K6JRF at the mic K6JRF's Page
formerly W6FZC

My Mercedes Benz
S500 Coupe

K6JRF's MB S500 Cpe
(Updated: Mar 5, 2016)

Analyze and Troubleshoot "Check Engine" MIL and Electronic Control Units (ECU)!

On/Off Ratio Testing (Lambda)

Air-Fuel Ratio

Testing Lambda control system with On-off ratio tester
The correct operation of the Lambda control system is the most important control function in the vehicle. The object of this control system is to keep the air-fuel mixture to within 2% of 14.7:1 ratio for all operating conditions. In this way, the catalytic converter can then process the three major pollutants, HC, CO and NOx. The catalytic converter burns the HC and CO to produce water vapor and carbon dioxide. The NOx emissions are 'reduced' and produce nitrogen and oxygen.

The operation of the Lambda system can be checked with the On-off ratio test. It can also be used to identify momentary faults not stored in fault memory. Faults are distinguished between those that occur with the ignition on and those that occur with the engine at idle.

Technic Tool Supply is now doing business as "GermanCarToolPlus". The direct link to their site is here. Their Lambda Tester's direct link is no longer active but appears to be the same as their previous "M0039" tester which I have tested on my S500. It also can used to adjust the CO levels in vehicles that have adjustment capability (S500 does NOT!) Please contact them directly with any questions you may have.

The On-off ratio can be checked with any suitable multimeter that has a "duty cycle" function. There are many availalble at various prices, so I won't recommend one here.

Set the meter for duty-cycle, put the postive lead into #3 pin of the X11 connector on the left side fender well. Put the negative lead into #2. Start the engine and let it warm up. Unless there are other problems the duty cycle should not vary much more than 7% either way of set center, 50% after the car is fully warmed.

For the X11 connector, the O2 sensor is on pin 3; pin 2 is ground and pin 6 is battery voltage (12.5Vdc). This connector is applicable to all Mercedes-Benz models starting from 1980. Note that in 4/94, MB discontinued use of the X11 connector. The signals were moved to X11/4 and are on pins: Pin 13, TN; Pin 14, OS1; Pin 15, OS2.

O2 Sensor Testing
If you suspect that you have a worn/bad O2 sensor, it can be tested both ON and OFF the car. The following test describes how to test it OFF the car. Doing it in this way is usually quicker since the test parameters can be manipulated easier.

Use a high impedance DC voltmeter (VOM) with at least 1M ohm/volt. This will not load the O2's output signal. Clamp the sensor in a vice. Attach the negative lead of the VOM to the case or the ground lead on the connector. Attach the positive lead to the output wire. If there's a "heater" wire (probably is), it does not need to be connected for this bench testing. Use a propane torch with a high flame and place the tip of the inner blue flame on the slotted or perforated area of the sensor. You should see a DC voltage of 0.6V MIN within 20 seconds. If not, the most likely cause is open circuit internally or the sensor has been lead fouled.

If OK so far, remove from flame. You should see a drop to under 0.1 volt within 5 seconds. If not it is very likely to be silicone fouled. If still OK, heat for two minutes and watch for drops in voltage; it should be steady with no drops. Sometimes, the internal connections will open up under heat. This is the same a loose wire and should be considered as a bad O2 sensor.

If the sensor is OK at this point and will switch from high to low quickly as you move the flame, the sensor is good. Any O2 sensor that can generate 0.8 volts or more when heated, show 0.1 volts or less within one second of flame removal, and pass the two minute heat test is good regardless of age.

However, I suspect that many are barely operable with 100K+ miles of on-engine use. I personally recommend replacing it after 60K miles. So does Bosch!

Lambda testing for 1995 and later MB vehicles
In early '94, the MB factory discontinued use of the X11 9 pin connector. For these vehicles, a special adapter cable/box is needed. PN is 900 589 0115 00. This adapter plugs into the 'A' an 'B' connectors on the Ignition Control Unit (N1/3) and allows the necessary signals to be accessed as they were in the discontinued X11 connector. The 'box' end of this adapter has a mating 9 pin plug compatible with the standard On-Off ratio's connector. Note that the needed signals are also available at the X11/4 connector as noted above.

A readout of 50% AND an oscillating needle (engine running) indicates that all input signals and the Lambda control system are ok. Readouts of 0% to 100% refer to particular fault source (see Fault Tables below).

On-Off Ratio Test Conditions for: Ignition ON
- Coolant temperature appox 80 deg C
- A/C Off
- Selector level in park 'P' position

- Connect ratio tester to Diagnostic connector (X11). Note this is NOT the X11/4 connector.
- Ignition: ON
- Engine: OFF
If any value other than 50% appears, refer to fault table for possible causes.

On-Off Ratio Test Conditions for: Engine at Idle and 2500 RPM
- Disconnect vacuum line at the Purge valve (Y58/1) and close it with a suitable plug for these tests.
- Coolant temperature appox 80 deg C.
- A/C Off
- Selector level in park 'P' position.

- Connect ratio tester to Diagnostic connector (X11). Note this is NOT the X11/4 connector.
- Ignition: ON
- Engine: Idle, then test at 2500 rpm
The needle should be at 50% AND should oscillate around that point (example: 42% to 55%). If not refer to fault table for possible causes.

Fault Table: On-Off Ratio Test: Ignition: ON
Ratio %
Possible Cause Remedy **
0% Voltage supply from socket 3 of data link connector (X11/4) open circuit Repair harness
10% Closed throttle position (CTP) recognition inactive Fuel Inj
20% Wide open throttle (WOT) recognition active Fuel Inj
30% Engine coolant temperature (ECT) less than 70 deg C or greater than 110 deg C Fuel Inj
40% Not used
50% Input signals ok
60% TN-signal (rpm signal) or camshaft (CMP) position sensor signal not present Fuel Inj
70% Starter engaged Fuel Inj
80% CAN data exchange defective Fuel Inj
90% Fuel safety shut-off active EA

Fault Table: On-Off Ratio Test: Engine at Idle and 2500rpm

* Needle must oscillate if all monitored signals are ok.
** Consult the appropriate ECU Service Manual for further fault isolation tests.

Ratio %
Possible Cause Remedy **
0% Short circuit to battery in wire to data link connector (X11/4), socket 3 Repair harness
10% CTP recognition applied constantly Fuel Inj
20% Output of fuel injectors or one or more fuel injectors have open circuit Fuel Inj
30% Engine coolant temperature (ECT) sensor (B11/2) faulty Fuel Inj
40% Hot wire MAF sensor (B2/2) Fuel Inj
50% * Oxygen sensor (G3/2) not operational or defective, open circuit Fuel Inj
60% CMP sensor (L5/1) Fuel Inj
70% TN-signal (rpm signal) Fuel Inj
80% CAN data exchange defective Fuel Inj or EA or DI
90% Vehicle speed signal (VSS) Fuel Inj
95% Deceleration shut-off active EA
100% No voltage at LH-SFI control module (N3/1) Fuel Inj

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