Friends - 1989 W124 wagon, 220K, well maintained, new fuel relay and pumps/filter (relay had failed) and OVP (precautionary) recently.

Car is dying at speed and once or twice at idle. Like it's losing spark (e.g. different than when fuel pump relay would cut out). Sometimes if you coast it will 'catch' and you keep going, more often I pull over, turn off ignition, sit for 10-30 seconds, and it starts right back up and off we go. Averaging 1-3 times per day of this happening.

Car has original O2 sensor, check engine light comes on then goes off the past 2-3 months, I have a replacement but haven't had/made the time to put it in. Can a failing/failed O2 sensor cause the above? Based on my experience I'm thinking either the coil or crank/flywheel sensor. Where would you start?

THANKS many times for any advice.

Brian

No I don't think a failed O2 sensor will cause these symptoms. An intermittent problem in the ignition system is a more likely culprit and the coil and crankshaft position sensor would be prime suspects.

A problem like this is very difficult to troubleshoot because by the time you get some test equipment hooked up it goes away.

Duke

Thanks, that's what I've been gathering since I've read some more posts. There is no CPS listed on FastLane, is it the reference sensor located at the crank or the one on the flywheel - IIRC one is connected to the diagnostic port.

THANKS

Brian

Don't know if 89 MB is similar, but replacing the Hall Effect transistor in an 89 Volvo stopped the same problem.
Mark

The crank position sensor, reference sensor - whatever it's variously called located at the left rear of the block that reacts to the flywheel segments is the one that is part of the ignition system.

The TDC sensor at the front of the engine serves no purpose whatsoever for engine operation. The lead terminates at the diagnostic socket. Its signal is purely for some piece of test equipment. I suspect that since it provides a TDC signal it is a reference so the test set can read out spark advance.

Any permanent magnetic device that provides a voltage signal will slowly lose magnetic flux over its life and eventually will not generate enough voltage to operate the system, so even if the sensor doesn't fail in some way, it will eventually effectively "wear out" electrically.

I recall from reading service information that there is a voltage spec range at a given engine speed, and if the sensor doesn't generate voltage within that range it should be replaced. One would need a sensitive inductive pickup to read the voltage signal as I recall it is not more than a few volts.

Duke

wear out?? Not enough so that you would notice, per what I read. In ten years, it MIGHT lose 1%, not exactly a big loss.

There's three main ways that magnets can degrade:
1) Heat: but it must be taken to its Curie temperature.
2) Demagnetizing field: As strong counter productive field could cause it to lose strength.
3) Shock: The magnet is violently struck to cause it to lose some of it's magnetic properties.

There's lots of heat in the engine but it doesn't get to . . . 1900degs F. . the curie temperature.

The magnet on the fly wheel can have a very slight effect, MAYBE the 1% degradation number.

The 'shock' levels in a car come nowhere close to changing the properties a CPS sensor.

You are adamant about NOT replacing a high mileage O2 sensor, yet you think that a CPS should be replaced. In the eight years that I 'put-in' at my tech's shop, I have NEVER seen a bad CPS.

For Taboo: " . . car has original O2 sensor, check engine light comes on then goes off the past 2-3 months". Check the codes from the DM.

__________________
Regards . . . . JimF
-------------------
'94 S500 Cpe

Visit my Mercedes Web Page

First Law of Thermodynamics: Energy is conserved!

Everytime the flux field is interrupted by the flywheel segments, the pickup produces a small voltage. The input impedance to the EZL is high, but not infinite, so a slight amount or current and thus, energy, is transfered and this will eventually consume the pickup's magnetic flux. "Permanent magnets" aren't really permanent, depending on how they are used.

Whether the reduction in magnetic flux in, say 150K miles of driving, is one percent, ten percent, or fifty percent I don't know, but it may be an issue.

I don't believe in replacing parts unless a reliable test shows that they are faulty, but it sometimes takes fairly expensive test equipment to run the tests that the average DIYer may not own or have access to.

A scope can be used to monitor the output voltage wave form of the 02 sensor, and given the cost of an O2 sensor, I believe they should be tested rather than replaced, apriori. A professional should have the equipment and testing the O2 sensor only takes a few minutes, but my suspicion is that many shops don't bother to look at the O2 sensor output waveform before replacing them. Many shops just want to replace parts rather than doing diagnostics. Even some dealers fall into this category. We here these stories frequently on this board.

Likewise, the Mercedes service literature has a voltage at engine speed spec for the crankshaft position sensor, and a scope should be able to handle this test, either with an inductive pickup on the CPS signal cable or directly connecting the CPS signal cable to the scope. And such a test will only take a few minutes.

I've never tested my CPS because I have no reason to suspect it may be faulty, but I have inspected the output waveform of my O2 sensor.

Duke