85 C10 No headlights or cluster lights

I have a 85 c10 and ran into an issue recently. When I pull the headlight switch knob no headlights or instrument panel lights will come on. I have no power at all at plug for the switch. Also my inst lps 5 amp fuse has no power on fuse box. Fuse is good but no power from box it’s self. Should I have power there and is that cause of this issue possibly?

Welcome to the forum 95.
There might be a relay in that system that has gone out.
Other members with the answers might chime in after they awaken in the morning.

The larger gauge red wire on the back of the headlight switch is the feed. I *think* it heads directly through the firewall without passing through the fuse block, then wraps around the back side of the engine and heads up by the alternator (but not to it) before joining up with another few larger red wires that head down to the starter to pick up power. There are fusible links protecting these wires down near the starter. Sorry that sounds a bit cryptic, but I don't run into problems that far upstream that often... My best guess is that you might have a bad fusible link or connection somewhere in the engine compartment.

The little fuse for the panel lights receives its power from the the parking/tail lamp circuit, so it only has power when those are on.

All wires from starter have been replaced.

So on Saturday I changed the the wires to the starter due to them being bare and just terrible. I had to drop starter to sue that due to long tube headers, it was just easier. Got everything wired up and no wires touching headers. Took it for a drive that night and everything was fine, headlights worked and all. Next day on Sunday I changed the plugs in it and later that day started it and no lights. Then last night I was looking for things I decided to look at my cargo light because when I bought the truck it hasn’t worked since then and the housing was melted a little bit, but lights worked with it at the time of purchase with bad cargo light. I decided to cut it out due to being bad and what do you know my headlights and headlight switch are working correctly. 30 min later I want to take the truck for a drive after days of frustration, start it up and no lights again. I have no idea.

Maybe you lost or broke a ground from the engine to the firewall??? Could've got bumped while changing plugs. I don't know how cutting out the cargo light would temporarily revive the headlight function, unless that provided a temporary ground path for your lights. I'm just spit ballin' here...

Your problem reminds me of years ago when I had a bad ground from the engine block to the firewall in my Monte Carlo. When that ground strap vibrated loose, it killed my headlights, interior lights, and radio.

Welcome to the forum, good luck, and let us know when you figure it out!

So I have found out that my voltage is low. Had battery tested it was good and alternator passed when I gave it gas. When it’s idle the alternator won’t charge. And it’s doing it to 2 alternators I have. One alternator is very old and the other I don’t know how old. The fan belt isn’t loose so what could cause low voltage when idle. That is also why lights will not work until I rev motor and voltage comes up.

How'd someone manage to melt the cargo light housing? Those are a fairly heavy pot metal casting. Dome light housings on 1984-up pickups tend to get pretty cooked... but none of that is going to affect your headlights.

All of the symptoms you're giving points to the problem being before the headlight switch. If you have no power at the plug going into the headlight switch (or at least an intermittent connection, since the lights work sometimes), start probing that red wire from the light switch and work your way back towards the starter. Double check your new wiring and connections... maybe flex and wiggle the wires a bit with your headlights on to see if they'll come on/go out/flicker and if anything is getting warm. Might need a spotter if you can't park where you can see what the lights are doing while you're doing this. Even if you did a bang up professional job on the wiring, stuff can get overlooked and weird little things can happen. I just had a fault where enough strands on a wire broke apart internally that it overheated and finished burning through while leaving the outer insulation looking like new. FML, eh?

^ Mmm... A lot of these trucks were equipped with alternators that won't keep up with electrical demand at idle... like when you're idling in gear (I'm guessing you have an automatic?) with your headlights and heater on. In that case, all the lights dim a little and the blower slows down, then everything brightens back up as soon as you step on the gas and start moving again. What is your voltage at idle? above idle? Your headlights will visibly illuminate at 6 volts or less, though they'd be useless for seeing where you're going.

So this thing has led lights in them (like that when I bought it). So it’s around 10.9 and 11.3 at idle. When I rev it, it goes up to around 14 like normal and it will stay like that at idle after rev. If I shut truck off and restart Volts are low again until rev. And until I rev and get volts up my lights won’t come on. Yes it is auto. And before this weekend it never did this that I know of. I have only had truck for 3 weeks. Is alternator supposed to be charging while running in idle? Most every truck I’ve checked does.

xreynolds95 said:

Is alternator supposed to be charging while running in idle? Most every truck I’ve checked does.

Click to expand...

Yes, your truck should be charging at idle. It should be 13 or 14 volts, whether at idle or highway speed.

It sounds like you have two bad alternators. Probably bad internal voltage regulators.

I'll give you two possibilities.

First, when having electrical problems on vehicles, people often fail to check their grounds. On a vehicle, the grounds are more often the problem that the hots, because they're a little more loosey-goosey in the original design that the hots. And most electrical problems in vehicles are connection issues, at least initially.

In particular, GM trucks have four separate parts that are insulated from each other by rubber bushings and the like and need to be wired together: the battery, the block, the frame, and the cab. Connecting four things to agree on where zero volts is takes three wires. One from the battery to the block. That's the big one. One from the battery to the frame. That's the smaller one from the battery to the radiator upper cross-member. The third one is from a valve cover screw on the rear passenger-side valve cover to the firewall. When that one is not there, you get all kinds of screwy stuff going on.

Second, you can replace the alternator with one that has the same housing and uses the same brackets but has higher output. That unit will have higher output all across its rpm range. We add stuff to our trucks and forget that the alternator it was originally equipped with wasn't specced with that additional load in mind. The original on the 85 C-10 was probably a 63-amp 12SI. You can get a 100 or 140 amp 12SI form alternator and just bolt it in. No bracket or wiring changes.

How did you fix your problem? I have the same problem right now?