Bad O2 sensor? Fault Light not on.
 

Hello,

I noticed an increase in gas consumption in the 300E '86, so I decided to check the O2 sensor and the diagnostic ports. After reading few posts and comparing the test results, I thick I have a bad O2 sensor. However The fault light is not on. It does light for few seconds when the ignition key is on, which proably means the light is OK.

Test results:

Key on , Engine off :
duty cycle 70%,
EHA +19 mA

Engine idling at operating Temp:
EHA gradually increased to +13mA.
O2 output fluctuates around 1 mA
Duty Cycle 92.3%
O2 heater plug : 14 V

The voltage at the ECU plug of O2 sensor is about 470 mV. Is this normal ?

I applied 1.5 V to ECU plug of O2 sensor (connected to pin 8 on ECU), EHA current gradually increased to -12 mA.


Either O2 is OK and my mixture is way too lean (but I think this would cause a rought idle which I do not have) or O2 sensor is bad, always showing about 0 mV, and the EHA is trying to correct it by sending +13 mA.

Changing the mixture with the 3mm allen key 1 or 2 quarters does not change the output at the O2 sensor (it remains close to zero Volt). Should I turn it more to see a chage?

I have left the O2 sensor disconnected and using the system in open loop state. The car nuns great but I have no way of knowing whether my mixture is close to optimum.

Unfortunately, I did the same tests on the 260E '88 and I have the exact syptoms.

Can I be sure that it s the O2 sensor before I spend $240 CDN on a new one?


Thank you.

I had a bad O2 sensor in my 89-300ce and no indicator lights showed at the dash. My symptoms where exactly as yours, specifically +13ma to EHA with no response to mixture changes (although the motor responded as expected).

After I spliced a 1990 Ford Mustang 302 O2 sensor ($70cdn) to the Mercedes connector of the original sensor my EHA signal would go to +13ma at start and then slowly drop to -3ma at idle when hot. The -3ma is set with the allen mixture screw and should result in slightly rich WOT (open loop) operation. When the system is working properly the signal to the ECU should oscillate between eg. -3.5 to -2.5 about each second.

Jorg

Thanks Jorg,

I guess I will get the Mustang O2 sensor as well as it is alot cheaper.

Thanks again.

It may not apply to you, since you say that you get a few seconds of the dash sensor light coming on, but on some pre-87 Mercedes, the factory procedure to "reset" an O2 warning bulb that went off via an odometer switch, was to remove the bulb. So, after that initial mileage based warning, you had to keep track of the sensor on your own.

Zorin,

No need to spend $240 Cdn on your oxygen sensor. I'm in Toronto and just bought one for my C280 from www.discountoxygensensors.com for $100 US, which included free delivery to a US address. Then had it forwarded to me by a friend who marked it as a gift with a value of $40 to get through customs. These guys *do* deliver to Canada, but there is an extra delivery charge, and I don't know how much that is. You can expect to get dinged %15 GST/PST at the border, but no duties. Even with that added on, it should still save you about $100, and if you have family or friends in the US who can forward it to you, you can save that much as well.

Good luck.

Jonathan