Fuel Cut-Off Relay - Honda Shadow Forums : Shadow Motorcycle Forum

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Old 04-27-2010, 12:55 PM   #1 (permalink)
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Default Fuel Cut-Off Relay

Hello all:

I've probably got a series of questions on this topic, but I'd like to start at the beginning. My bike has a fuel cut-off relay. From knowing how the bike operates and looking at the wiring diagram, it appears that this relay engages the fuel pump anytime either the start button is down or the engine is running (and would therefore kill the fuel pump if the bike stopped running, hence the name I assume). Stop me there if that isn't correct.

Next, this relay has 3 wires. One comes directly from the battery via the ignition switch and is always hot if the ignition is on. One goes to the fuel pump. These two wires would seem to be both sides of the switch in the relay. The third wire goes to the ignition control module if I remember correctly and would seem to be the one that activates the relay. Can someone tell me how this relay works? I've never seen one that didn't have at least 4 wires. It seems to be missing the ground on the coil to me, but it obviously worked at one time.

I'll stop there for now in the interest of not making this too long-winded. If I can get this cleared up in my head I'll proceed with my line of questioning.

Thank you for your help.
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Old 04-27-2010, 01:12 PM   #2 (permalink)
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Is it possible that the bolt that holds it on is the ground? I know some regulators rely on the bolts as ground.
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Old 04-27-2010, 02:00 PM   #3 (permalink)
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The relay is held in place by a rubber boot that encircles it completely. It doesn't contact the frame.

I've got some ideas about what might be happening, but I was hoping someone would come along that knew all about it and could lecture me on the subject.
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Old 04-27-2010, 08:17 PM   #4 (permalink)
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the relay is ground triggered. the third wire connects to the wire that runs from the ICM to the the coil (rear cyclinder i think, not that it matters). Look at the schematic and you will see that the coils are also tied to the battery full time. the coils fire when the ground from the ICM makes the current flow, then the ground is opened and the coil fires. this is just like a distributor engine except the points are replaced by the ICM. It is these ground pulses that the relay needs to switch power on. All these pulses are not very good for the relay and may also shorten the life of the pump.

I substituted a standard 30 amp relay that is triggered on whenever the run switch is in run. I just have to get to my stop switch in a crash to stop the fuel! So far so good.

For testing you can jumper past the relay. Just search, there was a good post with pictures.

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Old 04-27-2010, 10:23 PM   #5 (permalink)
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It's not really a relay it's a transistor and a thyristor

The Spark unit acts as a trigger letting battery voltage go to the pump every time there is a spark.
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Old 04-28-2010, 02:25 AM   #6 (permalink)
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All I can say is that I had the Fuel Pump and Relay both replaced a couple weeks ago, been having a problem for the last year off and on where the engine would just die while riding, sometimes after few try's it would restart, after several months of checking this and that it was determined that the pump was bad so replaced it and the relay just to make sure that those two things were good to go, after about 500 miles everything is working just fine............
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Old 04-28-2010, 02:39 AM   #7 (permalink)
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As was previously stated it is a transistor, transistors only use three pins... one for the voltage going in, one for the voltage going out, and the third causes a reaction in the semi-conductor that bridges the two together and causes the voltage to go through it.

Yes it confused me too, where is the ground wire?

If you jumper the always hot wire (when the ignition is on) to the wire that goes to the fuel pump the fuel pump should come on.
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Old 04-28-2010, 08:57 AM   #8 (permalink)
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Thank you all for the replies.

Thinking of it as a transistor makes a lot more sense. Its been a long time since I was forced to learn about them in college, but I remember the basic idea I think.

My fuel cut-off relay went bad and therefore would not engage the fuel pump. I shorted around it and the pump worked fine, so I just bought a relay from Radio Shack and put it in (the shop wanted $65 for the original relay!). I had to add a ground wire for one side of the coil, but other than that I hooked it up the same as the original relay. Now that relay engages as soon as the ignition switch is ON instead of waiting for the start button or a run condition. I don't have a problem with that other than not understanding why it is happening. Why did the original relay not activate until the bike was running when my Radio Shack relay engages the fuel pump as soon as I turn the key on?
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Old 04-28-2010, 09:51 AM   #9 (permalink)
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Quote: "I had to add a ground wire for one side of the coil, but other than that I hooked it up the same as the original relay. "

If you have one of your coils always connected to ground there is no way the spark unit will ever fire that cyclinder.

If you only grounded the relay it will run whenever the ignition switch is on. If your bike is dropped to the left side, how will you stop the fuel pump? You will have to lift the bike to reach the key.

On the other hand a normal relay will not like being pulsed at 60 times a second. And these pulses will be sent to the pump. It may not be able to switch that fast, then the pump will never run.

A standard relay can be wired as I described above; switched from the high side of the igniton coil and ground. Fuel will be flowing whenever the cuttoff switch is in run. At least this gives you a chance to kill the fuel pump after a mishap. My sabre has been connected this way for two years. The pump and standard relay are working fine.
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Old 04-28-2010, 10:07 AM   #10 (permalink)
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I meant the coil of the relay I put in. I grounded one side of the coil and ran the original wire to the other side of the coil. Currently, when I turn the key the fuel pump comes on. I don't know if the engine cut off kills the fuel pump, though. I haven't tried that. The relay works though, because I rode the bike 30 miles on Saturday.

How is this any different to the fuel pump than what was already occuring with the stock relay? Its just being activated by a different relay. I can see where the new relay might not like the conditions, but I can't see where it would effect the fuel pump.

I don't have the wiring diagram with me. I'll have to look at it again regarding the ICM.

I installed a button I can press that bypasses the relay and connects the fuel pump to 12 volts full time just in case anything happens.
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