Turn signal always illuminated when headlights on

This is a job for Qavion...


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Is it only headlights or sidelights, too?

 

Understood, but does the RH turn light come on with just the sidelights illuminated. I think I would have a hard time linking the headlight circuits (specifically) to the turn signals.

I can't think of anything obvious at the moment other than earthing issues. On the F355, I know where the earths are (in impossible to reach places) above the radiators. Looking at the 348 parts diagram, I see an earth in that general area.

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I have a picture of the LEFT hand earth on the F355 here

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Basically, it's under the rear buttress. There is one on each side.

Apart from that, I can only think of rear light assembly internal wiring issues.

 

You would think so, but I've seen it on my own car. i.e. unrelated lights illuminating or flashing due to a bad earth. Electricity tries to find a path back to the battery via a good earth and if it doesn't get one, it goes through the closest thing it can find to an earth (e.g. a bulb element).

Unfortunately, I don't have a wiring diagram which goes into that kind of detail.

Quite possibly. You would have to compare it with another 348. Unfortunately, I don't have a 348 to play on.

Have you tried shaking/flexing the wiring harnesses in the engine bay to see if the light goes out?

 

Most auto electricians in this situation would probably start with a visual inspection of any wiring that can be easily seen, looking for corrosion or fatigue points, then by pulling the front and rear lamp units containing the turn signal to check for continuity between the plug and each individual bulb sockets (switched & earth conductors) then checking for shorts across pins on the lamp units. Do a good visual inspection for corrosion, broken or loose conductors etc. as you go.

If the lamp units check out, front & rear, then it’s time to check the pins in the harness for each lamp unit. Make sure you plug the other lamp unit back in, eg. when you’re testing the rear harness plug, reconnect the front lamp unit and vice versa.

If you see a short in the wiring between the turn signal and position lights then pull the hazard & turn signal relays & check again, if the short has gone with the relay out then it might be the relay or wiring backwards from the relay. While you have the relay in your hand, check its function.

If you still haven’t found the problem then it could be in the column stalks. IMO, the best way to test it thoroughly is on the bench. If the column stalks are fine, then you are left with a short in the wiring.

If by now you haven’t found the fault then this is where the “fun” starts. You will need to isolate the issue as best you can before digging into the loom.

Hopefully, you find that it is either a problem in one of the lamp units (eg. corroded earth) or a simple pinched wire in a readily accessible area. All the best with it.

 

Yes. Not unusual. Problem is finding where the bad ground is. It's never where you think it might be.

Example, a bad ground in my pickup's tail light causes the instrument panel lights to come on when ever I stepped on the brakes.

 

Would this pic help?

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(Left side shown)

How hard is it to trace the wiring from the connector pins to the lamp holders?

How many pins are there on the connector? Is it a 9 pin plug or 10 pin?

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I hope the front bulbs fix it, if they do, celebrate.

otherwise, you should follow a systematic diagnosis approach like the best pros do. Internet guesses and theoretical postulating is unlikely to fix the issue (unless you get lucky).

 

I don't see how only one light would be affected if the stalk was faulty.

If the rear ground wires for the lights are the same as the F355's, they will be under the rear buttresses, not under the car.

F355's shown here (left hand one) in the forward top corner of the engine bay.

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I'd be interested to know if the 348's are in the same location.

 

Ah... ok... Seems we are running out of options.

 

Can you check what type of relay you have at the position where the "K" relay, seen on the picture below, is located.

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The relay at the K location should have diagram as on these examples:

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This relay is different from all other relays, it has pins 87 and 87b.

 

If you pull the bulb for the RH rear turn light, can you check to see if there is 12 volts on the lamp holder (centre contact) with the sidelights turned on? Don't use the lampholder as your earth. Use a separate chassis earth.

I'm just trying to make sure that there is not an issue on the power side of the turn light. I don't think there is, otherwise the turn arrow on the dash and the front right hand turn light would illuminate.

 

From this, I would definitely say the fault is on the power side of the wiring, rather than on the earth side. But... if there was, say, a short between the sidelight power wiring and the brake light power wiring, why don't the red sidelights illuminate (when switched off) when the turn signals are used.

 

What year/version 348? (Your Profile seems to have never been updated, and only shows a 308QV.)

Is this a problem that wasn't there and suddenly showed up, or something that has always been there during your ownership? 348 have an unusual hazard light/turn signal architecture, and there is a relay that only runs the RH rear turn signal light in the hazard function so, if that's wacky (or someone substituted the wrong thing), it could turn "on" the RH rear turn signal light yet not turn "on" the RH turn signal lamp in the dash nor the RH front turn signal lamp. Not yet saying that that's it, but just unplugging some of these additional hazard function relays might give a clue of where the trouble lies IMO.

 

Need to correct myself (and the editing time expired!). Not so sure that anything (stock) could just turn "on" only the RH rear turn signal so could be molestation, but I can't even find where these hazard function relays (shown on the 348 WSM schematics) are located .