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Old 08-11-2016, 03:58 PM   #1
glennr
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Question Auto Headlights Issue


2002 LW300. When I put my headlight switch in the Auto position, the lights will intermittently turn off for a half second and then back on. In the Park or On position everything works as it should.
What should my first guess be, the switch in the left column stalk or the ambient light sensor?

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Old 08-15-2016, 05:04 PM   #2
derfderf
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Prob issue with dirty contacts on switch.

Pull the low and high beam bulbs. Then work the switch on the stalk into and out of the auto position about 100 times. This should sweep the contacts clean of any corrosion. Wait till nighttime and see what happens
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Old 08-15-2016, 09:57 PM   #3
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Default Thanks.

I might give that idea a try but I'm nut sure it is worth the effort. I can get a new after market switch for under $20. And, according to you tube, it does not require a steering wheel removal on this model. I never had to replace headlight bulbs but usually car makers require you to be a contortionist with tiny hands to do it.
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Old 08-16-2016, 04:36 AM   #4
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5 min vs $20.
Plus it may not even be the problem.
Your call
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Old 08-16-2016, 10:48 PM   #5
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Default I gave wrong symptoms

I tried the pulling bulbs and rotating the switch 101 times but it did not help. However, it looks like I gave the wrong symptoms. What really happens is that day or night, with the switch in the Auto position, the DRLs come on. About once a minute the DRLs will go off and immediately back on with an audible click. The headlights do not come on even in the dark unless the switch is manually turned to ON. In the manual On position there is no flickering. I tried covering the ambient light sensor with my hand but it made no difference.
Any ideas on the new symptoms would be appreciated.
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Old 08-17-2016, 07:01 AM   #6
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First of all, let me say that I have never tried to sort out a situation with "automatic" headlights.

But I will help you all I can.

One of the most critical tests that the system has passed is that when set to manual "on", the lights stay on with no flicker.
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My next question is what are the DRLs (your high beams) doing when the switch is set to manual "on"?

If the high beams (DRLs) stay ON and do their once a minute check, and continue to stay on amidst the flickering....

I THINK(=GUESS) the issue is with the light sensor or the wiring going to the relay it activates. The explanation gets messy; I had to re read it 4 times before posting.......

Rationale: The BCM must send a request for a reading on the V generated by the light detector once a minute. If the voltage output (more light=higher voltage) is above a certain threshold value, the sensor assumes it is daytime and the DRL relay that enables the DRLs (high beams) is commanded closed by the BCM, turning on the DRLs.

With the once per minute check, it seems to always think it is daytime so the DRLs stay on even at night. The once a minute clicking is the likely the DRL relay engaging.

------------------

Whoa Nelly! if the assessment of what the light sensor signal does as discussed above, there should NEVER be a reason for that DRL relay to disengage. It should stay closed and the DRLs should never cut out.

POSSIBLE defective DRL relay

----------------------

However, at night, the light sensor's output voltage should drop below the threshold V, and the DRL Relay should open while the headlight Relay(s) should be commanded closed by the BCM. This is a sequential request, and I'm willing to venture that the BCM logic is written such that the DRLs must go off FIRST (= open DRL Relay) for the headlight relay to be closed, turning on the headlights.

Problem is, that happens only for a few seconds when the DRLs cut out.

When they do cut out, do the normal headlights turn on for that short window of time? This is critical to know.

That DRL click MAY be the DRL relay trying to disengage, opening for a tiny moment, then closing again.

POSSIBLE defective DRL relay

------------------------------

Photocells are strange beasts when you can't figure out why it is wired the way it is. It also makes it difficult to know where to put your multimeter.

I would trace back from the DRL relay to find which photocell lead is carrying the voltage. Then play cover the sensor for long periods while watching the voltage at the relay.

These devices usually have some hysteresis (delay) built in so they do not respond on a real time basis

If the voltage at the relay coming from the photosensor never changes, the photosensor is likely defective. If the voltage does change, but there is no change in the relay position, the DRL relay is likely bad.

Brain drained

Gnite

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Old 08-17-2016, 05:55 PM   #7
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Default Test results

OK, here is what I discovered.
The DRLs go off with no flickering with the switch in the ON position

When DRLs cut out the headlights flash on for less than one second then the DRLs come back on.

I pulled the DRL relay. Of course the DRLs are off but the quick flashing of the headlights persist. So does this mean there is actually something wrong with the headlight circuit? I swapped the DRL relay with the Rear Window Wiper relay. There was no change.

I don't know where the headlight relay is but since there is no flickering when in the ON position, is this even a possibility?

If I cannot find and fix the issue at a reasonable cost (for an old car), I may just leave the relay out and go 100% manual.

Last edited by glennr; 08-17-2016 at 06:14 PM..
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Old 08-17-2016, 11:23 PM   #8
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Default Latest Test Results

I located (almost) the Headlight relay. It is in the LH interior fuse panel. Unfortunately there are two relays that do not appear on the label on the cover panel. So I eliminated the two that were labeled and swapped the other two. No change.
I also noticed that with the switch in the Park Light position, the symptoms are the same with the flickering except of course the Park Lights are on.
In a dim garage, I covered the light sensor with a paint spray can lid and the Headlights never came on in the Auto position, even after 10 minutes.
I can see a broken sensor preventing the lights from coming on or keeping them on but I do not understand how it would cause the brief flicker.
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Old 08-19-2016, 02:05 PM   #9
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Nice. You troubleshot your way out of things to troubleshoot!

1) The fact that everything works as advertised on the manual "normal" setting tells me that the two basic light circuits (lo and hi beams) (NOT THE DRL PORTIONS) are just fine. There is no flicker because the DRL circuit is NOT commanded to do anything by the BCM or the bCM is not looking for info from the light sensor. It sits idle. There are no light level comparisons.

2) The fact that the headlights flicker on as the DRLs flicker off TO ME means the two "interacting" portions of the DRL light wiring systems are intact (NOT INCLUDING THE SENSOR), b/c if the DRLs turn off the reg headlights come on, then go off again which then has the DRLs turn back on, So all of THAT wiring and relay stuff is behaving as advertised. they are never on at the same time and switch correctly in both directions. They are just not operating as they should

3) The fact that w the DRL relay pulled, the headlights still only blip on and then goes out is KEY because it is telling us that the light comparison is still being made (which it SHOULD), and the BCM is commanding the drl relay closed; it's not there and therefore the DRLs are not lit up. However the blip of the headlights continues every time the light level is compared because the BCM is still requesting it or the sensor is send the BCM the information.

Even though the DRL Relay is NOT there, there must obviously be a DRL wiring loop for the lo and hi beams (INDEPENDENT of the circuit used for the Manual setting) such that a command from the BCM to turn off the DRLs still affects the low beam side of that DRL circuit to blip on before turning off again. Maybe 2 separate outputs from BCM. Dunno.

What I am QuITE confident about is that the paint can cover test, in the context of ALL of your troubleshooting, indicates that either the light sensor is bad or there is a broken wiring connection from the light sensor to wherever it feeds its output voltage (I'm thinking the BCM at this point).
--------------------------
The flicker is likely the result (an artifact) of some programming that says if the light comparison is screwed up (bad sensor), let it make the comparison, determine that, and then failsafe back to the DRLs -- maybe because it was a law for a while that cars MUST have running lights or headlights on. This failsafe would leave you with DRLs in auto setting and fully operational lights in Manual setting until you got it fixed.

Check wiring from light sensor to where it is going. I suspect you'll need a new sensor --but you will need to know where to connect it anyway so check the wire first.

I appreciate your attention to detail in troubleshooting. It's refreshing to work with someone who figures out the next four steps and does them before logging back in....
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Old 08-19-2016, 03:35 PM   #10
glennr
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Thank you for your help. I will try to check the sensor wiring and see if I can find a cheap replacement sensor. If it turns out to be the BCM, I will try to convince the wife to get rid of the car. It is coming up on 100k and I figure that with the timing belt, water pump and some fluid flushes, I will be looking at $1,000 repair bill which is bad. If I have to add a BCM and programming it will be prohibitive.
Thanks again.
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