Why are my headlights dimming when braking? - Page 4 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 03-12-2013, 03:40 PM   #31
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most people dont have those and its kind of expensive to get if the alt is going to be the same price. what i was saying was take voltage measurements and see what the values are at the point of the different loads to see if there is a actual problem. i never put one of those wires on it but i got my bonnie when it was almost new and it did it then and still does it. the alt is original with 180+k on it. the battery is on its 3rd replacement. nothing changed it they just suck at idle output. there are newer designs you can retrofit.
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Old 03-12-2013, 05:08 PM   #32
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I hope that made sense. Or you can read this one, but its using voltage which isn't a very good test. How to Test a Car Alternator
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Well, I dont really wanna buy a hundred dollar tool for this shitbox, so I did the voltage test.

With the engine off, the battery sat at 12.71 volts.
With the engine running, the battery was at 14.58 - In park, heat off radio on
With engine running in same condition, alternator was at 14.62 volts.

With engine running, heater on high, car in reverse, radio on, alternator dropped to 13.6 volts

I then got in the car to look at idle speed. When it was running, in park, heater off, it showed 900 rpms on the gauge. When I put it in reverse, foot on the brake, heater on high, radio on, it looked to drop to about 750-800 rpms.

So, What does this round of tests say? Anything conclusive?

Is the alternator supposed to see the extra load and bump up the idle?
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Old 03-12-2013, 05:15 PM   #33
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The alternator only sees one thing. Voltage. It is suppose to start charging when it sees the voltage drop down to a pre-determined value.

And no, the PCM is suppose to see the drop in voltage and increase RPM to compensate for alternator load, but only to keep it from stalling the engine. Try idling it in drive with all lights on, rear defrost on and foot on brake while monitoring voltage.
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Old 03-12-2013, 05:22 PM   #34
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The alternator only sees one thing. Voltage. It is suppose to start charging when it sees the voltage drop down to a pre-determined value.

And no, the PCM is suppose to see the drop in voltage and increase RPM to compensate for alternator load, but only to keep it from stalling the engine. Try idling it in drive with all lights on, rear defrost on and foot on brake while monitoring voltage.
Monitoring how exactly? Just let it sit like that and see if it changes? Monitor the voltage at what point? The battery or the alternator?
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Old 03-12-2013, 05:43 PM   #35
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A few other things to take a look at:

* slipping serpentine belt (Kinda a "duh" suggestion, but worth a look)
* oil contamination of alternator (Look around the alternator and see if anything'* oily)
* poor grounds (at tail lamps & headlights especially)
* failing heater blower motor, corroded/melted wiring

You can also check alternator output with an oscilloscope if you know someone who has one. Make sure the display on the 'scope is a straight line, if it'* jittery (especially with peaks/valleys) that will give you a surefire signal that the rectifier (internal to voltage regulator) is failing.

Also, a suggestion: If you plan on keeping the car for awhile, and you *do* replace the alternator, eat the core charge and keep your old alternator. Find a good shop to rebuild your old alternator, and keep it in a box someplace safe as a backup. I try to do that with starters and alternators so you can be back on the road ASAP after future failures. An added benefit is, if you live paycheck-to-paycheck, you don't have to scrape together the money for a new one every time one fails.

Never hurts to cover your arse.
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Old 03-12-2013, 05:52 PM   #36
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A few other things to take a look at:

* slipping serpentine belt (Kinda a "duh" suggestion, but worth a look)
* oil contamination of alternator (Look around the alternator and see if anything'* oily)
* poor grounds (at tail lamps & headlights especially)
* failing heater blower motor, corroded/melted wiring

You can also check alternator output with an oscilloscope if you know someone who has one. Make sure the display on the 'scope is a straight line, if it'* jittery (especially with peaks/valleys) that will give you a surefire signal that the rectifier (internal to voltage regulator) is failing.

Also, a suggestion: If you plan on keeping the car for awhile, and you *do* replace the alternator, eat the core charge and keep your old alternator. Find a good shop to rebuild your old alternator, and keep it in a box someplace safe as a backup. I try to do that with starters and alternators so you can be back on the road ASAP after future failures. An added benefit is, if you live paycheck-to-paycheck, you don't have to scrape together the money for a new one every time one fails.

Never hurts to cover your arse.
Thanks for the input.
Belt tensioner is new, so is the belt. no squealing and no signs of slipage.
Alternator is dry. No signs of oil or liquid contamination
I have already cleaned the grounds in the trunk (taillights) and up front on the fenders (headlights) I think I mentioned that in the begining of this thread. But, it helps to clear that up again. I have also cleaned connections at the battery and the alternator. The only ground I couldn't get to was the ground at the block on the front side behind the starter.
I have not looked at the blower motor. It works flawlessly so I guess I didn't think about looking there.
I dont have access to an oscilloscope unfortunately. But, I did notice that when measuring the voltage at the alternator it was bouncing up and down a little bit, probably within .1 of a volt. Is that an issue?
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Old 03-12-2013, 06:13 PM   #37
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0.1 volt fluctuation? No, that'* far from an issue. 3-4 volts would be an issue.
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Old 03-12-2013, 06:37 PM   #38
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Quote:
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Try idling it in drive with all lights on, rear defrost on and foot on brake while monitoring voltage.
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* failing heater blower motor, corroded/melted wiring
I did the idling thing. Not sure what I was supposed to find? For over a minute I just watched the voltage drop, it started at a good 14.6 and ended at 13.4. There was constently a .2 volt drop from the alt to the battery every time I checked it during that minute or so.

I looked at the wiring at the blower motor. Looked fine to me. I couldn't get to the resitor though, as all the fuses in that fuse holder are in the way. How do you get to that?

Again, as the blower works great at every step, I have a hard time believing it is worth the time to dig into that when it works. Am I wrong?

What next?
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Old 03-13-2013, 07:27 AM   #39
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those voltages seem fine to me. it will start out around 14 and go down to 13.4 to 13.6 as the battery charges. 14.6 seems a little high if anything, depends on how much its that high to determine if its a problem
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Old 03-13-2013, 11:56 AM   #40
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those voltages seem fine to me. it will start out around 14 and go down to 13.4 to 13.6 as the battery charges. 14.6 seems a little high if anything, depends on how much its that high to determine if its a problem
Alright. So if everything seems fine, what'* the deal??

Is there any way to bump up the predetermined idle under load in the PCM?
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