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41 Olds taillights only turn on with headlights.

Doctor's Pontiac

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Hope somebody can provide clues to further explore a taillight wiring issues in my 41 Oldsmobile. Car is stock except that prior owner added a NAPA turn signals to the steering column with 6 wires on it. This NAPA unit is f rom the 1980s and has a 3 prong flasher. He rewired car and wire colors are all over the place and useless as reference. 


The problem is that taillights only work when the headlights are on.   


I have an original headlight switch, with 2 clicks when the knob is pulled: Pulling the knob one click should turn on the front parking lights and taillights. Pulling to a second click connects the headlamps while the taillights continue on.  With the modification done in my car, the click # 1 energizes nothing (actually no wire is connected to the corresponding tab on the headlight switch) . The front parking lights have only 1 wire which is used for turn signal) and therefore no parking lights or taillights are active. With 2nd click headlights turn on and tailights do the same.




All  4 turn signals and flasher work fine. Taillights have 2 wires, one for stop light (white) and one for taillights (yellow), for whatever is worth mentioning.


I spent hours testing taillight wires circuit with multimeter and noticed the following:


1. Two wires from taillights merge at the rear of car and extend through the roof of the car to under the dash where connected to the turn signal additional harness that goes to the steering column.

2. From here, wire(s) go to the headlight switch (headlight wires come from front of car directly to the headlight switch).

3. I disconnected the taillight wire coming from back of car where it connects with the turn signal harness and plugged the taillight wire directly to the corresponding tab on the headlight switch (click # 1) and now taillights work with first pull of the knob but when knob pulled again to # 2, the tailights turn off (expected because tab for click# 1 has no continuity with tab for click #2

4. Then I spliced the tail light wire coming from back of car and divided in 2 wires, one going directly to the tab # 1 on the switch and the other to the turn signal as before. When doing this way, with the first click of the knob the tail lights turn on but also the headlights do the same!! Headlights have a totally different larger wire attached to a different tab on the headlight switch. Both, headlight and taillight wires are connected somewhere

6. Foot dimmer works well to alternate high and low beams

5. The schematics of aftermarket turn signals I find online shows no connections at all to the headlights.

6. Stop light works well


Don't think the headlight switch is the problem. I got a 2nd NOS switch installed and have same problem. 


Is the turn signal at fault? What would you do next?


I will welcome any comments how to check this further, thank you






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The turn signal is, or should be totally unrelated to the problem. Let's talk about that first because if wired correctly it shouldn't even touch what you are having trouble with.


There are 2 common types of switch, "4 wire" and "7 wire" as people seem to insist on calling them. They do not always have exactly 4 or 7 wires. The real difference is that a "7 wire" switch is capable of "sharing" the brake lights with the turn signals, while a "4 wire" switch cannot, and requires you to add signal bulbs to the back of the car.


Wiring for "7 wire" signals is as follows.


6V >> flasher input terminal,  flasher load terminal >> signal switch




6v >> brake light switch,  brake light switch  >> signal switch




Four wires, two for the signal bulbs in front, and two for the shared signal/brake bulbs in back. These may be 2-filament bulbs, and probably are in a 41 olds, but we are ONLY talking about the bright filament for now. The dim one is for the tail lights and is not involved.




One wire (the seventh) from the "P" terminal on the flasher to an indicator to flash inside the car. This is used when there is only one signal indicator inside the car for the driver. It is usually located in the signal switch housing or stalk on cars where the signals were added.


When there are separate indicators inside the car for right and left, they are piggybacked on the front signals and the "P" terminal and the seventh wire are not used.


Whew. OK, now let's talk for a moment about what had to change when they installed this, assuming they did it right. Originally, on most cars with no signals, the brake lights would be wired:


6v >> brake light switch,  brake light switch >> one brake light,  that one brake light >> the other brake light.


So, when you add a "7 wire" switch, the brake light wiring changes to this:


6v >> brake light switch,  brake light switch >> signal switch,  signal switch >> one brake light,  (the wire from that one brake light to the other goes away),  signal switch >> other brake light (a new wire).


Also, new wires had to be added for both front signals, because those didn't even exist when there were only parking lights in front.


If they tried to combine these with the parking lights in front, dual filament bulbs and sockets needed to be added to the parking light housings


Or they could have mounted whole separate signal light housings in front.


Or, they could have eliminated the parking lights altogether, put bigger single-filament bulbs in the sockets, disconnected the parking light wiring, and connected the new signal wiring to the parking light sockets.


This is what you should see with your signal wiring. You may notice I barely even mentioned tail lights. That is because they should not be involved with this at all, and if they are, something is miswired.


You probably do have dual filament bulbs in the back in a 41 Olds, don't you? If so, the tail lights use the small filament in the bulb. The shared signal/brake lights use the big filament.


This is getting long, so assuming this much is all wired correctly, I'll tackle the tail lights in the next post.


Edited by Bloo (see edit history)
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In the wiring, you should see only the changes I noted to accommodate the 7 wire switch.


Now, about the tail lights, and the license plate light and so on. They had to have their own terminal on the light switch, so that they could be turned on separately from the headlights.


Here is the electrical section of the 40 Olds manual on oldcarmanualproject.com:




There is a graphic of the switch (p. 15 of PDF) that is labeled in what seems to be an unhelpful way, but on p. 17 there is a wiring diagram, and it looks like if you blow it up you can probably see what physical positions the wires connect to in the switch. at least if a 1940 is similar. From your pictures, it looks like it might be the same switch.


It looks to me like on the wiring diagram the 3 terminals along the side of the switch, starting at the dash end are 4, 3, 2, and that 4 powers the dimmer switch (headlights), 3 powers the tail lights, license light, etc., and 2 powers the park lights. Blow the diagram up and have a look.




Edited by Bloo (see edit history)
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In the headlamp switch picture where are the incoming power wires connected?

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It would appear that the 3 terminals along the very back are all terminal #1, and that is the voltage feed from the ammeter. It would also appear they used those screws as a junction block to feed everything else that is hot. See P.17 of the PDF.


EDIT:  Frank, in re-reading this I see I completely misunderstood your post. I plead lack of sleep. Carry on.


Edited by Bloo (see edit history)
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