1994 C280 -- New Wire Harness -- STILL won't start
 

This all started several months ago. Occasionally, the car wouldn't start. It only has 60k miles and runs great. It cranks just fine, just doesn't "hit". This might happen once or twice a week. Generally, when this happens, if you try it a few times (sometimes 5, sometimes 30) -- it will finally "hit" and start up immediately.

It has NEVER stalled and run quality is superb. It is almost like you have all of the spark plugs unplugged -- then plug them in and crank it.

So, I took it to the dealer. After much diagnosis, they got a code for the crank position sensor and replaced it. All was fine for 3-4 days, then, same old thing.
Took it back. After 3-4 days, they said 2 things:
1 -- They were getting a code for the rear oxygen sensor.
2 -- The wire harness needed to be replaced at a parts and labor total of $2,200. :behead:

It is my opinion that the sensor would give your run quality issues but not the intermittent starting problem. The harness, though, could do just that.

This weekend, I replaced the entire engine harness. Part from Phil at Fastlane for $640 and 3 hours of my labor. Later in the day, after about the 10th start -- it did the exact same thing again!

Guys -- what can this be? I did read somewhere that it could be the OVP and that you might try tapping it when this happened. It happened again today and I tried it 10 times or so to no avail. Went and tapped the OVP and it fired right up!? :dizzy2:

Could it legitimately be the OVP or was that coincidence?

Any productive thoughts or suggestions are appreciated.

I'd replace the OVP if it is the original part #.

It is a known bad part.

http://www.peachparts.com/shopforum/showthread.php?p=732944&highlight=blues#post732944

If your relay is "Siemens" , change it.....

It is a Siemens, so I will.

Now, how about my O2 sensor question? Could that possibly give a symptom like I have? I can't imagine it would.

I'd replace the OVP whether it's a Siemens or not, old OR new part number. The "new" OVP can still fail, causing intermittent crank/no-start problems. I've seen this happen many times. Just because it's the lateat, greatest OVP doesn't mean it can't go bad.

how did the old harness look???

still got it?

It looked pretty bad. Very "crispy" and brittle. A couple of the connections cracked upon removal. Saw many exposed wires. This car has only 60k miles.

Only took me about 3 hours to replace. 1 hour of that was figuring out the big hookup just behind the left headlight. It fed 4 different wire pin connectors to the new harness. 1 of those 4 wasn't in the new harness and getting that one was a pain.

Yes, I still have it.

Can anyone comment on my question regarding the O2 sensor?

The rear 02 code should clear after you solve the problem and the cat heats up and burns off the mix. ..
Again, I would Change the OVP.,

Phil has the OVP on the way. New part #, Siebel brand.

Explain your...explanation, above, please. Are you saying the rear O2 is not now working due to the OVP?

Your cars Engine Management Sytem has not been working , either due to wire harness or OVP voltage supply interruptions..[ or both]...so, the cat gets soaked and the rear 02 sensors job is to monitors the cats output. It fails the test and pops the code.... when the engine management is to spec , the cat will burn off the remaining crap and return to a normal specs.

This can also be accomplished by a hard run , under load [ read hill] AFTER the cause/s have been rectified .. [ with the OVP being the FIRST suspect]

It is quite common to see many parts changed before the OVP gets even looked at. With their notorious fail rate, this has always amazed me .. Specially when the owners have used a Benz Dealer .. just too hard for me to comprehend how this can possibly happen........but if you do a search, one will see it happens consistantly. I would venture to say that the OVP has caused more guys $2K due to mis-diagnosis than any other simple $60 , plug-in part.
I am one of those old school techs who, when we percieve an electrical power feed malfuntion, start at the front of the feed line [ Battery supply voltage] and work foward. The next stop , of course, is the OVP.. Now , I guess we could start at the other end [ The ECU] and work backwards to the voltage supply, but I am just lazy and want to find the problem as soon as possible.........meaning I am not as concerned about racking up the bill as I am at solving the problem.
Any good tech dept not only knows of OVP problems, but also has a known good OVP in their toolbox for rapid check/verification.

Ok, Arthur, you were exactly right. Two weeks and no starting problems whatsoever. It was the OVP that was causing the problem.

This leads me, though, to a few questions:

1. The check engine light still hasn't gone away as you thought it would. When checked before, they said it was the rear cat O2 sensor. Runs perfectly now.
2. Mercedes Dealer charged me for r/r the crank sensor when they couldn't figure out what was wrong. As I mentioned earlier, that didn't help one bit. Now that I figured out what was wrong, how would you suggest I approach the dealer? I mean, I don't expect them to give me a refund necessarily, but a labor credit toward a next visit might be reasonable. $500 for the part and labor.

Thanks, P

Bump, anyone have any suggestions regarding my previous post?