[glhs#116]

1971 Trans Am.

So, I was busy doing some other things. Working with the heads and so forth. Was night and I was going to test driver. That's when I noticed. No headlights!

What I have:
* All lights work apart from headlights (side lights, brake lights, indicators)
* Low beam and high beam are as dead as each other
* Not seeing the blue indicator as I click high to low to high

Seeing as it is both lamps high and low I was thinking headlight switch. But I noticed I actually had a spare on hand. Swapping it changed nothing. I kinda thought if it was the footswitch it would only affect one of the other?

Also, I can see fuses for all sorts of lights except headlights. Can't see any fuses in the under dash that say headlight. Is there some other place where they are fused?

I really hate electrical stuff.

Sam

[johnta1]

The dimmer switch has both go through it.
Check the dimmer switch for 12 volts. Light Blue wire comes from Headlamp switch I think.

If it has 12 volts, then you'll have to follow the wiring to headlamps.

[glhs#116]

OK. This thing really wants to piss me off. I seem to have some sort of severe, I think they call it "voltage drop". Anyway, I'll explain.

I took off the dimmer switch. I have 12v to the centre (blue) wire. I tested the dimmer switch and it seems to do its job shunting the circuit either to the green or the tan wire depending upon its position. I then took the headlamps off. They are on a relay that triggers off the driver's side headlamp connector. No volts at the "relayed" headlamp connector terminals. I diconnected the relay stuff and measured straight off the factory headlamp terminals. I saw 12v there on either the high or low beam terminal depending upon the position of the dimmer. This was true for both headlamp connectors. Seemed good. So I tried the headlamps to see if they were bad. Nope. They light up if I wire them off the battery. I took the terminals out of the headlamp connectors and gave them a good clean. Still wouldn't light a bulb if a bulb was plugged in. Still show voltage if there isn't. Finally I tried measuring the voltage across the terminals with a bulb plugged in. I saw something like 3v and no light. Looked all over for a damaged wire or bad connection. All looks good. I tested the bulbs with one terminal plugged in and one on the harness. The earth side is good because if that side is in the harness connector and I jumper the hot side of bulb to battery it lights. However if the hot side of bulb is in the harness connector and I jumper the earth to the ground side of battery the light stays off. Having found that out I wondered how far back this went. I took off the dimmer switch again and took power straight from the blue wire feeding the dimmer switch and again I can measure 12v from the wire to a ground but I can't use it to light up a bulb. It's like it has power until it is under any load greater than a multimeter. The bulb doesn't even give a dim glow.

Now the other weird thing is this is out of the blue. Never had a problem with lights before. No dimness. No flickering. And all the lights are still just as good except the headlights. As mentioned before, changing out the headlight switch changed nothing. What is on the "feed" side of the headlight switch? But then again, if that were the case why do the side lights and tail lights and all continue to work fine?

Honestly. This is driving me mad.

Sam

[johnta1]

You have relays controlling your head lights? (not factory)

If so have a drawing of how it's wired?

One thing that may cause a problem is the connectors on the firewall that makes the connections from inside to outside. (below master cylinder)

[rustedgoat]

I think I remember a bad high beam switch or the connector caused no headlights (high or low beams) for me. Not sure if when you say dimmer if your talking about the headlight switch you pull operate with your hand or the high beam switch you operate with your foot. But I see you checked both of them.

I'd try by passing the foot switch with a jumper wire, sending 12v to the lights

[glhs#116]

So, this is how bad it is. I tried running a bulb straight off of the blue wire at the connector that feeds the footswitch in the driver’s footwell. Same result.

Sam

[glhs#116]

So, this is how bad it is. I tried running a bulb straight off of the blue wire at the connector that feeds the footswitch in the driver’s footwell. Same result.

Sam

[johnta1]

The switch I think gets its power from the soldered joint in the wiring loom next to the alternator. Quite a few connections there. Possibly it is corroded or loose there? Wrapped in tape or something. Red wires mainly. Should be easy to find. (laying on intake driver side)


Check that out.

[OG68]

You can always read voltage even if theres' a single wire strand remaining.
But as John mentioned, a corroded or loose connection will not pass the necessary current for the applied load.
I'm not familiar with Firebird wiring but in addition to the harness splice along the drivers side valve cover, check the starter and horn relay connections.

[Pepi]

If you say the blue wire going to foot switch has 12 volts, but will not light up a bulb, you have a bad connection or break in the blue wire further up. As stated, 1 strand will show voltage, but not carry current. You can verify that by running a new wire supplying 12V to where the blue wire connects to foot switch and if everything works, you know from that point it is all good.

Time to start tracing the blue wire back and see when it will finally light up a bulb.

[glhs#116]

I must say a huge thank you to everyone who responded. I was really feeling like nothing I was seeing made any sense at all and I started to see that not only was there sense in what I was seeing, there were also a lot of clues. I must admit I was also really grateful to have my trusty factory 1971 Pontiac shop manual which has a truly amazing fold-out electrical diagram.

Here's what I did. I made some jumper wires for testing. I first tested that the lights worked if I jumped to the wires at the high/lo footswitch. They did


After verifying all that worked both on its own and with the relays I was able to put the lights back in and work on where the fault was upstream of the light switch. I realised that the big distinction between the headlights (which didn't work) and the side lights and all (which did) was that those other lights are fused and the headlights aren't. That meant they had two different feeds.

I realised that since a bunch of the other stuff that was working was fed from the same splice I had better dig in and unwrap all the factory wiring bundle so I could see the state of the connections and fusable links and such. It was then that I narrowed my search down to a red wire that powers the headlight switch bypassing the fuse box. Protecting it is a green fusible link. I got the bundle open and I could clearly see where the fusible link was burnt out because I could feel a small section where the wire seemed to be missing. I cut the insulation here and decided the best way to deal with it would be to just join it back together here since the rest of the wire was intact.

The exercise was worth doing because I was able to find and correct some other connections that were less than perfect.

Of course, nothing could be that easy and I found that although I had lights the instrument panel lights were now not lighting. This turns out to be governed by a stupid fuse that is half the length of any normal fuse. Any idea where you get one of those? But apart from that everything is now working again!



Sam

[OG68]

The Fusible Link failure was the result of an electrical system failure. I would keep looking between the headlight switch and the link for whatever caused the fusible link to burn open. And replace the link. It provides protection for the rest of your harness.

Your local parts store should have the small fuse and the Fusible Link. The links are colored coded for the right amperage. If they don't have it, Amazon will.

[glhs#116]

I'm pretty sure I know what knocked out the link. I accidentally shorted the positive for the alternator briefly when I was unbolting the power steering pump. I had an inline fuse on one of the other wires which blew and everything seemed to work after I'd replaced that so I didn't immediately make the mental connection when everything but the headlights still worked.

I live in the UK and I've never seen fusible links in a store here. I don't know if they were used in any cars here. I have a box of glass fuses but, again, never ever seen the short ones. I'll look in the usual online places.

By the way, I didn't replace the fusible link. I just bridged over the tiny bit that was burnt. So I figure the link is still perfectly able to "blow" again if it needs to since it functions by being a "four gauges smaller wire" that can overload before the rest of the circuit (as the factory manual says). But, yeah, I should probably find a new link to make the repair fully proper.

Sam

[Pepi]

Glad you got it figured out and all working again!

[rolling money pits]

I’m all too familiar with that pesky fuse…

fuse you need is an SFE 5 amp. Do not get an AGC 5 amp, it will not fit.

I * think * the reason for a fusable link is that it won’t blow, like a fuse, if subjected to a slightly higher load for a brief moment. Thus, replacing a link with a fuse or breaker, will cause issues. I guess one could put a slow blow fuse? An electrically smarter person, hopefully, will opine if my info is incorrect.