Headlight Wiring Question

[Paranyos]

So I went to use the headlights on my 27 touring for the first time yesterday evening, Nothing! When I disconnect the headlight wire from the bucket, one post reads 6 volts and the other has 2 volts (when on high beam). On low beam, one post reads 6 volts and the other is 5 volts. I am assuming one should read 6 volts and the other 0. I hate electrical issues, they always confuse me. What do you guys think my problem is? I am ordering new Led bulbs and I don’t want to just plug them in as is for fear of blowing 2 new bulbs. Thanks in advance!!

[Steve Jelf]

My first suspect would be frayed wiring making contact where it shouldn't. It could be something else, of course, but I would start with a thorough inspection of the wiring. That includes making sure all connecctions are clean and tight.

[old_charley]

If you're checking the voltage at the contacts on the socket plug, try taking the plug off and take voltage readings on the bare wires. If it reads 6 volts and 0 volts I'd say you have a short in the plug. Easy to replace. If you still have the same readings try disconnecting the wires at the terminal block on the firewall and take readings on the terminals. If you still have voltage on both the high and low beam terminals then it sounds like a short in the headlight switch or in the harness from the switch to the terminal block. If the readings at the terminal block are O.K. then it would seem to me that current is leaking between the high beam wire and low beam wire somewhere in the wiring harness between the terminal block and the headlights. And, as Steve suggests, make sure all of you're electrical contacts are free of corrosion. Good luck.

[Novice]

Paul. Bryan and Steve are on the money with what to check. I would add that its helpful to first check out the problem with a test light such as a small 6/12 volt bulb with pig tail leads. Ground one lead and test the wire circuit with the other lead. This places a load on the circuit that a voltmeter does not do which can give a false reading with a resistive connection not making solid contact.and don't forget to check the ground to each headlight can.
Basic trouble shooting start at the voltage source and work through switches,wiring "Fuses" to the load / headlight. If the test light is normal brightness circuit is OK. If it is dim there could be a problem with a bad connection. When You fix the problem! The test light will be normal and the voltage will read correctly. and Your headlights should work too.
Good Luck

[Paranyos]

Thanks for the suggestions! Looks like I will be spending some time tomorrow trouble shooting. I never go out at night, but who knows I might get caught out later than expected. It would be nice to have lights!!!!

[Paranyos]

Am I correct in assuming that when one wire is 6 volts the other should be 0 and when I switch to the other beam they should reverse? Just making sure I understand this correctly.

[Novice]

Paul. It depends. If a light bulb has one contact the wire connecting to it would be Positive 6/12 volt the other wire would be ground to the base socket of the bulb and test 0/no volts. In the case of Headlight bulbs and brake/Stop light bulbs with two contacts.You in effect have two lights in the same glass bulb Hi/low with a common ground to the metal bulb base. one is high beam the other low beam. One high for Brake the other low for tail light. all should have 6/12 volts when turned on. High/low. Brake/Tail. the ground wire is 0 volts. Hopefully Not to confuse the issue but some LED replacement bulbs with twin tab 1157/1158 case. have one tab positive and the other negative/ground and the metal base is no connection You can use that type of bulb on the model T but You haft to do a wiring change at the firewall terminal block which can get involved I don't recommend it.
Not trying to start WWIII but if Your car does not have a main 25 amp fuse going to the main power connection at the floor starter switch You may want to consider it. I have encountered sparks coming out of the ignition switch and shorted headlight contacts. So I installed a inline fuse holder at the firewall terminal block in both of My Model Ts for ease of checking/changing and when working on the car You can turn off the power to the cars electrical system by just removing the fuse. In a perfect world with a new model T or all new electrical might not be necessary. and Yes there is a 1 in a million chance the 30" unfused wire going to the starter could short. My kind of odds. I Also added a Battery disconnect for when the car is at a show where people want to sit in Your car and can easily step on the floor starter switch.as well for winter storage.
Good Luck

[D Stroud]

I am not very electrically inclined, but I do know that a bad ground can cause all kinds of weird things to happen. Lots of juice going where it shouldn't and vice versa. Just something to consider. Dave

[Paranyos]

Ok, I guess I have it straight. When the high beams are on, that wire should be 6 volt and the other post should be dead. When the low beams are turned on, the order is reversed. How is the bulb socket grounded? If I remember correctly, I only have one cable going to the back of each headlight can with the two positive wires in it.

[cdromtom]

The ground path is from the socket to the headlight bucket to the fender to the frame to the battery. Any dirty or loose connection with give you issues. Make sure there is no paint where these are connected. You should be able to measure from the socket to each portion of the ground path to see any issues.

[Paranyos]

Great, thanks!

[Paranyos]

Thanks to all who replied! I have received a lot of help from this forum over the past year since I bought my 27 touring. You guys were right on, it was a grounding problem. I learned something very important to pass on to anyone new to this hobby. I had my light bar and headlights off a month or so ago. I figured I would paint it up real nice! What I didn’t know was that the bare spots underneath as I removed it were not there due to shoddy workmanship, the bare spots are needed for the grounding of the headlights. After removing some paint between the light buckets and mounting bar and fenders, all is well. Another lesson of many learned the hard way. Thanks again!! Now to learn how to remove my rear axel and drive shaft to look at pinion bearing and sleeve. I have a bad vibration to find.