1985 Nissan 720 4x4 battery light on and gauges not working.

[DankDatsun]

My 1985 Nissan 720 recently had the battery light come on while also having all the gauges read 0 except the speedometer. Tested the voltage while running was sitting at 14.72 so I’d assume it’s not the alternator. Can anyone help me trouble shoot this problem?

 

[datzenmike]

There is a slight connection between the charge light and the temp and gas gauges, so change the 4th fuse over from the left hand side (15 amp) See what happens.

 

I looked at this but can't see how a blown fuse can light the lamp. Give it a try.

[DankDatsun]

Yes that did the trick. Thank you Mike🫡

[datzenmike]

OK In the charge light circuit there is battery on one side of the lamp and a charging alternator on the other. With equal voltage on both sides... no lamp. If the battery fails, the charge will pass through the lamp to get to the battery and it lights the lamp. If the alternator fails, the battery pushes through the lamp to ground in the alternator lighting the lamp. Schematically the power on the battery side appears to come from the 4th fuse but if it's broken nothing can flow through the lamp. or shouldn't.

 

I suspect that there are other things on that same circuit, a load or something that the lamp uses as a path to ground. I don't know, didn't figure it out. Well I'd rather be lucky than good. Glad you got it working.

[DankDatsun]

Well I pulled out the 15a fuse and it was blown so I changed it and that fixed all my

gauges and took the battery light away. I believe it was the flasher and meter fuse.

[powderfinger]

I need to dig into something similar to this on my 720. My battery light is always on but very dim and the brightness pulses with the turn signals. It also gets brighter when in reverse. Basically with any additional electrical load. Actually that's not true. I don't think the wipers affect it, but the wiper are unusually slow. It's dim enough that you can't hardly tell its on in daylight. Alternator is charging fine, I have been figuring maybe it is a ground somewhere that isn't the best but haven't really dug into it other than checking the main grounds. Been like that since I got the truck on the road.

[datzenmike]

1/ Use jumper cables and ground from the block to the sheet metal body see if that helps.

 

2/ The back of the alternator should have a ground post on it with a larger Black wire. Make sure it's secure. This is the ground to the electrical system which is better than the mechanical mount to the block.

 

3/ Find the three fusible links attached to the positive battery post. Two Green and one Black. Jumper around the Black one. This is the charging circuit and how power is put back into the battery. Have a care as this is 'hot' don't touch to anything grounded, which is pretty much everything.

 

The above are easy and cheap to do but in the event you can't get rid of the glow, disconnect the battery and remove the alternator and take in and have tested. A bad diode or worn brushes can cause a low charging rate and the charge light to just barely glow. Replacement alternators are cheap and that's the problem... they are CHEAP and often bad in the box. If you can afford to, have it rebuilt properly. 

 

 

[DwayneOxford]

Had this a few times, always one bad diode in trio. Still charges but not good.

[datzenmike]

Well Hainz says to get running and then turn on all the lights, the heater on full and the wipers. If all are working it should be able to maintain over 13v charging. 

[powderfinger]

I'll try the Hainz method. Luckily I have a few spare alternators so it's pretty easy to just swap it out. Just haven't done it yet, been working on my Arrow.