Hi/low beam are flashing

[Juanon]

Hi, my c10 1987, oem headlights, when i turn on the lights after 5 minutes, the front and rear lights start flashing,

i change:

Low/hi beams headlights
switch control
switch socket
i clean all grounds
check all fuses

when the problem start, is like original alarm but not horn noise.

This problem is only when i turn the lights

please help!!!!

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[chengny]

It wasn't a police/emergency vehicle at some time in the past was it?

Do you mean the headlights flash lo-hi-lo-hi repeatedly and on their own?

Or are you talking about the front/rear parking lights and side markers?

I ask because I see this:

the front and rear lights start flashing

And, unless things are way different in Mexico, there aren't any rear headlights.

I am going to guess that you mean:
When you turn on the headlights that is when the front and rear parking lights start flashing (but not the headlights themselves) - correct?

Or do you really mean the headlights start flashing hi to low without touching the dimmer?

Come back and stick with us - someone will help you.

 

[Juanon]

hi, thank you for your help,,

my car is not an emergency or police car.

if i turn the lights low beams after a minutes the same lights start flashing like if not have power, if i change to high beam the lights flashing too, not at same time the 4 beams like a police.

When you turn on the headlights that is when the front and rear parking lights start flashing (but not the headlights themselves) - correct?

correct,

when the problem start, i unplug a socket and check volts and is correct.

the lights problem is only when i turn lights.

one thing, i dont change the dimmer switch at this moment.
i touch is hot, not too much for burn.

sometimes too, when the lights are on after minutes flashing or turn off, but i dont turn off the switch, is in on position.

the front turn signals is ok, stay on,, no problem.

 

[chengny]

one thing, i dont change the dimmer switch at this moment.
i touch is hot, not too much for burn.

I think you answered your own question. The dimmer switch should not be hot. Try pulling the connector from the switch body, look for broken wires and worn away insulation near the switch.

I can't imagine why the headlights cycle from high to low. The dimmer is a manual switch - loose connections and broken wires would normally just make the operating beams flicker on and off. How the switch is changing the power from highs to lows is beyond me.

If that doesn't help I would try a new dimmer.

As far as why the parking lights are also flashing ... I would have to think a while. Fix/replace the dimmer switch and maybe the other problem will clear up too.


Have you done any electrical repairs or modifications recently?

 

[Juanon]

I CHANGE The dimmer switch, connectors from the switch body.

i change the low to hi switch, the connectors.

i make good grounds.

my problem is only: when turn low beams after 5-10 min start, is begin to on/off off/on.........if change to hi beam is the same problem

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i make video, watch facebook

https://www.facebook.com/jucarespe18/videos/1725697730985280/?l=1219503960700796836

 

[chengny]

Here's what I think Juan;

There is probably a short circuit in the power to the low beam headlights (more about that below) between the H/L switch and the sealed beams.

In every other electrical circuit in the truck, a short circuit (or a short to ground) will blow the fuse associated with that circuit.

But, the power supply for the headlights has no fuse to protect against an over-current condition. It is protected by two other devices:

1. A fusible link located in the engine compartment - it's down near the starter solenoid. That link also protects some other circuits. But you can ignore that device because it is a piece of wire that actually melts when too much current is passing through it. Once melted, it never allows current to flow again.

2. The second current limiting device is a circuit breaker. This circuit breaker is located inside the actual headlight switch. It opens and kills power to the headlights (high and low beams) when current to the sealed beams exceeds 6.5 amps. What is important about this circuit breaker however is that it is a self-reset type. If it gets hot due to excess current flow, it trips. But then as soon as it cools off, it resets by itself and the contacts close (and allow power to flow to the headlights once again).


The result of this circuit breaker tripping and re-setting repeatedly is that the headlights will be flashing. But unlike a flasher that is intentionally installed in a lighting circuit (which goes on and off in a steady rhythm), the circuit breaker opens and closes with an uneven frequency. That is what appears to be the case in your video.


A couple of things:

1. The reason I believe it (the short) is in the low beam power supply, is because if it were in the high beam circuit, the lights would flash only while on highs. The lows would flash as well, but they wouldn't be the cause - just a symptom.

2. I have never actually had the headlight switch's internal breaker trip on me, so I can't say for sure, but you should be able to hear it click as it opens/closes.


Try this:

Unplug the switch from the harness, pull the switch out of the dash and reconnect it to the harness plug. Leave the switch hanging under the dash.

Turn the H/L's on and wait until they begin to flash.

Then, with the headlights still on, turn the engine off (to keep things as quiet as possible). Put your ear to the H/L switch and listen to see if you can hear the breaker snap open and closed.

If you can hear a click that is in time with the headlights flashing...I would say you have at least identified the problem. If that turns out to be true, come back and we will work on fixing the issue.

 

[Loren Bartling]

Did you ever fix the issue? I have the same issue with my 1986 C20 Silverado!!!!