'86 720 Dually; slow voltage leak
'86 720 Dually; slow voltage leak
I've got a ratty old 720 dually. It's got enough problems to keep the forum busy for a week. For today, I'd like to ask for some help figuring out a slow electrical leak. You know how there's a main cable coming off the positive side of the battery going straight to the starter. Then there's that little secondary connector that goes to all the other stuff. If I unplug that secondary connector the truck will hold a charge. If I plug it back in it's deader than a doornail about a week later. Anyone have a link with some good pointers? I've got an old analog multimeter, but that's about it for electrical stuff. No fancy digital Fluke or clip-on ammeter. I could probably borrow a clip-on ammeter if that would help.
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84Datsun720
- Posts: 12
- Joined: Thu Feb 17, 2011 10:54 pm
- Car: 1984 Datsun/Nissan 720 SC LB
Re: '86 720 Dually; slow voltage leak
Coming from somebody who had a similair issue, I would check the back of your alternator because there is a clip for the alternator, a positive wire and a neagtive wire. However, the positive wire also has a wire that is attached to a small cylinder that is attached to the alternator. I found mine was completely disconnected and may have been the cause of the failure of my alternator. I had a lot of suttle signs that my alternator was going, but never put the dots together until it completely failed. Just offering a suggestion. 
Re: '86 720 Dually; slow voltage leak
The alternator's still working. If it's running it'll charge. My problem (well, one of the problems) is a slow voltage leak. A guy at work had a good suggestion. He said take the auxiliary connector off the battery and plug a test light inline. Start pulling fuses til you see the light go out.
I coulda fabricated a test light, but the quickest thing to do was just plug my old analog multimeter in. The gauge jumped to battery voltage. When I pulled the third fuse from the front of the fuse block out the multimeter dropped to almost zero. I woulda rather seen it go to zero but anyway. That third fuse is labeled tail lights/cabin light.
I put the fuse back in. The cabin light hasn't ever worked. I popped the plastic lens off and pried out the dead bulb. No difference. I moved back to the door switch. Took out the screw, pulled the switch out of the frame. The wires had been cleanly cut off. Not chewed like a mouse, it looked like clean cuts. My guess is that some previous owner might have been trying to stop the voltage leak and cut those wires? Nothing I did inside the cab moved the multimeter.
Moved to the back of the truck. Disconnected the round plug that isolates the wire loom from the tail light bulbs. No change at the multimeter. That's all the further I got on it so far. I should probably buy or make a test device that makes some kind of noise when it's plugged in. That way I could move around the truck pulling wires and just listen for the sound to stop instead of having to spot the multimeter. Or get really long wires for the meter probes. That would work.
I thought I had an official Datsun manual laying around here somewhere. A wiring map might help me fish around for other connections to pull.
I coulda fabricated a test light, but the quickest thing to do was just plug my old analog multimeter in. The gauge jumped to battery voltage. When I pulled the third fuse from the front of the fuse block out the multimeter dropped to almost zero. I woulda rather seen it go to zero but anyway. That third fuse is labeled tail lights/cabin light.
I put the fuse back in. The cabin light hasn't ever worked. I popped the plastic lens off and pried out the dead bulb. No difference. I moved back to the door switch. Took out the screw, pulled the switch out of the frame. The wires had been cleanly cut off. Not chewed like a mouse, it looked like clean cuts. My guess is that some previous owner might have been trying to stop the voltage leak and cut those wires? Nothing I did inside the cab moved the multimeter.
Moved to the back of the truck. Disconnected the round plug that isolates the wire loom from the tail light bulbs. No change at the multimeter. That's all the further I got on it so far. I should probably buy or make a test device that makes some kind of noise when it's plugged in. That way I could move around the truck pulling wires and just listen for the sound to stop instead of having to spot the multimeter. Or get really long wires for the meter probes. That would work.
I thought I had an official Datsun manual laying around here somewhere. A wiring map might help me fish around for other connections to pull.
Re: '86 720 Dually; slow voltage leak
So it looks to me like I might have two voltage leaks. I hooked the multimeter in between the positive side of battery and the auxiliary connector again. This time I pulled every fuse out of the fuse block, then started putting them back in. I got a big jump when I plugged the "Tail lamp/cabin light" fuse in, just like before. I pulled that one back out and moved on. Got a big jump again when I plugged in the Horn fuse. Took the steering wheel plastic off and unplugged all the connectors inside, one by one. The voltage leak remained.
I'm getting my **s kicked by a 27 year old truck
I'm getting my **s kicked by a 27 year old truck
Re: '86 720 Dually; slow voltage leak
I think I found it. The room lamp timer under the passenger seat. The cabin light bulb has been burned out forever. A couple of days ago I found that the driver's side door switch had all its wires cut off sometime in the previous century.
So the room lamp timer had been waiting for at least a decade for the driver's side door switch to tell it to shut off. I don't know whether the timer has finally given up and burned itself out, or if it's still doing what it's supposed to do. It looks OK inside; nothing obviously wrong anyway. I'll have to fix the wires and connections (and possibly the switch itself) on the driver's side, then replace the room lamp bulb, then plug the timer back in and see what happens. This is a good example of a system that creates problems years later when small things begin to fail. If the door switch fails you'll have the timer running the battery down.
So the room lamp timer had been waiting for at least a decade for the driver's side door switch to tell it to shut off. I don't know whether the timer has finally given up and burned itself out, or if it's still doing what it's supposed to do. It looks OK inside; nothing obviously wrong anyway. I'll have to fix the wires and connections (and possibly the switch itself) on the driver's side, then replace the room lamp bulb, then plug the timer back in and see what happens. This is a good example of a system that creates problems years later when small things begin to fail. If the door switch fails you'll have the timer running the battery down.