Electrical Dim Headlights

Discussion in 'Classic Mustang Specific Tech' started by candphall, Jul 2, 2013.


  1. candphall

    candphall Member

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    I have just installed the headlights on my 69. Both high and low beams work but are extremely dim. I am thinking it is a grounding issue but I have not investigated it further. My question is this is is there a separate grounding wire for the lights, I think i may have read that their is one that grounds in the area of the radiator support, or is it included in headlight harness from the firewall. I have grounded the engine to the firewall and the new under dash harness is grounded, all harness are new. I have not replaced the headlight switch or the dimmer. Any help would be appreciated
  2. horse sence

    horse sence Please don't call me Mom SN Certified Technician

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    its been a while since i looked at the wireing on my 69 ,but i think the ground was in the harness and the main ground wire bolted at the voltage regulator .
    try going to halogen bulbs
  3. j69302

    j69302 Active Member

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    Have you measured the voltage at the lights? This will tell you if its lack of power or just dim bulbs.
  4. 66Coupe289POWER

    66Coupe289POWER Member

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    Try measuring the voltage drop on the ground circuit. Since high and low are dim its either the feed to the HL switch or the ground. Backprobe the ground wire at the light with them on and measure voltage to the negative battery cable. Any more than 0.5v indicates high resistance on that wire and is causing the lights to dim. Do the same for the power side but measure the voltage from the positive batt cable to the HL switch and from the HL switch to dimmer switch. The lights have to be on to measure voltage drop.
  5. candphall

    candphall Member

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    Thanks for the advice.
  6. candphall

    candphall Member

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    Are halogen lights typically dimmer than the original sealed beams when installed in the existing system?
  7. 66Coupe289POWER

    66Coupe289POWER Member

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    It could be the wiring is not capable of carrying the current the halogen lights want. The voltage drop test I described will be able to check that. You want the total drop to be as close to 0v as possible. The factory wiring has the headlights drawing power through about 15 ft of wire. You could install some relays into the circuit to shorten the length of wire the headlights draw from. Use the factory wiring to power the relay's coil and the load side of the relay gets its power directly from the battery.
  8. tx65coupe

    tx65coupe Active Member

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    The halogen lights should be brighter. I'm guessing that its a bad ground or high resistance in the old wiring.

    I suggest installing some relays to run the head lights. I got the setup from RJM Injection.

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