720inOlyWa Posted August 31, 2017 Report Share Posted August 31, 2017 I read, with great interest, the thread titled ‘No Voltage At Fuel Pump‘ because it dealt with the fuel relay and the fuel pump.- problems that I may be encountering. Many of the questions- and recommendations- are closely related. So I am going to start a thread as I try to figure out my problem, in hopes that the answers I find might help someone else who hears fighting bumblebees in a dixie cup, in the glovebox of their 720 truck. When I got my 84 King Cab running, I could hear something buzzing away under the dash, over by the glovebox. I did a little reading and even nabbed a replacement fuel pump relay out at the wrecking yard, but switching it out did not help. That‘s when I noticed, by feeling, that it was the little choke relay off to the side that was making all the racket. So I pulled it and the noise stopped. But so did the electric choke function. By way of background, a toggle switch has been added to the center console. It connects a hot lead from the fusebox to the fuel pump. I should say ‘connected‘, because I disconnected it from the fusebox, disabling the boosted juice to the fuel pump. Before I disconnected it, I flipped it with the ignition on. It made a strong fuel pump clicking, a bit louder than my daily driver. When I turned off the switch, naturally, the fuel pump went silent. With the fuel pump booster wire disconnected, I cannot hear any fuel pump noise at all. None. I have not taken this truck out on the road, so I do not know if it would suffer fuel starvation at some point. I do not know if a truck can start and idle without some fuel pressure to the carb. What I get from this discovery is that, at some point, the truck stopped running reliably due to poor fuel delivery. The ‘mechanic‘ tested the fuel pump with a jumper wire and saw that the pump was strong, so he just made the jumper semi-permanent. Junker mechanics. The choke relay issue and the fuel pump output issue are, no doubt, related somehow. I need to add one last observation: the gas gauge is not operating correctly. I have poured 6 gallons into the tank and when the key is turned on, it comes up to just a bit above E, and does so very slowly. No matter how much gas I add, it reads the same, as in barely and slowly. The truck goes up on jack stands tomorrow, for a lube job and to remove the tires and wheels for rehab and replacement. While it is up on the jacks, I will do my multimeter tests on the fuel pump and gas gauge sender unit and begin isolating the main suspects, including the relay, ignition switch, and fuel pump until I hit a winner. First up: all new fuses and fusebox cleaning. It can‘t be that easy, but you never know... 1 Quote Link to comment
datzenmike Posted August 31, 2017 Report Share Posted August 31, 2017 For the auto choke heater to work it needs a charging alternator. For what ever reason the voltage getting to the relay may be weak. When it clicks on the voltage drops below what's needed to keep it on and it clicks off, only to click on again causing a buzzing sound. This is a common problem with the 720s. The fuel pump relay also has a connection to the alternator, the oil pressure sender, and start signal. It acts like a room light delay. It is turned on with the ignition but will time out if the engine does not start or one or more inputs fail. 1 Quote Link to comment
720inOlyWa Posted August 31, 2017 Author Report Share Posted August 31, 2017 So you would also suspect that the Alternator isnt putting out sufficient juice when running? Interesting, since I could not find a short anywhere and replaced the choke relay once without solving the problem. I will look at that, too- and first. THANKS! 1 Quote Link to comment
datzenmike Posted August 31, 2017 Report Share Posted August 31, 2017 Possibly the alternator or possibly the power is not sufficient (weak) at the relay. 1 Quote Link to comment
720inOlyWa Posted September 1, 2017 Author Report Share Posted September 1, 2017 Possibly the alternator or possibly the power is not sufficient (weak) at the relay. In which case, the ignition switch may ve the culprit?... I measure the output of the generator at the battery, when running, correct? And it should read something a bit over 12 volts, right? 1 Quote Link to comment
datzenmike Posted September 1, 2017 Report Share Posted September 1, 2017 Just over 14 volts when running would be good. Under 13 would not be right. To keep a 12 volt battery charged you have to 'push' about 14 volts through it. All Nissan alternators after '77 are internally regulated and set for just over 14 volts. More than 14.5 is just not needed and not the best for a battery. I guess you could jumper your vehicle with a running vehicle. Turn ignition on (not start) and see if the relay is still buzzing. 1 Quote Link to comment
720inOlyWa Posted September 5, 2017 Author Report Share Posted September 5, 2017 I just checked it with a voltmeter and it is vacillating between 10.5 ad 11 volts, even with a belt that is a bit more snug than I would have it. On my Fudgecicle, the multimeter read 13.8v. and does not waver. I have a spare alternator sitting around here, all cleaned up and ready to go. I think I will slap that in there and see if it sorts things out for me visa vie the choke relay. If so, I am thrilled for the advice!!! 1 Quote Link to comment
720inOlyWa Posted September 25, 2017 Author Report Share Posted September 25, 2017 Well, Damn. I thought that I had it whipped, but I can see that this is going to be a real fight after all. I replaced the alternator, and reinstalled the (new) choke heater relay, and started the truck. It ran much better and no relay scream. I read the voltage off of the battery terminals while running and it read 14.35 volts. More good news. I basked in the glory of Ratsun salvation as I turned off the key and headed out to do something else. With that behind me, I went out to work on it the next day and it started up so nicely, especially for a cold start, and ran just great... until a couple of seconds in, when the battery light went off and the charging circuit took over. As the light on the dash went out, the choke relay began its primal scream again and the engine began running rougher. Oh man! I am thinkin‘ that the cab of that truck heard some brand new words. Which goes to show me that I should never count my salvations until about 10k miles have passed under the tires. Any thoughts? As of now, I cannot reason out a mechanical- or electro-mechanical- reason for this to happen, unless there is something wrong with the ignition switch, perhaps? Seems like the switch short perhaps. I haven‘t tried to begin testing voltages to or from the switch yet. I pulled the relay again, started the truck, measured 14.35 at the terminals and it was running nicely again. At that point, I realized that I was getting tired, and the dog needed a walk. So I think that is all the data that I have to share, right now. If you have any ideas, I would sure be happy to hear them. 1 Quote Link to comment
datzenmike Posted September 25, 2017 Report Share Posted September 25, 2017 What ever the alternator puts out should be felt at the relay. Like I said all 720s seem to experience this chattering choke relay. If putting out 14.35 maybe bad contact at fuse? 2 Quote Link to comment
Charlie69 Posted September 26, 2017 Report Share Posted September 26, 2017 I had a similar problem with my 86 king cab 720 and it took replacing the ignition switch to solve my problem. 1 Quote Link to comment
720inOlyWa Posted September 26, 2017 Author Report Share Posted September 26, 2017 I am not sure what could go wrong inside te ignition switch to do this, but I have tis feeling something is funky in there... I will measure voltages all over and look for any obvios issues in the wiring itself before tearing into the switch. Was it a bear to replace, charlie? 1 Quote Link to comment
datzenmike Posted September 26, 2017 Report Share Posted September 26, 2017 The actual switch screws on the back of the lock. Maybe hose it out with contact cleaner? I think the 720 has it's own fusible link. Measure the voltage on both ends of all three? fusible links. They should be the same as the battery 14.35 when running. Unplug and plug them back in to make better contact? Check the voltage on the fuse that supplies the relay. Again should be 14.35 or what ever you have when running. 2 Quote Link to comment
720inOlyWa Posted September 26, 2017 Author Report Share Posted September 26, 2017 Yes, yes, and yes. Thanks. Will do. I will douche it with DeOxIt, yes. 1 Quote Link to comment
720inOlyWa Posted October 13, 2017 Author Report Share Posted October 13, 2017 Okay, so the mystery continues.... and this is getting pretty old! I replaced the Alternator, replaced the ignition switch. Both of these moves helped the truck quite a bit, but did not stop what appears to be a serious ignition short whenever I plug the choke heater relay back in. The relay screams, the engine stumbles badly. I cleaned all contacts with DeOxIt. It measures the same voltage at the relay as at it does at the battery (when running). So I am kind of flummoxed here. What else can I replace? Check? Should I suspect the computer control unit, under the seat? I am not sure where to turn. As a side note, before I replaced the ignition switch, I didn‘t hear the fuel pump coming on until I turned the key to Start. And even then, the sounds was much weaker than my other truck. Now that I have replced the switch, as with my other truck, I can hear the fuel pump coming on when I turn the key to On, and the sound from the pump is more robust. So replacing the ignition switch certainly helped with fuel delivery, but it did not fix the screaming choke heater relay problem... Any ideas? 1 Quote Link to comment
Roadster-ka Posted October 13, 2017 Report Share Posted October 13, 2017 The voltage might be alternating current due to open diodes in the alternator. I know you replaced the alternator but there are very poor quality units out there. A scope pattern would show if this is the situation or a modern alternator tester. Alternating current would drive the choke relay crazy but would show up as the proper voltage. 1 Quote Link to comment
Charlie69 Posted October 13, 2017 Report Share Posted October 13, 2017 Mike you are correct the 720s do have fusible links, usually 3 of them. They plug into the positive battery cable end. 720inOlyWA if you disconnect the wire from the choke heater on the carb does the noise stop? 1 Quote Link to comment
datzenmike Posted October 13, 2017 Report Share Posted October 13, 2017 I suppose the choke heater may be faulty? I guess you could check the resistance but I don't know what a good one would be. You have another truck??? Swap the choke heaters and see if that clears it up. 2 Quote Link to comment
720inOlyWa Posted October 14, 2017 Author Report Share Posted October 14, 2017 Thanks, guys, I am on it. I will check / clean the fusable links and check out the choke heater and post my results. Is the choke heater wire te one that plugs into the bimetal choke spring, or something else entirely? 1 Quote Link to comment
datzenmike Posted October 14, 2017 Report Share Posted October 14, 2017 Usually a Blue wire clips to the back of the choke heater housing. May have black sheath on it. On the inside, just visible in the back, is a heating coil. 1 Quote Link to comment
720inOlyWa Posted October 15, 2017 Author Report Share Posted October 15, 2017 Thanks, Mike. Got that. I guess that I must be a hard case, learning-wise. I just dug out the Haynes manual, which I really do not use very much, and it has- for some reason- a bit better section on the choke heater relay. For some reason, I have had a hard time finding the assignment diagram for the connector at the harness, but I see it here in Haynes, which gives me a good starting place. I am going to measure the values all over the place with my HF multimeter and find this issue, one way or another. I feel like it is a simple crossed wire, or badly corroded connection somewhere. I had not looked at the fuseable links before, and am not sure what I am looking for there, but I will figure that out as well. the Alternator is an original Nissan issue, which came with another truck that I put 15k miles on without any charging / electrical issues. I had it checked out by my starter motor / alternator guy before I put it in surplus as a backup, and ten stuck it into this truck. So I am very sure the alternator is strong and working correctly. I think that I am getting close. Thanks for the help. I am going to keep posting my results and process here until it all works fine again, in the hopes that someone else can learn from my circuitous path to choke heater relay victory. (I get messages that tell me this is not an uncommon 720 problem.) I am hop-scotching back and forth, doing a brake job and replacing wheels and tires at the same time tat I am casing down this electrical issue in the hopes that I can get it driveable before the real bad weather sets in. So thanks for the help! 1 Quote Link to comment
datzenmike Posted October 15, 2017 Report Share Posted October 15, 2017 Well I guess we can't discount the relay is at fault. If you have one they are easy to swap. Not suggesting running out and buying one but they are easy enough to grab at a wrecking yard. 1 Quote Link to comment
Charlie69 Posted October 15, 2017 Report Share Posted October 15, 2017 I have had to clean the fusible link plug where it bolts to the positive cable. This is a bad area for corrosion. I clean the bolt and nut and added a small flat washer to help the connection. 1 Quote Link to comment
720inOlyWa Posted October 16, 2017 Author Report Share Posted October 16, 2017 The low tension wiring elements that come to the choke from the wiring harness connector is different from my 85 and 86 trucks, and I do not have a wiring diagram / shop manual for an 84 720. But I began sorting that set of wires out a little bit to see if I can match voltages with function. One thing I noticed with the screaming relay plugged in, that choke itself is still not reacting at all. So I will look at that choke heater itself to determine if it is getting juice, etc. I replaced the choke heater relay and, because I was struggling with this a bit, I bought a new one instead of a wrecking yard part. I tore apart the fuseable links and they were clean and looked fine. I sprayed them with DeOxIt anyway. I keep coming back to the fact that plugging in the choke relay creates a short that cause the engine run to go to hell without activating the choke in any way. I can‘t get my head around that. This is what I am dealing with now. The two wrapped wires were connected to a spade connector that was attached to the intake manifod, as if they were being grounded. (That can‘t be right) 1 Quote Link to comment
datzenmike Posted October 17, 2017 Report Share Posted October 17, 2017 If it were grounded the sudden voltage drop at the relay might be enough to cause it to open. Once open the voltage is restored and the relay trips on again. It would happen very fast just like described. 2 Quote Link to comment
720inOlyWa Posted October 17, 2017 Author Report Share Posted October 17, 2017 Well, removing them from their grounded attachment, and separating them, did nothing to fix the relay scream. Disconnecting the choke wire didn‘t change anything, either. I tink tat when I figure it out,it will be a real ‘Are you kidding me?!?‘ moment. Can you tell me what the two wrapped wires are for? any ideas? 1 Quote Link to comment
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.