Electrical help

[Bitch'nBronco]

Today my fan shut off going up a hill and I noticed the plug for the relay I have wired in was melted. I'm going to trace wires/grounds to see if there's something causing the heat (chaffed/damaged wiring) but Im wondering if I should add a second relay in series. The fan is a 12V 270W volvo 740 fan.

The relay that was plugged in was actuating but not making complete contact, I swapped in another relay that I had and the fan worked again, but could feel the wires getting warm within a few seconds of running it. Unsure of why this is happening now, since ive been running this setup reliably for 8ish years.

Sorry if the post is a little frantic, trying to figure it out before I drive 40 mins home thru hills and dont want my Bronco to have an electrical fire on the way home. (Standing by for the mechanical fan comments lol)

 

[ntsqd]

270W at 12 VDC would be 22.5 amps. More than the typical 'sugar cube' relay is designed to handle.

To gain more ampacity you'd need to operate two or more of those relays in parallel, not series. In theory two such relays operated in parallel would each see 11.25amps, well within their typical 20A rated capacity. Alternately can use a higher current rated single relay. Buy with some overhead. For a 22.5A motor load I'd be looking for at least a 50A rated relay.

HOWEVER, this is ignoring the likely cause of the problem. Electric motors have large so-called "in-rush" currents during start-up. Very briefly these can easily be up to 2 times the steady state running current. It varies from motor to motor, but 2X is the RoT that I use when laying out the circuit. Usually do not need to use a fuse that is 2X the steady state current, but other components will live longer if this RoT is used in their sizing.
Breakers can be a different story. Fuses and fusible links tend to tolerate brief over-current conditions better than do breakers. there may be a more scientific way to do this, but this is what has worked for me: If you want to use a breaker be prepared to change it as needed. I'd start out with a breaker that is rated 1.25X what the steady state demand is rounded up to the next size (so 35A in this case), and then see if it randomly pops. If it does then replace it with a breaker that has 5A more capacity. Repeat.

 

[Jdgephar]

Crimp connectors and interfaces between the spade and the connector can also overheat if they become oxidized (corrosion). 22 amps is a lot of current for those 1/4 spade connectors. 8 years of that takes it's toll on the connection. It doesn't take very much resistance at 10 amps of current to start creating 0.5 watt or more of heat at a connector.

I don't like the two relay option. If one fails, now one relay carries all the load again. I also assume you mean the relays would be wired in parallel, not in series. I would go with a single relay rated for the current you are using, and something better than a 1/4" spade terminal with that kind of current level, if you want it to last and not overheat again.

There are some 50amp relays with screw down terminals for the main power in / out. I would look at those.

 

[Bitch'nBronco]

Thanks, the brass contacts inside the plug looked corroded. I appreciate the detailed responses. I've definitely got some room for improvement, ill see what I can scrounge up on the way home and then go thru the harness and look into the different relay options,fusable link, etc

 

[ntsqd]

There's a Volvo relay on ebay that might be worth looking at. No idea what it's ampacity is, but it may be the part that originally switched your fan?
Kit: https://www.ebay.com/itm/1444108718...d=link&campid=5335857059&toolid=20001&mkevt=1
Relay only: https://www.ebay.com/itm/1772940192...d=link&campid=5335857059&toolid=20001&mkevt=1

 

[ba123]

Why don't you rewire it with constant hot to the fan and have the relay control the ground? Will that fan work that way? Then just add a beefy enough fuse inline to the direct power feed.

 

[Jdgephar]

Why don't you rewire it with constant hot to the fan and have the relay control the ground? Will that fan work that way? Then just add a beefy enough fuse inline to the direct power feed.

The fan should be able to work that way. However, You're still switching 22 amps of current. The connector can still overheat. You still want your fuse / circuit breaker as close to the battery positive as possible.

 

[ba123]

No you're not. You're switching a ground. The only power on that relay is power to control the switch. No load if done that way.

 

[ntsqd]

Amps without voltage = no watts.
That assumes that ALL of the voltage drop happens across the fan motor.

 

[73azbronco]

No you're not. You're switching a ground. The only power on that relay is power to control the switch. No load if done that way.

Minimal power at the switch side of a relay, so hot or ground does not change draw on acitvation circuit. Problem is draw side running full power of probably 100-200 amps.

 

[ntsqd]

OP says 270 watts, figuring worst case at 12.0 Volts DC that's 22.5 amps. Will go down as the system voltage goes up. Rough estimate of the fan motor's in-rush current at 2X steady-state worst case running current makes it 45 amps. Wild and crazy let's say 50 amps. In-rush current rarely lasts very long, measured in micro-seconds.

 

[Bitch'nBronco]

There's a Volvo relay on ebay that might be worth looking at. No idea what it's ampacity is, but it may be the part that originally switched your fan?
Kit: https://www.ebay.com/itm/1444108718...d=link&campid=5335857059&toolid=20001&mkevt=1
Relay only: https://www.ebay.com/itm/1772940192...d=link&campid=5335857059&toolid=20001&mkevt=1

I have the Volvo Relay, this is how I have it wired currently. I forgot to reply yesterday, I appreciate the detailed responses, the connector spades were a little corroded and I took the failed relay apart, the plastic melted into it and stopped the relay from completing the circuit.

 

[Bitch'nBronco]

Where it melted was the power from the battery (pin 87 on the relay, 12V in with a 30A fuze inline) and a little melting on Pin 30 (to the Volvo relay). The relay I had was an RL45 40A/12V. I'm going to bump to a bigger relay and replace all of the wiring going to/from the relay. The Volvo relay was replaced with a new OEM Volvo replacement about a year ago so I don't think that's the issue.

 

[Bitch'nBronco]

Why don't you rewire it with constant hot to the fan and have the relay control the ground? Will that fan work that way? Then just add a beefy enough fuse inline to the direct power feed.

I like that idea, then all of the load is on the Volvo relay

 

[ntsqd]

The relay that melted is the power supply relay, not the Volvo relay? 30A is right at max capacity for some, not all of what appears to be a Bosch "Sugar Cube" relay. Did the one that melted say "30A" on it?

 

[Bitch'nBronco]

Its the cube that melted, its a 40A/12V cube. I didn't find anything wrong with the Volvo relay

 

[Bitch'nBronco]

Kind of hard to get my phone to focus, but you can see the plastic melting into the relay (opened it up). All of the copper is discolored which makes me think there was a good bit of heat inside of it. Its not mounted near anything hot, its on the core support behind the headlight.

 

[Oldtimer]

Looking at the picture in the first post, the heat zone appears to be centered around the crimp for the yellow wire.

 

[ntsqd]

I was just taught to use the zoom for macro shots.

 

[Jdgephar]

Where it melted was the power from the battery (pin 87 on the relay, 12V in with a 30A fuze inline) and a little melting on Pin 30 (to the Volvo relay). The relay I had was an RL45 40A/12V. I'm going to bump to a bigger relay and replace all of the wiring going to/from the relay. The Volvo relay was replaced with a new OEM Volvo replacement about a year ago so I don't think that's the issue.
View attachment 949282

What's the purpose of the additional relay with the toggle switch? Are you using it just to be able to disable the fan from turning on? I assume it also makes sure the fan turns off with the key, right?

If that's the case, ba123 is right, just use the relay to open or close the ground connection to the temperature switch. Then the relay only has very low current through it. The battery 12V from the fuse can go direct to the Volvo fan relay 30 terminal. This will eliminate two high current connections (plus the relay contacts) in the circuit to fail.

You could change the toggle switch to have another position (or add a second switch) and use that to force the fan on (in low or high speed, depending on how you wire it).