst being aware of multi tasking.........most idiots who run you off the road are multi tasking whilst in IDIOT mode !


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[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

So who said looking at the radio while driving was okay? Once familiar with the radio, most common tasks can be performed without looking. It's hard to read a multimeter by touch.
Bullpoop. There are any number of methods for taking readings without harming the circuit. Damaging the insulation is just plain stupid.
If a million people do a foolish thing, it is still a foolish thing.

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1989 300 SEL that mostly works, but needs TLC

If you don't have the instrument ( s ) to obtain an O2 sensor reading via a plug-in, I have been tought, the correct way is to back-probe into the connector, ( a safety-pin works well ) or, if you absolutely must pierce the insulation, you should seal that spot after.

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2007 C 230 Sport.

Gee when I went to school and became and EE I was taught how to do these neat little tricks. It is called improvising. Also note that looking at a VOM while in your hand next to the cluster is the same as checking your speed, temp, fuel level, RPM's or even the clock. If you can not look at your speedo and still see traffic with your peripheral vision then you should not be operating a motor vehicle.

Sorry but any moron can check there speed and drive at the same time, so why not look at a VOM digital display. And I am not saying do this in rush hour traffic.....

As for piercing a wire, it is something that is common practive in the electrical industry, otherwise they would not make and sell regular wire probes that pierce a wire so you can check for voltage, like the standard little tester with a lightbulb in it. Now for higher voltage no you do not pierce the wire. Like when I was working on TV transmitters with over 5KV going through wiring, no you never pierce the insulation. At 12V it is ok to do without causing injury to yourself. Albeit I have been zapped by a wire with ****ty insulation on it and 7KV going through it. Those wire are tested by sliding the connector out a hair and probing at the connector. Unfortunately the way the MB connector for the O2 is you can not slide it out and probe without breaking the connection and I am not buying an MB Breakout Box to test the O2 once or twice. So you must improvise.

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

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1989 300 SEL that mostly works, but needs TLC

You're not following me.
When I say back-probe, I mean you leave the connector connected, slide a safety-pin into the connector WITH the wire, connect your DVOM to the pin and read the values.
As for re-sealing pierced insulation, it doesn't take much moisture to " creep " into the hole & spread up the wire.
Remember when you're dealing with VERY LOW voltage readings, 0.1 volt, etc. or computer GROUND signals, it pays to be anal about sealing any " wounds ".

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2007 C 230 Sport.

Well it is inside and I seal it well. As for back probing I know what it is. The connector on mine was just a pain in the ass to do it with. No room to get around the sealed connector they used on my 190E. On other cars I would have done this since it is normally easier. But alas MB used a connector that did not easily facilitate this. Also note that my wire was pierced inside the car under the floor mat where no ones feet or water should ever be near. And also why I used a smaller probe. Hell sometimes you have to do what you have to do to get the test results. Barring buying an MB break out box to test one wire. And it is very evident under the hood that I am not the only one that has ever pierced a wire with a probe. I had to fix many that whomever worked on the car had not ever sealed, let alone used a piece of shrink wrap or tape.... And hey my car has 172,000 miles on it and is 18 years old and I paid a whopping 800 bucks for it. So I am not too worried. If the wire fails I will solder and shrink tube another piece in there.....

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

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1989 300 SEL that mostly works, but needs TLC