MF switch or earth problem? SD23 1985 720

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

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michael3006
Posts:26
Joined:8 years ago
Location:Queensland Australia
MF switch or earth problem? SD23 1985 720

#1

Post by michael3006 » 8 years ago

My passenger side headlight has gone dim. But not all the time, once in a while the light is perfect just the same a driver side.
But most of the time it's dim.
Also when this happens I can see a very faint glow from both of my Highbeam head lights.
When I change from headlights to Highbeam only the drivers side one comes on.
What are your thoughts, before I start opening things up.
It's a 1985 single cab, when the pass side light goes dim the voltage at the light connection is 5v.

plenzen
Posts:869
Joined:13 years ago
Location:Cochrane Alberta Canada

#2

Post by plenzen » 8 years ago

Poor Ground is my first thought

Someplace where the body grounds to the battery
Try installing a new ground strap someplace with a direct connection to the battery.
Grind off some bare metal and use a jumper cable to see if that makes a difference.

If not then inspect the wiring from the rear plug on the headlight all the way back in the harness for abraded wires etc.
Check the plug that fits onto the dimmer switch

I do not recall if there is a relay between the switch and the headlights, and if right side and left side are separated.
Retired Pauly
Problem with being retired is that you never get a day off.
1987 D21-J SD25 KC
KJLGD21FN

michael3006
Posts:26
Joined:8 years ago
Location:Queensland Australia

#3

Post by michael3006 » 8 years ago

Thanks.
Next day off is monday.
I will post the results.
Mick

michael3006
Posts:26
Joined:8 years ago
Location:Queensland Australia

#4

Post by michael3006 » 8 years ago

Worked till today.
But no joy yet, I tried the extra ground and looked over the wiring loom as far as I could in fading light couldn'y see any problems.
Nevermind I will have another go tomorrow hopefully I will have better luck in full daylight.

Cheathu
Posts:5
Joined:8 years ago

Re: MF switch or earth problem? SD23 1985 720

#5

Post by Cheathu » 8 years ago

michael3006 wrote:My passenger side headlight has gone dim. But not all the time, once in a while the light is perfect just the same a driver side.
But most of the time it's dim.
Also when this happens I can see a very faint glow from both of my Highbeam head led tubes.
When I change from headlights to Highbeam only the drivers side one comes on.
What are your thoughts, before I start opening things up.
It's a 1985 single cab, when the pass side light goes dim the voltage at the light connection is 5v.
I think you need to check connection wire must be lose

Graeme
Posts:5
Joined:7 years ago
Location:British Columbia

Did you find a fix??

#6

Post by Graeme » 6 years ago

Hi Michael,

I am having the exact same problem with my 84' 720, except its the drivers side low-beam that is dim, and the drivers side hi-beam does not turn on. There is a faint glow in both hi-beams when the lows are turned on.

I have removed, sanded and replaced the ground connections to the block and body directly from the battery, as well as in the middle of the wiring harness to the body above and infront of the starter.

Did you ever fix your problem? I would be happy to hear any ideas.

Thanks!

plenzen
Posts:869
Joined:13 years ago
Location:Cochrane Alberta Canada

#7

Post by plenzen » 6 years ago

I'm still thinking its a bad ground someplace in the headlight circuit.

Are there grounds near where the headlight plugs are or thereabouts?

I know on my 87 there are NUMEROUS ground points under the hood.

I guess to check the harness where it plugs into the headlight switch but, I really feel that it's a loose/bad ground some place.

Try and follow the wiring harness under the hood down both fenders to the rad support and check for ground points along the way.
Check where the main harness goes through the firewall as well.

Other than that

You may have to wait for the electrical guru Al ( asavage ) to check in.

He usually comes up with the answer.

Paul
Retired Pauly
Problem with being retired is that you never get a day off.
1987 D21-J SD25 KC
KJLGD21FN

Graeme
Posts:5
Joined:7 years ago
Location:British Columbia

it works!

#8

Post by Graeme » 6 years ago

Thanks for the speedy reply Paul. I had a look around and couldn't find any more ground points from the wiring harness or around the lights. Then I decided to take apart the switch, which turned out to be the problem! I cleaned up the contact points under the black plastic cover with emery cloth and voila! Both the low and hi beams work normally!

plenzen
Posts:869
Joined:13 years ago
Location:Cochrane Alberta Canada

#9

Post by plenzen » 6 years ago

Yahoo
Retired Pauly
Problem with being retired is that you never get a day off.
1987 D21-J SD25 KC
KJLGD21FN

waynosworld
Posts:519
Joined:7 years ago
Location:Vancouver Washington USA

#10

Post by waynosworld » 6 years ago

Just for future reference, there are no grounds on the headlight side of the headlight relay in the 720, if you add one anywhere, it screws the whole circuit up, and the reason I know this is because I put a 720 diesel harness into my 521 diesel, when I tried using the brights indicator light for the brights in the 521 instrument cluster, nothing worked right, and the reason was that using that small light in the 521 instrument cluster was adding a ground to the 720 headlight circuit.

Another little tidbit of information, if you ever decide to put a 720 diesel wiring harness into another vehicle like a jeep or something with round headlights, you will have to rewire the headlight plugs, as the square 720 headlights are wired differently than the round headlights, the headlight ground is in a different place. Image
That little thing made me like this for a while. Image
I know the voices are not real,
but they have some really good ideas.

michael3006
Posts:26
Joined:8 years ago
Location:Queensland Australia

#11

Post by michael3006 » 6 years ago

Yep, I gave up and put in to the auto elec. He said I had bad connections.
Oh well!

Graeme
Posts:5
Joined:7 years ago
Location:British Columbia

#12

Post by Graeme » 6 years ago

I'm sure I have a bad connection somewhere; I was driving along with the headlights working fine then hit a bump and BAM, back to dim left lowbeam and no left highbeam. Additionally, the blue highbeam light in the dash was out as well, where it was working before the bump.

Planning on cleaning the terminals at the back of the blue light next, and again on the switches with the black cover on it. Any other connection points that may be causing this problem?

Thanks!

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asavage
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Location:Duvall, Wash.
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#13

Post by asavage » 6 years ago

For fun, MF Switch pictures.

Now, look at the Headlight circuit wiring diagram & schematic (for 1982 720).

Note that different fuses feed the left & right headlamps -- there's a big clue!

Fuse No. 14 looks to be for the left headlamps. Look hard at where the fuse plugs in.
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.

waynosworld
Posts:519
Joined:7 years ago
Location:Vancouver Washington USA

#14

Post by waynosworld » 6 years ago

One of the first things I would do is check all the fusible links and connections/plugs, if your lights are dim right now, grab the fusible links and start moving them around, listen and look for sparks, check both the positive and negative battery posts, all the while having the lights on to see if they flicker, my guess is that your issue is in that area.
Weird thing about the fusible links themselves is that I have had them back feed power, even though one was blown, most everything still sorta worked except the glow plugs, but it was warmed up, so it started anyway.
Have you ever had the battery connections fail when you tried to start your vehicle, but you lift the hood, grab the battery cable and twist, go back hit the key and have it start, bad connections do weird stuff, check that fusible link area and all the links themselves.
I know the voices are not real,
but they have some really good ideas.

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asavage
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#15

Post by asavage » 6 years ago

According to the wiring diagram (linked above), a single fusible link feeds both HL fuses. Graeme's only got one HL dim, so a fusible link problem is right out.

The GP fusible link connectors are the single most trouble-prone part of the Nissan GP system, and not just on the 720 SDs. But it's not going to cause one HL to be dim.

The return path to ground through the high/low relay is also shared. For a single HL problem, it's either in that single fuse path for the feed, a connector problem right at the back of the HLs, or an internal wire harness splice corrosion problem (like the famous Maxima harness break).
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.

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