Can a 4 prong relay be used to trigger a ground?
 

http://forums.pelicanparts.com/uploa...1714506468.jpg

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I’m trying to use a relay to trigger a ground connection.

In my hand drawing, the wires coming into 87 and 30 complete a ground (or a circuit) that triggers a separate fan relay.

So as I understand it, when I put 12v into 86, it should close the relay switch, and complete the circuit between 87 and 30.

When wired like this, it does work. For a while. But the relay fails after a short time.

I figure I must be missing something. I know it’s not the normal use of a relay, like shown in the diagram, in that it’s not triggering a higher load circuit, but in my feeble electrical mind it should still work.

How many amps does your fan pull? What is the relay rated for? Likely the relay is not rated for enough load.

Just to be sure I understand, you've got 1 relay triggering a second relay, not providing power for the fan motor, right?

That’s correct.

It is completing the circuit of the temp sensor on the radiator. Those are very thin wires, triggering the high speed fan relay (the fan uses huge wires).

The fan pulls big amps, the power wires for it are huge. But see above. That should make the fam motor amps irrelevant, right?

http://forums.pelicanparts.com/uploa...1714508221.jpg

The top sensor is the high speed fan sensor. The wires piggybacked onto it are going into 30 and 87.

So when the relay is activated (or when the temp gets high enough to close the sensor switch), the high speed fan relay is triggered.

Could be an arc is being created in the first relay points from the coil in the second relay, is this being switched a lot? Could try a small varister to ground to help the voltage bypass the relay. If that is what is happening. If you just touch the wire to ground to turn on the second relay and pull it away does it arc?

Edit, your last post just arrived in, so you are burning out the temp sensor contacts? If so same load related problem.

No arc.
(30 amp Bosch relay, btw)

How many milliamps does the relay coil use? Any idea what the current the temp sensor is rated for?

Idk if I’m burning out the temp sensor contacts, the car never gets anywhere near hot enough to trigger it.

I don’t know, but I can try to figure it out.

I’m guessing it has something to do with my piggybacking the wires like that.

Can you do a quick sketch of the entire circuit. Sounds like you have a manual switch two relays and temp sensor all working together?

http://forums.pelicanparts.com/uploa...1714511024.jpg

This is my understanding.
The X wires are added by me.
The Y are existing.

I do have a manual switch, shown on the left.

All of this complication is caused by my determination to have the switch light up when on. So the switch has 12v going into it for the bulb, when I press it in, it turns on the bulb, and also sends 12v to the relay.

It actually does work. I press the switch in, the internal light goes on, and the fan is triggered.

But it seems to kill the relay eventually.

That should work, I don't see why that relay would not last. Looking at picture of your temp switch I see a red wire, that is not a positive is it?

So you have 2 ways to trigger the fan, the temp switch AND a manual switch (parallel)?
Or is it that both have to be engaged for the fan to come on (series)?

No, that’s one of the wires coming from my added on relay. (Probably should have used another color),
In this picture, the 2 disconnected wires on top are the factory temp switch, the bottom are the 2 from my relay. They are piggybacked at the temp switch.http://forums.pelicanparts.com/uploa...1714513551.jpg

It’s parallel.
When I hit the dash switch the fan comes on, no matter what temp the radiator switch is at.

Is the contacts that fails on that first relay? If so I'm still thinking you are getting some voltage spike back from the fan relay coil.

Idk. It stops working, I change the relay and it works again.

One of the times it failed, if I tapped on the relay with a wrench, it would get going.

Is it possible I have the relay wired wrong?

If it was wired incorrectly, would it work at all (even temporarily)?

Your sketch looks correct. Could it work if wired wrong, well it would be able to work if the wire from term 85 (the ground side of your first relay coil) went to your other relay coil, then you would have both coils in series and your switch would energize both coils. Try a lower rated relay, there is a chance that the contacts in that 30A one do not get far enough apart. Check the wire diagram for the other relay though, does it only have 4 terminals to plug into?

That's what I'm thinking there's an issue with having 2 switches in parallel. I can't remember what the issue is, but I've run into this before. I think having 2 switches on one circuit requires some other mechanism to ensure that you don't have issues, which is part of the reason why there's more to wiring 2 way light switches than just sticking in an extra switch.

I don't think that will be a problem, as long as the ground is the same for both. Different with house wire because you want to be able to turn on and off from each switch.

http://forums.pelicanparts.com/uploa...1714523041.jpg

Here’s the diagram. It’s the high speed temp switch im piggybacking on.

80's BMW? Looks very similar to my E28 circuit.

Place an amp meter in series after your relay or at the coolant temp switch just to check how many amps it is seeing, that side is fused at 7.5A so should not be much.

E24, so similar to your E28.
I’ll check with the amp meter, will have to wait for my new relay to get here.

You added a physical switch, correct?

Why have a relay at all? you're "switching" the low current coil side of another relay, you can just put the switch in line and toss the extra relay.

Also, I'm concerned that what you're getting is a feedback loop. Are the new relays dying when the radiator is at high temp? You might be getting a loop where the temp switch and the relay flutter when the temp switch closes.

Yes, it’s a physical switch.

I added the relay because the switch has a light bulb in it. I rigged it so the switch lights up when it’s activated (like a typical AC switch).

I can’t see any other way to accomplish that without a relay, but I’m open to suggestions.

Just checking that you are not feeding the coil of the relay through the light bulb?

It’s possible that I am. It’s been a while since I made the modified switch, I don’t remember exactly what I did.

I did make a drawing at the time, but looking at it I don’t really understand the switch part. It of course made sense to me at the time, but now I’m not sure what it means. (But I suspect it may be feeding the coil through the light bulb).

http://forums.pelicanparts.com/uploa...1714569793.jpg

It’s an LED bulb, if that matters.

You show a ground wire going to the switch so you are probably ok but check it anyways. I was thinking that if you had the light in series with the relay that would limit the power going to the coil and could explain the short lifespan, light would work fine but relay would not.

Use a current rated PNP transistor circuit for ground switching.
That is how industry does it.

Interesting. Looks like I need to use a diode to protect it?
I’ll have to dig into this, on first read it’s above my current (pun intended) level of understanding.

https://electronicsclub.info/transistorcircuits.htm

One thing I do not like about transistors is when they fail they can remain on, relays usually fail off. If you are having problems keeping a relay working SSR will likely be less reliable.

Maybe I can eliminate my add on relay?

How about wiring like the bottom diagram?

http://forums.pelicanparts.com/uploa...1714596246.jpg

That should work, but the relay you have in there now should also work. With that new setup make sure you have a fuse where you pull the power for the light by the fan, or have wire rated for 30A. I still think you should try a lower 15 amp rated relay in the existing setup.

Ok, I’ll try a 15 amp relay tomorrow.

If that doesn’t work, I’ll try the alternative wiring with a fuse.