Expert: Dick Benjamin - 11/19/2008

Question
Hi Dick,
I am currently restoring my first classic mustang and am getting very close to the end. I have replaced the old console to engine wiring harness and everything so far has been working wonderfully, until I plugged in a new set of halogen headlight bulbs and they would not turn  on when the switch was pulled. I unplugged the bulbs and turned the switch back on and the headlight-on indicator came on and there is power all the way to the plug, but when plugging in the bulbs, the indicator light turns off and there is no power to the plug.

I figured that it was just the fact that modern halogens draw too much power so I bought some original bulbs and when I went to try them out, I plugged one of the bulbs in, pulled the switch and the bulb lit! So I figured that I had the problem solved. The next day when I installed the bulbs and did another test, The same problem, no power to the bulb plug and no headlight indicator lit. After about 5 minutes pulled the switch again with no bulb plugged in and power was back and indicator was on. I thought maybe it was the switch that was worn out because it was the original so I replaced the entire thing and am still having the same problems, and the bulbs have not lit since the 1st time.

Any insight into this problem would be greatly appreciated, Thank you in advance


Answer
This is typical of the sort of problems we get into when we change things from the original design.  The engineers who set up the electrical system in your car did not provide for Halogen headlights.  Thank goodness your circuit breaker is working well, it has prevented a serious fire.

If you want to put halogen headlights in your car, you need to buy a headlight relay, and mount it somewhere under the hood.  To connect it, run a very heavy wire through a fusible link to the + terminal of your battery, and connect that to the power input to the relay.  Run the factory headlight wires to the relay trigger terminals - one for high beams and one for low beams.  When you turn on your lights, you will hear a loud clunk under the hood, but now only the relay trigger current will flow through the factory wiring (thus preventing the circuit breaker from sensing a monster load), and the humungous load current will flow directly from your battery, through the fusible link (your protection against shorts in the newly added system), to through the relay, and out to the super-bright bulbs.

Just to add one caution, be sure to aim those super lights low, to avoid blinding folks coming the other way!

You may have to upgrade your charging system to keep up with the load if you are going to be driving at night very much.  Modern cars with these lights have 120 AMP alternators, usually.   I think your car has something in the range of a 40 amp generator.

The other thing you could do is just be happy with the lights Mr. Ford engineered into the car - it's your choice.

Dick