Is a relay just a device that turns things on and off? - PeachParts Mercedes-Benz Forum

Machnumber2 · #1 04-20-2001, 12:02 PM

For example, the OVP and the fuel pump relay are just switches that close and open an electrical circuit right?

mbdoc · #2 04-20-2001, 12:32 PM

yes!

stevebfl · #3 04-20-2001, 01:08 PM

Yes and NO!

They are each current carrying switches that cause an action. They both also have other functions. A fuel pump relay decides that a motor is off because the rpms are below 300; thus potentially saving lives in an accident. It also shuts down the fuel pump at a certain rpm to save the motor.

The OVP relay uses a zener diode to short circuit all power surges over some voltage. It clips all voltage spikes through the fuse. So no, it is not just a device that turns things on and off!

JimF · #4 04-20-2001, 01:38 PM

I'd like to expand on Steve's statement" They are each current carrying switches that cause an action."

Were talking now about the "current carrying portion" of the relay. What goes on here?

Relays, in theory, are the best way to switch two signals into one source. They have better than 10 Meg ohms isolation to the non-switched signal and 0 ohms loss to the needed signal. The best of all worlds!

BTW, practically speaking, "0 ohms contact resistance" is more like 0.01 to 0.1 ohms resistance during the relay's life expectancy.

But the realities of a relay make it not-so-perfect! Among the many things that they are rated for, is 'contact life'. In other words, it's good for a certain number of switches. That means that after those number of switches, its contact resistance now starts to rise from 0 ohms to as much as 10MEG ohms. Of course, at 10 Meg ohms, it's not a relay any more, it's a resistor! Under high contact resistance, the relay is said to be "OHMIC".

The ohmic condition is what affects a car's operation. When the relay contacts become omhic, electrical current is dropped across the relay's contacts 'robbing' the power from the intended device. That is why the car won't start or fuel pump won't run or runs intermittently. Remember, I'm only talking about the contact portion of the relay. In relays like the OVP, there are other functions going on but these can NOT work if the relay becomes ohmic!

Most military grade relays are rated for 100,000 operations. Commercial ones are rated for typically 10,000. But this does not mean that it will last for the rated number of operations. This data is gathered statistically so any given relay could fail immediately whereas the next one could last for 1,000,000 operations. It's kind of a crap shoot!

So not only can the relay fail by the zener shorting, using the OVP as an example, but can also fail due to becoming OHMIC. And this is the most common method of relay failure!

Things that you NEED to know. Hope this helps.

[Edited by JimF on 04-22-2001 at 11:26 AM]

Gilly · #5 04-20-2001, 10:57 PM

Let me try! I'll shoot for not too much info, and not too little. Yes, a relay takes a small input signal, lets say from a switch on the dash, and turns that signal into a closed circuit for a device that is a higher current consumer, say for example a headlight or a horn. The reason they came to be was to prevent running the higher gauge wires that the electrical comsumers need all the way to the switch. In the case of something a little more complex, say the example of a fuel pump relay, the main job is the same, turning on and off the fuel pump, but instead of a switch that the driver operates, there are many switches that decide that it is OK for the fuel pump to run, and none of them are really controlled by the driver, except in a very remote sense (turning the key on and off).....Gilly

mbdoc · #6 04-21-2001, 08:02 AM

Some times we try to go over board & get a bit too technical! A relay's job is to switch power from a remote trigger signal. It's job doesn't change regardless of other devices inside of the relay. ie: Overvoltage relay, it's primary job is to supply power to computers & yes it has protection to keep unwanted power spikes but but still if it doesn't do its main job & keep computer powered-up then nothing else matters. MR Webster defines a relay as an electrical device that responds to a change of current or voltage in one circuit by making OR breaking connection in another. Very good definition. MB has some very complicated relays on their cars, but the purpose of any relay is the SAME. TURNING on power.

#7 04-21-2001, 10:22 AM

As a shade tree mechanic, interested in learning, I appreciate the over technical discussion. There is relevant and new information in these posts. I learn a lot on this site and visit often. One reason I read posts that do not pertain to any problem I have is so that I can learn something useful. This has become a hobby. Not only fixing and fixing up my Benz, but also learning as much automotive mechanics as I can. Electrical is a weakness for me and sometimes I struggle to understand. Even if the thread is over my head I can learn from it and research if necessary and/or practical. Keep it coming. If you guys are willing to spend your time to explain these things in detail, I will be reading.

Thank all of you for spending your time and resources.

Steve
'85 500SL Euro

jeffsr · #8 04-21-2001, 09:47 PM

Here's a simple, easy to digest relay definition. For the most part, relays use lower voltages and currents to control larger voltages and currents, otherwise all the switches and buttons in our cars would have to be industrial grade and HUGE!! to handle current loads. Transistors are in a way like relays. They use a small voltage to control or valve larger currents and voltages. Relays can be simple or very complex, such as the OVP item in the MBZ's. As Steve stated this device "clamps" voltage to sensitive devices with a zener diode, which is a semiconductor that only passes it's rated voltage. Over that number, the fuse blows and the relay opens and protects the sensitive electronics from being baked.

Machnumber2 · #9 04-21-2001, 10:07 PM

This is great information, makes the inside of the car a bit less intimidating

#10 05-01-2001, 02:55 PM

OK. I'm going to replace this device on my 13 year old 190E. It exhibits all the simptoms described in this and many related posts. Now to display my ignorance. Where is it locate on the car and what is required to change it? Is any calibration necessary or will the new one just work?

Thanks for the wealth of knowledge.

Darrell

G-Benz · #11 05-01-2001, 04:34 PM

By the way, the solenoid on your starter is another word for "big damn relay"! Hate to have that much current flowing around my hand when I engage the ignition...

Anyway, the OVP is located behind the battery. There is a black plastic shield behind the battery (held in place by form only), that can be removed simply by pressing near the top of the lip and moving it out of the way of the firewall.

From what I've read in earlier posts, you would now be staring at three units, one that controls the fuel pump, another for the air conditioning, and the OVP. In early models, it ha a red top with one or two fuses (mine has a green top). I am waiting for my OVP to come in the mail shortly, so I can't tell you anything else about replacement until it comes in.

I am sure more replies from other more-experienced owners will follow.