Fuel pump not priming - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 11 (permalink) Old 08-13-2012, 06:35 PM Thread Starter
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Fuel pump not priming

Hey guys,
Bit of a head scratcher here for any electronics wizards out there.

I have a SP1 here that isn't priming the fuel pump when the key is switched on.
Several things of note here.
Battery is charged and in good order.
The fuel cut relay tests correctly.
The bank angle sensor and relay test correctly.
I have tried a different ECM on the bike, same problem.

I know there is a problem on the fuel cut relay signal line.
First test: You are supposed to have no continuity between the brown/black wire and ground with ignition off - I have continuity. I have disconnected the ECM and checked for a short, there is no short in the harness, it seems like the short is through the ECM.
Second test: when you turn the ignition switch on you are supposed to get continuity on the same (brown/black) circuit for a few seconds (this is what causes the fuel pump to prime when the key is switched on), - I have no continuity when the key is switched on.

From this I can see that the ECM is supposed to earth the signal line to allow the fuel pump to prime when the key is switched on, but for some reason this is working incorrectly. I need to know what tells the ECM to switch the signal line to ground when the key is switched on. I believe this is the key for finding the problem.

Now as a point of reference, if I short the brown/black signal line with a test light and turn the key on the fuel pump runs (and keeps running, no longer priming but just on full time) and the bike will start. I can then disconnect the short I created while the bike is running and the bike will still run. However once the bike is switched off I will have to repeat the short to get the bike running.


I know on some CBR's etc, there is a zener diode on the base of the ignition switch that can cause the fuel pump not to prime if the bike is hotwired, as it reduces the fuel pump voltage down to 9volts instead of the normal 12v. Is there something similar on the SP1's? I have taken the plastic plate of the bottom of the ignition barrell but couldn't see any zener diode there.

Any help would be greatly appreciated! Feel free to ask any questions, I am sure I would have left out some info you may beed.
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post #2 of 11 (permalink) Old 08-13-2012, 06:42 PM
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Not to sound like a smart ass but is the run switch on??

If it is, check out the internals and make sure the connections are okay. I have had a solder joint come off before in that area.

Next is the kick stand switch circuit.



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post #3 of 11 (permalink) Old 08-13-2012, 07:55 PM Thread Starter
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Originally Posted by madbuyer View Post
Not to sound like a smart ass but is the run switch on??

If it is, check out the internals and make sure the connections are okay. I have had a solder joint come off before in that area.

Next is the kick stand switch circuit.
Not smart arse at all! great suggestions.
The Kill switch is on, and I have had it apart, everything is functioning correctly in there.
Also, I have checked the side stand switch, it is fine, I even isolated that circuit just to double check.
Still no dice, keep the suggestions coming!
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post #4 of 11 (permalink) Old 08-13-2012, 08:13 PM
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https://www.rc51forums.com/forums/sho...k+angle+sensor

Take a look here...



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post #5 of 11 (permalink) Old 08-13-2012, 10:36 PM Thread Starter
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Thanks madbuyer, I have pm'd Makis in the hope he can help! Several things are very similar in the thread.
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post #6 of 11 (permalink) Old 08-14-2012, 05:28 AM
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Some thoughts....

You said that the Fuel Cut Relay is working fine. How did you check it? Did you force it to engage? Try swaping it with the High Beam's Relay.

The 30A fuse near the battery (not the one in the starter relay) is ok?

Does your fusebox have power on every single fuse's one end?

Does your Neutral indicator comes on when you turn the key to ON?

On the Fuel Cut Relay coupler, are the two Black/White wires live (with + current?)

When you turn the key to ON do you have + current on the Black/White wire exiting the Bank Angle's Relay?

The Brown/Black wire is a controlled ground by the ECM like you mentioned.
ECM engages the Fuel Realy for a few seconds, by default when the key turns to ON position but once the engine starts cranking and the ECM "feels" it through the cam's, cranck's sensors, it keeps the relay into an engaged position so that the Fuel Pump keeps working.
The ECM cannot sense a fuel pressure drop in the fuel line.

Since you said you tried another RC's ECM and you've got the same not priming results, I believe the problem lies somewhere in the harness or on an other relay since they are all connected somehow to each other.

Bank Angle's Relay is very important because it's the positive current's "first gateway" to everything else. And that's the Black/White wire that goes from that relay to the harness and two of it's ends go to the Fuel Cut Relay. One is grounded through the ECM through the Brown/Black wire and the current flowing inside the relay, engages it so the current can flow from the other Black/White wire to the Brown one that goes to the fuel pump.


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post #7 of 11 (permalink) Old 08-14-2012, 03:37 PM Thread Starter
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Some thoughts....

You said that the Fuel Cut Relay is working fine. How did you check it? Did you force it to engage? Try swaping it with the High Beam's Relay.
I have tried using a couple of different relays with no luck, also I have tested the relays already on the bike, 12 volts to the engage to the switch and then checked for continuity.

The 30A fuse near the battery (not the one in the starter relay) is ok?
Yes the fuse is ok

Does your fusebox have power on every single fuse's one end?
Yes, all getting the correct voltage, and all in good condition

Does your Neutral indicator comes on when you turn the key to ON?
yes

On the Fuel Cut Relay coupler, are the two Black/White wires live (with + current?)
yes, they both have battery voltage

When you turn the key to ON do you have + current on the Black/White wire exiting the Bank Angle's Relay?
yes

The Brown/Black wire is a controlled ground by the ECM like you mentioned.
ECM engages the Fuel Realy for a few seconds, by default when the key turns to ON position but once the engine starts cranking and the ECM "feels" it through the cam's, cranck's sensors, it keeps the relay into an engaged position so that the Fuel Pump keeps working.
The ECM cannot sense a fuel pressure drop in the fuel line.

Since you said you tried another RC's ECM and you've got the same not priming results, I believe the problem lies somewhere in the harness or on an other relay since they are all connected somehow to each other.


Bank Angle's Relay is very important because it's the positive current's "first gateway" to everything else. And that's the Black/White wire that goes from that relay to the harness and two of it's ends go to the Fuel Cut Relay. One is grounded through the ECM through the Brown/Black wire and the current flowing inside the relay, engages it so the current can flow from the other Black/White wire to the Brown one that goes to the fuel pump.
The only other thing I can think is that the ECM I have for testing is faulty as well, I will give a friend a call also with an SP1 and see if his ECM works on the bike.

Thanks so much for your help Makis, it really is appreciated. I will try this other ECM and get let you know how I get on.
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post #8 of 11 (permalink) Old 08-14-2012, 10:21 PM
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There is a Green/Red wire on the B18 pin of your ECM.
That should have positive current when you switch your key to ON.
That wire primarily is powered from the starter's relay harness plug.
If it doesn't then check it on it's other end on one of diode's pins in the fusebox. And another end of the same wire runs up to the clutch's switch.
If no end of it's has + you should start from the starter relay.

Also check the ECM's A11 pin for proper grounding (just in case)


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Last edited by MakisRC51; 08-14-2012 at 10:24 PM.
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post #9 of 11 (permalink) Old 08-15-2012, 10:08 AM
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Also, idk if it was mentioned but check the right hand control, on my buddies rc the hand control was causing the fuel pump not to prime. It would still spin over but not fuel pump. Replaced hand control and it was good. Just a thought.
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post #10 of 11 (permalink) Old 08-15-2012, 08:16 PM Thread Starter
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Well, I am happy to report we got the bike going. The fault did lie in the ECM. Funny, I had tried 2 other different SP1 ECM's in it, apparently all three were duds with the exact same earthing fault. I borrowed the other one off a friend who pulled it straight off a running bike so I knew it was right. Interesting.
Thanks for everyone who helped, it is very much appreciated.
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