ArdWrknTrk Posted April 10, 2021 Share Posted April 10, 2021 Just my 2 cents, but looking at the wiring diagram you will have 12v at T when the engine is running because it will backfeed from the fuel pump that is powered from the energized fuel pump relay. I don't understand why the EVTM is not showing the diode mentioned. I know it is there... Link to comment Share on other sites More sharing options...
85lebaront2 Posted April 10, 2021 Share Posted April 10, 2021 I don't understand why the EVTM is not showing the diode mentioned. I know it is there... There will be a voltage at T with the engine running, but it should be lower than 12V due to the resistance wire and the current load of the fuel pump. If the pump is not in the circuit due to failure or an open circuit, than you will show 12V. If you don't believe it, try this at the coil. Key on, engine off, unplug the "horseshoe" and measure the voltage on the + side, it will be 12V, plug it in and see what it reads, it will be closer to 6V. Since the wiring has been intercoursed with, I would see if the resistance wire is even there. The value is given in the EVTM. On a side note, I just found I still have the engine part of the fuel pump wiring. Link to comment Share on other sites More sharing options...
nic55kel Posted April 10, 2021 Share Posted April 10, 2021 There will be a voltage at T with the engine running, but it should be lower than 12V due to the resistance wire and the current load of the fuel pump. If the pump is not in the circuit due to failure or an open circuit, than you will show 12V. If you don't believe it, try this at the coil. Key on, engine off, unplug the "horseshoe" and measure the voltage on the + side, it will be 12V, plug it in and see what it reads, it will be closer to 6V. Since the wiring has been intercoursed with, I would see if the resistance wire is even there. The value is given in the EVTM. On a side note, I just found I still have the engine part of the fuel pump wiring. Jim, I had not copied that the diode was in the circuit, I should have read your earlier posts more carefully. As for the voltage that is present at the pump, I could be wrong but I think the main function of the resistor, .75 ohm, is to compensate for the difference in system voltage between a cranking and a running engine. A low pressure fuel pump shouldn't be drawing more than 3 - 4 amps and so with a running engine the pump should see ~ 14 - (~ 2.5 resistor drop) = ~ 11.5v. Flashbacks through the carb brings back memories of singed eyebrows trying to clear flooded cammed big blocks. Quickly learn to keep the head back as one holds the choke open. Bob Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted April 10, 2021 Share Posted April 10, 2021 Jim, I had not copied that the diode was in the circuit, I should have read your earlier posts more carefully. As for the voltage that is present at the pump, I could be wrong but I think the main function of the resistor, .75 ohm, is to compensate for the difference in system voltage between a cranking and a running engine. A low pressure fuel pump shouldn't be drawing more than 3 - 4 amps and so with a running engine the pump should see ~ 14 - (~ 2.5 resistor drop) = ~ 11.5v. Flashbacks through the carb brings back memories of singed eyebrows trying to clear flooded cammed big blocks. Quickly learn to keep the head back as one holds the choke open. Bob I can't get the Daffy Duck exploding cigar out of my head now! Link to comment Share on other sites More sharing options...
Ranchman Posted April 10, 2021 Author Share Posted April 10, 2021 I can't get the Daffy Duck exploding cigar out of my head now! Thank you all for the input. I now have Daffy Duck and the old exploding cigar in my head I'm not sure where this diode would be unless it's in the tank selector relay? To answer Bill The .75 ohm resistance wire is there and measured fine, and I can here the pump run. Jeff Link to comment Share on other sites More sharing options...
85lebaront2 Posted April 10, 2021 Share Posted April 10, 2021 Thank you all for the input. I now have Daffy Duck and the old exploding cigar in my head I'm not sure where this diode would be unless it's in the tank selector relay? To answer Bill The .75 ohm resistance wire is there and measured fine, and I can here the pump run. Jeff Now all you need is Wile E Coyote chasing the roadrunner.... The only diodes I see in that diagram are the two in the tank selector valve as part of the motor feed circuit. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted April 10, 2021 Share Posted April 10, 2021 Now all you need is Wile E Coyote chasing the roadrunner.... The only diodes I see in that diagram are the two in the tank selector valve as part of the motor feed circuit. You are right Bill! I is isolated from the starter when the relay is disengaged. I guess there's no reason to keep it from back feeding. Link to comment Share on other sites More sharing options...
Mike Fawcett Posted February 28, 2022 Share Posted February 28, 2022 You are right Bill! I is isolated from the starter when the relay is disengaged. I guess there's no reason to keep it from back feeding. ok, but after all this, you're still not getting 4.3 psi to the carb,,, why?? Link to comment Share on other sites More sharing options...
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