Wiring gremlin - Nothing works unless trans is in reverse

[Scottsdale87]

Hello all,

For starters, I have been working on a conversion project. I took a LWB 1987 Chevy v20 4WD with the 350tbi and swapped everything over to a 2WD SWB frame from a 1978. I went from dual fuel tanks to a single and am also using a SM465 trans out of a 1976. As my project has been nearing the end, I finally put a battery in to check lights and functionality before towing it over to have exhaust done on it before an initial start up happens.

I pulled the park light switch to check the front and rear lights, but nothing turned on. Fuses are brand new, bulbs are new, rear lights are new Grote sealed units, etc. Out of curiosity, with ignition on/engine off, I put the truck in reverse and miraculously most of my lights turned on. I had 2 bulbs on the cluster, interior lamp behind the dash, stereo, heater fan, and front side markers as well as headlights and front/rear park lamps (signals did not blink) and license plate lights come on. Fuel pump also came on until I unplugged it. I traced the plug from the transmission and it is connected to the Blue/Green wired connector that goes up and through the firewall with it's own grommet. Everything else seems intact under the dash with those wires. Out of the main bulkhead, the only wires I had spliced into were the dark green, light green, yellow and brown wires and followed the 81-87 schematics to wire in my rear lights correctly. The other two wires that followed down with that harness (wht/brwn?) terminated into a connector which I am sure is for the fuel pump/sending unit.

I am at a bit of loss. Could it be a transmission switch, a bad ground, something wired incorrectly, bad relays/flashers? Has this happened to anyone else before? Everything was pretty easy to follow, and all the wiring is still the same minus removing all of the garbled dual tank wiring.

Any help or guidance would be very much appreciated before I give up and drop $1k+ on an American Autowire kit.
Thank you!

 

[Ricko1966]

Sounds like the backup light switch is either providing power or a path to ground,for the items that aren't working. It is most likely providing a ground but not necessarily. Pull a rear bulb check for power at the contacts in the socket. If you have power stick the striped end of a piece of wire in the socket just 1/4" or so and replace the bulb. The bulb will hold the wire. Run it to a good known ground. If the light comes on you know the problem. Do you have all three grounds to the battery? You need a ground to the engine(usually the large main negative battery cable) you need a ground to the body(usually a smaller gauge wire from battery to radiator core support. And a ground to the frame usually a jumper from the body or engine to the frame.

 

[EvilGenius]

I'd check your grounds. Things get wonky when there isn't enough ground to go around for the desired current load. If you have wire hanging around run a jumper wire from the ground block under the dash direct to the negative battery terminal. The ground block under the dash is usually up on the parking brake bracket.

 

[edgephoto]

You have bad grounds. Clean and replace your ground cables/straps.

You could also do some voltage drop testing and know exactly what is wrong.

Or you could just drive around in reverse.

 

[Scottsdale87]

Hey all,

Thank you for the replies and suggestions. Today I bought a couple of extra ground straps from the local Napa and installed them, along with cleaning up the factory grounds and reinstalling. The factory grounds that go to frame/battery/radiator support are all in very good shape.

The new and fresh grounds helped in the sense that it stopped everything from running through the reverse switch. Now my next problem seems to been not getting power to the fuse block. We tested the rear wiring, and other than having the bright license plate lights run bright off the park lamp wiring, there was no power running to either signals or reverse lamps. We investigated the brake light switch (it had continuity) but it had no power. I tested the fuse block and almost everything other than the horn fuse had no power, again, brand new fuses. With the ignition on, I couldn't get my heater fan or stereo to turn on like I could when I put it in reverse before I cleaned up the grounds.

Is this starting to become a fusible link issue, or am I still needing more grounding? It's odd because I have front lights, and power to the passenger signal lamp socket (two new known working bulbs were not lighting up tho), and dim rear park lights.

Maybe if I run power though the trans again it'd be a pretty slick kill switch. Haha.

Edit: I removed the bulkhead on the firewall, used contact cleaner and scrubbed lightly with a brush to clean the contacts. Fresh dielectric grease was also used upon reinstalling.

 

[Radiohead]

Now that's an interesting development.

Some guys have a master switch on their positive side. You have one of those rare rpo codes for remote ground interrupt circuit.

No, you need to look up your wiring diagram here on the site and find the points where highly inconvenienced electrons are awaiting their destination and test for their presence there. Plus you will know which colors are going where.

 

[Ricko1966]

Fuse box gets power from the fusesble links I'd definitely take a look at them.

 

[Broken85]

Check the 10 gauge wires off the starter and check continuity to the fuse box. (Also check resistance) If no continuity (or high resistence), change the wires or replace the fuseable link. If it is good, start checking continuity across the fuse box. The below diagram will tell which order to go. If there is no continuity, check the engine side of the fuse box, and in the fuse holders too. You may have burned up circuit in there.

You must be registered for see images attach

 

[77Dmax]

I'd guess the linkage was probably providing a ground source when in reverse. The switch would melt with that much power going through it.

Check your fusebox again but this time have the ground lead on the battery itself. If voltage appears at the fusebox then you know for sure its a ground issue. It'll be a good time to check the cable itself also.

 

[75gmck25]

Large battery cable runs down to the large lug on the starter. From that lug you have two wires with fusible links.
#1 runs up the back of the engine through a metal tube, along the valve cover top, and connects to the bat terminal on the alternator. This is the battery charge wire.
#2 runs up through the same tube, across the firewall behind the engine, and up to the junction next to the brake booster. This is power to the truck.
(There is a third wire at the starter to activate the solenoid - that is not in play yet)

All the power for the truck comes from that junction next to the brake booster, so start there and work forward. In addition to the power in, it has one power wire running over to the fuse box, and another power wire running over to the HVAC relay on the evaporator plenum. On a newer truck like yours there may be some additional power wiring for the EFI, electric fuel pumps, etc.

When fusible links get shorted they may burn up inside and it’s not very visible on the outside. Insulation may be distorted from the heat, but I’ve had one burn up from hooking the battery up backwards and at first glance it still looked intact. It even powered the truck for a while, until I turned on the headlights and then it drew enough current to burn through inside.

 

[Jgonick]

It's odd because I have front lights, and power to the passenger signal lamp socket (two new known working bulbs were not lighting up tho), and dim rear park lights.

fuse block and almost everything other than the horn fuse had no power, again,

These are all on the same red wire. There are usually two main red wires coming from starter. -1st main red wire runs to light switch and part of fuse block. 2nd red wire runs to junction block another fusible link then to ignition switch and fuse block. The 2nd red wire & including the wires coming from ignition switch feeds most of the fuse block.

look at fuse block diagram @Broken85 posted above and it shows where the power is coming from to the fuse block.

 

[Jgonick]

I'll also throw this out here- lights can be confusing- they are fed power by multiple wires depending the situation- hazards, turn signals, brakes, parking lights.. ALSO LEDs add a whole new layer of confusion. However, Head lights are usually pretty straight forward though.

See attachment for main grounds

 

[Scottsdale87]

Thanks everyone for all the information! It is greatly appreciated. After removing the battery and throwing on the charger this morning before getting back at it yet again, I have some good news. I have power!
My initial worry after having the engine pulled and put back in was pinched wiring. I followed the positive battery cables as you all described above, following power all the way up to the junction box. Everything looked and test good, but I had a funny feeling, so I disconnected the battery and removed the main power wire at the junction box and the terminal was very corroded. I lopped it off and crimped a new connector on and finally received those electrons I was looking for!

My fan runs, radio turns on, wipers wipe and washer pump sprays juice, which is a massive win.

After receiving the picture of grounds, I ran one from my new Grote lights, to a bared spot on my frame and now I have reverse and park lights. My front passenger park light bulb also came on by putting that temporary ground on.

Still no brake lights, signals or cab lights.
Me thinks I have a bad flasher and/or brake switch. As far as cab lights (clearance), is there a common issue with them?

Edit:
Clearance lights on the cab

 

[Ricko1966]

Thanks everyone for all the information! It is greatly appreciated. After removing the battery and throwing on the charger this morning before getting back at it yet again, I have some good news. I have power!
My initial worry after having the engine pulled and put back in was pinched wiring. I followed the positive battery cables as you all described above, following power all the way up to the junction box. Everything looked and test good, but I had a funny feeling, so I disconnected the battery and removed the main power wire at the junction box and the terminal was very corroded. I lopped it off and crimped a new connector on and finally received those electrons I was looking for!

My fan runs, radio turns on, wipers wipe and washer pump sprays juice, which is a massive win.

After receiving the picture of grounds, I ran one from my new Grote lights, to a bared spot on my frame and now I have reverse and park lights. My front passenger park light bulb also came on by putting that temporary ground on.

Still no brake lights, signals or cab lights.
Me thinks I have a bad flasher and/or brake switch. As far as cab lights, is there a common issue with them?

Brake lights run through the turn signal switch,so no signals no brake lights I'd look real hard at my signal switch and it's wires.

 

[Jgonick]

Watch this video to understand brake and turn signals.-LINK-

Attachment has turn signal switch wires labeled. White is power for brake lights, purple is power for for turn signals, brown is power for for hazards. Does your hazards work?