Hello. I have a 1987 300D. With an inop tach, ac fan won’t turn on, low idle and abs light on, on the dash. I tested the OVP relay which appears to be good. ELR is also not working.
Speed sensor on transmission is attached and cleaned.

Before I removed the OVP to test it, the tach would come on after a while of driving it, most of the time.

I wondering if I have some sort of bad wiring connecting these system? I’m lost. Any help would be appreciated.

I would make an effort to sub another one in as a test. Depending on your background in Electrical/ electronics.

I do not have a schematic of the internals.

Know anybody there with another car that uses it? Or with a lot of luck where one is in a local salvage yard. Failure prone part makes it highly suspicious. Wiring issues in these older Mercedes not usually a big factor. Until they got newer.

I may be wrong but it is an over voltage protection unit. To test off the car you would probably require a dc variable voltage supply. If you have that your testing may be valid. Again I do not have a schematic. It is also fused on top I believe. Since it has a relay. I suspect it is an on off device. Rather than a variable ouput.One might test it in place by carefully winding a wire on the output pin by winding it around it.Keep it very high on the pin. Then seenig if there is voltage output with the car running. I assume it is too hard to get a reading from the bottom of the socket.

I might also check the output voltage of the alternator at the battery. Sitting at more than 14 1/2 volts with the engine running the OVP may just be doing it's job.

Bad voltage regulator might be causing an issue with it. You did test it off the car. Did you establish the voltage at the point the relay drops out? Or only that the relay was still closed at 12V? Caught a lot of vlotage regulators that were not regulating properly. Those cars were okay still as they did not have any over voltage protection incorporated. Plus many aftermarket replacements for thos regulators had so many issues I only use good origal types. Unfortunatly they are a lot more expensive.

Now I am also cheap.If I could not find a substitute ovp unit. If the voltage on the battery was in the normal range. I would locate the voltage input to the device plus the voltage output. Solder two wires to scrap pins and insert them into the socket. Without the engine running and the battery voltage at 12.75 or less. Connect the two wires. but since I do not know if there is something critically sensitive to the voltage. I might just put one pin in the output of the socket using an external voltage supply of 12 volts. or close. If tack and everything starts to work when you start the car. I asume then the device is defective. Of sourse I would have probed to make sure the device was being voltage supplied before doing that. Ignition key has to be on for that. Now my warning is I am capable of doing all kinds of tests because of my electronic background. Of over fifty years of extensive constant troubleshooting.

So i am just mentioning things I might do depending. So what you do is up to you and I cannot recommend my approaches to everybody. Any electrical or electronic issue that is not intermittent is easy compared to those that are. There are a lot of methods to deal with intermittent conditions as well. I am glad to be retired from that field for quite some time now. What they call the over voltage protection. Sounds very simple.

Yet without seeing either one internally or a schematic.I am not positive.I am curious about that ten amp fuse on top especially. I suspect they are just using a zenner diode inside but this is European electronics. Perhaps built by some horrific outfit like Phillips in Europe in the day. They sometimes engaged some really strange approaches in engineering design. There should only be three active pins on that device if the design is rational. One will be a ground.

Why I went into this is the device can be working as it should be, Youi have to check the output of the alternator voltage at the battery terminals with the car running..First before looking at the device. If the voltage is okay. Then the ten amp fuse is suspect on top of the device. They do seem to age out. you clear those two things first. Before actually dealing with the device, You mentioned the tach works sometimes. If that is only with the headlights on. Or the car is under a heavy electrical load situation is interesting. I still would have to see a schematic. Or examine an actual unit, Cutoff threshold is perhaps too low. Or even just dirty contacts in the relay.

Description was very long winded. Just never saw anyone dive into troubleshooting these before. So just thought it might be worth expanding as the part is expensive for what it probably only is. Plus failures have occurred often enough.

Barry12345, Thanks so much for your wealth of information.

Basically there are 5 pins. Unfortunately I’m unable to load a picture on here. The two bottom pins are a positive and a negative. Then if the top right pin also is supplied with a 12v supply it should click. Mine clicked so I assumed it’s working correctly. I did this test with the Ovp relay removed and using the cars battery with three separate wires. Apparently it could still be considered a bad OVP relay. I will do the other tests your recommended. I really appreciate your input.

There was a thread recently on a similar issue that may prove insightful. Barry has provided a lot of good advice. Prior to going into your diagnostics though, you really need to ensure that the electrical connections are solid. These issues often arise from broken, weak solder joints in either or both the OVP and ELR. My recommendation is that you open these two boxes first and reflow the solder joint from the pins to the board, and any other questionable solder connection. Inspect them really carefully under magnification. Once you are assured that the solder connections are good and solid, then diagnose as advised should the problem persist. Your flakey tach is a good indicator of a weak/poor electrical connection and likely in one of these two boxes. Details on the how to are in the thread below and many other that walk you through the soldering process.


https://www.peachparts.com/shopforum/diesel-discussion/414722-rough-idle-after-start-up.html

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

With no disrespect. What a load of nothing.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

https://www.benzworld.org/threads/ovp-relay-what-does-it-do-exactly.1562149/

There is a lot of info on YouTube.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Not sure but if you get the click by turning the key on with somebody listening carefully. It is probably bad. The way it should work is the relay contacts remain closed other than if the excess voltage the device sees activates the relay to open the contacts to shut the output voltage down.

Speculating here as even after all these years I have never seen even a picture of the internals. For example there may be two relays in there. The price of the replacement is so high there may be some kind of weird design in there.

I did not mention poor solder joints as I have no ideal of your background. If there is an early chip in there. Bridging contacts accidentally with solder is only one possibility.

Perhaps the gentleman that has reffered to my explanation. Rather than being critical of it can give better help. Various posts are helpful on your thread.

I may ask a member to mail me a bad one at some point to examine. As i mentioned some European electronic circuit designs can be really strange.

Why do really old solder joints go bad sometimes? Besides the obvious reasons Solder itself actually ages.

Even a roll of solder you buy has a shelf life. Best before something like seven years. Flux core solders only gauranteed two year by manufactures usually.

I believe the gentleman on site that rebuilds cruise control components. Or another one not on this site states clearly. If you have resoldered the component do not send it in for repair.

87tdwagen. What do you mean by open up the boxes. I took the plugs apart to make sure all those were still very strong and they seem to be fine. Are you referring to the fuse box?

Another issue I am having is I cannot find this Ovp relay anywhere online. Just one website but it’s back ordered.

Read over the posts in the thread I linked earlier for details.
Basically, you want to physically open both the OVP and ELR relays (boxes). Start with the OVP as primary culprit usually. It should consist of a metal cover held to the bottom by some depressed dimples, tangs, where the pins are. Spread the tang, and slide the cover off of the relay assembly. Now you will be able to inspect the innards of the OVP. The Pins that you plug the relay into the harness, are both physically and electrically stressed, over time, they can develop micro fractures on the solder join where these pins attach to the board inside the OVP, leading to flaky electrical issues. If you have a basic soldering iron and skills, try reflowing these connections to the board. Place solder tip on them until the solder liquefies and then remove heat. You can choose to add solder if needed, but generally not really needed. By reheating the solder that is there, it is effectively resoldering itself to both the pin and the board. This will create a solid joint and "should" solve most issues. You can do the same to the ELR if needed, meaning if you did the OVP and still have an issue try the ELR, that cover is plastic and only requires spreading the sides so that it can unclip and slide off revealing the assembly inside. The connections in the ELR are smaller than the OVP so a deft hand and or a fine tip on your soldering iron is required to do those pins.

There are no guarantees, the modules/relays "could" be bad, but from what I have seen and read on these forums, the vast majority or issues stem from poor electrical connections that can often be revitalized via resoldering these pins. There are threads that may have pictures on the how to, OVP resolder that could provide more insight, but once you have the OVP open, I think you'll see what I mean.

There have also been many parts updates made to these over the years, and the superseded parts should work in the event that the OVP is actually (rare) bad. I believe most all OM60x engines used the same, or can use the same OVP. That may open your parts search up a bit.

Good luck,

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

Allow me to make this easy for you: your OVP isn't working. It's almost certainly bad, the only other thing it might be is an OVP wiring problem. Testing by listening for a click?

It's not possible to test an OVP. There are hillbilly procedures for testing whether it has continuity, but even if it does, the semiconductor that provides voltage protection can still be bad. In addition, the fuse contacts can go intermittent, and can easily fool a tester. Just replace it, and everything should snap back in line. With one caveat: a lot of the cheap replacements are worth it at half the price, if you get what I mean.

Please read through the thread above that 87tdwagen recommends.

I am the original poster of that thread and resoldering the OVP and ELR fixed my problem. I just opened them up and took a hot iron to the solder joints to reflow them. Problem solved after months and months of trying to find and fix my idle issues.

I took the Ovp relay apart and all the solders “looked” good but I have zero experience. Are you speaking of reflowing the soldering spots on the board? Because I didn’t see any soldering joints where the pins, that come out of the bottom, connect to the board itself. Can you just clarify?

Im going to have to figure this out because I can not find another new Ovp relay for sale anywhere.

Yes the spots on the board'
Watch this video for a demonstration of what needs to be done, it is easy, and the video shows the OVP getting the pins connections resoldered:

https://www.youtube.com/watch?v=zPSSXSVMiJk

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

Like 87tdwagen says above. That is a very informative video tutorial.

I just touched each joint with a hot soldering iron and reflowed the solder that was already there. I didn't add any additional solder.

Sometimes the cracks in the soldered joints are too small to see without a magnifying glass. I touched each one just to be sure and it fixed my problem.

Good luck.

I gave it an attempt and I believe the soldering iron I borrows doesn’t get hot enough. It’s 30 watts. I think it needs more heat because the bigger solders won’t melt.