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Automotive relays and internal resistance?
Automotive relays and internal resistance?
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Old 9th February 2007, 01:00 PM   #1
pjpoes is offline pjpoes  United States
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Default Automotive relays and internal resistance?

Hey guys, I know this probably isn't the best forum for this, but I have an relay harness for my headlights I made myself in order to supply my driving lights and headlights with more current. I used all Bosch relays as I understood them to be among the best, all rated at 30 amps of current. The headlights are DOT legal hids, before anyone gets mad at me-it was an option on the car-I just changed the harness, and off road only HID based driving lights. One of the lights started acting up so I changed the relay, and suddenly the color of the HID changed on that light. It became more yellowish and actually looked brighter to some, less to others. I checked the voltage and current with the lights on and it was within 10% on each side. I would also imagine hids use voltage regulators before they up the voltage, but maybe not.

So does anyone know anything about the internal workings of relays enough to one why changing the relay would cause a color change. I changed the relay with another one and the color went back to normal, so its definatly something with that relay. I haven't checked internal resistance yet, I have to rig up a test system first to see, but I'm wondering if maybe the Bosch relay I had in it has either lower or higher internal resistance, reducing current or voltage enough to change the color. My method of testing voltage wasn't under load, and my method of testing current was the clamp on style, so I have no idea how accurate it was. I calculated that I should be getting about 2.5-2.7 amp draw from each hid, and I wasn't seeing that, but maybe I'm mistaken that its a constant 35 watt draw. Anyway, I would appreciate some thoughts.
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Old 9th February 2007, 01:43 PM   #2
tkifowit is offline tkifowit  United States
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Join Date: Jun 2004
Location: Atlanta, GA
The control side (85 and 86 on the bottom of the relay) should be 90 to 110 ohms. The work side, where the points are (30 and 87 on the bottom of the relay) sould be 0 when the points are closed.
If there is any resistance on the point side when they are closed that will change the current to the bulbs.
Normal watts for these are 37 to 45 per side (manufactures have slightly differant specs).
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Old 9th February 2007, 04:18 PM   #3
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
test your relays under load.

Measure the voltage to chassis at the input terminal and at the output terminal. They should be near enough the same. If the voltage is 10% lower the relay is faulty. If the voltage is 1% lower (-120mV) the relay is suspect.
If the voltage loss across the relay is <=10mV then OK.

The performance of your lights (and all headlights) depends on the voltage delivered to the bulbs and the age of the bulbs.

Compare the voltage at the input to the HID to the voltage available from the alternator. If you have lost more than 500mV (in total including flow and return) then start looking for which cables and which connectors are causing the problem. If you can get the total loss to below 100mV then you are doing really well but this may need separate 17A (2sqmm) cables to each relay and separate 17A cables to each HID unit.
regards Andrew T.
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