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1978 E350 7.5L Class C Motorhome
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Discussion Starter · #1 ·
I've been doing some refurbishing on this motor for a while now, and I'm almost done with the fuel system - one of the final pieces. Before I wire things up, I want to make sure that what Im doing is correct and makes sense (being the electrical illiterate that I am.) Here is the current blueprint.
Handwriting Font Rectangle Parallel Schematic


I'm not totally certain how/where to connect on the ignition. I'm also not to sure what relay to buy, if anyone has any suggestions? I'm certainly open to critique or better options.

The idea here is I have a toggle that controls which pump is on, or I have the option of leaving both off. There is a 2nd switch for the Sending Units to activate the gauge; I didnt feel like undoing all that wiring (the gauges work) just to make it 1 switch. Theft deterrent was one thought (along with a manual shut-off valve under the vehicle for when it's parked for any length of time) as well as full control of the fuel system separate of the ignition. I would like the option to prime the carburetor before cranking if the vehicle has sat for a while. Connecting to ignition AND Starter should allow me these options, correct?

BTW! This is a 1978 E350 Camper Special. It is an RV. Stock internals. Edelbrock 2166 manifold, edelbrock 1405 carburetor, timing set to straight-up, plans to get headers so it can breathe.

Here are pictures of the new fuel setup. Pumps are Carter 4600HP, all lines are Aeroquip Startlite PTFE hose.
Rear Pump
Motor vehicle Automotive tire Bumper Automotive exterior Automotive wheel system


Front Pump
Automotive tire Motor vehicle Hood Bumper Automotive exterior


Y-junction w/ pressure gauge, on/off valve, then fuel filter. Sending Units were rebuilt (brand new cloth filters) by Tri Starr Radiator - they did fantastic work on my old beat-up units.
Automotive tire Motor vehicle Bicycle tire Wood Automotive exterior


And the engine bay with another gauge and carburetor connection.
Motor vehicle Automotive design Automotive tire Automotive fuel system Rim


Thanks for looking, and TIA for any help!

Josh
 

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I never use the starter relay power side. Your carb already has enough fuel in it to run for a little while. Oil PSi comes up fast and will start the pump. Besides if it's having problems starting, why pump to much fuel? I also put in a manual bypass spring loaded switch. In case I want to use the pump with out the engine running.
 

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Your diagram is more complicated than necessary, and does not energize the power relay when oil pressure is low (for priming). That is irrelevant, as the relay is always on and will drain your battery (swap NC/NO). However, just swapping will mean the pressure switch is irrelevant and the system always on if key is not off (low psi = power from key sw, high = power from Bat).

The selector switch should preferably control relay signal, not outbound power (2 independent pumps, 2 relays). The diagram is missing fuses. With a few fixes it should work, but I would simplify your concept and wiring to match your purposes.

To design the circuit, use your drawing and ask yourself "where can the signal, and the power, go in this condition?". Reconfigure until it goes where it should, when it should. When possible, control the relay signals, with power switched/routed as an end-event. You'll get it. :cool:
 

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Discussion Starter · #5 ·
Your diagram is more complicated than necessary, and does not energize the power relay when oil pressure is low (for priming). That is irrelevant, as the relay is always on and will drain your battery (swap NC/NO). However, just swapping will mean the pressure switch is irrelevant and the system always on if key is not off (low psi = power from key sw, high = power from Bat).

The selector switch should preferably control relay signal, not outbound power (2 independent pumps, 2 relays). The diagram is missing fuses. With a few fixes it should work, but I would simplify your concept and wiring to match your purposes.
I see what you're saying about the relays: Each pump should have its own relay. Then wire those relays into the selector switch. As I design a new circuit, I'm not sure where to put the Pressure Switch. Will a 3 prong switch work? Will I need 2?

I never use the starter relay power side. Your carb already has enough fuel in it to run for a little while. Oil PSi comes up fast and will start the pump. Besides if it's having problems starting, why pump to much fuel? I also put in a manual bypass spring loaded switch. In case I want to use the pump with out the engine running.
I was hoping to use the selector switch as manual bypass, im just not sure how to wire that up. Honestly not sure how to wire any of this up. There are plenty of diagrams out there, but none of the ones I see for 2 pumps take a safety or inertia switch into account

Here's my 2nd attempt. This is the first time I've ever really dabbled with wiring something up, minus basic soldering (guitar) following a diagram, so this is a 100% learning experience for me. Excuse any ignorance lol
Handwriting Font Rectangle Parallel Paper
 

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Closer. The purpose of the relay is to control a higher amperage circuit with a low amperage circuit. So, 86 and 85 should be the low amp control side, and 30 and 87 (or 87a depending on usage) will be your load side. In the diagram above, you have your switch on the load side of the relay, requiring the use of a high amperage switch to feed power to the load side of the relay. So, both relays will be energized all the time, but only one would have power feeding to the pump. I would place the switch to close ground on the control side of the relay, and leave the load side wired hot constant, that way the relay would be off, and only allow the bat + voltage to pass when the switch is selected, and the relay energized. Also, skip the starter solenoid and go direct from bat + with an additional fuse.

Unless you were planning to pull fuel pump relay load power through the ignition switch, in which case...you shouldn't. Mark the 4 terminals of the standard relay on your drawing...it makes it easier to visualize.
 

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When selecting fuses and wire gauge remember E=A*R. E is electromotive force, or voltage as we all know it, A is amperage, and R is resistance. By measuring the resistance of a component, you can apply your voltage and calculate amperage (A= E/R) and select proper fuses and wire diameter. There are plenty of charts online to steer you in the correct direction, but remember, static battery voltage and running voltage are different. Calculating total circuit Amps and ohms varies from series to parallel, so I suggest brushing up on ohms law.
 

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The manual bypass switch, bypasses the oil pressure switch only for using the pump/s when the engine not running. As I look at your diagram, the manual switch you have will have all the amps for the fuel pumps running though it, appx 30 amps. You don't want that. You want it on the side of the lesser amps, that control the relays themselves. Power for the fuel pumps relays, to power up the pumps, should go right to the power source. Lesser power that controls the relays, is where you want you manual selector switch. It's hard to find, and I know I'll be corrected, a good auto 30 amp switch anymore, which is why we use relays now.
 

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Discussion Starter · #9 ·
Closer. The purpose of the relay is to control a higher amperage circuit with a low amperage circuit. So, 86 and 85 should be the low amp control side, and 30 and 87 (or 87a depending on usage) will be your load side. In the diagram above, you have your switch on the load side of the relay, requiring the use of a high amperage switch to feed power to the load side of the relay. So, both relays will be energized all the time, but only one would have power feeding to the pump. I would place the switch to close ground on the control side of the relay, and leave the load side wired hot constant, that way the relay would be off, and only allow the bat + voltage to pass when the switch is selected, and the relay energized. Also, skip the starter solenoid and go direct from bat + with an additional fuse.

Unless you were planning to pull fuel pump relay load power through the ignition switch, in which case...you shouldn't. Mark the 4 terminals of the standard relay on your drawing...it makes it easier to visualize.
If im translating this correctly, you're saying that in my current iteration the relays would always have power (which is unnecessary - they only need power when switched to on) and that, instead of using the switch as a 'trigger' of sorts to allow that constant power to come through to the pumps, I should use it as more of a 'gate-opener' to allow power into the relay/pump selected. If I have it correct in my head, closing the ground will allow the power to pass through (i.e. the switch is on the ground side as opposed to the power side...after the load), but im not sure exactly what this looks like as far as the actual wiring goes?

As for the numbers on the relay, I will make sure to start adding those! Im beginning to see that most (all?) Relays seem to use the same numbers as identifiers 🤓 Thank you for you patient suggestions!

As to skipping the starter solenoid, why do you suggest that? Constant power draw on the solenoid, 1 more thing to fail? Or just an extra unnecessary connection?

Thank you again for bearing with me! Lol
 

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Discussion Starter · #10 ·
The manual bypass switch, bypasses the oil pressure switch only for using the pump/s when the engine not running. As I look at your diagram, the manual switch you have will have all the amps for the fuel pumps running though it, appx 30 amps. You don't want that. You want it on the side of the lesser amps, that control the relays themselves. Power for the fuel pumps relays, to power up the pumps, should go right to the power source. Lesser power that controls the relays, is where you want you manual selector switch. It's hard to find, and I know I'll be corrected, a good auto 30 amp switch anymore, which is why we use relays now.
This makes sense to me; I had thought my bypass switch didn't make sense there as I drew up the 2nd iteration of the wiring, I simply wasn't sure where to put it. So I WANT it to bypass to oil pressure switch (so I can turn on the pumps whenever desired) but I DONT want it sending too much power through the bypass switch itself. This would look something like wiring the bypass switch up to the oil pressure switch on the NC side, yes?
 

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I'm concerned you may not understand how a relay works, which can be problematic when designing a circuit using them.

A relay is an electromagnet on the control side, and a switch on the other. When the electromagnet is energized, it closes the switch.

The electromagnet is called the "control" side (85, 86). The switch side is our "load" side (30, 87, 87a).

The idea is to switch to electromagnet on, causing the relay to switch on your load. Your fuel pump in this case.

Applying that theory to your diagram, your ignition switch will energize both relays, but your selection switch will only allow voltage to the load side of one relay at a time. Which, more than likely will cause issues...which we can explore later if you like.

I suggest visiting the website I posted earlier to gain knowledge about the subject, for I fear drawing this diagram for you will not teach you anything about electronics, which can lead to problematic circuits, wiring problems you can't solve, and worse ..like burning your rig to the ground.
 

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Discussion Starter · #12 ·
As for the relays:
86 - ground
30 - battery (incoming power)
85 - oil pressure switch
87 - pump (outgoing power)
87a - bypass switch, to allow power through regardless of oil pressure switch

Close?
 

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Discussion Starter · #13 ·
I'm concerned you may not understand how a relay works, which can be problematic when designing a circuit using them.

A relay is an electromagnet on the control side, and a switch on the other. When the electromagnet is energized, it closes the switch.

The electromagnet is called the "control" side (85, 86). The switch side is our "load" side (30, 87, 87a).

The idea is to switch to electromagnet on, causing the relay to switch on your load. Your fuel pump in this case.

Applying that theory to your diagram, your ignition switch will energize both relays, but your selection switch will only allow voltage to the load side of one relay at a time. Which, more than likely will cause issues...which we can explore later if you like.

I suggest visiting the website I posted earlier to gain knowledge about the subject, for I fear drawing this diagram for you will not teach you anything about electronics, which can lead to problematic circuits, wiring problems you can't solve, and worse ..like burning your rig to the ground.
You are absolutely correct on that. I'm beginning to gleam some understanding, and that website you linked is helping quite a bit. At first I was looking through the diagrams and was thoroughly confused, but I have since found the 'about relays' section and am slowly absorbing the info!

As to the problems occurring that you mentioned, it seems that frying a circuit due to constant unused power - and potentially a fire - is the most likely since the current would be flowing to basically a dead end. Just hazarding a guess there.

I don't want someone to do this for me either. I learn best doing it myself, but I do admittedly need a little hand-holding and guidance as I learn to navigate wiring and circuits in general. The reading can be a little terminology-heavy sometimes for someone with no electrical knowledge, so the diagrams and more layman explanations are really helpful. For example: a closed circuit sounds (to me) like it won't let power through while an open will allow it through; But it seems that, in electrical circuits, that is the opposite of how it works.
 

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OK, this is quick 'n crude (trying a new draw program :cautious:), but should give you some ideas of how to proceed. Note the power circuit is limited to the relays and pumps, while the control is everything else at low power. One pump at-a-time, and pumps only run with oil pressure, or when the momentary button is pressed (priming). There are many ways to approach this. Look it over and ask whatever, or change requirements.

Diagram is powered, but pumps off. If oil pressure up or bypass, pump 2 would run:
Rectangle Font Parallel Slope Pattern
 

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General fuse suggestions, remembering that only one pump is powered at-a-time. Holley recommends 3 to 7.5A fuses for typical street pumps, although I don't know what he's actually using. So, instead I maxed the rating to 10A in order to protect the wiring in dead-short—which is the purpose of fusing anyway, the wiring.

Likewise, the relays use less than 0.2A each on control signal, and 1A is generally smallest common ATM/ATO/ATC style fuse. 1A will protect the wiring and components. Do you have other suggestions?
 

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Discussion Starter · #17 ·
OK, so you just bridged the NC and NO sides with a 2nd switch (it took me a moment to figure out what 'mom bypass sw' was if I'm being honest 🤣) I think I was expecting too much out of 1 switch; Wanting it to turn on the selected pump at any point in time while also responding to the oil pressure switch.

I notice on most diagrams I see for pressure switches, the C usually runs somewhere other than battery - and on a relay, 30 (the (C)ommon) typically DOES run to battery. And I see here you wired the C of the pressure switch up to battery. Why is that? Specifically for the bypass switch?

As for the relays, does this sound right regarding your diagram:
30 - battery
86 - ground
87 - pump
85 - oil pressure switch
87a - unused


I'm also still a bit confused as to why the starter solenoid and ignition is able to be bypassed. Hazarding a guess that it's because the oil pressure switch does that work by reading the oil pressure?

General fuse suggestions, remembering that only one pump is powered at-a-time. Holley recommends 3 to 7.5A fuses for typical street pumps, although I don't know what he's actually using. So, instead I maxed the rating to 10A in order to protect the wiring in dead-short—which is the purpose of fusing anyway, the wiring.
The pumps I'm using are Carter 4600HP. I couldn't find any electrical specs on them surprisingly. In the manual they go so far as to suggest using 14ga wiring and that it...so I used 12ga, because I had it on hand...and I tend to be a bit extra.
 

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14ga wire is rated for 15 amp max. The Carter fuel pump in question with regularly pull 8-9 amp, and it dead head pressure situation can rise as far as 11. I'd run a 15 amp fuse for the fuel pump, and going with 12 ga wire will be fine.

As far as the relay is concerned, depending on where you are mounting it and what brand relay you purchase, 1 amp should be fine, but I typically choose 1.5 amp fuses to accommodate for heat, as they typically end up in the engine compartment, and heat makes everything worse.

I've seen heavy duty, cube relays with inline resistors require 3 amp fuses.

Ohm your parts out, and do the math. Best bet.
 

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I think I was expecting too much out of 1 switch; Wanting it to turn on the selected pump at any point in time while also responding to the oil pressure switch.
You could get that using the ignition switch, but then the trade-off is that the OP switch could not control turn-off. Turn-on/off with or without pressure is the primary safety goal, and a 2-pin switch could do that by jumping the two pins, but still needs the momentary control of a 2nd switch. I assume you already had the 3-pin OP switch, so I used that, and avoided other issues by not involving the ignition switch. There are always other options, and that diagram was only intended to spur ideas with different perspective.

I notice on most diagrams I see for pressure switches, the C usually runs somewhere other than battery - and on a relay, 30 (the (C)ommon) typically DOES run to battery. And I see here you wired the C of the pressure switch up to battery. Why is that? Specifically for the bypass switch?
Switches do not care the direction of electrical flow. Hook them up however they work like you want. On relays, it can matter (see below).

BTW - we don't like to hang more stuff on the solenoid stud, or more amps on the old key switches not designed for it, so an option is to have a separate heavy wire feed from the battery (+) terminal to an insulated stud, or bus bar, or mini fuse box where you can connect accessory loads. Lots of options, and your choices.

As for the relays, does this sound right regarding your diagram:
30 - battery …
The rule-of-thumb (RoT) for automotive relays was established by Bosch, and is only a standard for specific relay types. But, if you don't know the type you have, the RoT safely works for any of them. The loose rule is:
Power in on even-numbered terminals. Power out or ground on odd-number terminals.

Trivia background if you care: Relays are switches, and the power section can usually be wired any direction, as with most switches. The load power is only on 30, 87 (4-pin relay) and 87a added on 5-pin relays. The control signal coil is on 85 and 86, and typically uses less than 0.2 amps.

The issue is that almost all relays have a "snubber" circuit, which damps or "shorts" the high-voltage flyback spike produced by the coil when de-energized. The effect is exactly like a tiny ignition coil. This little spike can interfere or damage other components, so we want it gone. Some relays use a shunt resistor to do this job, and the signal can be bi-directional. Others use a diode, which can only be used one direction, or it will be damaged. Some use both.

If you don't know which you have in that little black box, wire it with the RoT. If you do know, wire it any way you like that will function properly. Using the RoT, I remember "in on 30, out on 87 when powered, or out 87a ("always") when off". Just how my brain remembers. Same for volts in on 86 (even) and ground on 85 (odd). I do detailed diagrams in a way that it doesn't matter which type of relay you have.

I'm also still a bit confused as to why the starter solenoid and ignition is able to be bypassed. Hazarding a guess that it's because the oil pressure switch does that work by reading the oil pressure?
Not sure I understand that one, but I assume you were simply trying to use a different 2nd switch. It appeared you wanted to primarily control fuel supply from OP, with control of which pump, and with override for priming. If you would rather control ignition or starter relay operation, or use for control, you can reconfigure to do that instead.

The pumps I'm using are Carter 4600HP. I couldn't find any electrical specs on them surprisingly. In the manual they go so far as to suggest using 14ga wiring and that it...so I used 12ga, because I had it on hand...and I tend to be a bit extra.
I found the instructions here, which included the 5A fusing on page 5. From the fuse rating, you can reference the preferred wire gauge for voltage drop, and yours appears plenty for minimum power loss. My RoT is conservative and follows aircraft standards for voltage drop of less than ½-volt for the entire wire path, 3 inches or 30 feet. On a 10-foot run, 18 AWG will handle it, and 16 AWG is under ½-volt. Bigger is OK, just more expense and weight. Do your thing! :cool:
 

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Discussion Starter · #20 ·
I found the instructions here, which included the 5A fusing on page 5. From the fuse rating, you can reference the preferred wire gauge voltage drop, and yours appears plenty for minimum power loss.
I've got one of the rotary vane pumps. Found the info for fuse rating on page 3, which I apparently over-looked more than once🦧 It suggests #14 or heavier wire with a 10-amp fuse (between the Oil Pressure Switch and the Pump).
Which brings me to the question: what is the suggested method for fuses - in-line fuse holders, mini Fuse box, or connecting to the vehicles existing fuse block? Is any one better than the others?

BTW - we don't like to hang more stuff on the solenoid stud, or more amps on the old key switches not designed for it, so an option is to have a separate heavy wire feed from the battery (+) terminal to an insulated stud, or bus bar, or mini fuse box where you can connect accessory loads. Lots of options, and your choices.
I plan to add a buss bar. The solenoid is already over-loaded and in need of cleaning up, so I decided to make that part of the fuel-system rebuild.

Not sure I understand that one, but I assume you were simply trying to use a different 2nd switch. It appeared you wanted to primarily control fuel supply from OP, with control of which pump, and with override for priming. If you would rather control ignition or starter relay operation, or use for control, you can reconfigure to do that instead.
It's simply that every diagram I've seen for wiring up an OPS involves the use of either the ignition system, starter system, or both. As such I assumed it was a necessity. Even the picture in the Carter manual has it connected to the solenoid. Then again, those are all for single pump setups, with no switch for priming/engine off use.


Thank you for taking all the time to explain this to me, background and all! It's nice having it in text to look over and really set it in my rock of a brain 🤣 Out of curiousity, do you work in aviation, or just follow those standards bc they are a high standard?
 
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