Help!! Need OVP plug schematic!
 

Does anyone have a schematic for the OVP connector for an 89 300E. I spent about two hours in the service CD and could'nt find it!!

This any good?

Looks good to me. Thanks Nick!! BTW, where did you get that if you don't mind my asking?

On the CD
go to wiring schematics/diagrams
go to page 117
go to frame 43

Here's a photo of one too, and by the way, if you want to buy it I'd sell it $20 + postage. Guaranteed working or your money back.

I replaced it trying to fix an intermittent fuel pump problem, turned out to be the MAS control unit instead.

It's from a '92 300E, PN #201 540 32 45

My goal here is to try to bypass the OVP to see if I can determine a way to test one. That would answer a lot of questions and save a lot of money for members. Instead of just going out and buying a new one every time a car hiccups, we could have a test to verify the OVP is/is not working. I bet there have been a million of these things purchased unnessesarily. I know I bought one and the car actually ran worse when I installed it!

Someone should start an OVP rental service. :)

I believe there's a test procedure outlined on the MB 124 CD-ROM somewhere, seem to recall reading it.

I was told that if you have power between the fuse posts, then the ovp was working. I was also told that you can remove the ovp and try the car briefly without it. It was worse without it on my car. (Although worse than dead is hard to imagine.)
I would like to know how to test the air flow meter rather than pay the big dollars to replace that part.
Good luck, (to us all).

What is the function of pin 30a?

One must ask ones self what does by-pass mean. Then one must find what an OVP does. AND, then it becomes obvious.

The OVP is simply a fuse like any other fuse except it is designed to blow on voltage changes not current changes.

It's most basic purpose is to connect various control units to the battery.

Based upon that concept what determination can be made of the case Stephen points out? That case being that a new OVP made the car run worse.

Its most likely that the old unit was bad. Think about it and tell me how I can be right. Unless you understand why I can be right you are not understanding how things work. I'm going to let someone else explain it. This is a challenge! All you techs take a break.

You got the by-pass right, but that wasn't my question.

Of course this isn't my thread, but my question is: why do I believe the original OVP bad if a new one makes the car run worse?

This question goes to the heart of the original question. The need to by-pass is dubious if the results of by-passing are misunderstood.

Nope.

Maybe some extra credit for true statements, but not the diagnostic insight I'm seeking.

Gonna give you another try, while we wait.

And BTW sixth post down last sentence:

"I bet there have been a million of these things purchased unnessesarily. I know I bought one and the car actually ran worse when I installed it!"

See now we are getting somewhere, but you flunk.

Let me correct a few misconceptions and see if we get some diagnostic hints from that special sentence.

First: With a good OVP a warm car set properly will notice no difference when the OVP is unplugged!!!!! can we say "limp home" While warm starting will not be affected.

Second: With a dead controller the idle valve goes to it's "limp home" position which is a little faster (on a 300E) than a proper running car at idle untill put in gear and the A/C turned on at which time the fixed setting is not enough for full idle speed.

I'd love to spell out my conclusions but think more time should pass to let others think this through. If you can't wait write me and I'll tell you how I see it. steveb@atlantic.net

Haven't got the email yet but to keep things from being too confused, one more point. I just read the cars you have and I must point out that we are NOT talking about your car!

Your car is a HFM SFI car and the OVP functions differently. You do not have an idle valve and because the system is electronic your car will not run without the OVP.

<<OR - the OVP may be defective with conditions such as zenor diode tripping waaaay too early or making intermittent connections. >>
I have read that this is one of the faulty parts that caused problems on the original and was replaced in the new style relays. For what it's worth...

Memory???
 

Bad OVP, going bad over time and the fuel computer continually adjusts to keep the car running. Until the old OVP is completely shot (although I do understand electronics tend to go out at once).

Now pop in a new OVP, the fuel computer has memory, trying to feed input according to the memory of the bad OVP resulting in running worse with the new OVP installed. Needs time to reset the memory.

Best guess.

Just for references, heres the star of the show;

and the other side;

Steve - I have just read your "challenge" - been outside installing a replacement window in the right rear door of a 300SD for a friend (made $50 - took 2 hours :):)).

Anyway, I know a little something (very little) about industrial controls and instrumentation (I know ALOT about pumps and pumping systems) my GUESS would be that a new - "good" working OVP relay could possibly put the correct voltage to a bad idle speed control valve which could possibly prevent the car from running the way it should. Just my GUESS :):)

I'll bet the majority of diyer's on here thoroughly enjoy this type of "challange", I know I do. First prize should be a free tour of Continental Imports in Gainesville, Fla :):) whenever the winner is in Gainesville.

Eng
Your E320 has an OVP and one of the original problem ones...

Might want to see if you have the upgraded one..

Real close Mike, actual you have the right answer for some of the wrong reasons.

This system is too stupid to make its own adaptations. It only works from real time.

Since you really have it, I am going to paste the reply I have already written. It can then be discussed. The good topic will be how I explain what you have explained and whether it is understood and of course whether it has merit. Here it is:

OK, here is how I see it. I can only get so much from a single statement so for it to give me a clue I must assume a couple things. Obviously that makes for errors and who can be right all the time. without investigation based on small hints I can't say much but occasionally these sets of circumstances clues me to things that allow me to get right answers to very little data.

The statement is that after replacing the OVP the car runs worse. Originally two conditions could have existed; one there is a good OVP and everything is OK or mostly OK (it did get worse). Two is a bad OVP and the car is purely running on the mechanical mixture setting with idle valve hung at a constant position. There are bushel baskets of other parts that we must assume are doing OK. If the second condition is the case two posibilities exist. One that the mechanical setting was done properly and when the OVP died it wasn't needed at idle and other warm running constant state conditions (very good "limp home" characteristics). The second condition here is the key. It would be that somewhere after the OVP died mixture was adjusted to achieve a balance of enough fuel to idle good and get by cold but not so much that it ran poorly warm (this would be the mechanical adjustment of course).

Now with this mechanically adjusted over ridden fuel system in limp home, the OVP is replaced with a good one and the management system sees the rich mixture and fixes it making a condition that doesn't work either beacuse of the other problems that were mechanically compensated for OR by making the electronics work again and their own malfuntion screws the mixture.

Try it from the other end. The original OVP is good. The engine is running with electronic compensation (closed loop). Removal of the OVP removes all electronic compensation. Similar to a bad OVP. Removing electronic comp from a properly set up car is un noticable while warm. So how can a new OVP change the way it runs, only if the OVP was bad in the first place and now electronic compensation is taking it somewhere it wasn't.

I see two ways to explain how a good OVP replacing a bad OVP can make things worse, but I see no ways a bad OVP replacing a good one could make it run worse (with everything else right). I had to throw that last bit in because I just thought of a way that it could happen it requires two faults.

Here it goes. Car has bad O2 sensor telling it that engine is lean. As a result the engine continues to go rich untill there is no more ability. Now someone comes along and mechanically adjusts the car back to useable mixtures (system is still eletrically pegged rich but over ridden mechanically to reasonable mixture).

Now a bad OVP is placed in the circuit and the electronic compensation goes away only leaving the mechanical which was brough lean to compensate. this will make the car run poorer with a bad OVP. Quite a stretch. I still go with some variation of the hypothesis that a bad OVP was replaced with a good and then taken back out leaving a broke car.

Thanks Arthur I thought about that after I posted it that is why you see the edit.
Thanks Steve - this is great stuff.

Now; If you feel the urge to point, feel free to do so.
Here's my story, sad but true.
Bought the car it was running fine, Used a good bit of oil though. Within 2000 miles the check engine light would flicker, still ran good. Then the ABS light would come on and go off. About three seconds on. Time on was within what the OM said was normal.
Within a couple of weeks engine began to miss while CE light was on. Problem was intermittant and might happen and then go a week without it. Tech at the local MB ******** told me to replace the OVP. I did. The frequency increased. Tech says "fuel pump relay". Did that. Again, increasingly worse. Next, he installs a resitor in the temp sensor. No change. I replace plugs with Bosch Platinums. Thing runs VERY bad. Go back to copper core and I'm back where I started. Replace O2 sensor. Frequency continues to increase. Finally got to the point where I would have to stop and unplug the CIS-E unit just to "limp home". Actually it ran pretty well like that. Hard to start cold though. Replaced the CISE. No change. Fuel pumps, no change. Car spends all day in the shop. Tech thinks its the contacts in the CIS-E connector. He spreads them and sends me on my way, after I paid him well! No check engine light! Hooray! But, engine ain't runnig smoothly! 50 miles later, yup, check eng liht! I'm about ready to scream now. So oil consuption is at a quart per 400 miles. I figure slowing that down might help. Had head reworked with all new valve guides, new chains, new flywheel position sensor and transmission(it needed one). Now car is very hard to start and gremlin is still there. Also starts in third gear, but that's another issue. OVP, power window relay top of CIS-e all get hot quickly. Voltage is at 13.65 / 13.66. And as you've already heard, idle valve "clicks" after it dies. Tech says, you guessed it... needs new OVP!!
That's why I would like to bypass it just to test for that!

Steven you need to try to find a nice 240/300D :):).

Yeah! A diesel will run without any electricity! Put the injector on manual and push it off!!:D :D

Looks like I picked a lot out of that innocent sentence.

I'm real sorry to hear such stories. I'm the first to defend other techs but I fear he is not working with proper diagnostic technique.

Which is a lot of what you are trying to find. The by-pass techniques here are acceptable but based upon this new info I would guess the OVP isn't the problem. The fact that you ran with the Lambda controller out to get home indicates the senario I posed and also indicated that the controller must be getting power.

You need to read my DIY article and bring what you don't understand to the table. Next stop EHA!

I think the next stop is Sears. To pick up a DMM with a duty cycle!! To be factual there are two techs in this story. One at the ******** was the one telling me to throw parts at it. The shop that I took it to was reputable European repair shop that specializes in Mercedes. Both use only OEM parts and have good reputations. Most of this happened before I found this forum. I had a very limited brain trust to pull from then! But there's always a challenge out there isn't there? I just wish it hadn't happened to me. A good friend of mine tells me that I know just enough to get into trouble, that often I attempt things that I would be better off leaving alone. I guess that's an accident of birth because it's my nature. And with the help of God and a few friends, I'll work this out. Hmmmmn... Gainesville is only three and a half hours from here!!!!;)

Just like school, right answer but showed the wrong work :eek:.

Actually, just after posting I took our sitter home, had a while to think on the drive back. I realized that I addressed only the computer side of the equation, and had intended to edit in the possibility of mechanical adjustments to try and overcome the poor run problem. But Steve had already covered that before my return.

Plus, I guess I'm getting adaptive and nonadaptive controls confused, there's just too many different species and the Rainman in me is starting to get confused :(.

Still, not a bad hip shot.

A little logic based in knowledge and a couple simple instruments covers a lot of territory with this sysetm. Parts used with logic can be efficient testing as long as the customer isn't keeping the parts.

I usually only go to the point of logic testing and basic test confirmation before I hang a part. I own many parts this way but there is a point of deminishing returns to total complete testing with labor as costly as it is. By working only on a couple types of automobile parts replacement testing can be used to great benefit. Once I own a control unit testing of such just became a piece of cake. I usually, when faced with a large diagnostic labor charge to prove a guess (some peoples guesses are better than others - and maybe not a true guess), try to find the unit used. This is particularly in the case of expensive control units. If I am right the customer is miles ahead; less diagnostics and a cheap part. If I am wrong I add to the list of easy to diagnose control units.

Good hip shop and right on the trail.

Let me tell you a bit about adaptation versus control. Closed loop control started in engines with lambda systems in 1980. The last year of 450s got such control. Very simple idea. Make a mechanical system to get close to a precise amount of fuel and then use an after the fact sensor (the O2 sensor) to evaluate how good the job was done. Then use electronics to correct, then sense the result and correct, and then sense the result and correct. The concept used with a system that could react in about 250 milli seconds was able to achieve an exact mixture by moving rich, lean, rich, lean, etc turning as soon as the midpoint is passed. the quicker the readings the narrower the swing rich, lean, rich. Placed in terms of percent CO (Carbon Monoxide the mixture gas in 5gas testing), the ideal is .5% CO. The O2 sensor reads from 0.0% to 1.0% CO correct running closed loop will have voltage swings from .2 to .8v on the O2 sensor. The faster the reaction the smaller the swing. From zero to one the percent O2 sensor voltage can be directly extrapolated to CO% .5v = .5% CO.

On these systems the base mixture had to be set as the systems ability to correct was limited. Proper mixture was to be set to .5% CO using an analyzer without the O2 sensor connected.

All of this sounds a lot like adaptation because mixtures are being adapted to a value. But it is really just feedback control. Adaptation started with the electronic systems which were no longer adjustable. The non adjustment was required to keep cars in closed loop without constant maintenance AND to prevent people from being able to over ride the system with mechanical adjustments.

So now adaptation is where the car changes its center of correction as the requirements change. If for example the car decides that 10% more fuel on average is needed to maintain mixture in the center of the correction capability it now moves that center ten percent. Current adaptations give the modern controller slightly less ability to go rich than to go lean but the original fuel calculation from algorithm can be moved from .68 of one to 1.25 of one, with one being the original calculation.

During warm closed loop considerations it really doesn't matter so much about adaptation. Where it matters is cold or during all fast activities. Because the car decides that due to wear and infirmity of components the current fuel needs are to be increased by 20% ( say due to restricted injectors) . The system now is running at 1.2 adapted. Now when you go start the car up cold its fuel calculation which was richer (for cold running calculation) anyway is now preset 20 percent richer. This is before the O2 sensor is warm enough to help out. The old car would have corrected rich when hot but when cold it would have went to its middle of the road setting made hot. If there were restricted injectors the car would suffer during cold running and also everywhere there was things hapeening faster than 250milliseconds. (snap acceleration comes to mind)

So I have about got myself confussed, I better stop.

[First: With a good OVP a warm car set properly will notice no difference when the OVP is unplugged!!!!! can we say "limp home" While warm starting will not be affected.]

So, in MY case OVP is not telling cold start mechanics to operate and may be screwing up the warm operation mode too?

Oops! That's not a valid question. I should ask is it possible and/or likely for the OVP to cause the controls to behave ths way?

Come on now Stephen.

The OVP doesn't tell anybody, anything, ever. Think of it as a fuse. Its either connected or it isn't or somewhere in between.

The car we have been talking about uses electronic control of a mechanical system that does pretty well without electronic help.

Your system on the other hand absolutely needs electronic help as thats all there is. No connection to the battery and you don't go period.

Now before you get perturbed with me, remember I'm the student seeking knowledge:) ! Most of this thread is way over my head, so I have to shoot in the dark on ocassion. And I'm sure I speak for everyone in thanking you for you're participation here. I hope this thread becomes a reference for the function of the infamous OVP.!

So, with the mindset of the OVP as a resisted fuse, what would be the results if it wasn't there at all? Assuming there would never be a spike in voltage.

Its needs to the system could be provided with a hunk of wire untill time came where protection was needed. Then the control unit could suffer.

The point to remember is that the device plays no part in decisions. It only powers up the system. Think of it as a UPS for the cars computers. As far as running the PC a wall outlet would be just as good, but we use UPSs to protect the thing from lightning/etc. Our software is totally not concerned with whether the power came from the wall socket or the UPS. Same holds for the OVP.

Now we're at a level I can understand. (sorry to have to bring you down here);) One last question and I'll go home, why is this thing blamed for so many conditions? It seems that every time the word "stall" or "erratic' comes up, the first "answer" is to replace the OVP.

Probably the key to most of its problems comes from the fact that the systems it powers on the KE and K-jet w/lambda cars work so well without any control at all. This leads to some weird, intermittant problems, especially if the OVP problem is intermittant as often is the case. The extent to which you see problems with it are probably also to to the many systems that get affected and the many ways things are affected. It pops its ugly head in where nobodyexpects all the time.

MB has so few bad connections and thats what this really turns out to be. If the car totally died everytime it went out like on the later cars it would be easy especially if it stayed broke long enough to look for it.

OK now its time to slap the teachers hand.

Go back a couple posts, I mistook who I was taking to when I stated: "Your system on the other hand absolutely needs electronic help as thats all there is. No connection to the battery and you don't go period."

The system I was refering to was the later HFM system not yours. I figured that mistake would really confuse everyone set on learning this so i needed to make the correction. I probably should go edit it. Hopefully anyone reading this as archives will read this far or they are going to quit.

Mr. Brotherton, my thanks for your perseverence. I know you had better things to do on a summer Saturday afternoon than to educate us about OVPs!! As promised, I'm through!:) I will however, monitor this thread in case someone else asks a question like "which terminals do you bridge to test the OVP? ;) ;) As best I remember, the only other answer was wrong and the student that submitted it had to stand in the hall for 15 minutes!!:p :p

Its been raining all day and I was set to go out to the local stock car races tonight (haven't done that in years and they have improved the track this year). Just seemed like the day to do it, but now its rained out.

Oh well, I had fun.

I wish I was there. I would take you and your family up to Alachua to Sonny's BBQ. You deserve something for all this!!:D

Well you guys across the pond have been busy while I've been asleep! A couple of observations.

1. The function of the OVP is to shut off power to the ABS and fuel computers to protect them if the car system voltage gets too high (for example if the alternator's voltage regulator screws up).

2. As Steve said, the KE fuel injection system is a mechanical one with electronic aids. If the engine and ignition system are in good condition it will run without any of these aids - I know because my 230TE (UK petrol prices are such that most of us can't afford to run 6 cyl cars) ran with a failed OVP on and off for months 'till I fugured it out. My project this sunny Sunday morning is to see if the HFM equiped E220 will run without it - I guess it won't.

3. The car that ran worse when the OVP was replaced must have had the mixture and idle (I think earlier ones had manual idle adjustment?) when the electronics were not working.

4. The common problem with the OVP is bad soldered joint on the PCB. It is easy to repair, just open it up and run a hot solder iron over all the joints.

Another couple of thoughts!

1. Checking voltage at the fuse terminals will not tell very much as this is on the 'live' side of the relay.

2. To bypass the relay bridge the 30, 87l and both 87e terminals. Beware, the ingition switch may not shut off the engine if you do this. You could use the 15 instead of the 30 but this may overload the ignition switch.

I was playing around with this old OVP and noticed that there is no continuity from side to side of the black resistor in the pin 15 ignition circuit. Car wouldn't start when this one was in. Another cause for failure? This one would be immediately noticeable since the car won't start. It's the black one in the middle of the picture.

<<
I was playing around with this old OVP and noticed that there is no continuity from side to side of the black resistor in the pin 15
??>>

I believe that is a diode, not black resistor...

Switch your meter/leads polarity and re-check for foward bias...

Current will pass through it but it doesn't have continuity either way polarity is checked. Does that mean it's ok?

If it shows no cont. both ways [ polarity wise], it is open.
If so , go to RS and get one [ for about 20 cents] and solder it in there , and go give it a try..

Edit:
Make sure of polarity of the new one . This is what the little stripe on one end of the diode is there for . Put new the same position [polarity] as the one you take out...

Electronics 101 question: No continuity but will pass current? How can that be? I'm lost here unless that means there is a "gate" in there that closes upon current application. By the way, the ID of the whatyoumaycallit in 1N 4003.

Well I did pull the OVP relay fuse on my HFM equipped E220 and guess what? No change to the engine, it started and ran fine, idle speed compensation worked in the normal way, only difference was the ABS light was on.

So I conclude that the HFM engine management computer is not fed from the OVP relay, presumably it has it's own protection from high system voltage.

Best discussion on electronics I've come across here in a LONG time. So, allow me to toss in a couple questions for the pros:

1 -- Get out on the Interstate and hit the brakes at speeds in excess of 70MPH in my SL. I get a slight pull to the right front and the ABS light comes on. After that it brakes w/o any pull. Turn the ignition off and the ABS light goes off. Got an ABS computer problem you think?

2 -- Just replaced the Environmental Control Unit (000 822 1203)awhile back ( a $1,500.00 part through Fast Lane and the MB dealership). Managed to find one on the internet for $125.00 so felt much better about doing it. Installed it and it worked fine for about six weeks. Now when I turn the A/C on and pull up to a stop light, the ECU makes a bzzzttt noise, the RPMs surge about 300RPM. You can put your hand on the ECU and feel this is where the relay(?) is sticking. So, the questions are: (1) what's causing me to continue to have this problem with the ECU, and (2) what's the connection between the ECU and the RPM surge? I'd think the latter would be more an idle control valve/unit problem and don't understand what's going on here.

FYI, I replaced the OVP at the same time I replaced the ECU. I've also replaced the idle control valve and idle control unit.

Any assistance here will be GREATLY appreciated.

[QUOTE]Originally posted by sublettd
[


2 -- Just replaced the Environmental Control Unit (000 822 1203)]



What the heck is an environmental control unit? I hope I ain't got one!!:eek: If so it too will fail or already has!