Dimming headlights while driving

[KMD88 2]

Hi everyone,

Just replaced my headlights in my 69. Drove it for about 40 minutes at night, then hit the freeway. Was on the freeway for about 5 minutes or so, averaging 50-60 mph. Right as I was exiting, I noticed my headlights begin to dim, then shut off completely. Then they came back on.

Right after, I drove down a main street for about 2-3 minutes with the lights still going in and out; fighting to stay on. Finally I flipped the light switch in and back out; which seemed to do the trick. No problems for the rest of the night, but I didn't get back on the freeway.

What could be the issue? The car drove fine the entire night; even during the headlight issue. My interior lights stayed lit as well; never dimming.

Thanks,

[barnett468 418]

could be the headlight switch or the headlight dimmer switch on the floor . if it happens again try depressing the dimmer switch just a tiny bit. if nothing happens try dimming the lights for a few seconds a few times.

 

.

[RPM 1,190]

I'd install a relay for the headlights. Takes the heat off the switch and let's the relay do the work. It also increased the brightness of my stock lights. If I can do it,  anyone can. I believe this is how I wired mine. 

While I was at it, I also did the horn which Mustangs have a history of fire issues with especially after market steering wheels. 

Just a thought. 

[capemustang 15]

As RPM said, you need a relay.  I upgraded the lights  to halogen on my 69 and needed to install two relays.  The older wiring harnesses are not able to handle newer headlights which is why they cut out.  I wish I could explain it better. I suggest taking a look at Daniel stern lighting.co
m. He explains it so much better than I ever could.  Also, vintage mustang forum has some great  threads that discuss why the headlamps cut out, the need for relays, and provide solutions.  

[KMD88 2]

On 10/19/2019 at 9:50 PM, barnett468 said:

could be the headlight switch or the headlight dimmer switch on the floor . if it happens again try depressing the dimmer switch just a tiny bit. if nothing happens try dimming the lights for a few seconds a few times.

 

Thanks barnett468. Are you referring to the high beam switch on the floor?

 

[KMD88 2]

On 10/20/2019 at 6:13 AM, capemustang said:

As RPM said, you need a relay.  I upgraded the lights  to halogen on my 69 and needed to install two relays.  The older wiring harnesses are not able to handle newer headlights which is why they cut out.  I wish I could explain it better. I suggest taking a look at Daniel stern lighting.co
m. He explains it so much better than I ever could.  Also, vintage mustang forum has some great  threads that discuss why the headlamps cut out, the need for relays, and provide solutions.  

Thanks for the info CapeMustang. Looks like this could be the culprit. I'll look into installing relays with the info provided here.

[KMD88 2]

Is this what I basically need?

 

https://www.cjponyparts.com/redline-lumtronix-headlight-power-upgrade-harness-for-four-headlights-1969/p/HVU2/

[barnett468 418]

1 hour ago, KMD88 said:

 

yes, and if that does nothing try pushing the headlight switch in and out.

[barnett468 418]

2 hours ago, KMD88 said:

Thanks for the info CapeMustang. Looks like this could be the culprit. I'll look into installing relays with the info provided here.

You do not "need" a relay with stock lights. You need to find the cause of the problem and fix that first before installing any relays etc, otherwise, it can reappear at any time. Your lights went out due to a bad/worn part or loose connection etc, a relay will not repair a bad/worn part or a loose connection.

 

 

[capemustang 15]

11 hours ago, barnett468 said:

You do not "need" a relay with stock lights. You need to find the cause of the problem and fix that first before installing any relays etc, otherwise, it can reappear at any time. Your lights went out due to a bad/worn part or loose connection etc, a relay will not repair a bad/worn part or a loose connection.

 

 

I believe you are correct that you don't need  a relay with stock lights. I should have been more clear in my earlier post regarding the need for a relay.  I decided to upgrade my headlights (4)  from stock to halogens (Hella H1 and Koito).  Because of the upgrade, it was strongly recommended that I install relays. Other friends (1967 and 1968 Mustangs) had upgraded their headlights to halogens and did not install relays.  They experienced what KMD88 described in his earlier post - the lights cutting out and then later coming back on again.  I do agree with barnett468 that you should check for bad/worn parts or loose connections.   I am new to this and could be completely wrong about the relays.  I hope you figure out what it is and don't have a repeat.  Not fun going dark unexpectedly while going down the highway.