StewyN Posted October 21, 2014 Report Share Posted October 21, 2014 Hey guys, another problem, seems never ending. Anyways when the truck is Running the fuel pump relay buzzes, and slightly after I shut it off. It did it once for a long time one day, then quit. Its done it a few times since then, on start up before, but only for a few seconds. Any ideas? Quote Link to comment
StewyN Posted October 21, 2014 Author Report Share Posted October 21, 2014 Actually i think it may be the auto choke relay Quote Link to comment
85"720 Posted October 21, 2014 Report Share Posted October 21, 2014 Check belts and battery cables. Might be the alternator going out. Quote Link to comment
StewyN Posted October 21, 2014 Author Report Share Posted October 21, 2014 what makes you say alternator? Quote Link to comment
StewyN Posted October 21, 2014 Author Report Share Posted October 21, 2014 So long story short if i disconnect the auto choke itself and the buzzing quit. hook it back up..buzz. it might be obvious that the choke is bad, but ive been wrong before Quote Link to comment
85"720 Posted October 21, 2014 Report Share Posted October 21, 2014 Because mine did that when my alternator started to die. Does your battery light glow dim? Quote Link to comment
Charlie69 Posted October 21, 2014 Report Share Posted October 21, 2014 Check all you grounds . Must be clean and tight. It is like others have said it is your alternator going out Quote Link to comment
85"720 Posted October 21, 2014 Report Share Posted October 21, 2014 Get your alternator tested. Does it buzz when your driving or only at a stop sign? Quote Link to comment
izzo Posted October 21, 2014 Report Share Posted October 21, 2014 It could very well be the auto choke relay. But the buzzing usually is the FP relay due to an alternator going out. The reason this is... The fuel pump relay gets a signal from your oil pressure, accessory and also your alternator. In the event of a crash, it will get shut off to prevent fuel from spewing out just in case of fire. When the alternator starts shitting out the relay starts buzzing from a half assed signal. I've had the buzzing relay before. Mine was always the fuel pump relay. But! The fuel pump relay is INSIDE the cab, passenger side under the dash against the kick panel. The auto choke relay is in the engine bay passenger side. You can run a voltmeter across your battery when it's off... Should be 12.x volts. When running 14.x volts (rev it once to around 2500 in case alternator is old and has to spin a little faster to kick in). And see if it's charging. Home depot sells a reliable and non fancy multi-meter for 20 dollars. Quote Link to comment
datzenmike Posted October 21, 2014 Report Share Posted October 21, 2014 Tim the '80 720 might be under the hood but all other 720s, the relay is beside the fuel pump relay. Might be a good idea to touch them and be sure which one is buzzing. The buzzing 720 choke heater is a common occurrence and usually means the alternator is not charging or is charging at a reduced output. It could be anything that interrupts the battery on one side and alternator charge on the other. As long as these two are equal the relay is quiet. So... Does the charge light in the dash light up or faintly flicker at idle? Check the charge voltage across the battery terminals when idling. Should be 14+ volts. Turn heater fan and headlights on. What does it read? Quote Link to comment
izzo Posted October 21, 2014 Report Share Posted October 21, 2014 Aye maybe I was thinking of the 510 when I said it was under the hood :rofl: owned too many datsuns :| Quote Link to comment
datzenmike Posted October 21, 2014 Report Share Posted October 21, 2014 I didn't check, but the 620 choke relay was. First year ('80) 720s were totally different to '81 and on and more like the 620.. Quote Link to comment
Charlie69 Posted October 21, 2014 Report Share Posted October 21, 2014 1980 720 L20B were mechanical fuel pumped from the factory. Quote Link to comment
StewyN Posted October 21, 2014 Author Report Share Posted October 21, 2014 well i disconnected the wire on the choke itself and the buzz quits, would this still point to the alternator? also recently found out it was only firing one one coil, fixed that and now has an extremely high idle, coupled with the buzzing relay. and it is the auto choke relay, held it while it buzzed. and as said above i unhooked the auto choke and and the buzz quits Quote Link to comment
datzenmike Posted October 21, 2014 Report Share Posted October 21, 2014 If choke is disconnected it will stay on or at the very least part on. Choke on will also engage the fast idle circuit. If you disconnect the choke heater you will need to find a new power source for it. I just spliced it into the idle cut solenoid right beside it. Red wire. Fast idle is likely caused by the timing being now too advanced for the twin plugs firing. Check and set the timing to 3 degrees BTDC. Quote Link to comment
StewyN Posted October 23, 2014 Author Report Share Posted October 23, 2014 I had figured on the timing, but after i spliced the wire to the idle cut, it jumped back up to 8 so after resetting it to 3 it seems to be doing alright. check back these next few day tho, i might still need ya lol. thanks guys Quote Link to comment
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