More 380SL Blues ... Lambda

[ctaylor738]

My on/off ratio on my '81 380SL sits stubbornly at about 10%. This occurs whether the engine is cold or hot and does not change with RPM. When the engine is cold, it should read a constant 60%.

In addition, the air pump does not engage.

I have verified the following:

- the control unit is getting 12V
- the control unit is grounded
- the O2 sensor is putting out .4V
- the oil temperature sensor is working correctly
- the throttle switch is working correctly
- the frequency valve does buzz

Adjusting the mixture has no effect on the ratio.

The car is running well and getting reasonable gas mileage.

My conclusion is that the lambda control unit is dead. Any other ideas?

TIA

[Pili380SL]

Chuck, sorry to bother you again...one more question pertaining to the control unit. I looked under the dash, passenger side above the footwell, and I don't see anything quite like the control box in the picture. is it about 3 1/2 to 4 inches across? 12 pin, two rows of six? I see the designation 2,4,6,8,10,12 for the pins in the row closer to me as I'm looking at it. The mounting of the box doesn't look the same...it's mounted right at the firewall, connector side down? Thanks again
Paul

[ctaylor738]

On the '81, it is in the passenger footwell floor under the carpet on the slanted part - not sure where they put in the '83. The CD should have Component Locations in the Electrical Troubleshooting Manual, though.

[Pili380SL]

Ah yes, component locator. I brought the CD to work today as my PC last night was giving me fits. I see it now. Thanks again, sir.

[fahrgewehr2]

I know about 10%

[ctaylor738]

Thanks for the suggestion, but you are one CIS generation ahead of me. This one is almost completely mechanical. There is no airflow pot and the on/off ratio does not indicate fault codes.

The electronics consist of a idle speed relay that manages an air valve to control idle speed, and a lambda controller that controls the mixture via the frequency valve.

[brewtoo]

Originally posted by ctaylor738
There is no airflow pot and the on/off ratio does not indicate fault codes. The electronics consist of a idle speed relay that manages an air valve to control idle speed, and a lambda controller that controls the mixture via the frequency valve.

Hmmm....must be a FOREIGN car.

[fahrgewehr2]

Thanks for the suggestion, but you are one CIS generation ahead of me. This one is almost completely mechanical. There is no airflow pot and the on/off ratio does not indicate fault codes.

The airflow pot had nothing to do with my 10%. The DC fluctuated at 10%, so it was not a code. It was my ECM that was bad.

[brewtoo]

Originally posted by ctaylor738
My on/off ratio on my '81 380SL sits stubbornly at about 10%. - the O2 sensor is putting out .4V

Does it vary around 10% or is it fixed?

The O2 sensor...does it put out a variable voltage or is it fixed at .4V all the time?

[nthach]

Originally posted by brewtoo
Does it vary around 10% or is it fixed?

The O2 sensor...does it put out a variable voltage or is it fixed at .4V all the time?

O2 sensor reading should fluctate depending on amount of oxygen in the exhaust. Is your DMM on mV mode?

[ctaylor738]

fahrgewehr2 -

Sorry about that - the link you sent me was a VERY long post and now that I have made it to the end that I see what you mean about the ECM.

I believe that the sensor voltage was fluctuating, but will re-check the today. It was also suggested that I check to see if I get the 10% reading with the sensor disconnected.

[ctaylor738]

I got out my DMM to re-test the O2 sensor voltage last night only to discover that it would not read DC volts even with a new battery. So off to Sears, where I discover a DMM with Duty Cycle on sale for $29.95.

Take a break from my day job and test the O2 voltage - very low, fluctuating around .005v. No change in the on/off with the sensor disconnected. Still sits at 10%. This reading is coming from the dwell setting of my Sears engine tester which I have used to test my 300E, Hbofinger's 560, and several other cars.

But, since I have this nifty new DMM, I decide to try out the Duty Cycle function. Lo and behold, I am getting a fluctuating 55-65%. Still no change with the mixture adjustment, though.

So now I am thinking 1) the dwell method does not work on this system for some reason, 2) maybe the brain is still alive, 3) the O2 sensor is possibly/ probably dead.

So - I will order a new O2 sensor and try this again. It's not clear why this one would fail in just over a year and a couple of thousand miles.

[porkface]

before you buy a sensor, check the voltage at the unplugged sensor lead, from the brain. key on should be 5 volts, if 0 then the 02 has nothing to work with.


off topic, where in fc? me, refugee from ffx, grand parents in brilyn park from 1947 to 1998. 66 cut their street in half. good luck, chuck.

[ctaylor738]

That's a new one on me. I thought that the chemical reaction in the sensor created a low voltage and transmitted that to the brain.

Please explain further.

I did ground the wire to the oil temp sensor, simulating a cold engine. This caused the brain to go to a constant 60% like it should, so maybe it's still got some intelligence. It warms up and then (today) fluctuates between 60 and 70%.

We are just off Sleepy Hollow Road, close to Congressional School, just west of Seven Corners.

[porkface]

the o2 needs a reference voltage to start with. the reaction in the exhaust just makes the voltage change. the o2 itself doesn't generate voltage, otherwise, we might have a new power source.

ok, off annandale rd past 50. what was the name of that ice cream stand on the corner? go by on the way to 7 corners at any time and there was always a line, day or night. bunch of us used to hang out in a little bar at loehmans and run down there for a cone. closed a few years ago. pity. good luck, chuck.