Bizzarro OVP problem solved!
 

Just wanted to post the ending of my hard start problem for anyone else that might run into the same that I did. Just to review, the car was very hard to start when cold, fine when warm. When I checked the fuse on the OVP, it was blown. After I had replaced it with a good fuse, the car would not start at all.
As Gilly had guessed, someone had probably tried to fix the hard start problem by fiddling with the mixture adjustment, but to no avail because the fuse was blown and the car was running in closed loop. I adjusted the mixture screw under the airfilter cover, first one way and then the other until I found which way made a difference. It was a good 3 complete turns out I think from where it should have been at, but now it runs perfectly and was very easy to do. Actually the biggest pain was trying to get the bypass and vacuum hose connected back on the engine while at the same time connecting it to the bottom of the air filter cover. I found that it was actually easier to connect the hose on the airfilter cover first, and then connect the others on the engine, as I had a difficult time trying to push the breather hose onto it's much larger mount for some reason. Thanks for all the help.

Steve

Steve:
Glad you got it running better. Let me explain just a few things though:
You stated the car was in "closed loop". This is incorrect. If anything you should consider it open loop. When it is "open loop" it means the car hasn't warmed up enough for the O2 sensor to properly work, and the engine is operating with the mixture set almost exclusively by the engine coolant temp sensor. Once the engine warms up sufficiently, the mixture is being set by the O2 sensor, this is known as "Lambda". What that means is that when the O2 sensor senses "rich mixture", the control module drives the mixture to the lean side. When the O2 sensor then senses "lean mixture", the control unit then enriches the mixture. This is Lambda, once the engine warms up the mixture is set using this Lambda procedure, contantly switching between rich and lean. Being in Lambda operation is also called "closed loop", the relationship between the O2 sensor and control unit is "closed". If the control unit isn't operating in Lambda, because the engine temp isn't warm enough, it is "open" loop, no relation yet between the O2 sensor and control unit.

However what was going on with your car was different. The control unit was essentially "dead" because of the blown fuse in the OVP, so the system has sort of an emergency running mode, which is called "fixed operating mode", or FOM for short. This is different than being in open loop. It's not even taking the engine coolant temp into account, the mixture is a constant, which is why it would start OK warm, but not very well cold, the mixture is set to run best at normal operating temp, that's how it's set up to run in FOM, at normal operating temp, no variation.

Gilly

Did I say closed loop? Of course, I meant open loop. And yes, I do understand the FOM mode, but thought that the 02 sensor still would be sending a signal to the ECU. I guess that without the fuse, the control unit is as you stated, "dead" and no sensor information is being used to determine the air/fuel mixtures. But while in the FOM mode would making mixture adjustments then have any effect on the running of the car? The adjustments that I made were mostly mechanical, not electrical from what I can tell, as it looks like it just mechanically adjusts the air sensor plate height to determine the air/fuel mixture. So I am thinking that it should still effect the running of the engine to some degree even while in FOM.

Steve

No, there is no power running to the air flow sensor when the control unit isn't powered up. The control unit has to provide power to the air flow sensor for it to operate, so it's dead.
To illustrate this, go out to the car now with it working right, with it running and press down on the air flow sensor plate. The car will die. Then do the same with the fuse pulled out of the OVP. It won't change it at all.
When you adjust the mixture screw, you are making a mechanical adjustment to an electrical part (the air flow sensor). If there is no electricity making it through the sensor you aren't doing anything but screwing it up for whoever is the poor sucker who gets the power eventually flowing through it. Sound familiar?

Gilly

PS It's kind of like the old trick of unscrewing the cap on the salt shaker as you leave the resturaunt ;)

Gilly, a followup question. I had my duty cycle on when I depressed the air flow plate. The reading increased before the car died. Could you please explain why the car dies & why the reading increases? Thanks.
Richard

Normally air flow is what causes the air flow sensor plate to go down. The mixture is enrichened due to the increased airflow. If you depress the air flow sensor plate, the engine doesn't "know" it was your finger that did it, it assumes that the airflow actually did increase, the amount dependent on how far you pushed the plate. It incorrectly enriches the mixture, enriches it enough to stall the engine. The duty cycle changing is a reflection on the enrichment that is being done by the EHA (Electro Hydraulic Actuator).

Gilly

So, would the stall suggest that the EHA is being properly powered by the OVP, & that the ECU is getting correct information?

Mmmmmmmm, it would tell you the control unit is working and the air flow sensor is probably "OK", if not perfect. I wouldn't say it's an indication that the on-off ratio (mixture) is set correctly. What did you set the on-off ratio to? Was the adjustment tower "uncorked" on top? You're supposed to replace the adjustment tower if the ball is gone.

Gilly

What would cause the fuse to blow on the OVP. Isn't this an indication of another problem?

TIA,
Bud

The OVP itself is bad, or there was a surge or incorrect polarity when jump starting the car, usually somehting along those lines.

Gilly

Gilly,
I've been unable to set the mixture because regardless of my adjustments to the screw (I haven't done more than a total of 1/2 a turn at any one session), the duty cycle reading stays fixed at 90%. I should add that before I bought the car the OVP fuse was blown, & I had it replaced. Can I safely try to attempt resetting to a 45% mixture by major screw movement/enrichment, which I believe involves turning the screw clockwise? BTW, the tower is open on top, & I use a 3mm allen wrench for adjustments. Thanks.

Gilly,

Through this whole thread no one has said what year and model were are discussing, but the EHA gives me some clues. I never heard the term" fixed operating mode. I'm guessing it would run rough cold because there would be no reduced control pressure like the basic KE which had a warmup compensator and also didn't use a temp sensor. It would be operating just like the KE system and have to be running lean when cold with the electronics dead. Now I'm curious enough to pull the EHA plug on one of my cars and see how it runs cold.

Richard28,
That constant 90% you are seeing is not a duty cycle. It is either saying you have a car with onboard testing system or you have a fault code stored and you will have to look it up in the code book. I don't have my books at home to look it up for you. Sorry.

Peter

You can unplug the entire control unit and the engine will STILL run. This is called "Fixed Operating Mode".

Peter is right, this is the very early version of a car having a diagnostic capability, although still no memory. I don't know what a fixed 90% is either, I'll see if it's on my 124 disc and report back later.

Gilly

A 90 reading is the dreaded "Current to electrohydraulic actuator implausible." This is accompanied by the following footnote:

"The plausibility of the road speed signal can only be checked by the KE control unit when driving. If an implausible road signal is recognized when driving, the control unit "sets" the on/off ratio of 60% and stores this. The fault is not erased unit the ingnition is switched off. A "60% on/off ratio" test is performed, e.g. in dealing with the complaint [of] jerking when vehicle moving and throttle valve closed."

I got this reading once on my late 300E. I concluded that it had something to do with the road speed sensor. But then it did not reappear so I never pursued it further. I have always been curious about what it really means.

The info I have on my 201 disc is that 90% is not an assigned code. There is an 80%, and 100%, but no 90%.

Gilly

Gilly,
My MB factory Diagnostic Trouble Code (DTC) Reference Guide says that there is a 90% code for some cars, but unfortunately Richard was not courteous enough to include us in on what model he is talking about. Can't help someone if they rudly neglect to describe the year and model.

Take care,
Peter

Peter:
This is sort of like a continuation of a previous thread. Here it is:

http://www.peachparts.com/shopforum/tech-help/76935-bizzarro-overvoltage-relay-problem.html

It's an 85 190E, I assume that since it's a gasser, in 85 it must be a 2.3.

I'm sure he wasn't intentionally rude.

Gilly

Thanks Gilly,

Read the thread, and now I'm locked on and tracking. By the way good diagnostics and scenerio development on your part.

Peter

Peter:
Given that you now have all the info, what do you think of the fixed 90% reading he has?
Gilly

Gilly,
That thread is about ------69. I still have no clue what year and model Richard23 is talking about, and he is the one with the 90% reading.

Peter

Peter:
Sorry about this, I'm a little confused now. Based on this post:

I assumed you now knew that it's an 85 190E, assuming that since it's a gasser that it's a 2.3.

Gilly

Sorry for the delay in replying, I was out of town since Thurs. PM. I wasn't being rude, but yes, I should have identified the car early on in my question -the car is a 1987 300e, a non-Califormia car. I hope you can shed light on what the code means-I swapped in a new EHA & the code still appears. Thanks.

???
 

Did you guys read my response about the "implausible EHA current?" That is right off the CD in Section B, which covers the early cars.

Richard,

Now that we know what car we are working with, my MB factory diagnostic code book says the same thing for the 87 as Gilly's CD. 90% not used, however, the instructions for my Baum tool code reader say 90% means EHA current implausible. The factory code book says that code isn't use****il 1990. Im going to have to either pose this dilema ib the techs only section or phone the mavins at Baum tools. I''l have to get back to you. If the EHA answer works out, remember that the EHA doesn't generate current so swapping it is futile. You need to isolate the reasson the current sent to the EHA is not correct.

Peter

Chuck
Are you using the 201 chassis CD?

Gilly

Gilly -

No, used the 124. He said it was an '87 300E.

CT

power supply to O2 sensor
 

I was under the impression that it was powered by the fuel pump relay (if you open it you can see the 2 seperate relays).
The ovp supplys the power to the ecu meaning that even though the fuse on the ovp is blown the o2 sensor will still be getting power and sending a signal to the ecu, as will the air-flow meter and the coolant temp sensor. The ecu will not be able to adjust the mixture accordingly however as it is not powered up.
The Idle control solenoid and eha refer to a constant mixture setting.
That is my understanding.

Chuck
Here is the original thread, he never referred to it as a 300E

http://www.peachparts.com/shopforum/showthread.php?s=&threadid=76935

AF300E:
On some models (whenever they went to a heated O2 sensor), the fuel pump relay also activates the O2 sensor heater as directed by the engine control module.
If the OVP fuse blows and the engine control module is dead, the EHA is not powered at all and the fuel injection system is then in Fixed Operating Mode. There is no Lambda in Fixed Operating Mode. In fixed operating mode the engine control unit can be removed and placed in the trunk, and the O2 sensor is doing about the same amount of work as the spare tire.

Gilly

Gilly

Been following this thread and there seems to be some confusion over the type of car.

I believe in the above thread you referenced, the car is an '85 190E and the owner of the car is a different member - classic69.

Cheers.

Azhari

The whole first thread, as I see it, was started by Classic69 about his 85 201 chassis.

Nowhere in this thread (which was also started by classic69) or the previous thread, do I notice an owner with an 87 300E:

Richard28 has no car listed, and doesn't mention what car he has in this thread

Bud has a 91 300E

ctaylor has about 4 cars, none that is a 124

AF300E has some type of 124, no specifics

Peter (autozen) is just posting answers, no questions, and has no cars in his profile

so WHERE is the documentation that this is a 87 300E? On the previous thread? It's ALWAYs been a 201, 1985, which I believe narrows it down to a 2.3 by default, assuming it's a US spec car.

Gilly

ps: coffee time now:p

Dunno where I got it, but I always thought it was an early 300E.

And I recently sold my '86 300E, having spent a fair amount of time fighting with the injection. But I kept the CD.

Half the people on this thread are not reading the thread and adding to the confusion. Some are trying to solve Richard's problem wilth his 85 190E. Richard doesn't have a 190E. In fact he never said what he had until I posed the question. About 9 posts up from this one he states that he has a 49 state 87 300E. Also I believe the thread has switched from the original question posed by 69 and shifted to Richard's question about only getting a constant 90% reading istead of a 50% duty cycle reading. End of Cliff notes.

Peter

Yep, I'll admit I was screwed up, I see it now.
I was screwed up from Richard28's first post, I guess since he said he had a "follow up question" I thought I was still dealing with classic69's 1985 190E, then totally blew off the one where he said he had an 87 300E. Oh Boy, sorry guys. I even went through LOOKING for a model listed other than a 190E and missed it. Of course up until 10pm last night, no one had a clue on what the new questions were about. It sure does help to have your year and exact model on your profile, maybe I could have been on track alot sooner.

Gilly

Gilly,
I again apologize for not including in my followup questions the model/year, which wasted the time of you & the others. It won't happen again.
Sincerely,
Richard

Richard
It would also help to include your vehicle description in your profile. Just simply put "87 300E", that's enough.

Gilly

Gilly,
Based on CTaylor's post on 90% duty cycle, it would seem that the first step should be to test /replace the OVP since it powers up the EHA. Is this correct?

Richard,

You can change a lot of things that provide current, but if you take the time to read the earlier posts again you'll see that you may not even have an EHA current problem. If chuck read the year model right on his CD, 90% deals with EHA current. My Mercedes Benz factory code book published in 93 says 90% isn't used til 90 year model. You could also just have a dead O2 sensor that is reading lean. You could unplug the O2 sensor, go into open loop, and check the output of the sensor with a 10 meg voltmeter. If you read about .2 or .3 volts steady signal, that would send a lean signal causing the mixture to go steady rich at about 90%. We need to find out who has the misprint in their reference material and go from there

Thanks Peter. I'm standing by. And sorry again for the earlier confusion.
Richard

90% constant
 

In my experience, if you get such a reading, you need o richen the mixture almost as far as you can using the lambda adjustment allen screw. They tend to jam at 90% if the mixture gets too lean (unknown why)

Got this reply in the techs only forum.

RPM55
Member

Registered: Jun 2002
Location: Cumming Ga.
Posts: 86


Tell him to try slightly pushing down on air flow sensor plate to see if readout responds. From what I've seen in the past the reading is 92% and an o2 sensor usually corrects the problem.

__________________
Mercedes Benz Master Tech

Peter

Pushing the plated was suggested in an earlier post, and when I did it the duty cycle reading did increase immediately, to about 91.4%, before the engine shut down in what seemed like one second. So, summing up, since there was a change in the duty cycle reading, the problem is the constant false signal from the O2 sensor telling the EHA that the mixture is too lean, and by responding appropriately the EHA actually causes an overrich mixture which can't be effectively overridden by adjusting the mixture control screw. Thanks so much. I'll try to take care of this over the weekend and report back.

Richard,

Good luck and please be sure to post the results, because that will confirm a viable diagnostic tool for all of us.

Peter

rookie question
 

Hi All,

This was an interesting thread. Since I have an '87 260E, I was wondering where the "Duty Cycle Readings" and/or "90%, 100%,etc...) comes from. Is this from a multi-meter?

sorry...

thanks...

From the O2 sensor

Gilly

Gilly,

That smart ass answer wasn't nice of you.:D

sjs from Fiji,

To answer your question, the % codes and duty can be read a couple of ways. Supposedly you can use a dwell meter, but I've never tried it. You can use a digital volt meter. If you can find an analog meter with an impedence of 10 meg or more, that would be excellent. You need extremely high o\impedence test equipment here for two reasons. The voltage signal is so low from the sensor that you can load it down and get incorrect readings. If you use a low impedence meter in closed loop, you can load and damage the computer. There are also special pieces of test equipment designed for o2 sensor circuit testing. that is what I use. Maybe others on this thread can let us know what they use.

Peter

thanks...
 

I knew I was on the right track...

cheers,

Sorry Peter, it wasn't a smart-ass answer, just the opposite as a matter of fact.
I believe you can use a voltmeter to read this if it has a duty cycle position.

Gilly

Where would one find the OVP fuse on a 260e? I replaced my OVP a few months ago, and I don't recall seeing a fuse in the device.

Thanks in advance.

The OVP has a clear flip open lid on top, there is a red 10 amp fuse under the lid.

Gilly

Thanks Gilly, I'll have to take a look in the morning. My ABS light has been coming on, and initially I had thought it was my OVP, but it is still happening, so I'm going to check my ABS sensors for grease/dirt. I've been having pwr loss problems lately as well, and I never thought that the OVP could be the culprit. See http://www.peachparts.com/shopforum/tech-help/78020-260e-power-loss-odd-smell-post493520.html#post493520

Do you know if the 260e has a similar fuel mixture adjustment as is discussed in this thread?