New tranny and now I have wiring issues?

[Jayred]

I need some help. I have a 71c10 with ls and now a thm350. I took out my 700r4 and just finished installing a thm350. Pretty simple swap, no wiring, no big changes except for moving the crossmember forward. I hooked everything back up exactly the way it was before-- said a prayer, started the truck, ran for 5 minutes to get the new fluids circulating and when I put it in gear the whole thing died. I would try to restart several times and it would begin firing as the starter was turning but once I stopped cranking it would immediately die. I tried that 3 times then out of nowhere smoke is coming out of the dash and I stop everything. I jumped under there and noticed two wires from the back of the ignition were shinny and hot like they were getting ready to melt. Nothing is rubbing on the headers, my grounds are all hooked back up, I have no clue what is happening here. I have never had any wiring issues ever. The only weird thing was the neutral safety switch was not letting me start it in park, I had to put it in neutral to get it to crank. The starter isn't the best, it's hard to start when the engine has warmed up.

So now every time I turn the key to acc the lights go super dark in the cab and those wires start to heat up again. I disconnected the battery and walked away for now. I don't want to keep fn'ing with it and cause more damage. Any thoughts on where to start. Wiring is my weakness

[VetteVet]

You have a short to ground, probably a pinched wire in one of the circuits that the 12 gauge accessory wire feeds.
Pull the negative battery cable off and connect a test light or a multi meter between the battery ground post and the negative cable. See if the light lights up or the meter reads more than 3 milliamps .003. I suspect that you won't get a reading until you turn the key on, and then it will exceed the 10 amp maximum setting of the meter, so you don't want that, or you might damage the meter. If you use a light then you should first remove one of the fuse panel fuses, then quickly turn on the key and watch for the light to come on. If it does then quickly turn off the key and repeat the process with the other fuses one at a time until the light doesn't light. when it doesn't then you will know which circuit the short is on and you can trace the wires in that circuit until you find the pinched or shorted wire.

If the short is not fused or if it is ahead of the fuse, then it may not blow any of the fuses, which should have already been blown if the short was after the fuse.

The accessory wire powers the wipers, the heater switch, the turn signal flashers, and the hazard flasher, the turn signal switch,and one other thing. I'll have to check.


You might also have forgotten the 700r4 compressor clutch switch wire, which is hot with the key on. It may be touching a ground. See if the clutch power was taken off the fuse panel from one of the fuses or from the IGN Unfused terminal on the panel.

The hard starting is probably caused by the large drain on the key ignition wire, and causing low voltage to the distributor. If you still have points ignition you will get a full 12 volts bypass to the coil, which would let the engine fire, but when you release the starter, the drain on the ignition would cut the voltage and cause the engine to die or misfire.

This thread will probably get moved to the electrical section where it should have been posted so I will monitor that.

Here's a diagram with the accessory wire marked with the black arrows.
It also powers the backup light switch fuse.

[VetteVet]

The difference in the linkage mounting for the 700 and the thm 350 is probably why your neutral start switch wouldn't start in park. The pivot bracket mounting holes are different. Mine were drilled so that the shifter bracket would line up with the trans shifter.

Check the bolt holes in the left frame rail where the bracket mounts to see if new ones were added.

[Jayred]

After getting into all the wiring two wires have melted and taken out some others. My 12r from the fuse block by the battery has been spliced by the driver headlight and after it is spliced it is melted all the way back to the fuse block. There is a brown wire I think it's 12BRN that is melted from the fuse block to the back of the ignition switch. Also from the back of the ignition switch is a red wire that is melted, but I can't find it in the wiring diagram. It looks like 12BRN is used for the front flashers and those wires are melted and frayed in the loom up front behind he tail lights. I'm assuming that is where my short is and it fryed the 12BRN and 12R. If those were fused I don't understand why it shorted though? Wouldn't it have blown a fuse? I'm going to replace the melted wires with overlay wire and resplice wires that were damaged nearby. What a cluster. Whoever had the truck before me added so many new accessories, I'm afraid is overloading everything.

[VetteVet]

In order to understand how this happened it is helpful to know how the wiring is set up and the direction of current flow. Electricity flows from positive to negative so the direction of current flow would start at the positive terminal of the battery and flow through the fusible link to the terminal on the right fender. It is a junction not a fuse block however your fusible link should have blown, if it had one.

Here is a view of the factory routing of the wiring showing the 12R wire by the battery and the splice you refer to by the left head light.







If you follow the 12R from the splice it goes to the firewall block into the cab where it makes a junction with the other 12R wires that feed the fuse block, the horn relay, the headlight switch, and as you can see the one going up is the one you questioned that goes to the ignition switch. When the switch is turned on, this 12R hot wire feeds the 12BRN which returns to feed the turn signals and hazards,along with the wipers and the heater switch.

This is visible in the first diagram above and in this one showing the key switch wiring.

Originally Posted by Jayred

After getting into all the wiring two wires have melted and taken out some others. My 12r from the fuse block by the battery has been spliced by the driver headlight and after it is spliced it is melted all the way back to the fuse block. See the first diagram

There is a brown wire I think it's 12BRN that is melted from the fuse block to the back of the ignition switch. Also from the back of the ignition switch is a red wire that is melted, but I can't find it in the wiring diagram. Both are shown in the second diagram.

It looks like 12BRN is used for the front flashers and those wires are melted and frayed in the loom up front behind he tail lights. I'm assuming that is where my short is and it fryed the 12BRN and 12R. If those were fused I don't understand why it shorted though? Wouldn't it have blown a fuse?

The brown 12 R feeds the flashers which run through the turn signal switch and are not fused.
I'm going to replace the melted wires with overlay wire and resplice wires that were damaged nearby. What a cluster. Whoever had the truck before me added so many new accessories, I'm afraid is overloading everything.

You certainly should install a junction block in place of the spliced 12R wires and take your accessorie feeds off that. Replace the alternator with a CS144 100 plus amp model, and use an 8 gauge power feed wire from it to the junction, and you will be much better off.

Here is what I started with on my truck. It's the splice you mentioned.



And here is what I ended up with which is not completely done.

[Jayred]

Thanks for all the help! Gonna spend a few weeks taking it slow and doing everything right.