Hi
​​​​​​So long story short. Was riding home the other day when fuel pump on my 97 f3 died. I bypassed it by side of road and rode back on gravity feed.

However, when I got back and tested it with a multi meter, the pump was only seeing 6 volts at idle. I then assumed the 'cut off' relay was faulty, bought another and saw the same thing. I then checked the pump direct to a 12v battery and it's dead. Should have checked this first but hey ho (it's less than a year old but was a cheap eBay affair).

What I now realise is the volts from the cut off relay to the pump increase with rpm as there is an input wire coming from the ignition module which it must detect rpm from I assume.

Therefore my question is - is this expected? Ie it's a variable output system and the 'cut off' relay is actually a control module? Has anyone else tested voltage at the pump at idle? The manual doesn't say anything about it other than 'check for battery voltage' at the pump etc

Thanks
Dan
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Yes it is expected. The Fuel Cut Relay is a solid state relay. It's output is controlled by a signal that is received from the same signal that controls the ignition coils for cylinders 2-3. The Fuel Cut Relay only receives power when the Engine Stop Switch on the right handle bar is in the Run position. Testing the operation of the pump is done by performing the Flow Test. This is done by disconnecting the Fuel Cut Relay, putting a jumper across 2 of the pins and measuring the flow rate for 5 seconds.

Since the pump receives on/off pulses tied to the ignition, you have to have a meter that can measure peak voltage due to the nature of the signal. And the engine has to be running.

 

You're a star thanks for that info! Now why can't Honda put that in the manual.

 

Well, I don't know about that, but I've certainly dealt with enough fuel pump issues on F3's to be intimately familiar with it's operation. I've done many a test ride with one hand on the throttle and one under my left leg with my hand inside the cowl checking to see if the pump was running. lol

Also keep in mind that the pump will stop if it develops pressure. That is by design since it's only trying to keep the carburetor bowls full. If the float valves close, the fuel has to stop coming in. It doesn't recirculate like a fuel injected engine. So the only time you're going to see a continuously running pump is during that flow test where it's just pumping into a container and not developing any pressure.

 

🙂 Been there
Mine just seems to have given up the will to live. Even connected direct to a battery it doesn't do a thing. I would get another cheap eBay one but that ended badly this time. Wemoto usually sell good gear but there's is listed at £190! Maybe I'll just have to bite bullet...

 

If you're in the UK or EU I understand that the cost can be quite large for the OEM. I've purchased a couple myself, but at around $150 US it's at least a little tolerable.

 

Ouch, I feel your pain. We do certainly seem to pay through the nose for OEM parts here
Unfortunately sometimes you just have to bite the bullet as going the alternate route is sometimes more hassle than its worth.

 

New Honda mitsubishi pump fitted and it runs again. Much quieter than cheap one. Exhaust is now blowing though