Alternator or Battery?

Dealing with all subsystems specific to the diesel powered Datsun-Nissan 720 pickup trucks.

Moderators: plenzen, Nissan_Ranger

Post Reply
User avatar
scottyk
Posts: 22
Joined: 12 years ago
Location: Northwest Ohio

Alternator or Battery?

#1

Post by scottyk » 12 years ago

Ok guys, I have a scenario for you.

I drove the truck to work on Friday. I got new tires at the tire place. By the time I got to the place to pick the truck up, it was dark. The truck wouldn't start. After I got a jump, I was on my way home. The battery light stayed on. On the way home, the lights got dimmer and dimmer until I could barely see. However, the truck never died and the headlights never went out. About a 20-25 minute drive home.

So what was it, the alternator or the battery? Would the truck continue to run and the lights stay on if the alternator was bad and the truck was running solely off the battery? My thinking is that the battery is bad, BECAUSE the truck kept running. Would the alternator keep the truck running by itself?

Of course, I plan on getting a multimeter tomorrow and testing the alternator when I get the chance.
1982 Red Diesel KC w/ A/C

after oil
Posts: 173
Joined: 12 years ago
Location: powell river, BC, Cascadia

#2

Post by after oil » 12 years ago

i understand that because diesels run off compression, rather than spark, the motor will go w/o alternator or battery. (but it wont start)
"... the rare and elusive 1987 D21/SD25, I have come to believe these trucks exist ONLY in the minds of people who own them. " phillip R.I.P.

1987 nissan d21 SD25, canada/2 tank SVO
1993 mitsubishi delica chamonix 4X4 diesel van

User avatar
asavage
Site Admin
Posts: 5329
Joined: 14 years ago
Location: Duvall, Wash.
Contact:

#3

Post by asavage » 12 years ago

Once running, the SD22 requires no electricity at all and will happily run until out of fuel.

Your charging system is not working. Probably the alternator, but bad wiring can cause this as well, especially the fusible links (right off the positive battery terminal) and the connections right at the back of the alternator.

If the wiring checks out, check the brushes first. I'm not certain about the A/C version . . . well, I guess I am: no direct access to the brushes cover.

Image Image

The compressor may be in the way, but on the non-A/C models you can inspect and replace the brushes without removing the alternator.

After checking brushes:
Image

you'll want to remove the alternator (leave the vacuum pump with the engine: unbolt it from the back of the alternator, it's only three bolts) and have it tested at your FLAPS or wherever.

If they condemn it, consider buying a used one from Matt in TN. I've already offered him $20 so you'll have to go considerably higher ;)

If you are interested if trying to repair the alternator yourself, check out my post on the LR150 alternator, it's chock full of pictures, part Nos. and prices.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

User avatar
philip
Posts: 1494
Joined: 14 years ago
Location: Southern California, USA

#4

Post by philip » 12 years ago

after oil wrote:i understand that because diesels run off compression, rather than spark, the motor will go w/o alternator or battery. (but it wont start)
"After Oil" has the exception. :wink: The Bosch VE with its electric fuel control solenoid. At some low voltage level, the solenoid will close off the fuel supply ... shutting down the engine.
Last edited by philip 12 years ago, edited 1 time in total.
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

User avatar
scottyk
Posts: 22
Joined: 12 years ago
Location: Northwest Ohio

#5

Post by scottyk » 12 years ago

Hmmmm, I may have solved the problem. See, the end of the fusible link that connects to the battery was very brittle. It came apart yesterday. This may have been causing a loss of power to the electrical system. I got a different fusible link from Auto Zone and spliced that onto the end of the original fuisble link metal. Tonight, I started the truck and put on the headlights. They stayed on!! Drove it around the neighborhood and also let it sit in the driveway for well over 15 minutes, and the lights stayed on!! The battery light didn't go off, though, so I may still have a problem with the battery. But it doesn't look like the alternator is bad.
asavage
Your charging system is not working. Probably the alternator, but bad wiring can cause this as well, especially the fusible links (right off the positive battery terminal) and the connections right at the back of the alternator.
Al did mention the fusible link and wiring. Should have listened to him. :D
1982 Red Diesel KC w/ A/C

User avatar
asavage
Site Admin
Posts: 5329
Joined: 14 years ago
Location: Duvall, Wash.
Contact:

#6

Post by asavage » 12 years ago

If the alternator light is ON, there is no voltage at the field/diode trio of the alternator: the alternator is not working.

Use the voltmeter, and you'll see that the voltage is not going above 13.5v when off idle. No charging.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

User avatar
scottyk
Posts: 22
Joined: 12 years ago
Location: Northwest Ohio

#7

Post by scottyk » 12 years ago

I also happened to be looking under the hood, and noticed that the alternator was not turning, or was barely turning every so often. The belt was slack. After loosening the top and bottom bolts, wriggling my arms around the A/C stuff, and adjusting the alternator so it was tight, I locked down the bolts. Started it and it ran. But I'm not sure if that fixed all my problems, I have to wait until I have some more time to mess around with it and drive it on a few shakedown cruises.
1982 Red Diesel KC w/ A/C

User avatar
scottyk
Posts: 22
Joined: 12 years ago
Location: Northwest Ohio

#8

Post by scottyk » 12 years ago

Well, it was the alternator that was bad. I decided to upgrade the alternator to the Maxima diesel alternator. Once I got everything figured out and on right, the new alternator fit right in. It even came with the vacum pump, all for $123 shipped from a rebuilding place in Columbus, OH off of Ebay. Seems to start up and charge better now. :D
1982 Red Diesel KC w/ A/C

User avatar
asavage
Site Admin
Posts: 5329
Joined: 14 years ago
Location: Duvall, Wash.
Contact:

#9

Post by asavage » 12 years ago

How close to being a "bolt-on" was the Maxima diesel alternator?

If you're going to recycle it, I'll pay shipping on the old alternator, for parts use.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

User avatar
scottyk
Posts: 22
Joined: 12 years ago
Location: Northwest Ohio

#10

Post by scottyk » 12 years ago

Hey Al, the Maxima alternator was exactly the same in size as the 720 alternator. Bolted right in. I think I am going to hang on to the old alt for now, if I decide to get rid of it I will let you know.
1982 Red Diesel KC w/ A/C

User avatar
asavage
Site Admin
Posts: 5329
Joined: 14 years ago
Location: Duvall, Wash.
Contact:

#11

Post by asavage » 12 years ago

scottyk wrote:Hey Al, the Maxima alternator was exactly the same in size as the 720 alternator. Bolted right in.
That's what Philip thought; I wasn't so sure. Thanks for the confirmation.

And the plug-in connector? Does the 720 harness plug right in?
I think I am going to hang on to the old alt for now, if I decide to get rid of it I will let you know.
Just so you don't throw it away. The rotors, in decent shape, are getting rarer. You can have them re-ringed, but that's more $$$.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

User avatar
scottyk
Posts: 22
Joined: 12 years ago
Location: Northwest Ohio

#12

Post by scottyk » 12 years ago

Yes, the Maxima alt has an extra connector than the 720, but it slips right on.
Back:
Image

Front:
Image
1982 Red Diesel KC w/ A/C

Post Reply

Who is online

Users browsing this forum: No registered users and 0 guests