76 Eldorado headlights stopped working

[Ranger magnum]

Hello all,

76 Eldorado convertible with auto dimming and twilight sentinel. Headlights stopped working one day, so I pulled the lower fascia panel off, and looked around. Did not see anything amiss, but did check all related connectors. Suddenly, they worked again! Which was a good thing, as we were planning on driving to Las Vegas from Socal, and would be completing the trip in the evening.

All went well until we got to the hotel. Turned off the car, checked in, and when I went to fire it back up, headlights did not work again. Mind you, we started and stopped the car numerous times during the trip, and the headlights worked fine.

Since we were driving only during the day, it was not an issue. When the time came to leave the following week, I made sure to leave in the am so as not to travel in the evening.

Got the car back to my shop and began work. I am a skilled mechanic, and collect and restore GM cars as a hobby, so trouble shooting is not new to me. However, the twilight sentinel is, and here is where I am unsure. I do not yet have a FSM, but will order one next week.

I have checked everything from after the headlight switch, and all checks out. When I disconnected the two connectors from the headlight switch, and jumped a hot lead to the blue wires, headlights work fine. Both high and low beams, and using the dimmer switch, I was able to cycle between them. (as an aside, all other lights work as they should. This issue is only with the high and low beams). Again, to reiterate, everything works beyond the headlight switch.

I removed and inspected the headlight switch, and disassembled it as well, and found no visible issues within. A continuity test with my multimeter on the right side connector shows that when the rod is pulled all the way out to activate the headlights, voltage flows from the AC pin to the #6 pin.

On this headlight switch, there are two separate connectors. The drivers side one (black in color) seems to function only for the parking, marker, and interior lights (which all work). The passenger side connector (Brown in color) is for the headlights and the some twilight sentinel system. Again, this is only my observation. I have no manual to go off of. (there are of course two more connectors that come out of the switch that activates the twilight sentinel, but in this instance, I am referring only to the main light switch)

Here is where things begin to get weird.... the parking light connector has two constant power-in wires: green with a black stripe, and a black with a white stripe. This powers pin #5. When the rod is all the way in the switch body, a continuity test shows that there is no voltage going anywhere. When the rod is pulled to the first detent, voltage flows to pin #4. A test light hooked to these two wires from the harness also shows voltage.

On the passenger (headlight) connector, a continuity test reveals this: with the rod all the way in the switch, there is continuity between the pin marked "AC" a red wire and a solid black wire coming out of the connector, and  pin #1, which has a single yellow wire coming out of the connector. When the rod is pulled all the way out of the switch, we have continuity between pin AC, pin #1, and pin #6. Pin 6 has two blue wires coming out of the connector, and those are the headlight wires. (these were the wires I ran a hot lead from and got the lights to work as mentioned above.)

Here is where I thing the beginning of the problem is: I believe that pin AC should be constant hot, and it is not. I tested for voltage at the harness, and there was none. Tracing the red wire leads me to the white sentinel box. Looking at the sentinel box, I see among all the other wires, a black wire with a yellow stripe. It is a larger gauge than all the rest, except for the red wire that leads back to the headlight switch. Based on its gauge (and my experience with other cars) this usually indicates a hot lead. Which all makes sense to me.

So it appears to me that I am not getting constant power to both the sentinel box and the headlight connector on the headlight switch.

And this is where I stopped earlier today. I will chase the black and yellow wire over the weekend, and see where it leads me to. The question I have is this: Does the constant power come from the battery to the sentinel, and from there to the headlight switch? Am I correct in assuming pin AC should be a constant hot?

If the sentinel box goes bad, do the headlights go with it? I found an NOS amplifier and photo cell on Ebay for $250. Should I buy it? I dont see them often, so maybe as a spare? I like all the options in my cars to work, so installing a non sentinel headlight switch (assuming it would even work) is not an option for me.

One thing that needs to be noted is that the constant hot wires in the drivers side headlight switch connector (pin #5) at some point became hot enough to melt is receptacle in the connector.

Excuse the biblical length post, but hey, for my first post as a new member, I should make it count!

Thank you in advance for any light you can shed on my problem. (pun intended)

Jason Dave

[bcroe]

I have had a few dimmer switch somewhat intermittent failures. 
Any contact getting hot is a serious problem, and warm is just the
same thing in an earlier stage.  good luck, Bruce Roe

[76eldo]

Try activating the foot dimmer several times. Your lights may come on.

[Ranger magnum]

While I understand my post is lengthy, in it is an accurate description of the root of what I believe to be the problem.

*from* the headlight switch, everything works as it should. That means the relay, the dimmer switch, the ground, and the lights themselves. Each of those items were checked and double checked, and in the instance of the dimmer, replaced with a known operational unit.

So, just to reiterate, when I remove the headlight connector from the switch, and run power to the headlight lead, all lights work.

The problem lies from the battery to, and possibly including the headlight switch.

I’ve disassembled the switch, and all looks as it should be inclusive of the fuse within.  I suspect that the switch is not receiving its constant hot. It appears as though that hot lead comes from the twilight sentinel amplifier. But without a wiring schematic, and information from someone with greater knowledge than myself, I am at a stopping point.

A new headlight switch is on the way, and I’ll begin with that and work backwards.

If anyone with intimate knowledge of the twilight sentinel system could chime in, we can take this off the board.

Thanks

Jason Dave

[35-709]

There are three fusible links coming from the starter, one to the ignition switch, one to the fuse panel, and one to the headlight switch.  These wires live in automobile hell --- wind, heat, sandblasting, forceful water, etc.  Might be worth checking them closely.  I have posted a picture of mine from my '73 Caribou (nee Coupe deVille) before but here it is again.  Car was still running!
   

[bcroe]

Here is what I see on a 76 wiring drawing.  Twilight Sentinel is wired in parallel
with the switch, so it can turn the headlights on, but not off if they are already on. 

Constant 12V comes into the headlight switch on pin 1, looks like yellow.  It
goes through a circuit breaker, comes out on pin 6 with headlights on, BLU-
LGT.  2 wires, one for twilight sentinel and one for headlights.  With GUIDE-
MATIC auto dimming the headlight wire goes to a power (dimmer) relay,
probably pin 1.  Power comes out on pin 2 GRN high beam or on pin 3 TAN
low beam, depending on the relay state.  These wires go to the headlights. 

I suggest taking your test light or meter (other lead grounded) and follow this
path till you see where the power stops.  This will lead you directly to the fault.  Swapping parts does not seem to be working here. 

The dimmer switch interacts with the relay, allowing a forced low beam.  But
that circuitry is incapable of turning the lights off.  good luck, Bruce Roe

[Ranger magnum]

Thanks Bruce!! This is quite helpful! I’m thinking it’s the relay too. I found a schematic for a later year Brougham, and it closely mirrors your description.

Any idea on the location of said relay?

And the the person the posted about the fusible link, at some point in my cars life, this links were moved to inside the passenger compartment, just below the fuse box.

Thank you both. This is why I love the internet.

I’m heading out of town tomorrow for 5 days. I won’t deal with this until my return, because if I do, and do not solve it, it will be on my mind the whole time.

I’ll get back at it next week, and report back.

Many thanks.

Jason Dave

[bcroe]

Thanks Bruce!! This is quite helpful! I’m thinking it’s the relay too. I found a schematic for a later year Brougham, and it closely mirrors your description.  Any idea on the location of said relay?

And the the person the posted about the fusible link, at some point in my cars life, this links were moved to inside the passenger compartment, just below the fuse box.  Jason Dave 

OK, at least you did not waste money on GUIDE-MATIC or TWILIGHT
SENTINAL parts which cannot turn off the lights, once the switch is
in the on position.  My 76 book says the G-M relay is to the left of
the fuse block, not a bad place to start looking for +12V or warm
connections.  HINT get a 76 book. 

I doubt its a fusible link because usually they just fail once, but a
loose connection is possible.  Yellow wire down to starter it says
here.  Do not forget the switch ckt breaker.  Your test light will
lead to the fault.  Bruce Roe

[Ranger magnum]

Problem solved!

With help from bcroe, (and the procurement of a FSM), I traced the issue to two problems:

First was a dodgy connector for the hot wire into the headlight switch.

Second was an issue with the headlight switch itself. a quick disassembly, some cleaning, adjustment of the circuit breaker, and liberal application of dielectric grease seems to have solved the problem.

But of course while I was doing all this, and had the dash partially apart, I figured why not take it all apart, change the telltale bulbs, replace the front speakers, and go through everything.

So thats where I am as of today.