Relay controlled ground?

[ftpiercecracker1]

No thats not a typo/mistake. A relay that controls a ground. 

 

My old toy tercel's electric cooling fan has suddenly stopped kicking on when the temp starts climbing. 

 

After some investigation ive confirmed its the thermal switch in the radiator.

 

The odd thing is the fan runs continuously when (key run) it is unplugged.

 

The plug for the fan has a constant 12v supply and the ground is switched to turn it on and off.

 

The relay clicks when plugging in and when i jump the terminals on the thermal switch.

 

 

I know relays can be controlled via ground or energized wire, but ive never heard of a relay that itself controls a ground. Does this make sense? How is this even possible? 

 

 

Recap:

Toyota electric cooling fan stopped working

Cooling fan plug has constant 12v w/ key on, but no grd.

Relay clicks and fan turns on when thermal switch in radiator is unplugged

Meaning grd circut has been closed by unplugging thermal switch. 

How and Why?

 

 

 

I want to assume they designed it this way such that if/when the thermal switch fails the fan will just run continuously, saving your engine. But that is not what has happened.

 

[derf]

A relay is just a switch.  You can have it interrupt a ground wire just the same as a +12V wire.

[ftpiercecracker1]

29 minutes ago, derf said:

A relay is just a switch.  You can have it interrupt a ground wire just the same as a +12V wire.

 

Right. I was thinking 12v from the battery had to pass through, like to some device.

 

Pwr grd for activating the relay, then whatever circut you want open/closed be it a grd or a pwr circut.

 

But why have the relay setup normally closed? I mean when the relay is energized the connection to the fan is open. When the relay is denergized the connection to the fan is closed and thus turns it on. Does that make sense?

[derf]

Speaking as an engineer myself, I can say with some authority that engineers make some weird decisions for all kinds of reasons.

 

It made sense to someone to do it that way.  What that reason was I have no idea.

[ftpiercecracker1]

1 hour ago, derf said:

Speaking as an engineer myself, I can say with some authority that engineers make some weird decisions for all kinds of reasons.

 

It made sense to someone to do it that way.  What that reason was I have no idea.

 

 

The ole "because they could".

 

I generally associate toyota with robust simplicity. I imagine there was a reason, but i doubt ill ever know.

[Ωhm]

9 hours ago, ftpiercecracker1 said:

I imagine there was a reason, but i doubt ill ever know.

Is it possible that the Radiator Temperature Switch being used has inverse logic, of say an MJ Radiator Temperature Switch? In other words, is the Switch closed when COLD and opens when HOT. As for switching B+ or GND to/from the FAN, that one works either way.

[silvertwinkiehobo]

The inverse logic is correct. If you have a failure mode, in this case, you want the fan ON, not OFF, if it fails. And doing so on the ground side makes the relay last a lot longer, because it minimizes the induction arc from the motor start-up as the contacts close on relay de-energizing.

[ftpiercecracker1]

7 hours ago, silvertwinkiehobo said:

The inverse logic is correct. If you have a failure mode, in this case, you want the fan ON, not OFF, if it fails. And doing so on the ground side makes the relay last a lot longer, because it minimizes the induction arc from the motor start-up as the contacts close on relay de-energizing.

 

None of this makes much sense to me, so ill surmize the whole system once more for myself and others.

 

Car is cold > key in run position > install relay > relay clicks > 12v at two wire fan plug, but no grd

 

Car cold > key in run position > relay installed > unplug thermal switch > same relay clicks again and fan turns on.

 

Jump wires for thermal switch and fan turns off.

 

The fan must be receiving B+ from a different source and not the relay.

 

 

Looking at just the relay and thermal switch.

 

When there is a jumper wire connecting the two leads of the TS plug the relay is energized. But with how this circut is configured, energizing the relay opens the controlled circuit.

 

The TS is a normally closed postion switch. Car is cold TS is closed and relay is engergized. Car warms up, TS opens, relay is denergized, grd connection is made and the fan turns on.

 

The relay being pretty much constantly engergized would explain why it gets a million degrees. Like can't put your finger on it hot. 

 

It also seems it failed to fail as designed too. As metioned it seems they tried to configure it with a fail safe in mind.  Should the TS fail it should fail in such a way that the fan will just run all the time, this is not the case for me. Car just got hotter and hotter. 

[MJCARENA]

Sounds like your temp switch fails to open at the correct temp or at all. Maybe that the switch contacts welded themselves together.

[gogmorgo]

A standard automotive relay can function as either or both a normally open and normally closed switch.

It doesn’t only “turn on” current between pins 30 and 87 when it’s energized. It swaps pins 87 and 87a. When the relay isn’t energized, pins 30 and 87a are connected.

Its not entirely common to run a circuit off 87a because you typically want things to turn off when the key shuts off, so this means adding another switch into the circuit to control b+ to pin 30.

More commonly the function is used to alternate between two accessories off the same power source – imagine a high beam/low beam switch where power comes from the headlight switch and is directed at the low beam until you hit the high beam switch, energize the relay, and power swaps over to the high beam circuit. Your fan is the “low beam” in this context, and you don’t have a “high beam”.

You can take advantage of this if you’re running dual-speed fans, it will run low speed until it hits a certain temperature and then switch over to high speed. I’ve also been thinking about incorporating it into a headlight relay harness to allow for “normal” DRL function.