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low beams both work, switch on the high beams and the passenger low beam goes out , drivers side both on, go from high to low and the passenger side low beam comes back on. 71 supreme. Thoughts appreciated.
Check your connector and the contacts on the bulb. If either or both have any sort of build up or corrosion, clean them up. Reason being is that when you hit the high beams, you're drawing additional power and with build up or corrosion on the low beam, it can't keep up with demand when you switch to high beams. I know this because I had the same issue on my '72 Supreme. Some sandpaper will work on the bulb contacts, and IIRC, I used an emery board for the connector.
One more time, if ANY lights work on high and low beams, the switch is fine. There's only one wire each for high and low coming out of the switch. How would a switch problem cause only some of the lights not to work?
To the OP:
Have you verified that the low beams have TWO good filaments? Each low beam bulb has separate filaments for high and low. If one burns out, the other still works.
One more time, if ANY lights work on high and low beams, the switch is fine. There's only one wire each for high and low coming out of the switch. How would a switch problem cause only some of the lights not to work?
To the OP:
Have you verified that the low beams have TWO good filaments? Each low beam bulb has separate filaments for high and low. If one burns out, the other still works.
Joe, are there two filaments in a low beam bulb for a 4 headlight setup?
If the low beams work, then you know the bulbs, wiring, and ground for the low beams is intact and functional. If you switch on the high beams, the high beams come on, then you know the wiring and bulbs related to the high beams is good. If the high beams DONT work (but the high beam indicator in the speedo comes on) then chances are the wiring from the high beams from the headlight switch, dimmer switch, and under dash wiring is good, the problem is most likely under the hood.
If the low beams work, until the high beams are switched on (the high beams come on) and the low beams go out (as the OP states) then I would have to agree the dimmer switch is most likely the problem. For whatever reason, the dimmer switch is removing the power from the low beams when the high beams come on. This would be easy to verify with a test light. With the headlights on, you should have power on the blue wire (headlight power feed from the headlight switch) on the dimmer switch. You should have power at the tan wire on the low beam, and power on the green wire on high. The low beams are suppose to stay on (should have power on all 3 wires on high) when the switch is in the high beam position.
Last edited by matt69olds; May 17, 2021 at 09:55 AM.
If the low beams work, until the high beams are switched on (the high beams come on) abd the low beams go out (as the OP states) then I would have to agree the dimmer switch is most likely the problem. For whatever reason, the dimmer switch is removing the power from the low beams when the high beams come on. This would be easy to verify with a test light. With the headlights on, you should have power on the blue wire (headlight power feed from the headlight switch) on the dimmer switch. You should have power at the tan wire on the low beam, and power on the green wire on high. The low beams are suppose to stay on (should have power on all 3 wires on high) when the switch is in the high beam position.
Sorry, but please look at the wiring diagram and tell me how the dimmer switch is at fault if at least ONE of the four high beam filaments comes on.
Again, if the low beams (only) go out when the dimmer is on high beam, the problem is a burned out filament in the low beam bulb.
I stand corrected. I re-read the original post, I thought he said BOTH LOW beams went out when turning on the high beams.
I guess I’m at a loss as to why a perfectly functional light fails to work when a unrelated bulb comes on. The ground wire for both bulbs are the only common connection. On my 69, both pairs of headlights bulbs share the same screw for ground.
Last edited by matt69olds; May 17, 2021 at 09:58 AM.
I stand corrected. I re-read the original post, I thought he said BOTH LOW beams went out when turning on the high beams.
I guess I’m at a loss as to why a perfectly functional light fails to work when a unrelated bulb comes on. The ground wire for both bulbs are the only common connection. On my 69, both pairs of headlights bulbs share the same screw for ground.
The low beam bulbs have TWO filaments. The high beam filaments are burned out. When you switch to high beams, the low beam filament in the low beam bulb is turned off and the high beam filament in the low beam bulb is turned on - along with the high beam bulbs. If the high beam filament in the low beam bulb is burned out, that bulb will turn off when you put the high beams on. Look at the wiring diagram.
Am I missing something here? The low beams are the OUTER bulbs. The high beams are the INNER bulbs (closest to the grills). On my 69, when you turn on the high beams, the low beams remain on. I understand the low beams have 2 filaments. Is the OP saying that turn on the high beams shuts off the passenger side bulb completely (no high beam, abd the low beam goes out). Power feed from the headlight switch on the blue wire to the dimmer. Low beams on the tan, light green high beam. Headlight wiring copied from the sticky. Low beam headlight on tan, high beam light green. They share the same ground. The only thing I can figure is a defective ground on the passenger side. Low beam passenger bulb is getting ground thru the high beam filament in the driver side. Turn on the high beams, you now have power on both filaments on the passenger side (no ground) and since the drivers side has an intact ground, the high beam works on the drivers side.
If the passenger ground was the issue, I would expect the pass low beam and drivers high beam to be on at the same time (but dim due to them being technically wired in series). Am I missing something?
I stand corrected. I re-read the original post, I thought he said BOTH LOW beams went out when turning on the high beams.
I guess I’m at a loss as to why a perfectly functional light fails to work when a unrelated bulb comes on. The ground wire for both bulbs are the only common connection. On my 69, both pairs of headlights bulbs share the same screw for ground.
Matt, go back and check again..passenger side only is the issue. I still say his contacts are dirty/corroded.
Ok, now my curiosity has peaked. On my 69, I’m almost positive the low beam lamps remain on with the high beam on. If that is not the case, I question my perceived reality since about 1988 (when I bought my car).
I thought all the quad headlamp vehicles were designed this way?? I would assume the low beams remain on to illuminate things closer and around you, the high beams were designed and aimed to provide additional lighting BEYOND the range of low beams.
When I get home from work, before I do anything I’m turning on the headlights in my car just to see.
Ok, now my curiosity has peaked. On my 69, I’m almost positive the low beam lamps remain on with the high beam on. If that is not the case, I question my perceived reality since about 1988 (when I bought my car).
I thought all the quad headlamp vehicles were designed this way?? I would assume the low beams remain on to illuminate things closer and around you, the high beams were designed and aimed to provide additional lighting BEYOND the range of low beams.
When I get home from work, before I do anything I’m turning on the headlights in my car just to see.
Joe, why would the low beam bulb have two filaments? Just trying to understand...
Because the federal laws for headlights have different brightness and aiming requirements for high beam and low beam. The high beam filament in the low beam bulb is 60W, the low beam filament is 37.5W.
I’m not crazy. Here are the low beams. Here are the high beams. Please note both low beam and high beam are on at the same time
You're not crazy, but you are getting beat up by bad nomenclature. There are six filaments in that car. The outside lights have 4 of them (2 highs, 2 lows) and the inner lights just have 2 highs.
When on low, the outside two lights have their lows on. When on high, all 4 lights have their highs on. The low beam filaments are not on when the switch is set to high in the outside lights, although some think that is the case because it is not as easy to tell if the lows are in the pattern as it is with the highs.
And just to use this as a teaching moment, here's the entire headlight circuit. Note that there is no fuse, the switch has a built in, self-resetting circuit breaker. The dimmer switch has three wires, with all high beams fed from one and all low beams fed from another. If ANY headlights work on HIGH and LOW, the dimmer switch is working fine. There is nothing else in this circuit, save for the high beam indicator that is fed by the green wire coming out of the dimmer switch. The red wire splits off inside the headlight switch and powers a totally separate set of contacts that operate the Park/Tail lights. This is a really, really simple circuit.
I'll also point out an example of how a schematic can differ from real life. Note the green wire running to the high beam filaments. In the actual harness, that single wire runs directly from the firewall connector to the connector for the LH low beam lamp. That particular terminal in the connector has three wires crimped into it, the one from the firewall connector, the one to the LH high beam lamp, and the one that runs along the core support to the RH lamps. If you ever have a situation where some or all of the high beams don't work (especially the ones on the RH side), check this crimp. Ask me how I know.
Same goes for the low beam wiring. The tan wire runs from the firewall connector to the LH low beam lamp. That terminal has two wires crimped into it, this one and the one that runs across the core support to the RH low beam lamp. If the RH low beam doesn't work and you know the bulb and ground are good, check this crimp.
And just to use this as a teaching moment, here's the entire headlight circuit. Note that there is no fuse, the switch has a built in, self-resetting circuit breaker. The dimmer switch has three wires, with all high beams fed from one and all low beams fed from another. If ANY headlights work on HIGH and LOW, the dimmer switch is working fine. There is nothing else in this circuit, save for the high beam indicator that is fed by the green wire coming out of the dimmer switch. The red wire splits off inside the headlight switch and powers a totally separate set of contacts that operate the Park/Tail lights. This is a really, really simple circuit.
I'll also point out an example of how a schematic can differ from real life. Note the green wire running to the high beam filaments. In the actual harness, that single wire runs directly from the firewall connector to the connector for the LH low beam lamp. That particular terminal in the connector has three wires crimped into it, the one from the firewall connector, the one to the LH high beam lamp, and the one that runs along the core support to the RH lamps. If you ever have a situation where some or all of the high beams don't work (especially the ones on the RH side), check this crimp. Ask me how I know.
Same goes for the low beam wiring. The tan wire runs from the firewall connector to the LH low beam lamp. That terminal has two wires crimped into it, this one and the one that runs across the core support to the RH low beam lamp. If the RH low beam doesn't work and you know the bulb and ground are good, check this crimp.
I'm not a sparky, but even I can follow that diagram, thanks for posting.
Just change out the passenger side outer lamp. Problem will most likely be solved. Make sure you get one with THREE prongs.
Please don't overthink that job either, the way the diagnosis in this thread has gone.
Remove headlight trim, remove the screws holding the headlight retainer ring to the bucket (NOT the two aiming screws!), remove and unplug the lamp and install the new one. Test operate it before buttoning everything back up.
Spraying a little electrical contact cleaner into the plug terminals before plugging in the lamp is a good idea, as is dabbing some light-bodied grease into them for corrosion protection.
*edit* don't waste your time googling for these headlights, because most of what you get is for LED conversions. Go to the parts store.
Last edited by rocketraider; May 18, 2021 at 08:08 AM.
Side question, can you change the lamps and the adjustment will hold true?
Yes, within tolerances. The bulbs have locating features that maintain their orientation within the buckets, but obviously there are manufacturing tolerances on that. Alignment is determined by the adjusting screws between the buckets and the mounting plate.
Ok. Thanks. I will be upgrading the H/O from T-3s which are failing to halogen equivalents so I am sure both what you said and different bulb designs will need a bit of tweaking.
Ok. Thanks. I will be upgrading the H/O from T-3s which are failing to halogen equivalents so I am sure both what you said and different bulb designs will need a bit of tweaking.
Just change out the passenger side outer lamp. Problem will most likely be solved. Make sure you get one with THREE prongs.
Please don't overthink that job either, the way the diagnosis in this thread has gone.
Remove headlight trim, remove the screws holding the headlight retainer ring to the bucket (NOT the two aiming screws!), remove and unplug the lamp and install the new one. Test operate it before buttoning everything back up.
Spraying a little electrical contact cleaner into the plug terminals before plugging in the lamp is a good idea, as is dabbing some light-bodied grease into them for corrosion protection.
*edit* don't waste your time googling for these headlights, because most of what you get is for LED conversions. Go to the parts store.
I ordered 4 new t-3 from lectric limited. I have no problem changing them out I put the car together from fully gutted. If I'd of only learned in high school of the double low beam filament I would be golden. Thanks for the input.
