Jeep Hauler 1990 K3500 Dually

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Previously I had asked what the third relay was on the firewall. After looking at some more wiring diagrams I'm thinking that it is for the electric cooling fan. Is that correct?
Nope, I was wrong. It is the transfer case relay.

I'm pretty sure that the windshield wiper harness isn't supposed to look like this. I will have to dig into it some more.
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Here are a few more things that came in. Thanks for the lead on the nice connectors.
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For those of you using the Painless Wiring TBI kit, be aware that I have found three typos with it. Two are in the instructions and one was on the harness label itself. I had to make a couple of phone calls to Painless to verify, but yeah. There are typos in the kit. It is not detrimental but it is enough to confuse you. Two are with how you wire the NSS (two different methods both refer to the same diagram) and one is with the AC power wire (it is called "C/A Power" on the harness).

I got more clean up done and found more of Bubba's work. Melted wires and connectors, bare wires, etc... I'm pretty sure that the purple wire here is the "crank" wire for the starter solenoid.

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Three butt splices in 12 inches.... Why, just why?
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In the driver's kick panel I found the remains of a security system. Three relays were taped together and spliced into the driver's door circuit. Those got removed and I had a chance to use my new crimp tool and marine butt splices. It is still far from perfect, but it is much better than the rat's nest that it was before.
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So I'm not going to get it running temporarily off of the new TBI harness. The knock sensor is MIA and so is the oil pressure sensor and oil pressure switch. That explains why the oil pressure gauge doesn't work. I'm just going to pull the engine and deal with it all at once.
 
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Wow, that wiring is downright scary, and to think it was connected to a battery. Any one of those connections could have caused a fire at any time. You will certainly have a good understanding of GMT400 wiring circuits by the time you have finished.

Yep, the underside for the flatbed and trailer wiring is just as bad, so is the engine bay. As an example, the engine temp wire for the gauge is bare and corroded so I don't trust those readings at all. Which could be another reason it would over heat. It was probably hotter than the gauge was indicting.

This came in the mail along with new speakers. Pretty cool little unit. I'll put an empty "cubby" pocket where the cassette player is now.
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Perhaps you guys can help me out since I am finding conflicting information as to what these relays are. See pic below.

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Just for fun, here are some more pictures of the wiring.
Ah, the three sea shells. I have the same truck and year. Same problems, bad bad wiring, overheating. I never knew what the third relay was for until I saw your post. (XFER Case)
On mine, I think the overheating is due to a bad sensor and bad wiring and bad dash connections - but I will never know because I am ripping it down to the frame and starting over.
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Was able to clean up some of the wiring yesterday. Trying to remove any of Bubba's wires and just get it down to the factory wires so I can evaluate those more. It is looking a lot better but unfortunately I'm finding many of the factory wires are damaged (cut, spliced or the insulation has been scratched to where the metal wire is exposed). Also, I found the issue with the reverse lights. The wire was broken right at the connector for the NSS. Since I was already under there with a new NSS in hand, I changed out the NSS and saved the old one. Instead of fixing the broken reverse light wire, he had wired in a toggle switch on the dash to turn on the reverse lights by cutting into a factory hot wire on the fuse panel and then running a separate wire clear to the light. So every time you backed up, you would have to manually flip the switch to get your reverse lights to come on. Not to mention he ruined another factory wire in doing that. The whole wiring harness is that way. I found three other abandoned toggle switches under the dash. No idea what they were used for in the past but they are all coming out.

Previously I had asked what the third relay was on the firewall. After looking at some more wiring diagrams I'm thinking that it is for the electric cooling fan. Is that correct?
@Off Grid Access - Can you post a picture(s) of the pully system on your truck? Mine has a hodge podge of parts from other trucks and I can't figure out what i am missing.
 

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Nice project! Half the fun is building it yourself. A couple of other things that I'm still looking into are the Hot Fuel Handling Module and the Check Engine Lamp Driver Module.

I think the relay is actually for the front axle 4wd engagement and not the transfer case since the t-case is all manual. When I re-wire mine, I will mechanically bypass the front axle disconnect (insert a pipe "slug" in front of the actuator and unplug the actuator). The dash light for the 4wd indicator on the Dakota Digital gauge cluster will be wired to the switch inside the transfer case. I don't mind having the front axle engaged all the time. Most of my Jeeps are that way already and this thing gets such horrid gas millage any ways I don't think I will even notice.

The Painless Wiring harness for the TBI has its own check engine light, so I don't "think" I need the Check Engine Lamp Driver Module, but I need to do more research on that.

The Hot Fuel Handling Module is an interesting one. I don't NEED it but it sounds like it could be useful. It just lets the fuel pump run longer than the ECM defaults when you first turn the key on. That way it can purge out any vapor lock from the system. The downside is that these have been know to burn up fuel pumps or even catch fire when they fail and there are no OEM units any more. The Painless kit does not have provisions for it. I found where a member here was making new and better modules a few years ago and messaged him. Not sure if he is still doing that or not though. One other option is to remove the module and just cycle the key a few times and pause about 5 seconds each time with the key on. I may just get an industrial 12V off-delay timer instead like the one linked below. It's only $12 on Amazon with the socket base.

DC12V H3Y-2 Delay Timer Time Relay 0-60S Second & Base Socket DPDT 8-Pin (DC 12V)​


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@Off Grid Access - Can you post a picture(s) of the pully system on your truck? Mine has a hodge podge of parts from other trucks and I can't figure out what i am missing.
Sure. I hope to get the grill off it sometime next week. I'll get a picture for you once I have a clear shot at the front of the engine.
 
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Here is the area that I am currently looking at. Does anyone know what the inline resistor is for (Number 3 on the left hand schematic)?

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