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does anyone know where the diode, in the fuel bypass wire, (The red with brown stripe wire) is physically located on a 1984 Econoline?
Had a fire and have rewired everything but that last small pass side corner. Im afraid if it was right there at the Starter Solenoid, it may be gone!
Thanks for the reply!
Yes. E350 camper. 460 with electric pump(s)
I would not have had any idea there was one, until I looked at the schematic. after the fire, all I had left was two stubs of wire, which I striped back to establish their colors. From the schematic, I found these were fuel system circuits. One, black/yellow, was the fuel system power circuit, via the inertia switch. After some searching, found the other, red/brown, is a "by-pass" circuit that allows the fuel system to operate in "start" mode. (some schematics I found online from F series or older trucks show this as a pink/black wire). But all show a 20 gauge fuse link and a diode. Here is a scan of the area of the schematic...
The table says the fuse link Splices 150 and 151 are located "near the starter motor relay". I found a piece of BK/Y wire with the fuse link in a junk yard here, but none had the exact R/BR wire, (but the vans were different years, and didnt have the "I" terminal for the R/BR wire)
I have seen those things before, while strolling through the junkyard and looking at stuff that's already been taken apart; in fact I have one from some 90s pickup truck, it's an almost thumb-sized, yellow device with that "diode" depiction on or molded into it.
Anyhow, for yours... notice that one end is blue, before it hits Splice 150 and that goes to one of the big lugs on the start motor relay; the other end is red/brown and leads off to wherever it goes. My guess would be that device was located someplace up very near to the starter relay in the same general area as the fusible links.
An EVTM for the vans would possibly/probably help but I don't have such a thing, I guess I'd follow the red/brown from the fuel area and install the new one someplace easily accessible (which is a misnomer, there is really no such location in a va but I digress) up near the starter relay.
I'd follow the red/brown from the fuel area and install the new one someplace easily accessible (which is a misnomer, there is really no such location in a van but I digress) up near the starter relay.
Can you trace the rest of the circuit and isolate both ends? Then you'd have a length of wire that may or may not have an inline diode. From there, use the diode check function found on some multimeters. That would tell you if the mystery diode is present.
If your meter doesn't have a diode check function, use a 12v test light and some fused test leads to route battery power in both directions through the isolated wire. If the bulbs lights in both directions, then you know the diode is missing and you can add one.
As far as I know my '86 (considered an '87) E350 does not have the diode. May be because it's a single tank and not a dual tank system.
With the engine running I can read (the stepped down, post resistor wire) fuel system voltage feeding back through the bypass wire at the starter solenoid.
And speaking of the starter solenoid, mine has both the small S (starter signal wire) and I (hot in start, fuel pump bypass) terminals.
Be sure you use the 20 gauge fusible links for both the main and bypass wires. In my case a prior owner used a heavier gauge fusible link on the bypass wire and it caused melted wiring when the starter solenoid failed in such a way that the I terminal (and thus the fuel pump bypass wire) remained hot all the time.
It looks to me like the diode(really a electrical check valve) is there to keep the electric pump feed from going backwards up the wire and making the starter motor turn all the time.
The older 460's used the starter relay with the "I" terminal and used the "I" terminal for sending voltage to the fuel pumps for priming the fuel system during cranking. Looks like this later version doesn't have the "I" terminal, so they tied it directly to the large terminal that feeds the starter. But when they did that they had to install the diode, so the starter connection can feed the electric pumps, but the electric pumps can't feed the starter.
I did a google search for "20 amp diode" and a bunch of hits came up, a lot of them on ebay. Seems they use them on solar panel arrays for blocking, just the application you would use them for. I thought 20 amp should be enough but you can get larger ones also.
If you are having problems finding large enough diode you could use a relay to do the same job.
A little more work setting up the wiring but will work fine.
Cheers Steve
My starter relay has both the small "S" (start signal wire) and "I" (auxiliary cranking power supply) terminals. My fuel pump bypass wire is attached to the small I terminal and I can detect the fuel system power on my bypass wire.
My wiring is original as far as I know.
I have seen other ford wiring diagrams (for different applications) where the I terminal is used to boost the coil voltage for starting, or energize the Dura-spark cranking retard feature.
Without looking anything up, if you don't have an I terminal on your starter relay I'd be surprised.
Another thing about the starter solenoid. Get a good one and make sure the battery cables and terminals are heavy duty (you're turning over a big block after all) and up to the job.
Cheap starter solenoids (and bad wiring) are notorious for causing the contacts inside the solenoid to weld together causing your starter to run on, even with the key off! It that ever happens you can try whacking the solenoid with a 1/2 wrench and that often will make it stop. Otherwise you better use the wrench to disconnect the battery. Quick.
Thanks for all the input! Think I got headed in the right direction here.
Truck is up and running tonight! No smoke! No sparks!
Decided, if there is a diode hidden in there somewhere...no matter. Doesn't hurt to have two! So i found a 6 amp diode. (With a 20 gauge fusible link...I saw that somewhere else...there could no way have needed to be a 20+ amp diode!)
My thoughts were similar about the relay. But...why would you need a diode with a relay in the first place?. Once the start function is released there is no circuit for power to back feed through? Right? Perhaps, the moment the start function is released, there could be a spike or surge back through the relay or something? I dont know. But as I said, I did put the diode in. And everything works!
Mine was originally connected to the "I" terminal. I decided to just connect it to the starter side of the solenoid. Shouldnt make any difference and all the later vans I looked at didnt have the "I" terminal. And there was still some kind of wire connected to the starter side of the solenoid on these later models. (Same bypass function? Or perhaps, as OP said, to boost the coil voltage for starting, or energize the Dura-spark cranking retard feature?)
X2 on quality Electrical components!! Im a firm believer in this!
Again, Thanks for all your input. We'll see how my rewire holds up!
Diodes are usually place across relay coils to prevent the back EMF spike which occurs when the relay is de-energized. It's an inverse polarity spike which can be several times the applies voltage through the relay.
Here's a thread from an electronics geek site about it:
How much power does the fuel pump draw through the bypass wire? I don't know but it you have the bypass connected to the S terminal you're using your start signal wire as the power supply. But if the bypass wire was connected to the I terminal it would receive power from the battery side. That dedicated fusible link on the bypass is designed to handle power surges regular fuses can't take. What fuse powers your start signal wire?
I'm sure you already know this but just in case, there is a resistor wire in the main fuel pump power supply circuit so in normal operation the fuel pump receives reduced voltage. However the bypass wire feeds full battery voltage to the pumps so it should not be used as a long term power supply.
Mine was originally connected to the "I" terminal. I decided to just connect it to the starter side of the solenoid. Shouldnt make any difference and all the later vans I looked at didnt have the "I" terminal. And there was still some kind of wire connected to the starter side of the solenoid on these later models. (Same bypass function? Or perhaps, as OP said, to boost the coil voltage for starting, or energize the Dura-spark cranking retard feature?)
The red/light-blue stripe wire - leads from the ignition switch to the NSS (or clutch) switch to the starter relay - is what actuates the relay... it supplies power to the electromagnet inside the relay, which closes the connections which supplies power to the starter motor and fuel pump relay.
How much power does the fuel pump draw through the bypass wire? I don't know but it you have the bypass connected to the S terminal you're using your start signal wire as the power supply. But if the bypass wire was connected to the I terminal it would receive power from the battery side. That dedicated fusible link on the bypass is designed to handle power surges regular fuses can't take. What fuse powers your start signal wire?
The fusible link is also the only protection against (accidental) grounding that wire has:
Granted it has power only in START but if it was to be (mistakenly) grounded to the frame and somebody was cranking the engine for an extended start-up attempt, that wire will melt and possibly/probably start a fire.
... the bypass connected to the S terminal you're using your start signal wire as the power supply.
No. I have the wire attached to the start cable lug. Not the "S" terminal
Originally Posted by Brnfree
...
I'm sure you already know this but just in case, there is a resistor wire in the main fuel pump power supply circuit so in normal operation the fuel pump receives reduced voltage. However the bypass wire feeds full battery voltage to the pumps so it should not be used as a long term power supply.
The fuel bypass circuit wire (red/brown stripe) that is attached to the solenoid only triggers a "bypass" relay located on the drivers side inner fender next to the main fuel pump relay. (if I remember, the bypass relay is the "green" relay)
The rest of the fuel pump circuit is original, as it was.
If you have dual tanks the second relay is the selector relay.
This is why there is a diode...
So the fuel pump relay cannot back feed the 'pump run in start' bypass wire to the starter solenoid.
The fuel pump circuit would then be trying to turn the starter motor, and would blow the fuse or have a meltdown.
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