[DeTomaso] Techno question: Headlight motor relay

[Mike Drew]

Chuck,

Slow down!

The big clear relay is a fiendishly complicated thing that you never have to touch. Stay away from it at all costs. 

Dumb question first—is the ignition key on? The lamps are ‘hot’ and will turn on and off with the ignition off, but the headlight motor requires the key to be turned on. 

If turning the key on fixes your problem, have a good laugh. 

What if the key was already on? There are only two possible causes of headlight buckets that will only operate in one direction—bad headlight switch, or bad limit switch. 

If your headlights are up, the forwardmost limit switch actuates to shut off power to the motor with the headlight switch on.  If the other switch has failed, then power doesn’t flow to the fiendish relay when you turn the headlight switch off.  It is fooled into thinking the headlights are already down. 

Disconnect the two wires from the rear limit switch. Turn the key on, and headlight switch off. Momentarily connect the two wires together (bypassing the switch) perhaps using a paper clip as a connector. If the headlights start coming down you have found your problem. Don’t let them travel all the way down because there is nothing to stop them, and the motor has a lot of torque. It will attempt to keep driving the headlight buckets past closed which is bad.  As soon as you separate the wires they will stop. 

What if the test above doesn’t work? Then the problem is likely the headlight switch. If you have been using high powered lights without the Moseley relay kit or equivalent you probably roasted the switch.  Others smarter than I can describe how to troubleshoot that. 

Limit switches respond favorably to WD40 baths combined with digital manipulation. Carefully remove the offending switch and literally submerge it in WD40 for awhile, periodically cycling it with your fingers. 

(This assumes the contacts where the wires connect to the switch are clean. If not, that’s another possible ‘duh’ diagnosis. 

Let us know how things progress!

Mike

Mike

Sent from my iPad

> On Jun 22, 2018, at 17:17, Charles Engles <cengles at cox.net> wrote:
> 
>   Dear Forum,
> 
> 
> 
>                       No good deed goes unpunished.
> 
> 
>                       The late 74 Pantera I have was working fine.  I
>   decided to upgrade to LED headlights---"plug and play".    I installed
>   them without much trouble and they worked.  All was well until I tried
>   to put the headlights down and off.
> 
> 
>                      They won't go down.    Nothing was touched with the
>   installation other than the headlight buckets.
> 
> 
>                      Today I removed the new LED headlights and the dash
>   switch still refuses to actuate the headlight retraction.
> 
> 
>                        The #10 fuse is good and the dash fan blower works
>   fine on that circuit.
> 
> 
>                      My dumb diagnosis is that the Headlight motor relay
>   either coincidentally failed or it didn't like the new electron
>   arrangement and failed.
> 
> 
>                         By the driver's left leg is the relay panel.  I
>   see a column of rectangular boxes and to the inside of that there is a
>   clear box thing, then a black box thin and finally, the cylinder thing,
>   which is the buzzer, I think.
> 
> 
>                        Which one is the HL motor relay?    Are new ones
>   available?
> 
> 
>                         I don't do electricity well and I am open to
>   advice.
> 
> 
> 
>                                     Frustrated,  Chuck Engles
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