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Headlight Voltage Drop

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A

aroostok

Jedi Hopeful
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Had some recent problems with the B following being out in a real heavy rain storm (even though top was up I was still out in it, if you know what I mean). Problem was with the lights. Had turn signals that weren't working. Also had a blown 30 amp fuse in box.
To make a long story short, got the flasher and turn signal problems taken care of puting in new flashers and making sure wiring was okay. But I have a real problem with the headlights now, that I did not have previously.
I have upgraded lights (h4s) that are connected by relays so as not to put all the strain on the switch. I replaced the relays but I've got a real strange one taking place now.
Power into the relays (brown wire - 10 gauge) is 14v measured both prior to the fuses in line and also at terminal #30. Ground is wired to terminal #85. Low Beam (blue/red) is terminal #86 as is the high beam (blue) on its own separate relay. Power to the lights then is transmitted through terminal #87. This all checks out with 14 volts to both #86 and #87 when the appropriate dimmer position is switched. But here's the kicker. Lights still do not work. And here's the real strange thing: voltage at the light socket into which the light would be inserted is only reading 8.5 to 9 volts. Wire from relays to headlights is 14 gauge. Any ideas why/
 
Make sure all the grounds are secure. Most electrical problems with lighting are bad grounds. PJ
 
I had a similar issue w/ my one very dim headlight. Check the harness that feeds the headlights. I cleaned the bullet connections, added some dielectric grease and viola, both lights worked.
 
You have brought power to the relays with 10 gauge wire, but you don't say what gauge you run from the relays to the light sockets. The oem wiring uses 14 strand wire (British designation), which is somewhere between 16 and 18 gauge in American wire measurement. If you are using the original wires to the lights, you are essentially negating the benefits of the 10 gauge line to the relays. Neither do you mention what gauge ground wire you are using. Again, the oem wiring has 14 strand wire, and while it has a shorter run, in order to take advantage of the benefits of the larger power wire (the ground wire carries as much current as the power wires do). As Paul suggests, check the ground circuit to insure that it is making good contact between the light sockets and the actual ground connection. Make sure that the ground connection to the body of the car is bright shiny metal under the ground wire lug and all surfaces are coated with dielectric grease. For you to be drooping 5 - 5.5 volts without any current draw from th elights, there has to be an extremely bad connection in the power - relay - light - ground path. Even with the lights connected, that much of a voltage drop is very excessive.
Cheers,
 
One thing not clear to me, is the 14V measure at the input and output of the relay with the wire connected to the relay and the headlights in circuit?
With no load, you will read 14V.
However, if you have 14V in and out of the relay on both high and low, and low voltage at the headlight, the wire in between is bad, an in-line connector has failed, or the connectors on the end are bad (relay and or headlight).


I have a letter in my files from NASA assuring me this is not rocket science.

We'll figure it out!
 
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