Low Beams Inoperable

bbayne35 · Jan 19, 2010

The low beams (both sides) on my 2000 GT stopped working. Parking lights and high beams both work. The bulbs appear to be fine. I am diagnosing per the service manual. Both low beam fuses in the Central Junction Box (in the car) are fine. I have ~12V at these fuse locations. I checked for voltage at the bulb connectors, and I have no voltage. According to the wiring diagram, the low beam wires go from the Central Junction Box through the C105 connector to the bulb connector. My assumption is that the problem is at the C105 connector, but I really do not know what the problem could be. This connector is visible and difficult to access from the inside of the car. It is not visible on the other side of the firewall in the engine bay.

Any suggestions? Do I try to inspect that C105 connector? Can the light bulb connectors fail? Can there be a grounding issue? Anything else I can check?

I have searched the forum and have not found any solutions.

Thanks.

wmburns · Jan 19, 2010

Trouble shooting low-beams

Based upon the tests already done, going to assume the headlight switch and multi-function switch is OK. Also going to assume that both bulbs have been tested OK.

Check fuse F2.4 (10a) and F2.10 (10a) in the Central Junction Box (CJB). Remove and test with an Ohm meter. Post. If the fuse is bad, STOP. Replace.

With the head lights on and fuse F2.4/F2.10 removed, test for +12v at the fuse holder (make note of which side gives voltage) and a known good ground. Post the results. Leave the fuses out. If no voltage, STOP. The problem is upstream.

Go to the head light socket and disconnect the socket (either side). Measure the resistance (Ohm) between the headlight connector wiring harness pin 2 (BK) and battery negative. The value should be low. Post the results. If high, STOP. The grounds are bad.

This next test is going to require a long piece of scrap wire. With the key off and headlights off, we are going to measure the resistance of the low beam wiring harness back to the CJB.

Using a long piece of wire to extend one of the test leads, measure the resistance between the headlight connector pin 3 and the open fuse circuit. On the CJB, use the terminal with no voltage (opposite side where +12 was measured before). The challenge is to get a good connection at the fuse holder and the headlight connector. This should be a low Ohm value. What we are testing is the body wiring harness from the headlight connector back to the CJB. Post the result.

Again, be sure to get a good connection as a bad test is going to start a wild goose chase.

Right side HD-connector low-beam pin 3 color=DB/LG Fuse=F2.4
Left side HD-connector low-beam pin 3 color=WH/LG Fuse=F2.10

IF the harness is bad, the first place to look is connector C105 (location 151-2 A7).

Has any work been done on the car? Accident? Water damage? After market headlight install? Note, some after market headlights use considerably more power than stock. This opens up the possibility of an overheated/burned wiring connector.

The attached wiring diagram is from a 2002. The connector numbers are different but it does show the circuit flow.

Based on the above test, it should be possible to exactly nail where the problem lies.

Good luck.

bbayne35 · Jan 19, 2010

Thanks. I will do the resistance checks when I get home from work and post my findings.

I replaced the headlight housings a few years back with OEM equivalents. At that time, I installed Silverstar bulbs. The passenger housing will occassionally accumulate moisture. I have not had any recent work done to the car.

bbayne35 · Jan 19, 2010

The problem turned out to be two faulty bulbs. I was unable to see the broken filaments in my driveway at 10:00 last night. I checked the resistance over the low beam filaments, and both were bad.

I also realized why I was getting a 0V reading at the bulb socket low beam pin last night...I had left the high beam switch on.

It seems strange that both low beams would go out simultaneously. Hopefully, there is not a short in the harness somewhere and new bulbs are all that is required.

I do appreciate the quick and very detailed response!

Kilgore Trout · Jan 19, 2010

Get yourself a set of Sylvania Silverstars, $20 each at AutoZone.

Ateup · Jan 20, 2010

Can we get wmburns on payroll here so we don't lose him before I have any problems with my car? Thanks for all your advice burns.

Low Beams Inoperable

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