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I appreciate the information provided and think it will work for my application as well but I was wondering if you could label the diagram you made with all the relay pin numbers. I am just confused as to how you actually wired the two relays up. I appreciate your help.
Originally Posted by cstephens
I am going to be making two minor changes to this circuit. I haven't yet found a DPDT relay made for automotive. And the industrial ones I have here at work are too big to fit in a box cleanly/easily without going to a pretty deep box. They're made for mounting on DIN Rail. So I am going to use two SPDT relays instead.
However, this presents an interesting issue that I did not notice the first time and would actually probably be a non-issue with a proper DPDT relay. If you look at my most recent wiring diagram with the DPDT relay, you will notice that there could possibly be a circumstance where, IF one contact made, and the other did not, there would be a short.
Thankfully, I think the solution is simple. Turn the relay around and have the E & D terminals on the commons (30 pin). This way, even if one of the contacts didn't close, it would simply be GND to GND or 12V to 12V. The only possible way the 12V and GND could ever come into contact is through the load...which might be an annoyance if I am trying to switch the tanks because one is empty, but certainly not going to burn up wires or pop fuses or anything.
This is the circuit under normal, unpowered conditions. No issues there. If BOTH contacts make at the same time, no worries, life is good.
This is the circuit if only one contact makes. This is if the contact was stuck in it’s NC position…maybe a wire to just one of the relay’s coil were to come undone or the relay gets old and can no longer hold the contacts. Whatever the case may be. As you can see, there would be a short from positive to negative. I planned to fuse that wire anyway, but I’d rather avoid popping it.
This is the new circuit under normal, unpowered conditions. No issues there now. I am still reversing the polarity of the switch, just doing it in a different place.
As you can see, even IF only one relay makes contact, the 12v and GND are isolated to only contact themselves.
Here's a idea I came up with and the diagram for it. Any way you do it, you are going to have to coordinate everything together. In other words when you plumb the valve in, and it's in certain position, you will have to make sure the correct pump is being activated.
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Is there anyway to make this circuit you have with two SPDT relays? If so how would the pins be wired? My 77 Ford F350 has the single wire going to the original solenoid switch.
I don't want to confuse the issue or anything as it seems you guys have a plan.
My mind is always geared such that I forget not all trucks have diesel engines.
An unmolested factory-issue gas burner truck of this vintage, does it have a fuel return or not ?
If no return, then a dual tank gas burner would not have the troublesome six-port tank selector valve, but a simple three-port valve instead, right ?
On a twin-tank gas burner truck that I have, no provision for return lines, it has a very simple Standard brand one-wire tank selector valve that "On" switches to the second tank and "Off" defaults to the first tank.
Unlike the problematic six-port valves that I abhor, this simple one-wire valve has never been a problem.
I don't want to confuse the issue or anything as it seems you guys have a plan.
My mind is always geared such that I forget not all trucks have diesel engines.
An unmolested factory-issue gas burner truck of this vintage, does it have a fuel return or not ?
If no return, then a dual tank gas burner would not have the troublesome six-port tank selector valve, but a simple three-port valve instead, right ?
On a twin-tank gas burner truck that I have, no provision for return lines, it has a very simple Standard brand one-wire tank selector valve that "On" switches to the second tank and "Off" defaults to the first tank.
Unlike the problematic six-port valves that I abhor, this simple one-wire valve has never been a problem.
460's and supposedly some 351w trucks did have a 6 port valve with a return if they had the hot fuel handling system on them. And of course diesels had the return system.
Is there anyway to make this circuit you have with two SPDT relays? If so how would the pins be wired? My 77 Ford F350 has the single wire going to the original solenoid switch.
Your diagram on the orange piece of paper should be it. With 85 and 86 being the coil. Yes, if you have a relay failure you will have a short, but the fuse should catch that if it ever happened.
P.S. Your diagram is not drawn correctly as far as relay function. Both relays will always be in the same position. Both with be swung to 87 or both will the swung to 87a. They will always work together.
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