weldingrod Posted December 26, 2018 Author Report Share Posted December 26, 2018 1 hour ago, datzenmike said: On the 720, the accessory side of the ignition switch powers two 15 amp fuses. One for the radio and cigar lighter, the other for the wiper/washer and air conditioning as well as the fuel pump control. So wiring the pump directly is no problem for it. You could do this with on Nissan relay. Wire the starter signal to the fuel pump. Wire the ignition through a normally closed relay switch also to the fuel pump. On the coil side of this relay wire the one side to the ignition input and the other to the oil pressure sender. Turn ignition on, the oil pressure sender is grounded and the relay it tripped to open position, so no power to the fuel pump. In the start position power is supplied to the fuel pump and it runs as long as the key is in start. Engine fires up and the start signal removed but now the oil pressure sender is not grounded and the relay trips to it's closed or rest position and power is supplied to the fuel pump as long as there is oil pressure. Crude but would work. Only down side is if the carburetor is completely empty... you would have to hold the starter on till the fuel reached the carb. Most carburetors hold enough gas to start without any pump and drive down the street. This relay is already used on 720 so just get one off a wreck. The auto choke relay is perfect for this. Quote Link to comment
weldingrod Posted December 26, 2018 Author Report Share Posted December 26, 2018 Seems other relay with fuel pump relay is the automatic choke relay. Seriously thinking of relocating pump relay if I have to go in there again. Quote Link to comment
weldingrod Posted December 26, 2018 Author Report Share Posted December 26, 2018 PS. Mike, haven't seen an early 80's era 720 at a wreckers for years. Salal pickers bought all small import 4x4's up years ago, beat the hell out of them and then bought newer ones when they could afford to. Had 4 in the yard here a few years ago and it appears I should have saved way more parts than I did. Quote Link to comment
weldingrod Posted December 26, 2018 Author Report Share Posted December 26, 2018 And it may not even be relay. If alternator wire or oil pressure switch wire not powered when running shuts off relay. Can't run truck, so can't test for power. Quote Link to comment
weldingrod Posted December 26, 2018 Author Report Share Posted December 26, 2018 Or will just cranking the engine power those wires? Quote Link to comment
datzenmike Posted December 26, 2018 Report Share Posted December 26, 2018 On 12/25/2018 at 2:37 PM, datzenmike said: Jumper the L (Blue) wire to the WB (White/Black) wire. This will send 12 volts to the pump any time the ignition is on. Probably the easiest thing to do Quote Link to comment
Charlie69 Posted December 28, 2018 Report Share Posted December 28, 2018 On 12/26/2018 at 2:40 PM, datzenmike said: Interior light. Comes on when you open the drivers door. Stays on for about 20 seconds after closing the door an leaving unless the ignition key is in. Not all trucks may have it but the amplifier is under the passenger side seat although the cruise control has something there as well.. Not all models and years of the 720 have the dome light timer. Just higher end models. Quote Link to comment
weldingrod Posted December 28, 2018 Author Report Share Posted December 28, 2018 Guess not an issue with me because mine doesn't work but maybe a good idea to check that circuit while I have the passenger seat out to remove fuel pump relay. 1 Quote Link to comment
weldingrod Posted December 28, 2018 Author Report Share Posted December 28, 2018 On 12/25/2018 at 2:42 PM, Crashtd420 said: Wont that cut off during crank? he may have to give it a 3 count and let the carb fill up, and then crank the engine.... since it is a daily driver for him there should be enough fuel in the carb to get it started.... Using that schematic of wiring plug, for bench test, oil switch power should power pump. accessory power should power pump. power from fuse 5 should power pump. power from alternator should power pump. This should diagnose all circuits in relay. Is that right? 1 Quote Link to comment
Crashtd420 Posted December 28, 2018 Report Share Posted December 28, 2018 On 12/25/2018 at 5:37 PM, datzenmike said: Jumper the L (Blue) wire to the WB (White/Black) wire. This will send 12 volts to the pump any time the ignition is on. That pic is probably showing you the orientation of those wires in the plug.... Ground = ground Oil pressure = ground , but when it activates it shuts the ground off to turn the idiot light off.... This could be different on the 720, I'm not sure.. so verify polarity and function of your oil sender..... Acc= 12v switched from key.... Fuel pump= where your sending the power...... Fuse #6 = this is the power for the pump..... Altenator =positive, I think it's the lamp system so its 12v while truck is running... So the goal is to connect fuel pump to fuel #6..... if you connect ground to the ground and power to the acc you should hear a click, if not add power to the alternator then, then add in the oil pressure switch, I'm not sure what the relay is looking for from the oil pressure switch, maybe if it sees ground it wont activate.... so maybe that stays off... check continuity between the fuel pump and fuse wire.... that's how I would go after it.... Feel free to comment and correct this if I'm off track.... 1 Quote Link to comment
Charlie69 Posted December 29, 2018 Report Share Posted December 29, 2018 Also the dome light timer on the 85/86 720 is in the passenger side panel under the King cab 1/4 window. Quote Link to comment
datzenmike Posted December 29, 2018 Report Share Posted December 29, 2018 Black wire is almost always ground. I think the alternator input is connected to one side of a relay coil, the other to a reference 12v from the battery 12v battery........ relay coil.... 12v alternator (running) no current flow) 12v battery>>>> relay coil>>> alternator (not charging) current flow to ground turns a relay to ON. The same applies to the oil pressure switch and another? relay for it. Starter input Ignition ON input. These two probably start a timer. The other two keep it going or turn it off. 1 Quote Link to comment
weldingrod Posted January 14, 2019 Author Report Share Posted January 14, 2019 OK, new relay installed, checked pump with 12volt source and it works, checked wiring from relay to pump and getting 12 volts, checked fuses all good, still no joy! Jumped relay plug, no action from pump. Hate to tow it to a shop but looking like only option at this point. 1 Quote Link to comment
datzenmike Posted January 14, 2019 Report Share Posted January 14, 2019 You have 12v at the pump? It should run. Do you hear it running? If you have power at the pump and it isn't running it can only be a bad pump or the ground is no good. If pump making sound but no gas, the tank is either empty, the line to the pump is plugged or the line to the carb is plugged..... this isn't that hard. 1 Quote Link to comment
weldingrod Posted January 14, 2019 Author Report Share Posted January 14, 2019 Pump runs off battery, new relay, tank full. Many years ago my Father taught me if something won't start/run the first thing you check is the fuel. New lines pump to carb and return. According to the idiot test jumpers at relay plug are side by side, right? 1 Quote Link to comment
datzenmike Posted January 14, 2019 Report Share Posted January 14, 2019 Looks like the W (white) to WB (white/Black) will send 12 v to the pump. Yes Have you made this jumper? Does the pump make any sound? Is it running or not??????????? 1 Quote Link to comment
weldingrod Posted January 14, 2019 Author Report Share Posted January 14, 2019 Used jumper and pump not running. Pump runs when hooked up to a battery. All fuses good. New relay installed. Test shows 12 volts at pump connection. Pump has been stripped, filter cleaned, reassembled and bench tested before reinstall. All new fuel lines tank to carb. 1 Quote Link to comment
datzenmike Posted January 14, 2019 Report Share Posted January 14, 2019 Is there 12 v at the relay on the jumper wire? W and WB ? If yes then there should be 12 v at the pump. The WB wire goes to the pump. Is there 12 v at the pump when the jumper in place? If no then the wire is broken or the 10 pin connector under the passenger seat isn't connecting. This 10 pin connector joins the external harness under the truck to the rear, to the cab harness. 1 Quote Link to comment
spddm0n Posted July 28, 2019 Report Share Posted July 28, 2019 Wow! Reading this thread made my head hurt! I have some intermittent fuel issues all of a sudden. Looks like I'm about to visit a lot of these same systems. I hate it when there is a gremlin in the system. I had hoped to rid my truck of most of these by now, but I do know I've found some bad "connections" on other devices. These are now so old, sometimes it can be an intermittent off/on bad connection (after manipulating) a component during install procedure...even after a bench test. I found the windshield washer fluid reservoir pump was coming on randomly and when I inspected this, I found the wires shorting out when someone bolted the reservoir down on top of the wires and crushed them. Over time, it wore through the wires and shorted them. 🙂 Quote Link to comment
weldingrod Posted July 28, 2019 Author Report Share Posted July 28, 2019 I started that thread and went through all you read. Problem turned out to be a small leak in lower right windshield gasket. Intermittent at first because relay was damp. finally failed when it got soaked. Just enough of a leak nothing showed anywhere else. Didn't discover that until I finally pulled the relay instead of testing circuits. I hate electrical!! Quote Link to comment
spddm0n Posted July 28, 2019 Report Share Posted July 28, 2019 2 hours ago, weldingrod said: I started that thread and went through all you read. Problem turned out to be a small leak in lower right windshield gasket. Intermittent at first because relay was damp. finally failed when it got soaked. Just enough of a leak nothing showed anywhere else. Didn't discover that until I finally pulled the relay instead of testing circuits. I hate electrical!! Interesting...Wow. Yeah, what a headache that must have been to go through. Gremlins. Thanks for reporting back on the ultimate cause. That, in turn, really helps people in the future when searching out gremlins. 🙂 I'm getting into mine this afternoon. Fingers crossed... Quote Link to comment
datzenmike Posted July 28, 2019 Report Share Posted July 28, 2019 According to the FSM... Ignition ON or OFF or ACCY but engine not running, no charge, no oil pressure..... no fuel pump Ignition START, engine cranking, no charge, low/no oil pressure............................. fuel pump * seems the start signal keeps the pump on Ignition ON, engine running, charging or no charge, oil pressure............................. fuel pump * seems the oil pressure keeps the pump on Ignition ON, engine running, charging, no oil pressure.............................................. fuel pump * seems the charging alternator keeps the pump on regardless of loss of oil pressure which is odd. Why not have loss of oil pressure also a loss of fuel pump? Quote Link to comment
spddm0n Posted July 29, 2019 Report Share Posted July 29, 2019 I found the problem with my truck... Bad fuel pump relay. Has anyone ever opened up the relay? I blew a resistor out on the circuit board, and another one looks like it's been heated up a bit too. I saw the magazine article where they repaired the contacts on the bottom of the circuit board. I made that repair just in case too (I didn't look cracked under my magnifying glass). Does anybody know what size these resistors are? I can't tell, since the burned one is...well..burned up. 🙂 1 Quote Link to comment
spddm0n Posted July 29, 2019 Report Share Posted July 29, 2019 3 hours ago, datzenmike said: According to the FSM... Ignition ON or OFF or ACCY but engine not running, no charge, no oil pressure..... no fuel pump Ignition START, engine cranking, no charge, low/no oil pressure............................. fuel pump * seems the start signal keeps the pump on Ignition ON, engine running, charging or no charge, oil pressure............................. fuel pump * seems the oil pressure keeps the pump on Ignition ON, engine running, charging, no oil pressure.............................................. fuel pump * seems the charging alternator keeps the pump on regardless of loss of oil pressure which is odd. Why not have loss of oil pressure also a loss of fuel pump? Hmmm...where in the FSM are you reading that? Which FSM do you have? I've found it annoying my 1985 FSM only states a lot of information in the EF & EC section, but only in the part of that section for Z24 for Canada. I just looked at the table in the section for Z24 for Canada (the US part says nothing about the fuel pump control). I calls the "relay" a Fuel Pump Control Unit. It says the fuel pump is not operating when there is "No" pressure. It says it IS operating when there is LOW pressure. 1 Quote Link to comment
datzenmike Posted July 29, 2019 Report Share Posted July 29, 2019 I have what you have posted. '84 FSM in the section with the electric fuel pump. Quote Link to comment
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