The Check Engine Light came on my 1990 300E Base Model, 3.0L.

I have a California Model so it consistantly blinks Code 17 or Code 18, occassionally both.

I replaced the Oxygen Sensor and the Idle Air Control valve - nice new components. This didn't fix it. I adjusted the air fule mixture with a DVM until I got half the battery voltage. This worked fine for about 10 months, no Check Engine Lights.

This week the Light came back on with the same 17 or 18 code displays. However, this time I only get 6.12 volts between pins 2 and 3 of the X 11 connector with the key on and the engine running or key on with engine not running. The mileage is pretty much the same, about 17.8 mpg and runs fine.

I swapped over voltage protector, as I had a spare and nothing changed in the voltage.

Has anyone seen this before and what can be causing the voltage to drop in half like that?

Thanks

Code 17 is for the O2 sensor, & power from the MAS relay to the heater was an issue on many cars.

Check to see if you have 12V to the heater with the car running.

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Thank You MBDOC. I am getting 13.38V DC across the pins of the two white wires in the sensor connector with the engine running.

Also cleared the code 17 and took the following voltage measurements at connector X11, pin 2/3:

Battery Volts 12.3V
With Key off, X11 pin2/3 voltage was 8.48v
With Key on, X11 pin 2/3 voltage was 6.94v
With Engine running in open loop X11 pin2/3 voltage was 6.94v for about a minute.
With Engine running in closed loop mode x 11 pin 2/3 voltage dropped to an oscillating 3.7 to 4.2v

Tracks with 50% of high volts?

Ran the car today. The check engine light stayed off while driving in town. Within five minutes of getting om the interstate, the check engine light came on. Code 17.

Still looking for the cause of the problem.

Any ideas?

For the benefit of other, I will pass on the solutions to my code 17 problem.

I decided to clean the contacts with contact cleaner and 800 sand paper on the ECM and connector. I did the same with MAS.

After doing this I again found 12.74 volts at the X 11 connector, pin 1 and 2 with the key off.

When the key was turned on voltage decreased to 6.74 volts. I started the car and after about a minute of 6.74 volts, the O2 sensor kicked in and the voltage oscillated between 1.2 and 2.4 volts. I adjusted the air fuel mix until I obtained about half of the high voltage of 12.74v. I set the air fuel mix voltage at 6.35v. I ran the car for 300 miles without the check engine light going on. I think I'm done.