Alternator problems on a 1981 diesel 720

SD diesels were widely available in the US in the 1981-86 Datsun/Nissan 720 pickups, and in Canada through '87 in the D21 pickup.

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indogwetrust88
Posts: 12
Joined: 5 years ago

Alternator problems on a 1981 diesel 720

#1

Post by indogwetrust88 » 4 years ago

Hi folks, this post is for a 1981 Datsun 720 diesel. SD22

I got alternator problems. Here's what's up:

I got my 720 about 6 months ago. Shortly after buying it the battery died on me. It was an old battery so I had it replaced under warranty. After driving it a few more times though the battery died again. I got the alternator tested and folks at the auto parts store said it wasn't sending any charge to the battery. So, I bought a new alternator. After I put it in I figured it was fixed. I drove it around for a bit, pulled into the driveway happy as can be, and turned my key to the off position but, when I did the car kept running. I was freaked out like my car was about to go "Christine" on me and kill everyone I loved. But I kept my cool, popped the hood, and got out to have a looksee at the engine bay. Everything looked normal, nothing weird happening. But I needed to shut the truck off so, I disconnected the battery. And then I was really concerned. It kept running. What the DUCK!? My car is alive! :shock:

I scratched my head for a second and thought deeply, "what...the....duck?" Frantically, I got back in the cab, put it in gear, and let off the clutch to kill the engine. To my relief, my truck was not a sentient psychopathic killer and it cut off. :lol: But I still had the problem of it running when I turned the ignition switch off. I figured there might be a bad connection in the old key/ignition switch so I ordered another one offline. It came, I installed it, and again it wouldn't cut the engine off when I turned the key to OFF.

I called my car Dr. pops and we realized if the car is running and the alternator is sending a charge to my mechanical injection pump, that'd be enough to keep it goin since it's diesel and it doesn't need a spark to ignite fuel. It was time to trace the wires. I traced a wire off the alternator, a blue & white one, to the mechanical injection pump. Bingo! And on talking with my knowledgeable father, we decided we'd detach that wire from the terminal and see if the truck would cut off without it. It worked!!! I E-taped the blue & white wire up to stop current and then taped it down to the wiring harness so it'd be secure.

I drove it a few more times and again the battery died. This time on the winding mountain road to my parents' house. :cry: I pushed the car around with help of a passer by, ran it down hill, and popped the clutch. It turned over but didn't have enough power to propel me across the terrain. I hitched a ride home with a cop. Not the kind of ride I like taking...Nonetheless, I got home, went and took my battery out, charged it over night, and retrieved my steed in the morning.

So this is where I'm at. I'm lookin at my alternator, there are three wires comin off it goin into a three prong terminal that hooks to my truck's wiring harness. (See Photos 1 and 2 below)

The terminal on my truck's wiring harness, the one I pulled the fuel injection pump wire from, now has one prong left. That wire is not leading to the positive battery pole. (See photos 3 and 4)

Doesn't there have to be a wire that goes direct from the alternator to the battery to charge it? On top of that, when I pulled the makeshift tape wiring harness apart that my lovely car salesman had installed, a lot of the wires were in bad shape. (See Photo 5) Some wires are just twisted together, not even soldered.

I'm thinkin, "where should I start?" I'm gonna put the alternator back in tomorrow, hook my blue & white wire back up to it, and test the alt with a digital voltmeter to see if the blue & white also carries voltage to the battery. Does anyone have a wiring diagram for a 1981 diesel 720 SD22, in case I have to fix the electrician's handy work? Anyone ever had this kind of thing happen?
Attachments
Datsun 5.jpg
Photo 5: Here's some of the electrician's handy work where a wire was split and soldered back together.
Datsun 5.jpg (1.46 MiB) Viewed 1050 times
datsun 3.jpg
Photo 4: Here you can see the janky wiring set up that came with the truck. The blue & white wire, mentioned above, is in the bunch to the right that goes down into a relay instead of connecting to the battery.
datsun 3.jpg (1.56 MiB) Viewed 1050 times
datsun 2.jpg
Photo 3
datsun 2.jpg (1001.11 KiB) Viewed 1050 times
Datsun 4.jpg
Photo 2
Datsun 4.jpg (1.01 MiB) Viewed 1050 times
Datsun 1.jpg
Photo 1
Datsun 1.jpg (1.81 MiB) Viewed 1050 times

waynosworld
Posts: 517
Joined: 7 years ago
Location: Vancouver Washington USA

Re: Alternator problems on a 1981 diesel 720

#2

Post by waynosworld » 4 years ago

The white wire with a blue line goes to the charge light in the dash.
The big white wire goes to the battery thru a fusible link.
None of these wires control the engine, what controls the engine is the engine controller below the injection pump that the rod connected to the injection pump(IP) goes to.
Sometimes the IP controller gets locked out or there is a bad connection, and what I have done for others is to unplug the IP controller and then plug it back in, the plug id just under the battery on the firewall side, now when you started messing around with the wires near the battery you could have moved the IP plug wires and made it connect again and start working.
Get the alternator working again first, then start dealing with the engine shut down issue, there are a few things/reasons why the engine might not start or shut down, they are the DPC Module, the IP controller, and the oil sensor are just 3 of them, it's a complicated system, there are several diesel 720s out there that are controlled by a choke cable because the owners could not get the electrical start/stop system working properly.
Your engine don't need 12 volts to run, once it is running it will run for as long as it has fuel, it only needs 12 volts to warm the glow plugs, and make the starter work, oh and to control the stupid system it has to turn the injection pump controller on and off to start and shut down the engine.
Where are you located?
I am sorry I didn't see your post till today, I will PM you to let you know I posted to your thread. wayne
I know the voices are not real,
but they have some really good ideas.

indogwetrust88
Posts: 12
Joined: 5 years ago

Re: Alternator problems on a 1981 diesel 720

#3

Post by indogwetrust88 » 4 years ago

Thanks Wayne!

Turns out the person who I bought it from or someone before them rewired a bunch of stuff and the new alternator wasn't matching up with the terminal on my wiring harness to charge my battery. I put the blue and white wire back in the truck's wiring harness terminal but switched the wire from the terminal hole it used to be in to it's opposite. Now I got a charge going to the battery from the alt. For now that's all I need to get to where I'm going. But I'm gonna consider rewiring everything in the future because my dash lights cut out when I switched terminal holes for the blue and white wire. Even with it being as much as a pain as it is, I think I ought to, considering the crappy job the last person did on it.

Thanks again.
M

waynosworld
Posts: 517
Joined: 7 years ago
Location: Vancouver Washington USA

Re: Alternator problems on a 1981 diesel 720

#4

Post by waynosworld » 4 years ago

indogwetrust88 wrote:Thanks Wayne!

Turns out the person who I bought it from or someone before them rewired a bunch of stuff and the new alternator wasn't matching up with the terminal on my wiring harness to charge my battery. I put the blue and white wire back in the truck's wiring harness terminal but switched the wire from the terminal hole it used to be in to it's opposite. Now I got a charge going to the battery from the alt. For now that's all I need to get to where I'm going. But I'm gonna consider rewiring everything in the future because my dash lights cut out when I switched terminal holes for the blue and white wire. Even with it being as much as a pain as it is, I think I ought to, considering the crappy job the last person did on it.

Thanks again.
M
Sometimes the new alternators are wired different, some even come with 4 wires, you got lucky it even had the same plug, I hear about new alt wiring issues quite a bit.
I am happy it is charging again, good for another 200,000 miles. :lol:
I know the voices are not real,
but they have some really good ideas.

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