View Full Version : Great, no headlights...

03-18-07, 11:59 PM
My 79 SDV has no headlights now! When I first picked up the car it had not high-beams (although the switch did work once). Last night I pulled the ights on and they went out.

I have replaced the dimmer switch (easy:thumbsup: ) thinking that might ifxit, but no dice.

I searched the forums and found a thread about a 79 FLeetwood limo that had a similar issue. Someone mentioned a fusible link down by the starter that might cause this, but his fix had something to do with the guidematic relay by the firewall or something.

What does the fusible link look like? What's the best way to get to it?

I really want to learn to fix this car myself, as it's fun, and it can save me lots of $$$$$.

Any help would be greatly appreciated!

03-19-07, 08:40 AM
Does your DeVille have Twilight Sentinel? Do any exterior lights work when you pull the headlamp switch on?

These electrical issues can be tricky. A Cadillac tech once whispered to me, "it's easier to diagnose an electrical problem in the Space Shuttle than a Cadillac."

03-19-07, 06:03 PM
What is Twilight Sentinel?

All the lights work except the headlights. I've got brakes, taillights, blinkers, etc.

03-19-07, 06:04 PM
I looked it up, and no, I do not have twilight sentinel.

03-19-07, 08:24 PM
It sounds like the headlight relay now. I think it's on the firewall but I don't know which one it is. Are you sure the high/lo switch is good? was it a new one? My 78 Seville is having trouble with the lights and I replaced the dimmer switch with a used one and it worked for a while but eventually died again. I had melted wires around my plug so I'm pretty sure my problem is down there but haven't fixed it yet. I just mentioned that because you may want to double check that area. Good luck.

03-19-07, 09:55 PM
Thanks for all the replies and the advice.

I am as sure as I can be that the switch is good--it is brand new.

I'll check the relay, if I can find it. I would assume it is on the driver's side, around where the switch would be if looking from that direction?

03-20-07, 03:26 PM
Well, I am a moron. Right there, next to the switch, it says "sentinel". Duh. I guess that is what it is, because I have headlights (the ring for sentinel was loose), but still no high beams. I bought a new dimmer, so I think the problem is elsewhere.

Is there any online resource for where that fuse might be? I can't see it on the interior fuse block. Also, something that shows where the relay is and what it looks like would help me immeasurably.

Thanks guys!

Old Fleetwood
03-20-07, 08:28 PM
If you can't get a Factory Manual right away, can you find a Chilton's, Haynes or similar book at your LIBRARY for that year car?
That might be your next line of attack since it looks like you need hands-on help; something to TRACE the line.
For example, my 1995 has two sets of electrical panels - one at the left side of the instrument panel that you find when you open the door. The other one is under the hood on the passenger side.
But yours is probably COMPLETELY different, so anything I'd tell you would send you on a wild pig chase.

03-20-07, 09:29 PM
Way back in your very first post, you may have been on to something with the "guidematic" reference. If you had both Guidematic and Twilight Sentinel, you would have two rotating knobs (rings, as you stated your Twilight Sentinel ring was loose) in addition to you regular headlamp switch and your foot switch for the hi beams.

My manual says "The GUide-Matic is a semi-automatic electronic device that controls the hi and lo beams in response to light from an approaching car."

There would be a light sensing unit on the radiator cradle tie bar just to the left of the center of the car. This thing is set by the factory and sealed up with epoxy and if a failure occurs you have to replace it. It would have light blue, dark green, gray and yellow wires going to it.

"The power relay switches the headlight beams in response to the signal from the photo-amplifier unit. The relay is mounted on the toe pan just above the headlight dimmer switch (below carpet and jute)" [The relay would have yellow, violet and light blue wires connected to it.]

"The foot switch is a special dimmer-override type that provides either 'automatic' or 'low beam' control of the headlights. It also contains an override section for obtaining an override high beam when in automatic low beam position, if required."
"The override section functions as follows: with the foot switch in 'Automatic' position, a slight downward pressure on top of the switch provides high beam regardless of the amount of light on the photo-amplifier unit."

Maybe you have Guidematic, and when you replaced your dimmer switch you didn't use the Guide-Matic type, or maybe your power relay is stuck, or somethings wrong with the photocell. If you have Guide-Matic, that is. It sounds like maybe both Twilight Sentinel and Guidematic are working against you.

03-20-07, 10:57 PM
I only have one ring around my headlight switch, not two. I am going to go and see if I have the photocell or not.

I remember the guy at the auto parts store asking me a question, to which I answered "no" but I don't remember if he asked me about guidematic or not. Ugh! I do still have the old switch, though. I'll look at it again and see if there is a difference.

I went to the library, and the manuals they had suck. None of the info I need is there. I am going to get a service manual ASAP. I found one that is complete but the spine is broken for $35, not too bad.

03-21-07, 01:49 AM
Ok, I am off to the local wrecker tomorrow to see what I can learn.

I do have twilight sentinel (it doesn't work), but I don't have guidematic. The dimmer I took out did not have the fourth connector coming out the back. I looked at the switched at RockAuto and the guidematic ones did.

I am going to spend my day pulling apart the cars to see what is what.