Turning on lights kills engine, and hot taillight relay?

[60tex]

Guys - hope all's well. Trying to ferret out a wiring issue on my new '87 60 project, where my tail, brake, and license plate lights won't turn on. As background, I bought the 60 "not running" a week ago, but with a new battery, oil change, and some manhandling of the gas pedal, it fired right up, but has always required goosing of the throttle to keep it idling. But then I went to start it last night in the garage, and it settled into the smoothest, most beautiful idle, with no throttle inputs required. After 30-45 seconds, I went to turn on the headlights and it immediately faltered and died. Restarted with the usual jamming on the pedal, and needing to keep some extra input to keep it from dying at idle. Whether I had the lights were on or off, it wouldn't idle.

Headlights work just fine and always have, but I've only gotten the taillights and brake lights to work once. I didn't change anything: they just worked one time, and then next time I went to try them, they didn't. Also, when the light switch is turned on, the taillight relay gets pretty hot. All signs point to bad ground/short somewhere, right?

Facts:
- bulbs are good
- Light control switch is good, as I tested continuity there at all three positions (off/parking/headlights)
- TAIL fuse is good
- At the relay connections on the relay block, I'm not getting any voltage on the upstream side of either the low power or high power circuit, whether switch is on or off. But when I turn the light switch on, I get 0.1v reading on the downstream side (I think) of the high power circuit.

Questions:
- If there's a short or bad ground, are there some known spots to start looking? (Truck is pretty dry with very minimal rust).
- Could the short have engaged the alternator (which is old), making it so hard to spin that the engine can't idle without additional throttle? Anyone experienced this?
- FSM mentions a fusible link. Where is this? And is there a link for each circuit, or one overall? All other electrical on the truck works, except the instrument panel lights, which I haven't started digging into yet.

Thanks a ton!

 

[60tex]

Ah, could the dying be the result of the taillight short plus a fusible link past its prime? Finally found the FAQs (sorry, noob), and it looks like the sudden draw from the short could be causing the fusible link to cut voltage. Which in turn would be less voltage available for ignition and give me my super-rough idle?

 

[NeverGiveUpYota]

Yeah my first guess would be the fusible link.

 

[60tex]

Thanks Yota!

Just thought of something - can I assume that since my taillight fuse is staying intact, and the relay gets super-warm but will not actuate (although it bench tests perfectly), that the short is probably in the low power side and not the high power side?

 

[NeverGiveUpYota]

I'm so not an electrical person. I did pull apart and rebuild my fusible link and replaced the middle section but I did it because my wiring was ugly.
@Coolerman, @Spike Strip... you guys have any suggestion for this guy?

 

[60tex]

Alright, mystery solved! Somewhere along the line, someone made a poor splice of both the white charging wire and the thinner white wire that terminate together into the ring terminal on the back of the alternator. Over time, the splice got super corroded. Wonder if this helped kill the alternator too. The bad splices were buried in the 1/2 plastic protective cable wrap going between battery and alternator, so everything looked good visually. I didn't take a picture of all of the green crustiness that I unwrapped, but you can still see it in the thinner harness wire.

Jumpered the wires, and everything lit up like it should. Will be doing some slight modifications to allow for a higher amperage alternator, while keeping the body harness protected by the fusible link.

Thanks guys!

 

[60tex]

Alright, so slowly I'm getting this beast reliable. Fixed the bad splice above, and now that I have nice clean power, the engine purrs and everything on the TAIL LAMP circuit (bunch of lights: tail, dash, license plate, side markers, glove box, etc) works. However, I am getting intermittent fuse blows on the circuit though, within 3-6 minutes of driving, but it won't blow if car is stationary and the lights are on. Sounds like a short somewhere?

Need to figure that out, but a couple questions in the meantime:

1. I notice that the tail light relay builds heat, and within 20-30 mins of the lights on, is almost too hot to grab onto. Any of you guys know how hot the relay should normally get, if everything working correctly?
2. Could a hot relay be the cause of the blown fuses, or a symptom of whatever's causing the fuses to blow in the first place?
3. Has anyone had luck fixing the blown tail lamp fuse issue (which isn't uncommon?) by replacing the relay?

I think someone mentioned that the ground for the tail lights is right behind the rear bumper, center of the truck. I cleaned up the connection (white/black wire, and one other - forget the color - going into the eye terminal), but not sure that was the correct one because it didn't shut the lights off. Where should I be looking?

Thanks guys!

 

[jawjatek]

If the load contacts (i.e.the "switch") of a relay are normal, they don't get hot even with significant current because there is very little if any resistance (and hence voltage drop) in the contacts. Heat = power dissipation in watts = current x voltage drop. If voltage drop is near zero, watts are near zero.

The actuating coil of the relay is usually where the heat comes from, but its not a lot unless the coil has shorted turns - then it gets hot due to increased current. This coil takes the full 12V drop, so heat = watts = 12 x current in the coil. If the current is high because the coil is going bad (shorted turns) it usually gets worse and worse over time until it finally cooks "open" (no worky - no click) or blows a fuse.

If the load current the relay is switching is normal, I'd say the relay itself needs replacing. I would measure the voltage drop across contacts, the coil and load currents, to determine the issue, but I'm an EE and this is easy for me, but hard to explain.

 

[60tex]

Makes sense to me! So logic side is likely fried. Thanks @jawjatek. Will start there

 

[Randy88FJ62]

Open up the rear quarter panel on the drivers side and remove the tail light assembly as well. Peer down there and verify the wiring is good. This wiring in the back driver's side area is prone to corrosion and deteriorates due to water getting in. There could be a short building up resistance and heat in your wiring.