Question about fuel cut relay


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Question about fuel cut relay

Postby WingAdmin » Mon Aug 26, 2013 1:36 pm



I'm working on my wife's PC800, which is a 1989, and shares the same type of fuel system with the GL1200. I've got a problem with it that I'm trying to diagnose.

The bike starts normally. It will at times start running lean, like fuel starvation, but then will come back to life. Eventually, it will start running lean, and will quit from fuel starvation. Hitting the starter, it jumps to life again and rides off like there was no problem. This happens every five minutes or so.

Checking the fuel pump, when the bike is started, it runs for a few seconds - click click click, maybe 5-6 clicks per second, and pumps fuel normally. It will then stop, until (I assume) the carb float needle valves drain off some pressure, at which point the fuel pump will start back up again for a few more seconds.

Eventually, the fuel pump will NOT start back up again, it will sit for a minute or more without running, and the bike will die from fuel starvation. Hitting the starter, the fuel pump jumps back to life, and the bike starts normally.

The battery/bus voltage is a solid 13.8V. I checked the fuel pump relay, which appears to be a potted, solid state relay, and it is a fluctuating 6-8 volts being supplied to the fuel pump. The primary of this relay is driven off one of the spark coil primaries, so I suspect there are pulses of 12 volts coming off the relay going to the pump, but my digital meter isn't capable of resolving that into +12v fast enough, so instead it's fluctuating around 6-8 volts.

Running +12V directly into the fuel pump causes a solid buzz and fast pumping, not at all like the "click click click" it is doing while the bike is running.

I checked the fuel cap to make sure it is not clogged and causing a vacuum - running the bike with no fuel cap in place, the problem still occurs.

The fuel cut relay, without question, is definitely solid-state. And it's not just a relay - it's modifying the signal. Here is the pulse coming from the ignition control module, which is sent to one coil, the tachometer, and the fuel cut relay:



Here is what is coming out of the fuel cut relay. Keep in mind this is with the fuel pump attached, so this signal is being attenuated/affected somewhat by the pump:



Obviously the two are not even remotely similar, apart from the timing. The very quick, relatively high-voltage pulse being generated to send to the coil primaries is necessarily fast, in order to excite the coils properly and generate a spark.

The output of the fuel cut relay rises quickly to 12 volts when triggered by the ignition pulse, but then tapers off down to zero just before the next pulse. This is shown at idle - if the engine speed is increased, the taper does not reach zero before the next pulse occurs. This leads me to believe that there is a capacitor inside the fuel cut relay, that is discharging to extend the pulse. If the engine speed is increased high enough, the pulses are almost invisible, and essentially a continuous +12V is applied to the fuel pump.

So this is all well and good, but none of it tells me why the bike is randomly dying of fuel starvation. As luck would have it, while I had the scope hooked up, the bike was operating perfectly, with the pump clicking away every few seconds, keeping the bike alive.

From research I did tonight, I found that the pump is quite reliable, and the fuel cut relay is usually the failure item. I've ordered a replacement relay, and we'll see if this solves the problem. I've also ordered a petcock rebuild kit, as it has the same type of vacuum petcock in it as our Goldwings.


So my question for GL1200 owners is this: on the GL1200, which uses the identical fuel cut relay/pump layout, what do you measure on the output side of the fuel cut relay? If you look at the fuel pump, there is a white wire that leads to the fuel pump connector, where it connects to a Black/Blue wire that actually runs to the fuel pump. When the bike is running, what voltage do you see there?



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Re: Question about fuel cut relay

Postby virgilmobile » Mon Aug 26, 2013 2:27 pm

I vaguely remember using a analog meter on the output of the relay.there was voltage only as the pump energized and it peaked near supply volts.
Verify the supply stays near 12 volts during pulsing.
Theoretically 12 volts can be applied directly to the pump for a test.
When it reaches full line pressure,it stops triggering the mechanical switch in it.
Mine would pulse fast till the carbs filled and then slow to a pulse every 5 -8 seconds as the bike would run.

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Re: Question about fuel cut relay

Postby virgilmobile » Mon Aug 26, 2013 2:48 pm

While reading again...."Running +12V directly into the fuel pump causes a solid buzz and fast pumping, not at all like the "click click click" it is doing while the bike is running."

I wonder....is the pump built exactly like the 1200???
My pump is just a large coil with a plunger inside attached to the diaphragm...
When the spring pushes it back and the fuel out,it triggers a switch contact....when it contacts...Pop goes the plunger to the other end...unlatching the switch and disconnecting the coil from 12 volts till the fuel is squeezed out by the spring pressure.
After I repaired mine,I tested it by attaching 12 volts direct to it(via a battery and #14 wire) and watching it pulse away....no buzzing...just click click click...about 4 per second dry test.
Do keep in mind,this takes a lot of power to pulse the pump.The surge could easily be 10 amp or more.

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Re: Question about fuel cut relay

Postby WingAdmin » Mon Aug 26, 2013 3:15 pm

virgilmobile wrote:I vaguely remember using a analog meter on the output of the relay.there was voltage only as the pump energized and it peaked near supply volts.
Verify the supply stays near 12 volts during pulsing.
Theoretically 12 volts can be applied directly to the pump for a test.
When it reaches full line pressure,it stops triggering the mechanical switch in it.
Mine would pulse fast till the carbs filled and then slow to a pulse every 5 -8 seconds as the bike would run.


The supply was dipping down around 8-9 volts during the pump stroke, which I would expect as it is pulling a fair amount of current.

With the analog meter on the output of the relay, were you seeing a constant 12 volts, or was it fluctuating on ignition pulses like I see?

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Re: Question about fuel cut relay

Postby WingAdmin » Mon Aug 26, 2013 3:18 pm

virgilmobile wrote:While reading again...."Running +12V directly into the fuel pump causes a solid buzz and fast pumping, not at all like the "click click click" it is doing while the bike is running."

I wonder....is the pump built exactly like the 1200???
My pump is just a large coil with a plunger inside attached to the diaphragm...
When the spring pushes it back and the fuel out,it triggers a switch contact....when it contacts...Pop goes the plunger to the other end...unlatching the switch and disconnecting the coil from 12 volts till the fuel is squeezed out by the spring pressure.
After I repaired mine,I tested it by attaching 12 volts direct to it(via a battery and #14 wire) and watching it pulse away....no buzzing...just click click click...about 4 per second dry test.
Do keep in mind,this takes a lot of power to pulse the pump.The surge could easily be 10 amp or more.


The pump operates just like you say. I wonder if the contacts might be worn in it? I should open it up and have a look at it.

Now that I think about it, It's possible the wire I used to jumper it wasn't capable of supplying enough current to actuate the pump.

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Re: Question about fuel cut relay

Postby virgilmobile » Mon Aug 26, 2013 4:11 pm

With the analog meter on the output of the relay, were you seeing a constant 12 volts, or was it fluctuating on ignition pulses like I see?


The ignition pulses just give the ability for the circuit to trigger..there may have been some pulses there but the analog meter didn't register them.

There wasn't any measurable voltages...Voltage only appeared when the pump would pulse/demand it...
It acted more like a current demand switch...no voltage/current on the line till the pump switch made contact and applied the coil load.

Even a test lamp would not light when attached to the relay with the pump unhooked.
Not enough demand I suspect....Perhaps a larger load...A headlight...but then I don't think the relay was designed to supply a sustained high current feed...Just a few millisecond pulse...enough to activate the pump coil...Sustained current draw could destroy the "relay" for sure...

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Re: Question about fuel cut relay

Postby virgilmobile » Mon Aug 26, 2013 4:24 pm

It's starting to sound like your on the right track in the first place....A malfunctioning pump relay..

As long as the pump is not binding ,the plunger moves full stroke freely,the contacts are good and the switch does trip at each stroke.

It does have that rubber bumper in there(loud ticking) and a bronze bushing...If anything should bind during a stroke and not release the switch contacts,it will continue to draw current from the relay...Then the relay goes pu pu.
It's worth a few minutes to pop the outside cover and inspect/manually operate it a few times just to be sure....
Sometimes it's just a bad part...other times is cause and effect...

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Re: Question about fuel cut relay

Postby WingAdmin » Mon Aug 26, 2013 9:05 pm

Well I've solved the fuel starvation problem tonight.

I started by jumpering the fuel pump, taking the fuel cut relay out of the picture. This made sure the pump had +12V at all times. This did not make any difference.

Next I got a 5/16" brass "T" fitting, to put in place of the petcock. If the petcock was the problem, I figured this would solve the problem. I didn't have to however. I unscrewed the petcock from the filter housing and started to remove the clamps holding the fuel lines in place, when I saw it - a giant crack in the vacuum line running to the petcock. Enough to make one of the carbs run lean, and to give only sporadic vacuum to the petcock, causing fuel starvation.

I cut the vacuum hose back past the crack, refitted it to the petcock, and the fuel starvation problem disappeared. Victory!

Still waiting on the rebuild kit for the clutch master cylinder however. That's the last remaining problem.

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Re: Question about fuel cut relay

Postby BikerBuck » Mon Aug 26, 2013 10:29 pm

Isn't it great when a plan comes together!

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Re: Question about fuel cut relay

Postby virgilmobile » Tue Aug 27, 2013 7:14 am

Who would have guessed that a 24 year old vacuum line could possibly have hardened and cracked....
Go figure.....Makes you wonder about the rest of them...hint hint. :D
Sounds like another project(for your apple) while your waiting for the brake parts to arrive.




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