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Blower motor wiring/basic wiring question


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Hi everyone,

I have a rough 1980 720 King cab and I would love to start making some real progress on its many many issues. I mean, this thing has really got some issues. Actually some of it's issues have their own issues. The wiring is a complete mess and I'd like to - one circuit at a time - start getting everything working nicely. In order to replace the blower motor circuit, I need to figure out where in the circuit everything belongs. I've looked at wiring diagrams but they don't really give me an accurate understanding of how to simply replace the whole circuit, which is really the route I want to go. I don't want to bother with trying to figure out where shorts or bad connections are, because I think they are so plentiful that I'll be too old to be allowed to drive it by the time I get everything working. So I just want to disconnect power from the old circuit and run it through a correctly rated fuse, through an appropriate relay, through a switch and through the three speed resistor thingy, and then to the motor. But I don't know the order of those things. Is the relay supposed to be before the switch?

Can anybody lay some pearls of wisdom on me?

Thanks folks,


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It would probably be easier to find out why the old one isn't working... or just get a new dash harness if it's hacked to pieces.

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The 720 wiring harness is a weird animal, the start of a new way of making wiring harnesses for datsun/nissan trucks.

Your grounds are everything, if you don't have a proper ground, your screwed, and I am not talking about your battery ground, I am talking about the ground in each stand alone circuit, as that is what the 720 wiring harness is, a bunch of stand alone mini harnesses/circuits.

So first off, make sure the ground wire is tight back in there between the glove box and the heater controls, I am not sure if you can get back there without removing the dash, and that part of the harness is not part of the dash harness, it is part of the main harness, I could be wrong here, but I believe that the heater motor has a relay on the fuse block, make sure that is good, that thing will really fuck with you.

If you are having headlight issues, change the fuses, if that don't work, change the relay, if that don't work then I would just go find another 1980 wiring harness.

I have been dealing with these harnesses in my datsun creations, as it is my favorite harness for my 520/521 swaps.

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Power goes directly to the fan motor from a relay and then a fuse. What turns the fan on is connecting a ground to it for high speed or through several wire coil restances to give various speeds.

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  • 1 month later...

I found the wiring in mine was OK, but the switches weren't. It was pretty simple to just bypass the stock fan speed switches and put a toggle switch in the side of that box. There's only one speed now, 100%.

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there is a free workshop manual for 720's that probebly has a good diagram for ya, its http://asavage.dyndns.org/nissan/display1.php?Path=720%2FFSM_1984&db=Nissan&table=720_FSM_1984&Page=20-001


probebly your in dash speed selector, easy to fry, lol... had 2 with fried switches. could easily just be your heater relay though

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  • 7 years later...
On 4/11/2013 at 9:05 AM, datzenmike said:

Power goes directly to the fan motor from a relay and then a fuse. What turns the fan on is connecting a ground to it for high speed or through several wire coil restances to give various speeds.

datzenmike (others):

You've helped me (and others) tremendously - and I thank you, sir. I've been scrolling through the site in search of answers to no avail.

I have an '82 720 kingcab and the blower motor gave-up on me. I started checking all of the fuses and they're fine.  I've checked the fan (seperate power and ground) and it indeed works.  I then bridged to a seperate ground (blue wires) and still nothing. Using my Fluke - I checked for power at the red wire and nothing. Checked the fusible links and all appears fine -  Finally, out of frustration I called a friend who also has an 82 standard cab 720 with no electrical issues came by and we swapped  relays - still nothing. Then, I made certain to clean ALL contacts on the fan speed selector switch though they appeared to be fine - I cleaned them anyway. Finally, I checked the fusible link under the hood and find no issue.










What should I look at next to diagnose the no-power-to-the-red-wire issue?


Thanks again for any and all help.



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A problem I had with my 86 720 king cab is at the positive battery cable where the fusible links connect that plug showed power to all the fuse-able links but the problem was it was dirty enough to reduce the power to the point that it would not power the heater fan. after disassembling it and cleaning it all everything worked fine.  It actually helped my dash light to be brighter. 

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The red wire goes from the fuse block to the relay, the red/yellow wire goes from the relay to the blower motor, the blue wire goes to the fan switch which when grounded out should make the blower motor run full blast, if you ground out the blue wire and the blower motor does not run then I suspect you have no power getting to the motor, don't just check that fuse, replace it, if that don't fix it then check your fusible links, to check them you have to completely unplug them wires from the harness, then check for power one at a time, if they are good then remove the heater relay and try another one.

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Isn't there a module with a bunch of coils providing various levels of resistance for the different fan speeds?

I'm sure there is. Mine were all burned out when I bought my 720, which made me think if was something else. I bought a brand new module, and it worked great. The resistance coil for the highest blower speed is already burned out again.

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