83 VF750F fuel pump relay issues

Discussion in '1st & 2nd Generation 1983-1989' started by CWB, Aug 10, 2010.

  1. CWB

    CWB New Member

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    As the title says, I have an 83 VF750F with a fuel pump issue. I assumed a few things in this process so please correct me if Im wrong. I couldnt get it to pump fuel so per the service manual, I jumped the black and white wires and it pumps. Next, rather than check the continuity of the blue wire from the ignition module, I assumed (maybe my 1st mistake now that I think about it) that since I measured 14 volts at the relay connector with the voltmeter connected between the blue and black wires with the ignition key on, that the wiring is good and it must be the relay (assumes black is the ground). Rather than spend $40 on a new relay, yesterday I tried wiring a $10 4 pin 12v 40 amp, normally open until its energized automotive relay instead but didnt have anymore luck in making it work. To do so I simply wired the grounds together, then used 3 jumpers to connect into the stock relay connector/harness. Can anybody help with any suggestions as what else to try/check? Id like to attempt to fix my wiring and use a relay before modifying it too much. Also, will it hurt the pump if I simply jump the circuit on a permanent basis between the white and black wires? Does the pump on these ever actually shut off during normal operation because of lack of fuel demand? Thanks, Carl
     
  2. 83 Geezer

    83 Geezer New Member

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    Ever fix your problem?


    I am having the same problem, can be riding on the freeway and it will just cut off.
     
  3. squirrelman

    squirrelman Member

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    Just eliminate the relay by bypassing it, using an insulated jumper wire at the relay plug.
     
  4. Glenn Lilley

    Glenn Lilley New Member

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    Did you sort out your fuel pump relay problem CWB?

    I am slowly working my way through my VF750F cleaning the contacts and link plugs using a product called De-oxit (fearfully expensive, but seems to do a good job!). Ran through the diagnostic tests, pump works fine with a jumper in the relay plug, Continuity exists between blue wire at relay and spark unit, but relay is non-functional when plugged in as fuel pump doesn't function! According to my manual, that means the relay is stuffed.

    Any suggestions other than buying genuine? Obviously this relay is solid state as evidenced by the circuit diagram so a standard relay probably doesn't work as you found out.
     
  5. Glenn Lilley

    Glenn Lilley New Member

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    Unfortunately, when I pulled everything out to bench-test the pump, it crashed and burned- ordered a terrific new pump from the USA (~$40) plus Del.) which works a treat. Maybe the original pump itself was dodgy or removing it caused a wire short .

    On closer scrutiny, the relay in fact was fine!
     
  6. Fats

    Fats New Member

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    Hi Glen. I'm in Sydney and re-birthing a 750 as well, and like you are at the dreeaded fuel pump stage. First attempt at start up I only succeeded in frying my relay and blowing my pump. I picked up a second hand pump which I have checked and works fine and I have ordered a second hand relay from the US that i'm waiting for. I did a wiring check to see if there is a fault that caused the relay to fry and discovered that both the blue and the black wire have 12 volts to them. I'm not sure if thats correct but it points to a dead short to me but maybe thats what the relay needs? I get 12 volts from the kill switch on the black wire and 12 volts from the coil on the blue. Is your bike the same?

    Any help would be hughly appreciated!

    Cheers, Fats.
     
    Last edited: May 4, 2018
  7. Glenn Lilley

    Glenn Lilley New Member

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    Hi Fats, exactly my experience or so I thought. But definitely had to replace the pump... it turned out that the relay was functional even though it seemed not to be...read on!

    Just to make sure that I also understood the system, I just went out and checked the voltages at the plug that connects to the relay with the relay in place. You are correct in that with ignition on, the BLACK wire and BLUE both measure at 12Vdc (0V on white wire). The question is, what happens next? How does that result in relay function to operate the fuel pump.

    My conclusions based on the manual, a bit of further reading, and measurement....

    It seems that the expensive Honda relay (genuine $100 plus) is definitely NOT a simple switch relay ($~10) such as is used in a headlight circuit, so beware the idea that it can be circumvented. Neither is the flasher relay (@$75) a simple unit, although that can easily be replaced with a basic two wire Narva brand flasher can.

    Ok, the fuel pump relay firstly senses, with fairly basic electronics, ignition pulses from the CDI (blue wire). Internally, a transistor and SCR are then periodically switched on, which shunts power from black wire to white, and activates the fuel pump. Ie. the SCR mimics a simple relay, but is different in that it produces pulsed DC to the WHITE pump supply wire. My measurements with a DVM show me that voltage averages out to somewhat less than the supply voltage too- my bike only measures around 8.5VDC at idle rising to an average of 9.5VDC with engine running at several thousand RPM. That might explain why I fried the fuel pump by bench testing it with constant 12.5V (it would be useful too if anyone has an oscilloscope to actually look at the waveform of this system). Furthermore, I seem to remember reading later that continuous operation of the pump with the jumper in place is also not a good practice...indeed, the Honda pump might only be rated for intermittent operation (a clue, they recommend only testing for 5 secs in the service manual). Which brings me to a WARNING: some guys suggest you can connect with a simple headlight relay and get away with it! This is probably reasonable, as in normal operation the pump only pulses now and then to top up the fuel bowls, and then mechanically switches off by pressure switch anyway. I would definitely be very careful bench testing the pump at full voltage- I think that was my mistake!

    The strange outcome of all of this is that unlike car pumps, which will prime/pressurise the fuel circuit immediately on ignition activation, the Honda relay doesn't appear to do this- apparently however, some M/C relays do have timers which immediately turn on the pump for a few seconds to prime the fuel bowls (not the VF relay). This seems to be a bit of a catch 22, so how do you prime a dry system without running the engine? It turns out that even a few seconds of cranking on a dry engine will bring enough fuel in to get it firing in my experience- it clearly is a fairly efficient high flow pump, it only takes a few seconds to draw fuel from the tank to the fuel bowls.

    Hope that rather long explanation helps, I am no expert though, I just started with a little prior knowledge and Google and worked through it in my restoration. I wished I had understood it more before I charred my fuel pump though...lol! The circuit below sort of explains things....due acknowledgement to the circuitry author.
     

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  8. Fats

    Fats New Member

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    Wow, thanks heaps Glenn, that's a brilliant explanation to the fueling system. Your advice is invaluable for the correct operation and also confirms my wiring is fine, good stuff. It all makes sense as the pump can't be on continuously as it ends up just flooding the carbs as I found out by bypassing the relay. At least I know my pump works well! I will just wait till I get my replacement relay and try again.

    Thanks again for your advice.
     
  9. squirrelman

    squirrelman Member

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    NO, fats, the pump can have constant power but never flood the carbs IF the float needles are in good condition, which, apparently, yours are NOT.

    Glenn may be the very best #1 clear technical explainer on this channel ever ! Great work, sir, exemplary writing skills !
     
  10. Fats

    Fats New Member

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    Thanks Squirrelman, good point. I did expect that I would have carb issues and you have just confirmed this. More work to do!
     
  11. squirrelman

    squirrelman Member

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    \
    Just doin my job, Bunky ! :rolleyes:
     
  12. Glenn Lilley

    Glenn Lilley New Member

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    That did occur to me re your carbs needle/floats Fats. I also discovered as Squirrelman suggested, once the relay (internal SCR system) activates to allow the pump to run, it should remain in an active state until ignition power is removed- pump otherwise only stops pumping if the needles close and supply line pressurises- I think the pump pressure sensor switches out at about 2 psi. Its a bit harder to test, but it might also deactivate if the engine stalls! There is possibly more to that relay than the simple circuit I showed, but knowing is a bit academic, and if it work OK, thats problem is over. Although, if someone bequeathed me a blown relay, I probably could determine its internal circuitry- nothing better to do except ride bikes..haha. My bet is an equivalent could be made from a few bucks worth of standard parts.

    Squirrelman thanks for the kind words, its born of a certain frustration with incomplete explanations found so often. :)
     
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