Please HELP...560SEL W126 Stalling: Oxygen Sensor Reads Negative Voltage........

[Dionysius]

PLEASE HELP.

1986 560SEL W126 Vehicle developed stalling problem. Drives perfectly but intermittently loses power and stalls. RPMs drop way down. Gassing has no effect. Highly dangerous and could be a road rage inciter.

Replaced fuel filter with no change. Then disconnected the oxygen sensor in the RH footwell. Installed voltmeter between signal wire from the O2 sensor and ground. Grounded the signal wire going to the injector computer module. Car now drives in open loop (since O2 sensor input is grounded) without any problem and I am monitoring the output voltage from the O2 sensor. Believe it or not the voltage ranges between -0.1 (lean) and -0.9 (rich).

NO....the leads are not reversed on the meter. Trust me on this.

I have NEVER heard of an O2 sensor outputting the correct range but with negative polarity or even with neg polarity period.

Can anyone here enlighten me as to what is going on. I will in time change out the sensor to see if it has been poisoned or has otherwise failed. The sensor heater is working to spec.

The sensor has only 5000 miles on it although my Bosch time warranty has expired timewise.

Engine emissions and condition are all to specification.

PLEASE HELP.

[Elektri]

And what is the EHA amperage doing (mA)?

Lambda (Duty Cycle %)?

[With oxygen sensor connected / disconnected?] *

Is that a heated Oxygen Sensor with separate wires for the heated portion?

* The car should still run with the oxygen sensor disconnected. Or with other CIS-E sensors not functioning/wires shorted/disconnected - Called "Limp Home Mode". The computer replaces the "improbable value" with a midline value which should allow the vehicle to still operate. More and more of these features were added to later model cars. I don't know which years had which limp home features? (Or if earlier years had no such features on the CIS-E computer or not?)

[Dionysius]

And what is the EHA amperage doing (mA)? It is correct at 8 mA mid range

Lambda (Duty Cycle %)? It is correct - 70% idle, goes to lean as it should with O2 disconnected (open loop). With O2 connected when it switches to closed loop it stalls. See my description above.

[With oxygen sensor connected / disconnected?] *

Is that a heated Oxygen Sensor with separate wires for the heated portion? Yes....2 white wires for heater, 1 black for signal.

* The car should still run with the oxygen sensor disconnected. Or with other CIS-E sensors not functioning/wires shorted/disconnected - Called "Limp Home Mode". The computer replaces the "improbable value" with a midline value which should allow the vehicle to still operate. More and more of these features were added to later model cars. I don't know which years had which limp home features? (Or if earlier years had no such features on the CIS-E computer or not?) It appears that the negative sensor readings will not allow the limp home to operate. It runs in open loop until heat up but after switch to closed loop it loses control and will stall. If I disconnect the wires to the EHA the car will now do the limp home. Like I state this strange negative response from the sensor kills the limp home capability and this surprises me. I will know the picture when I change to a new sensor.

[Elektri]

Well EXCELLENT troubleshooting! You've got this problem "surrounded" and limited to the sensor/CIS-E/EHA.

And good next step trying a different O2 sensor - what I would try next. Correct part number parts?

Be aware the power for these things probably comes from the Over Voltage Protection Relay - A "voltage surge" protective device.

P.S. So far as I know, the power to the O2 sensor should ONLY heat it up so it starts working quicker - should have NOTHING electrically to do with the electrical signal it outputs?

[pmckechnie]

Dionysius,
I saw a problem like this one time (many years ago) that drove me crazy. O2 readings were negative. I have also seen them too high. I got the car from another shop that just gave up on it. It turned out to be a bad ground for the ECM. Now this was a Buick if I remember correctly. GM back in those days got the ground for the ECM from the engine. The ground at the ECM itself was a few tenths of a volt higher than at the engine. The ground for the O2 sensor was at engine ground.
I am just mentioning this as food for thought.

PaulM

[JamesDean]

Try disconnecting the o2 sensor from the CIS unit completely. Connect your meter ground to chassis ground and then observe the sensor output, see what you get.

[oldtrucker]

Yep, I think that some of the inputs above point to actually a floating ground, or somewhat at least.
Completely isolating the O2 should reverse polarity.
In this case the new / replacement O2 sensor would do the same thing.
Got to find the bad ground, good luck on that one....

[Dionysius]

I do concur that a floating ground could be the cause so I must next eliminate that. I will first rework the engine chassis ground lead and verify good metallic continuity. Next I will ohm out the sensor body to both engine ground and chassis ground at the neg batty terminal and look for any differences. I will then loosen/tighten the sensor threads and remeasure. If all this does not yield success I might jump direct copper from sensor body to neg batty and repeat ohm test.

If all above leave me still with the problem I will pull the sensor and bench test it with a propane torch plus stress it some with a little vibration.

At that point if sensor itself proves bad I will pony up to the bar and install a new replacement.

It might take me a couple of weeks but I will report back my findings.

I thank all of you who have posted so far and please chime in if I am omitting any step. This is a great site and I am so impressed at how helpful all of us owners are to each other!!!

[porkface]

what is the voltage at the green wire when key is on and sensor unplugged?

any change in sensor reading after unplugging the 2 pin heater?

the unplugged sensor should make the duty cycle freeze at 50%. good luck, chuck.

[Dionysius]

First to answer some questions. Disconnect of the heater wires had no effect on the measured signal from the sensor. Thus no cross-noise present. Next I verified the chassis-engine grounding and it proved to be very good.

Next step was to remove sensor and propane torch test it. The propane bench test repeated the findings of negative readings: -0.1 to -0.9. The sensor "battery" has shifted its "ground" reference by approx -1.0 volts.

A negative sensor signal would drive the CIS-E to a hard rich condition and this is the problem I experienced. This was causing engine RPM reduction and power loss to the point of stall.

I have never seen this failure mode reported for an O2 Sensor but I am not an expert here. Any Bosch technologist out there care to confirm that this does occur and indicate how/why it occurs??. Also could the computer module have been designed to be smart enough to recognize this condition (continuous hard neg O2 sig input in presence of high RPM and at operating temp with forward motion) and force it to the limp home mode. What it did could have caused a major catastrophe were I in the Lincoln Tunnel or other such compromised situation. If Bosch wants the failed sensor I will send it to them. I have verified "limp home" mode by pulling the input lambda sig to the EHA. That is what I am running with now. What I got from the neg failure mode was NOT limp home....it was a complete stall. This is a drive critical and a safety concern. IMHO an O2 sensor should NEVER send a vehicle into a stall. Let me know if I am alarmist here.

More to come after I install replacement.......

[pmckechnie]

I can't say what would happen if the O2 would go negative as I have never seen it. I have been working on these types of problems since around 1980 when most domestic car started using them and I have never seen a negative voltage come out of an O2 sensor, but one thing I did learn is "Never say that is impossible, because if you do, you will be proved wrong". Again most of my experience is with domestic and very little on euro cars, but most cars used Bosch sensors, like yours. A domestic car won't go into closed loop mode until the O2 sensor shows activity of some sort. I would think staying at or below 0v would keep closed loop from happening.

If you reconnect the EHA but unhook the O2 will the car run reasonably well?

[Elektri]

The CIS-E computer is "analog" (as opposed to digital). And perhaps this would allow backwards or upside down signals to do strange things? There is mention of a microprocessor below, but that may be something added as the years went by? (More and better fault detection - learning curve thing.)

Also I can imagine they would not initially think to test for such a situation because they would never imagine it happening.

So far as "implausible results" (term they use in the factory service manual and Bosch KE footnotes on this [barely mentioned anywhere]), I've not been able to find very much written on this, but from what I have read, mostly it seems to be open circuit, short circuit, or a sudden change in value / not possible [intermittent connection].

Anyway following are a couple of footnotes I've found on this. Keep in mind that the notes below may only apply to later year KE control units and some things mentioned may not have been implemented in earlier units???

A footnote on this...
[y) Emergency running (plausibility)
The signals supplied by the peripheral components
to the KE control unit are continuously analyzed. If
the composition of the signals is no longer logical, a
substitute value (emergency running property) is
formed automatically for the non-plausible signal by
the electronic control.
If, for example, a rapid change in temperature is
simulated - engine at normal operating temperature
and coupling of coolant temperature sensor then
unplugged, the microprocessor in the control unit
compares the momentary temperature with the
stored temperature. The control unit detects open
circuit (short-circuit) and programmes emergency
running mode. The control unit does not detect a
slow change in the characteristic curve
(characteristic curve drift).]


Another footnote...
[With the ignition switched on, the
input signals to the KE control unit are tested in the
static state of the components. The faults are not
stored and can only be displayed immediately (i. e.
only if the fault exists at that moment) by measuring
the on/off ratio at the diagnostic socket (X11). This
output mode can also be used for a rapid check of
the signals monitored in this case. For measuring
on/off ratio and fault table see Diagnosis Manual
Engine.

When the engine is running, faults which occur and
are recognized (e. g. open circuit, short-circuit and
implausible values) are stored in the KE control unit.
Loose contacts are also recognized as faults and
stored. The stored faults can be read at the test
coupling for diagnosis (X11/4) with the pulse
counter. A condition is that the battery has not been
disconnected in the meantime otherwise the stored
faults have been erased.}

[Dionysius]

I replaced the O2 sensor and problem resolved. I repeated all measurements and even replaced the failed sensor to verify my findings.

I want to thank you all and a special thanks to Elektri for the info in last post. I always assumed that the CIS-E module was microprocessor driven. Analog computers were vintage technology by the time 1986 had rolled around. It is indicated from what I have discovered in my 1986 unit that no range limiting is done on the O2 input signal because the negative values sent it into a stall when it switched into closed loop.

[Frank Reiner]

Originally Posted by Dionysius

I replaced the O2 sensor and problem resolved. I repeated all measurements and even replaced the failed sensor to verify my findings.

I want to thank you all and a special thanks to Elektri for the info in last post. I always assumed that the CIS-E module was microprocessor driven. Analog computers were vintage technology by the time 1986 had rolled around. It is indicated from what I have discovered in my 1986 unit that no range limiting is done on the O2 input signal because the negative values sent it into a stall when it switched into closed loop.

With all respect to Elektri, KE-Jetronic does use a digital controller. K-Lambda (with the remote frequency valve) was the only K-Jet system that used an analog controller.

[Elektri]

Originally Posted by Frank Reiner

With all respect to Elektri, KE-Jetronic does use a digital controller. K-Lambda (with the remote frequency valve) was the only K-Jet system that used an analog controller.

"The engine control unit (ECU) may be either analog or digital" [KE-Jetronic]

Above from...
Jetronic - Wikipedia, the free encyclopedia

Note: I would be happy to learn more about the various KE-Jetronic control units, but very little is written about this. I even have a Bosch KE-Jetronic video and a Bosch KE-Jetronic technical instruction paper, but most of the information in those is on the mechanical parts of the system.

I've also been keeping an eye out for a spare CIS-E control unit (ECU) as I get parts at wrecking yards and it seems EVERY Mercedes I come across has a different part number control unit.

I have no idea what the differences are in the various control units - ECU's?