Testing OVP
 

Hi

I'm new to this board but I'm grey hair to troubleshooting.

I was coming to this board because wifey Benz suffer from a gremlins.

You know the stalling problem when you decelerate at a stop. Wifey benz does it when the engine temp reach 85C and you decelerate. She have to fully depress the trottle pedal and crank it for about 30 secs and it will re-start at that point the idle will play from 1000 to 500rpm after a minute or two it will set up and the engine temp will rise to about 90C. She could drive the car for 100 miles after without any stalling unless she stop the car and let it cool down.

So now wifey is like Cendrillon she have to be back home before 85C. :-)))

When I do a search on this forum there is a lot a post pointing tower the Over Voltage Protection relay(OVP).

Being familiar with relay, I know they can be tricky but we can test them.

First this relay is not an over voltage but an over current relay it is built for 10 amp if you ask more than 10 the fuse on the top will blow this will protect your component from over current.

Why are we using relay?
To protect the ignition switch from overheating. The ignition swtich just activate an electromechanic plunger(few amp) to close the circuit and let flow the power from the battery to the component.

How can we test it?
First test the wiring harness:
Ignition switch OFF
Unplug the connector, use a test lamp or a multimeter check all the pin from the connector for power. You should have one pin with battery voltage this is the pin 30(always). If you can't find one, this is a wiring harness problem check for broken wire or bad terminal.
Check on the relay to find the pin 31 this is your ground find the connector terminal who plug to that pin and check it for ground this can be done with a test lamp or ohmeter.
Ignition switch on
Again check the connector pin for battery voltage you should have two pin with battery voltage the pin 30 from our previous test and the pin15. If you can't find two pin with battery voltage again check the wiring harness.

What we achieve with does test is:
Make sure we have battery power to the relay
Make sure the ground is good
Make sure we have power from the switch to activate the electromagnetic plunger in the relay.

Testing the relay:
We might use jump connector or just plug the relay into the connector so we can touch the pin with our test lamp or our multimeter.
Ignition switch ON
Now you should test for battery voltage on pin30, pin15, and all the pin87 should have battery voltage. if not throw the relay in the glove box and buy a new one.

This is a standard test, if you think your relay do not work under certain temperature just use an hair dryer to overheat it or put in the freezer to cool it down and re-do the test. You may do a load test using a 55 watts sealed beam on any pin87, do this one on the bench with jump wire.

Now that you read all this here a hint I have 3 different diagram of this realy for different Benz model all diagram show that if the realy do not work the abs light will come on.

Well I still searching for the gremlins in my wife car my relay is good.
If you're a tech and know where I should look next let me know.
Hope this post will help some of you.

Max
John Deere Combines Specialist

The OVP relay has a zener diode in the circuit (see diagram on relay case) which means it is an over voltage device as zeners fire on a preset voltage not current...the fuse blows when the zener fires as it (the zener) now presents a short to ground so drawing excessive current..so... the fuse blows because there is an overvoltage condition...a zener and a fuse always go together in any over voltage protection circuitry...hope that clears up the function of the OVP...note that the OVP RELAY when removed should not affect performance (bUt u then don't have OV protection!!) I HAVEN'T TRIED THIS THO..
Peter
300sel

I took my OVP relay out, applied 12 vdc to pin 30 and pin 15 (with pin 31 to ground) and nothing to pin 87, unless I used a toothpick to move the relay contacts closed, then I had voltage to pin 87. One thing though, when I first applied the voltage to pins 30, 15, I had voltage to pin 87 for a few seconds, then I heard the relay open, and no current to pin 87. This happened a few times, then nothing to pin 87 at all unless I did the toothpick thing mentioned above. Also the diode checks good with a multimeter set on diode test, got a reading with the meter probes connected one way, but not the other like it should. Maybe the diode breaks down with current?

I tried calling an electronics components place withthe number on the diode, but of course the number didn't cross over to any thing!

M.B.Changed the components and design on the OVP and FPR a few times during the lifetime of use.

These components are now up to and above 20 years old and more.

The components can change their electrical characteristics over that time especially being subject to extremes of heat or cold as the whole vehicle is subjected to.

Finding new individual electronic components e.g. Zener diodes that were in production 20 years ago is not the easist of tasks - especially when they were produced in third world countries in a limited run basis.

The diode is probably not your problem - the coil inside the relay is (the electromagnet that makes the relay contact move).

A bad relay coil will do exactly what redlaser said above the way it's wired.

Since the coil isn't moving the relay contact anymore, it seems that would be the culprit.

I just did an OVP test on a spare one I had that didn't work. The zener diode was fine as well as the regular diode near the relay's coil. The problem with this OVP was the coil itself, it had an open thus it's fubar.

Upon further inspection, I found that it is the coil, not the diode. What's weird is that after sitting on my back porch all night (humidity) the relay works fine for a while, then back to no action from the coil, so as a temporary "fix" untill I replace it I used solder to make contact between two points on the circuit board, next to the pin that has constant voltage. When the coil is working correctly, and the ignition is turned to the on position, which gets juice to pin 87, it seems that one of these points is from the coil that would supply the voltage to pin 87. So I just bridged them together. I know it's just a "jerry rig", but untill I can cough up $90.00 for another relay it will have to do. Anybody know a source for relays that is cheaper than the dealer?

Thanks,
-DDaymen

Fastlane?

Speaking of OVPs, anybody have the skinny on the (at least) two pinouts?
For instance, the difference between a 560SEL and a 190E 2.6?

Thx -CTH

If you are tight for cash.
Why don't you get one from a wrecking yard.
Their plenty there.

Regards.
aam.

Already checked out the wrecking yards, no 560sl's, but I'm sure other models use the same relay, I just don't know what they are. Funny thing happened this morning on my way to work, stopped to get coffee, turned off the ign. switch, removed key, nothing happened, the car was still running, no dash lights came on, it was like I had not turned it off or removed the key!, after inserting the key again, turning it to the run position a few times then turning off and removing, same thing, no effect. but then after driving the rest of the way to work, parking, then turning off the car, all was normal again!

Strabge things these cars can do!

When the OVP is removed, the system runs in open loop. There is no computer controlled adjustment.

This is my understanding of how a Zener diode works and the function it plays in the OVP relay. The OVP relay contains what is known as a Zener diode which has a reverse bias (the diode's anode connects to the negative supply and the cathode connects to positive supply). When the reverse voltage across the diode exceeds the rated voltage, then current starts to flow in order to limit the increase in voltage. What is great about this diode is that it maintains almost a constant voltage over a range of current values as long as the current remains within the diode's breakdown range (between the current min and current max rating for that specific diode). What this does, and why it's important, is that it can maintain a very stable voltage level over a widely varying load. With DC circuits as the load varies, so to does the average output voltage. By adding a Zener diode you can stabilize the output voltage. Why is this important? Well, there are certain components that are very sensitive to voltage changes, and in an incredibly complex set of circuits such as a modern automobile with all sorts of various components that may or may not be operating at any given time, that may be operating at varying load levels, and with just good old physical wear and tear...there is going to be some serious variance in the load. Adding the Zener diode makes sure that this load variance doesn't affect the voltage output, and thus you don't fry your components & fuses or generally experience frequent malfunctions. A zener diode, and the OVP relay for that matter, does not regulate the current to my knowledge. It regulates the voltage as the name implies. Are you thinking about the saturation current?

Oh, and one handy little bit of information to have about the Zener diode in the OVP relay is that it is actually incredibly easy to test. Set your DMM to the diode setting, and connect the positive lead to the pin that corresponds to socket 31 and the negative lead to the pin that corresponds to socket 30. Do this with the relay disconnected from the vehicle. You should receive a result between .4V - 1.5V. There could certainly still be other issues with the relay, but at least from that point on testing is the same as any other relay...which is pretty much what your procedure outlines. Oh, and one other thing...given the nature of this diode, you might be able to guess why it's a really bad thing to reverse the leads when attempting to jump start a vehicle, or why you really don't want to create an electrical arc at the terminals. This sort of thing tends to kill your OVP relay.

Let’s get this clear. There is no effective test for the OVP relay. I don’t care who posts what here, and I don’t care what may or may not be in the FSM. There is no effective test. If you have symptoms related to the OVP, substitute another unit.

As to whether it’s over current or over voltage regulation, it’s both. Primarily it’s there for overviltage protection (thus the name). This may be accomplished with a zener. While you can test the blocking ability of the zebra as mentioned above, testing the breakdown voltage would require isolating the diode and using a variable voltage power supply.

As for over current protection, the OVP is one of several relays that is incompetently placed in an unfused power source. In order to protect the wiring harness, it has one or two ATE fuses in the lid. If one of these fuses blows, it may indicate a short in a downstream circuit. It may also indicate an internal short. The OVP would be suspect in either case. The peculiar mounting for these fuses can also cause intermittents.

The other thing to consider with the OVP is poor solder connections internally. There isn't any realistic way to get inside the OVP without breaking something, so when issues arise with the OVP, you're time and money ahead to substitute a known-good unit, or install a brand new one.