The Stovebolt Site Forums The Shop Area The Electrical Bay Rear Turn Signal/Tail Light Crossed Wires

I've got a head scratcher on my 1959 Chevy 1-ton pickup that I hope someone can help me with. When I turn on the left turn signal, I have unwanted continuity between the turn signal wire and tail light circuits. Left turn indicator on the dash flashes, left front directional flashes, but in the rear the park light portion of bulbs pulse on both sides. When I check voltage on the left turn signal wire, I get a good reading that's similar to right side. Right directionals work fine front and rear.

There's a three pin connector that appears to be original by the master cylinder under the cab, and I've redone all of the wiring from that connector back to the lights but that didn't solve the issue. With connector unplugged, multi-meter still buzzes when checking between light colored turn signal line and dark tail light line coming from cab.

The shop manual wiring diagram doesn't show any turn signals, so I'm wondering if anyone has a wiring diagram that shows directional lights?

I've stripped the harness covering up to the fire wall and nothing changed when spreading the wires apart so wires weren't touching etc. Guessing something is wrong under the dash, although everything looks pretty much original and not butchered up by a PO. Anyone have any suggestions on likely spots where the tail light and turn signal wires could be crossed or shorted? Thanks for any help, and let me know if I need to clarify any of the details.

Check the bulbs for shorted filaments. I had that issue on my F250 a few years ago, and the turn signal refused to flash. The right bulb had a filament that was broken and shorting to the taillight filament.

Thanks for the reply. I swapped the rear bulbs left and right and that didn't make a difference. I can check the front park/turn lights to see if something is wrong with them, but they seem to work fine.

Kerwin, need some answers first. Then we will get it fixed. Unless you already have it fixed.
1. Is your turn signal a clamp-on aftermarket switch (part no.? and brand?) or a GM accessory switch which is part of the column "bell"?
2. Does the turn signal switch have 6 wires going into it?
3. Describe what happens when switch lever is push down for left turn. Describe what happens when left lifted up for right turn. What do front and rear light do or not do?
4. Do brake lights work properly? Do tail lights come on with headlights?
5. Were all lights and turn signals and brake lights working and this just happened? If not, explain history of this issue.
Please don't guess on any of this or use previous observations, please test again. Describe carefully.

The directional switch wiring is very simple.
Get an ice pick style voltage tester. Forget the multimeter. After questions answered, we will see if it's all wired correctly using the ice pick tester. Then begin to identify the problem. One step at a time. So far sounds like a bad bulb or bad ground somewhere. Not a wiring problem. Not sure yet. Wire colors is not a good way to describe wires at this point. The to-from routing explanation is better.

Answers to your questions:
1. Is your turn signal a clamp-on aftermarket switch (part no.? and brand?) or a GM accessory switch which is part of the column "bell"?
GM accessory switch part of steering column.

2. Does the turn signal switch have 6 wires going into it?
Yes

3. Describe what happens when switch lever is push down for left turn. Describe what happens when left lifted up for right turn. What do front and rear light do or not do?
With all wires connected, when switch lever is pushed down for left turn the left front turn signal flashes normally. Left rear turn signal barely flashes and uses filament in bulb that tail light would use. Bulb is 1157. Verified that wiring to light is the same as right side that works OK.

When lever is lifted up for right turn, all lights work normally. Right rear light uses filament to indicate turn signal, tail light uses other filament. Right front directional works OK. No change if headlights/park lights are turned on.

4. Do brake lights work properly? Do tail lights come on with headlights?
Brake lights work. Tail lights come on with headlights and work OK until left turn signal is used (as explained above).

5. Were all lights and turn signals and brake lights working and this just happened? If not, explain history of this issue.
Truck has been in storage for several years. I had previously driven it but can't remember any issues with the lights. However, it wasn't a daily driver and used primarily during daylight hours at the time so might not have noticed any issues. As I said, can't recall.

Please don't guess on any of this or use previous observations, please test again. Describe carefully.

My money is on a bad ground on the left tail light. The circuit can’t complete thru the left side ground so it’s back feeding thru the running lights getting it’s ground from the right side running light. Since both running lights are connected together, both tail lights are flashing.

There you go bartamos. That’s gotta be one of the best most complete responses you have received here in a long time. Thank you Kerwin.

I’m gonna throw this out there, just because, GROUND!

Take left rear bulb out of socket. You will see two spring loaded contacts. Use an Ice pick tester. Clamp tester alligator clip to a good bare metal ground place near by. It's VERY important you are sure that this clip has good ground. Turn on ignition switch, push lever on column down. Go back and touch contacts down in socket, one at a time. See if one of them blinks the ice pick light. If one does blink nice and steady, move ground clip to touch just the socket while touching same contact with pick. Be careful not to touch clip to pick.Report.

You have provided good info for us to determine the probable cause and now you can pin point and confirm.

The fact that the brake lights work, which use the same filament as turn, and tail lights work, seems to show it's a weak blink input problem, not a ground problem. These test may help determine that. You switched bulbs, so not that.

Other causes for your issue: (blink input/output)
1. Dirty/corroded/broken socket contacts somewhere, including turn signal switch
2. Loose socket input wires or partially broken wire inside insulation.
3. That 3 wire connector, you mentioned has bad/corroded contacts.

If the ice pick test at socket shows the same issue, next test:
Unplug the 3 pin connector that has three wires going to rear. Do the same test procedure on the three contacts in the live half of connector. Look for good strong steady left rear blink signal on one of the pins. Report. If this test has the same issue, we will have more tests/recommendations. No sweat.

Just to be sure, not being picky, but your comment "Verified that wiring to light is the same as right side that works OK." Verified by what method? Wires are not the "same", they are different wires with different colors. from different connection points along the way and different switch feeds. This has nothing to do with you, it has to do with troubleshooting from our house....BUT... We mostly have to ignore statements like that or we will miss something. Let's stick to testing a little at a time. Concentrate on doing good thorough tests and we will catch the gremlin. Did you get the ice pick tester yet? Get a voltage tester, not a circuit/continuity tester. CLICK

Bartamos, thanks for the info and things to check. I'll start with the first check that you suggested tomorrow. Right now I have the three wires to the rear disconnected at the three pin connector, which basically broke apart when trying to disconnect. I'll have to put things back together with a new connector if you want me to start at the rear lights and work forward. Otherwise, I can start at the wires ahead of the three pin connector. You're the person giving suggestions so your call.

Also, to clarify my statement about verifying the wiring to the rear lights. What I meant was that I verified that the contacts in each socket, left and right, were similarly wired. There are red and brown leads coming from the each of the lights that are connected to the vehicle harness. I was attempting to convey that the wire from vehicle harness for left turn signal was going to the red wire from the left light and the wire from vehicle harness for right turn signal was going to the red wire from the right light. Additionally, the red wires for each light, left and right, went to the similarly placed contact within each light.

Excuse my ignorance, but what's the difference between using the voltage reading function on a multi-meter compared to an ice-pick tester? Multi-meter has ice pick leads on it that I've already been using to check the voltage etc.

Ok I understand the connector is all busted. That's OK. So strip the ends and test just the hot side first. If that shows good, twist each pair back together tightly and proceed with socket test. After the problem is found, and if wires are good, you will do a proper splice as referred to earlier. That connector is not needed and adds resistance and corrosion points. Any connectors along the way back there should eventually be eliminated. They were only to facilitate the assembly line.

I understand you did a visual color or wire observation. Some logic to that. No issue.

My take on buying a $10 tool: Many times we need to do a good easy troubleshoot to find out where voltage is or is not. For lighting issues, starting issues, switches, electric motors, gauges, lighter, heater, wiper.......... We know it's 12V. We don't care about amps or ohms, we are not looking for continuity or any AC. Just looking to see if voltage is reaching a certain point.
Multimeters are good tools, amps, ohmage, continuity, voltage drops, spikes. Gives specific values for testing certain components, battery and charging.

If you want to do the test for a blinker, which is an on/off flow, with a multimeter, that won't simulate a bulb flash, go ahead. I will work with that as best I can.

The ice pic is WAY better, faster, easier, more effective and accurate for lighting issues and many other issues. We need to chase the voltage up stream from the failure to find where it starts and stops flowing. As well as testing and confirming the ground. Get the ice pick tester and use it as described. Nothing to read, no mode to select, no range to select, no mistakes, no delicate cumbersome meter to keep from dropping. You will like it. When using it to test for voltage, you get continuity for free. With this issue, you are not looking for "readings" or to "buzz out" a circuit.

Let me know what you find out. Your issue, while fairly common, is difficult to find sometimes. Grounds are very weird. It's not a wire to check, it's the whole truck!!! We don't really know if it's ground yet. Waiting on you

I'll pick up an ice pick voltage tester later today and start the checks. Hopefully will have some results late today or tomorrow. Thanks again for the help!

Phak1 wins the prize for suggesting the cause to my problem.

Started at the connector wires by the master cylinder with my new voltage tester. There is a purple wire, white wire, black wire coming from vehicle harness. With left turn signal turned on, I had a good flash on the voltage tester on the white wire. With right turn signal on, had a good flash on purple wire. Black wire had steady light on tester with headlights on.

Twisted the three wires to the leads going back to the tail lights. Connected white wire to the wire going to left rear light for brake/turn signal. Connected purple wire to my wire going to right rear light for brake/turn signal. Connected black wire to the wires going to each rear light for the tail lights.

With rear lights back in the circuit, I checked the voltage at each contact within the left rear light. Had good flash with left turn signal on at one contact, other contact indicated voltage with headlights on. I then inserted the bulb back in socket and tried the left turn signal, same issue with poor flash and both sides flashing on wrong filament. I then connected an alligator clip jumper wire from left light housing to good ground and the left light WORKED FINE.

Thanks to all, especially bartamos for the help and detailed steps to take for diagnosing the problem. Now, I need some more detail on how the left turn signal and tail light circuits, which really should not be connected, get crossed when the light housing doesn't have a good ground? Is it happening within the light bulb somehow?

I’m glad you got it fixed and happy to help.

My questions:
Just unsure of your status on this:

1. Do you now understand what the problem was?
2. Did you fix it or want to know how to fix it?
3. I don't think a theory or two of why and how it created the "phenomenon" you observed is useful. We have said that ground will try any path available. Thru another filament or whatever.
4. If you have not fixed it permanently, I can help do that after a question or two.

If you have fixed it please advise and we can close this out........

Commentary and theory:
An 1157 is a dual filament bulb. Each filament has a voltage input stem and a ground stem. Both ground stems are connected to the bulb case. The bulb case is supposed to touch the socket and socket touches housing and into the chassis ground path, eventually back to the battery negative post.
It's possible if the left bulb case losses ground that the signal comes up the turn filament stem, thru that filament, into the base, can't find good enough ground for such a high resistance/bright filament. Current keeps going thru the case, backfeeds up the tail light filament ground stem, thru the tail light filament backwards, then down the input stem of that filament, into the tail light wire, goes over to the right tail light input stem, thru that tail light filament to a good ground. This lights only the two tail light filaments because they have way less resistance than the brake/turn filaments. I can't say why the left brake light works on that same filament and finds a good ground except to guess that the flash is a short duration intermittent signal and the brake is continuous.... or the flasher output is weaker than the brake switch output. Both of those go thru the turn signal switch so maybe there is a weak contact in there for the flash. That is the part that throws me. Same filament, same bulb case, same socket, has good ground for brake, no good for turn.

The loss of ground in the rear of a dual filament light is fairly common. lots of different things happen. Some filament backfeeds somewhere and lights up dimly. If you try to figure out what the current may do to find ground, good luck. Everyone else just fixes the ground issue and has a sandwich.

1. Do you now understand what the problem was?
I think that I have an understanding of the problem, the left rear light housing is not receiving a good machine ground, most likely from rust etc. from where the housing connects to bed support.

2. Did you fix it or want to know how to fix it?
Did not have a chance to permanently fix it last night. My thought is to remove light housing and make sure there is bare metal on both housing and bed support for a good ground connection when light is re-installed.

3. I don't think a theory or two of why and how it created the "phenomenon" you observed is useful. We have said that ground will try any path available. Thru another filament or whatever.
OK, but thanks for providing the commentary and theory.

4. If you have not fixed it permanently, I can help do that after a question or two.
Any additional help would be appreciated. You'd mentioned providing some info on how to properly join wires etc. That would be good to get. Normally I use the tube-type butt connectors that you crimp, with heat shrink tubing if the connection may be exposed to the elements.

Sounds like you really have everything under control and know what you are doing. I agree with all your comments.

To be honest, the "proper" way I mentioned to splice is not the way I have done it for 50 years. I just use the same way you do. There are "better" ways like the "western union splice". There are butt splices like the crimp one but you heat melt solder inside them. The crimp with heat shrink over it is just fine for 12V DC lighting. You may get other opinions so you can decide. I should not have mentioned it because I don't do it. Sometimes you can get brainwashed on here. It's important to dress, route and clamp the wires along the way. Also, you can add a ground wire from light housing to frame as redundancy.

You have really made this an enjoyable effort. You followed everyone's suggestions, used your own experience and ability and did the work properly. You found the problem, not us. Good job.