79 620 harness into a 74 620?

[snapp_hunter]

i can't imagine there are many differences, but i have a 79 parts truck, and a 74 truck that the P.O decided to cut both the engine bay and under dash harnesses apart. i pulled the engine bay harness out of the parts truck, and i am going to get the under dash one out tonight. my question is, are these harness compatible with each other? the P.O has the ignition system wired up to its own stand alone fuse box, so the truck starts, but nothing else electrical works; dash, headlights, tail lights, radio, etc. if I swap these harnesses and leave the ignition stuff untouched i should get power to all systems right? wiring is my nightmare, I have never done it, and I've always been afraid to.

[UnderControl]

The electrical systems in these trucks are pretty simple and should be mostly the same between them. As I recall a few wires may have changed colors and the later model had an electronic dizzy and probably an internally regulated alt.

 

Grab wiring diagrams for both years, print them out if you can, and compare the two. If you can help it don't take apart the interior loom, that should keep it pretty simple to find what you need.

[datzenmike]

The 74 may have different shaped plugs for side marker and parking lights. Not sure how the EI distributor wiring will work for a points set up. If you have the '78 distributor and alternator I would swap them along with the wiring.

[cruznude]

I dont know about what you have but the harness I replaced in my truck, some of the plugs required the flat plug type where as I had the pin type and had to acquire the right switches.

[datzenmike]

Yeah you have a points distributor that needs ON and START signal to the coil and the ballast. The '78 has none of that.

[datsunaholic]

Every year 620 has a different wiring harness.  In fact '74 and '79 is about as different in the wiring harness as you can get. 

 

Almost every electrical component uses a different type of plug.  Only the bullet and blade SINGLE connectors (like the oil pressure switch, starter solenoid, temperature sender, and horn) are the same, and a few small components like the ignition switch that still used the flat rectangular plug.  Most everything else changed from blade-type plugs to pin type plugs.  Plus the relays are in totally different locations between the 2, and the relay types are incompatible for the headlights.  The '79 harness has no provisions for the voltage regulator, so you'd need the '78+ alternator.  The fuse box is wired differently even though the plugs are the same..  The turn signal switch is electrically different (not just the plug shape, but the number of wires).  The plugs that connect the various harnesses are completely different.

[HOGIE]

Now that you know the harness is going to do you no good you can send it to me. I have a 79

[snapp_hunter]

Every year 620 has a different wiring harness.  In fact '74 and '79 is about as different in the wiring harness as you can get. 

 

Almost every electrical component uses a different type of plug.  Only the bullet and blade SINGLE connectors (like the oil pressure switch, starter solenoid, temperature sender, and horn) are the same, and a few small components like the ignition switch that still used the flat rectangular plug.  Most everything else changed from blade-type plugs to pin type plugs.  Plus the relays are in totally different locations between the 2, and the relay types are incompatible for the headlights.  The '79 harness has no provisions for the voltage regulator, so you'd need the '78+ alternator.  The fuse box is wired differently even though the plugs are the same..  The turn signal switch is electrically different (not just the plug shape, but the number of wires).  The plugs that connect the various harnesses are completely different.

crap. thanks for the information before I got too far, i have a lead on a 1975 complete harness for like $60, so Maybe that would fit the 74...

 

Now that you know the harness is going to do you no good you can send it to me. I have a 79

im not pulling that harness out from under the dash unless you make it really worth my while, I have the hood compartment one already pulled. PM your offer

[UnderControl]

'74 Wiring Diagram

http://www.davidcmurphy.com/olddat/data/dat620-1974.pdf

 

'79 diagram

http://www.davidcmurphy.com/olddat/data/dat620-1979.pdf

 

There is a lot more stuff on the 79 than I thought. From the looks of it you pretty much have to start from scratch on that harness anyway.  If it were me, I'd take the 79 harness strip it down. lay it out as close as you can to the 74 layout and swap over whatever connectors you need.  I don't remember your truck having a voltage reg, but the alt was also on the other side of the engine, so you may want to confirm that it is or isn't an internally regulated alt.  It also looks like your carb doesn't have any of the solenoids that the stock carb did so there are a few less wires to worry about. As far as head light relays go, if you are redoing the harness, might as well do the bosch style upgrade.

[cruznude]

don't forget while doing all this electrical to do a little up grade on the alternator. In the long run you'll be so much happier that your not dealing with electrical again. Just saying.

[snapp_hunter]

'74 Wiring Diagram

http://www.davidcmurphy.com/olddat/data/dat620-1974.pdf

 

'79 diagram

http://www.davidcmurphy.com/olddat/data/dat620-1979.pdf

 

There is a lot more stuff on the 79 than I thought. From the looks of it you pretty much have to start from scratch on that harness anyway.  If it were me, I'd take the 79 harness strip it down. lay it out as close as you can to the 74 layout and swap over whatever connectors you need.  I don't remember your truck having a voltage reg, but the alt was also on the other side of the engine, so you may want to confirm that it is or isn't an internally regulated alt.  It also looks like your carb doesn't have any of the solenoids that the stock carb did so there are a few less wires to worry about. As far as head light relays go, if you are redoing the harness, might as well do the bosch style upgrade.

 

 

don't forget while doing all this electrical to do a little up grade on the alternator. In the long run you'll be so much happier that your not dealing with electrical again. Just saying.

i did forget to mention though, it has a weber carb already on it and a GM internally regulated alternator already on it. this is why the previous owner hacked the wiring I believe, I appreciate the wiring diagrams, but that is straight jibberish to me lmao. i guess it might feel better if I had a wire harness in front of me to compare it to, but I can't make heads or tails of it, thats what worries me about attempting any wiring to begin with

[Lockleaf]

Pick a system and trace it. How about turn signals. If wires cross and they have a dot on the intersection, Those wires connect. If they cross without a dot, those wires do not connect. Don't look at the harness, just take it one simple wire at a time. You can figure it out.

[snapp_hunter]

Pick a system and trace it. How about turn signals. If wires cross and they have a dot on the intersection, Those wires connect. If they cross without a dot, those wires do not connect. Don't look at the harness, just take it one simple wire at a time. You can figure it out.

thanks for the vote of confidence, I'm going to have to do something eventually aren't I?

[Lockleaf]

This isn't amazing, but it could help get you started

 

[datzenmike]

I think it's safe to say that a '79 harness is pretty much an impossible swap for all the above reasons. It may help repair the '74 harness.

[snapp_hunter]

talked to a man that claims to have done many late model wiring swaps into early model trucks. he claims that if you switch the gauge cluster as well as electrical components under the dash that everything else matches up. i figure since I don't have another option right yet, i may see what lines up under the dash and see whats going on. at least its something to do and then i can confirm or deny the validity of the idea.

[datsunaholic]

Actually the gauge cluster is one of the few things that DOES swap over.  Other than the speedometer switch existing/not existing they're electrically identical.  But the main plugs between the harnesses are entirely different.

 

Sure, it's doeable- it's a lot of work, but I stuck a '79 harness in a '76 cab, but that's because I did a cab swap. The rest of the truck stayed '79- engine, frame, bed, and the rear wiring harness was untouched.  I used every single electrical component from the rotten '79 cab, since the truck is a '79 with a '76 cab on it.  Only a few small things like the dome light and door switches stayed original to the '76 cab.

[Lockleaf]

It is definately doable. My 71 510 wagon has an 82 720 wiring harness in it. I have had to repin connectors and straight up change lots of stuff though to make it work. I'm running the 510 gauges though.

[snapp_hunter]

well its my best option as of right now, so i'm gonna give it a shot! ill post progress here, as well as my build thread in the 620 section, not that there's anything really going on in it yet haha/

[snapp_hunter]

so i pulled both harnesses and they are actually pretty similar. im thinking im just going to use the 79 parts harness to repair the 74 harness. my other option is to switch everything out that matches the 79 harness into the 74 truck. thoughts? it seems like the second way would be less time consuming, but the first way seems like the correct way to go about it to me.

[Seeker > 620 KC]

Late last year I picked up a handful of dash's from a guy in Oregon who wanted them all gone. The pads were all badly trashed and I didn't have the room to store them, so I dismantled them all and have been selling the pieces off by themselves at the Eagle Rock swap meet in Los Angeles and on CL. One was an early model with the flat plugs, the others were all 78-79, some still had parts stickers on them. I can look and see if I have still have the early one left if you still need one, but I think it might have been missing a few plugs or had been altered.   You and anyone who needs one of them can PM me if interested.

[cruznude]

I have an diagram that I had blown up for me of a 79 kc from kinkos. its 30 inches wide and almost 3.5 ft long. It makes things so much easier when the letters about a quarter inch. cost was about 16.00 and no eye glasses to see what that is.

[Jersey]

I had a '79 KC parts truck that had a perfect harness, so I decided I'd swap the complete harness into my '77 KC. I swapped everything over except for the guage cluster. It took me about 4 hours to do it.

[snapp_hunter]

well ive been sorting through the 74 harness, and I am just going to use wires off the 79 harness to patch it back together. for example: the P.O decided he wanted to install a foot switch for the headlights and decided to use some thick housing wire and cut the plugs off the headlights and ran his own system, all the wiring for the headlights is gone from the factory harness, but the plugs are here, so I am just going to take the correct color matching wire from the 79 and re wire my headlights. I have a question, there are alot of wires that are cut that went to the original carb and emissions control electricals. how can i remove those pins from the plugs on the terminal ends? i want to delete them completely the right way, instead of leaving the bare wires under the dash.

[snapp_hunter]

never mind, just decided to pull em straight out, seems to be the way to do it. just wasnt sure there was a special tool to remove them at first haha.