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powering the fuel pump on a 308

Discussion in 'Technical Q&A' started by caponome, Nov 12, 2012.

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  1. caponome

    caponome Rookie

    Dec 30, 2009
    34
    #1 caponome, Nov 12, 2012
    Years ago, I bypassed the seat switch for the fuel pump.I know this is old stuff but as I am installingf an XM radio I have had the ignition sw in the on position which is fine for verifying the radio connect but of course I have the constant hum of the fuel pump. I called my mechanic asking how production cars handle the fuel pump when the ignition is on and engine not running and he said they do it through a relay connected to the ECU. I have a time delay relay that will emulate this but I need to know definitively if the engine is running or not after the time delay expires. I spent most of the weekend tracing the wiring diagram and cannot find a lead that will be charged (or not ) relative to the engine running. Any suggestions? Thanks for the help.
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
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    #2 Steve Magnusson, Nov 12, 2012
    Can you be more specific about what you are trying to do? On a carbed 308, when the key is "on", not only does the fuel pump run (assuming that you've bypassed the seat switch on a US version), but the ignition system will also be "on" (which can be fatal for a points-based ignition if the engine isn't running and draws a significant current).

    Are you trying to have only the radio "on" when the key is "on" and the engine isn't running?

    Also, please give year/model/version -- and ignition system type, if modified.
     
  3. caponome

    caponome Rookie

    Dec 30, 2009
    34
    #3 caponome, Nov 12, 2012
    I have a 79 308gtsi it was the last year for carbs and dists in the 308. I am running the original ignition. More specifically I was trying to stop the pump from running when the engine is off but the ignition is on. Your right of course about the ignition coils heating up so it might make sense if I incorporated more than just the fuel pump in this exercise assuming it is possible.

    My intent was to power a time delay relay from ignition on to run the pump for say 30 seconds. A second relay would power up when the engine was running and transmit power to the n/c post of the timer relay so the pump would continue to be powered after the delay expired.

    I have found over the years that there are several times that I need the ignition on and do not want the engine on. I could use simple cut out switches, but that would be too easy for me. Your input is appreciated.
     
  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    #4 Steve Magnusson, Nov 12, 2012
    Last edited: Nov 12, 2012
    The way they did this on the later injected cars was to use a "tachometric" relay to control the relay running the fuel pump. You might be able to do the same thing -- i.e., use the tachometric relay (where the light blue wire, from the ignition switch terminal 15, normally connected to the top of fuse #2, is connected to terminal 15 on the tachometric relay) to control a (high-current capable) relay that puts +12V onto the top of fuse #2. (In this case, terminal 87 from the tachometric relay would be used to close the high-current capable relay so ignore some of the TR-specific jargon in this jpeg):
    Image Unavailable, Please Login

    This would give the result:

    key "on", but engine not running = top of fuse #2 at 0V (which leaves the fuel pump and ignition system unpowered)
    key "on" and engine running = top of fuse #2 at +12V (which runs the fuel pump and the ignition system)
    key in start position = top of fuse at +12V (which runs the fuel pump and the ignition system)

    (If you need a more detailed possible schematic just give a shout. What I'm not sure about here is that the tachometric relay may "glitch" when going from mode 1 to mode 2 so this could be trouble if the ignition drops out for a moment and this causes the tachometric relay to fall out of mode 2.)
     
  5. Brian Harper

    Brian Harper F1 Rookie
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    #5 Brian Harper, Nov 12, 2012
    Use the now unused seat switch to pass power to the radio. The radio will work whenever you are sitting in the car. That's what you want, right?
     
  6. caponome

    caponome Rookie

    Dec 30, 2009
    34
    #6 caponome, Nov 12, 2012
    Brian my issue is about needing accessory power when the engine is not on and I need to test stuff that requires me to be out of the car. The radio is not the problem, cutting power to the fuel pump and as Steve pointed out the ignition coils as well when I want to power the accessories only is the issue.

    Steve, I will layout the circuit you outlined. I looked at the tach circuitry and the wiring for the oil pressure sending unit and gauge as well. I didn't see a relay for the tach. My wiring diagram '78 WSM doesn't show the seat switches either. I will look again. I don't know enough about how these senders and instruments work. My concern was that since these instruments are electronic and I believe they rely on voltage variations (I think) nor could be sure that the voltage would be zero, I was reluctant to add the load of the trigger voltage my relay requires into the circuit for fear it may distort the instrument reading, if there is a relay for the tach that might do it. I will be back with questions for sure.

    Thank you both for taking the time to help.
     
  7. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,804
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #7 Steve Magnusson, Nov 12, 2012
    Terminal 1 on the tachometric relay is connected to terminal 1 on either coil (it will have no effect on the tach gauge behavior).

    There is no "relay for the tach" in the stock configuration -- the tachometric relay is a component that you would add yourself (along with another ...113 relay).

    The schematic you should be using is in the 150/78 OM (covers 1978-79 US 308B/S) -- the seat switch will not be present on euro 308 schematics.
     
  8. caponome

    caponome Rookie

    Dec 30, 2009
    34
    #8 caponome, Nov 14, 2012
    Steve Thanks again, I found a schematic for the relay to identify the functions of pins 1 and 50 for my own understanding of how the relay works. Not surprising, you are spot on. Too bad for me but these relays are not readily available. I think my problem comes down to a replacement for pin 1 in the form of a continuous signal. In that case I can use a configuration of 2 relays to accomplish the task.
     

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