OVP help,, simple question
 

I know there's a lot a treads for ovp issues, but i can't find one specifically for me. My 89 300E is running like GARBAGE right now. It has the roughest idle all the time when i stop at a light, the dash starts shaking and everything vibrates. It hardly ever dies though, maybe like once parked on a hill when i first started it. THen when i drive it acceleration is so messed up. I'll floor the pedal and i wont go anywhere then i'll let off a bit and it'll start climibing. Ive gotten pretty good at brake torquing constantly. Also i wanna ad that the other day i was coming home and the ABS light came on, then flickered, then the engine light came on for like a second then shut off. Currently no warning lights are on. So OVP or something else?

OVP are notorious for doing these sorts of things. I'm sure you've read this already, check and change the fuse on top of the relay. If you have someone with a similar car that works, try their OVP relay out. Only takes a few minutes to swap out.

Yeah, the engine and ABS lights and cruddy running are classic OVP symptoms. Your motor is running with no computer, a testament to the overall design of the thing. I don't know too many cars other than that vintage MB that will run without an ECU.

Check the fuse on top, and pull it out and inspect the blades. I have seen OVPs with burned up fuse contacts, while the fuse was fine.

Swapping it out for a friend's will be a good test. It's a quick and easy swap.

i already looked at the fuse its not burnt and the contacts look like in good shape. Now my mom has a 91 300E you think i can try her relay. Is tehre a difference between 89 and 91 ovp relay, because i really dont want to wreck her car. Also any steps for taking the ovp relay out? thanks

Same OVP in both years.

Alright mine in the 89 is very loose, all i have to do is pull off the electrical contacts at the bottom?

If it's the same as my 300CE, there should be one metric hex head screw that's holding it in place. Remove that screw (however big it is) and lift up on the OVP relay. It's a very simple thing to do that takes about 5-10 minutes each.

On my 300E, the 'ears' on the OVP fit into a bracket without using other fasteners. It just tilts [only one way] until one ear pops free, slide it to the side [to clear the other 'ear'] and lift out.

just tune the engine up...if the OVP or O2 sensor are bad they will trip the check engine light on the dash...throw some new plugs and a rotor and filters at it and see what happens. I swear, everyone thinks this too far through...just start at the beginning...tune it up....you would have to do that anyway regardless of something more serious....the OVP is much overrated as far as a problem area...classic

I had the same ABS light and then flashing check engine light... I suspected my ovp for months now. Was never able to catch it in the act.. Finally just changed it, 3 days, and no symptoms..

Plugs and wires may cause some symptoms, but the abs light would lean me towards the ovp or elsewhere.

What years models have OVP ?
 

I have never dealt with OVP. What years and Models have OVP ?

If the ABS light is on and your ABS is functional -- It's the OVP. I've never heard of the light come on for any other reason. I had it on for BOTH of my w124's I own. 300TE required a brand new OVP, 260E just needed a new fuse for it.

on the 89 you will get a 'Check Engine' light...if not, it's something else...

this OVP is very overrated...really...

Maybe, but it still bit me in the ass twice on the same car.....

I've replaced three in the past 2years (different cars), it cured the symptoms each time.

Very common failure, there's even an engineering change to the original relay (different part number).

My 89 300sel has had the engine light on for awhile. The other day, after reading about code( mine was 4)...I pulled it off its mount, tried to unplug it(did not want to break) moved it around and checked fuse. Replaced it and the engine light went off!!! For 2 days no problem...ran great, but I turned on a/c which does not work just to put a load on and bam the light came back on. Went thru the same exercise and light went off again.

Any ideas, suggestions as to why. I do have issues with a/c...heater is always on it seems......
Thanks and the lil box is important!!!!!!!!!!!!1

Is this so? Since the OVP is the power source for the ABS controller, if the OVP is bad and the ABS light is on, it's because the ABS controller (and, therefore, the ABS system) is inop. Isn't that so?

My ABS light was on because of a bad speed sensor on one of the front wheels. In my case, the OVP was fine.

It would take a bit of research to get a complete and precise answer, but generally speaking, most MB chassis during the 80s and up to about model year 1995 had an OVP, although it seems to have been more common on 6-cylinder motors than on V8s. I don't know its usage on pre-80s cars, but probably not, inasmuch as the electronics on earlier chassis were simpler (no computers), and the OVP is there basically to protect some expensive, sensitive control units. Maybe a professional tech can give a more complete answer.

Alright guys i swapped the Ovp from the 91 300e to my 89 300e and still broken. I then took the ovp and started the car and it took longer to start and the ABS light was on and ran even more rougher than right now. So OVP is out of the question whats next?

Allen, you don't say what maintenance history is or what else has recently been done to the car since your symptoms first appeared. It's time to haul out the multimeter and the CIS fuel pressure tester and start systematic diagnostic testing of the ignition/electrical and fuel systems. The tests themselves are surprisingly easy and quick to perform. That will begin to narrow down the list of possible causes. Then post back any results that are questionable and you'll no doubt get knowledgeable help.

OVP data
 

OVP relay, Over Voltage Protection Links
http://www.peachparts.com/shopforum/diy-links-parts-category/209638-ovp-relay-over-voltage-protection-links.html


Code data..

Jim Forgione's DIY articles
http://www.k6jrf.com/MB_S500.html




Have a great day.

The car had all the necessary tune ups 1 or 2 years ago, plugs wires distributor cap/rotor, full tune up. Heres the story. The problem where i press the gas and go no where (flat spots) happened awhile back. So i brought my car to my mechanic and we put a gas analyzer in the exhaust. The car was running as rich as possible. So buy turning the screw over the intake manifold we brought down the Ho, Co2, then i drove around for about a week and was noticing that when i stoped at a light the whole car started shaking and rpms droped, vacuum was at the middle/red like it does now but when the car reached optimal temperature it stopped doing it, , . So i brought it back to the mechanics and his solution was to up the richness of the fuel basically to mask the problem. It worked though for about a year all of a sudden it happened again in like June of this year but now it happens all the time even at optimal temperature. So im kind of hoping for something to break or coming up with a real solution. I dont have the necessary tools to diagnose the car..

Have you checked: Temperature sensor, fuel pressure (filters and pump operation).

The temperature sensor enables cold enrichment and if reading cold, could cause the car to run rich.

The fuel pressure is an important part of the fuel mix also: When fuel pressure is low in a CIS system, the counter-pressure on the metering pin will be low and allow the air-flow sensor plate to rise too far, injecting too much fuel. If the fuel pressure is lower still, the car will not run well due to inadequate fuel flow and pattern. Fuel filters can be a culprit here and are a good place to start if they haven't been serviced.

temperature sensor?
 

I replaced the fuel filter thinking it was the culprit but no luck, the fuel pressure is fine also. How ever my car has the strangest symptoms from different temperature like it took almost the entire battery to start the car every day in the winter, also in the rain its hard to start. Where and how do i check the temperature sensor because there is a good chance that something is wrong with it. Please help thanks

ovp
 

inside the OVP the solder connections near the large diode open up .resoldering works ,but i belive the best is to replace the OVP and the fuel relay togather at this stage for dependability
mak
300se

OVP
 

I am getting the same symptoms with my 1989 300E. Intermittent ABS light with check engine light. Goes on and off! I also suspected the OVP relay so I finally ordered a new one. Hope it solves the problem...............

The temp sensor is located on the top of the engine block on the left side of the valve cover, all the way at the back of the engine. There's a large black electrical connector plugged onto it, and its wire runs over to the left inner fender and ends in a 4-pin connector plugged into the ignition control unit there. The sensor is tested with an ohmmeter at various engine temperatures, and the results compared against a resistance-to-temperature chart printed in any good service manual. To perform these tests, you need a multimeter, available for about $25 at Sears, and a good service manual to walk you through the test procedures and give you the resistance specs for the relevant electrical component you intend to test. As already mentioned, the tests themselves are quite easy and quick to perform, anybody can do them. Of course, the alternative is to simply replace the sensor and see if that corrects the symptoms.

Correct me if I'm wrong, it's been about 13years since I last had a 103-powered Mercedes, but from memory it seems that you can see the airflow-meter plate with the air-filter assembly removed.

Anyway, gain access to the airflow-meter, take a magnet (one of those magnet-on-a-pen things usually works well if it's strong enough) and touch it to the bolt in the center of the airflow meter plate, see if you can lift it. It should lift smoothly, without scraping the sides of the bore. Also, it should rest with the top surface just about the same level as the bottom of the tapered part of the bore. If either of these things are not true, it needs a simple adjustment to correct it.

I have had several cars (mine and friends') which for no reason I know, have gotten to a point where the metering plate will touch the sides and drag a bit. When this happens, the cranking vacuum will not lift the plate enough to provide adequate fuel, even with a wide-open throttle. VERY easy adjustment but be careful and ask questions first.

If it got that way after needing some throttle to start, sounds like a possibility. This also affects mixture at idle some times.

The other common problem in that system for hard-starts is the cold-start injector or the temp sensor, not providing the extra fuel shot to start the cold engine.

Alright you guys are awesome thanks for your help. I do have a voltmeter and I know a bit about about electricity but i will ask if i get into a problem. Today it was pouring rain and the car took 3 long cranks to start, now some people were skeptical about me saying that it is moisture/temperature related but i honestly am 100% sure it is. I have another tread about hard starting but it was never resolved because summer came and i never had a problem since, but I am thinking that the rough idle is some how inter related and winter is around the corner so I need to get this fixed. Could someone get me a link or a page of the multimeter testing for the temp sensor and hopefully a walk through. Also the magnet check seems simple enough so I will give that a try as well. Thanks ill keep you guys posted.....

Allen, this should get you going. The first page is the testing proc and the second is the resistance-temperature scale to compare your results to for specification at several temps.

With the 4 pin style temp sensor, I believe that the resistance is measured across opposite pins [e.g. 12 o'clock and 6 o'clock]


My M103 has a two connectors, matching the Bosch unit in the online catalog described as
[interesting how vBulletin added the smily in the quote - maybe it's trying to tell me something]


Does this sensor just have one circuit [measure from pin to pin] or two circuits [measure from each pin to ground]?

Time to go to the store an get a better meter anyway [next steps are to look at the 'duty cycle' readings and the output voltage from the O2 sensor - can't do either with my cheapie meter]

If you're talking about your '89 300E, the sensor is actually two identical sensors in the same component. One sensor supplies signal to the KE controller for fuel management, and the other supplies signal to the EZL for ignition management. The 4 pins are indeed measured on the diagonal, pins 1 and 3, then pins 2 and 4. Pins 3 and 4 are ground. The measured resistance for each pair of tests should be identical at the same temperature point shown in the chart above.

My 89 300E has a two pin connector [two circular posts, with o-rings on each one]

Image from the catalog [if this works] http://12.153.160.115/images/catalog...0280130044.jpg

Measuring from each post to ground shows about 2 ohms [cheapie analog meter] at current ambient temp [80F] - that should be closer to 1.7k ohms. If I'm reading things correctly it's time to order a new sensor.

2K Ohms is just about dead on for 80F. The important thing is that both thermistors give similar values at every temp point. I'd take 1 or 2 more readings at higher engine temps, before decididng that the sensor is kaput. So far it looks OK to me.

I'm going to check it again cold, then warm up the engine and test it again.

If I'm reading my meter correctly [log scale] it is showing 2 ohms - not 2k ohms. [time to get a better meter]

I took apart the intake manifold and i checked the intake flap and there is no binding or scoring against the sides,it moves freely i dont understand what you ment by putting a magnet over the bolt cuz theres already a magnet over it? Could you explain further, also i dont know which sensor is the right one for the temp sensor is it one unit or 4 little ones. PLease help, pictures would be great thanks

Allen, the temp sensor involved in these tests is located at the very back of the engine, on the driver side of the valve cover. It's either connected to a big black electrical connector as shown in the photo below, or it's connected to two wires and looks like the photo dlevitt posted in post #34 above. There was a production change in '88 or '89. The big black one is referred to as the 4-pin sensor, the green one is a 2-pin sensor; it's one or the other, but they operate in exactly the same manner. The procedure for testing the 4-pin sensor is what I gave you in my last post above. The test for the 2-pin is slightly different, as shown in the image below.

I don't really know why your '89 has the old style green sensor. Those were upgraded with the black 4-pin component, which my '88 has. Oh well.... Yeah, 2 Ohms isn't right. Is it possible you just weren't doing the test right? Try the test I posted above for the 2-pin, before condemning your multimeter.

It's a crappy meter - one log scale from < 100 ohms to a megohm.

I bought a new dvm [compleat with duty cycle] at Sears this morning. I'll see what readings I can coax out of it this evening.

Had the ABS and check engine light and related issues and ordered a new OVP! Took a 250 mile trip today and had no issues! Guess the OVP was the culprit!

Alright i checked out the temperature sensor, my 89 has a 2 pin, its green like the picture above now i didnt know the exact temperature of the engine i used the temperature gauge in the car and i found that the numbers didnt match up like at 80 celcius i got dead on 0.6 kohm which on the page says should be for 60celcius. Also i want to know does the temperature sensor make the car start hard, rough idle, run like garbage?

Yes, it will affect driveability/idle condition. Using your numbers, the fuel and ignition control units would be interpreting the sensor signal at 80C as if the engine were not yet warmed up (60C), and would adjust lean/rich mixture accordingly. Also important that both sensor pins measure close to the same resistance values, so that the CIS controller and the EZL controller are getting the same information.

Alright so it sounds like a possibility, the way i tested the sensor was i took my volt meter put it on kohm and that was the result. I put both prongs into each end of the sensor, how do i measure each sensor pin separately. I also suspect that my ignition coil could be the culprit because it give me different readings from day to day which means it could be shorting out inside. Im going to swap my 91 300E coil into the 89 or with a 90 190E (2.6L) , are they the same one? both M103 engines only difference is that the 89 is set up front to back while the 91 is set up side to side. Ive been reading about coil problems and thats about the only thing left that hasnt been replaced since ive had it and the problems consist of intermittent starting, rough idling, low torque...which im experiencing all three. Thing that puzzles me is the coil is within spec so we'll see from the swap.

I still didnt have time to change out the ignition coil, i was wondering where and how do you test the crank positioning sensor, any input. Also i know that if the sensor is broken the car wont start which is what happened to me, but i changed it with a used one, can it cause idle, drivability,hard starting problems?

I finally exchanged my ignition coil with another working 300e and i couldnt test the cold start but it still had rough idle and flat spots throughout the whole accelration band. Im suspecting fuel now forsure, EHA possibly?? On first start up dead cold the car, once started runs awesome, no idle problems, no sputtering, accels perfectly but as soon as i shut the car off and come back and drive it minutes or hours later, all the problems come back

Testing the CPS is what I posted for you earlier over here:

http://www.peachparts.com/shopforum/tech-help/263045-89-300e-crank-sensor-help-post2313926.html#post2313926

As for the issue you now have of good cold starts but hard hot starts, leaking injectors would give you those symptoms, but so could failing electrical components, by flexing solder joints, etc. and losing electrical contact as they heat up. The EHA could be an issue, but the EHA is also involved in cold starts, and they seem to be OK, so that's a long shot. I would just do systematic troubleshooting on all of the electrical components, following the FSM diagnostic procedures, and a fuel pressure test too.

You have it mistaking
 

I have the opposite, my car has hard cold starts, perfect hot starts, never had an issue with hard starts. Any time its raining, or moisture in the air or cold temperature it takes like 10 long cranks to start, i suspect the EHA what do you think/??

when i restart the car when its hot it starts right up, but then i get rough idle (rpm drops to around 500, vaccumm goes to the middle and the whole ddash starts shaking) and i get those flat spots where i press the accelerator and i have no power, doesnt go anywhere until i let off and press again

Did you ever finish troubleshooting the coolant temp sensor (CTS) at various other temps? From your initial test some time ago, you already know that it's giving a bad reading at 80C. If I were you, I would complete this test first, before moving on to other things. The CTS is an easy, low cost fix, and definitely affects driveability issues. Doing a systematic diagnosis of the ignition and KE systems following the FSM troubleshooting procedures, is not difficult, and it'll point you in a better direction for a fix than jumping around, and in a lot less time. :)