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Voltage regulator causing dim headlights and slow windows

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
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1997 - 2000 V70, V70 AWD
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FireFox31
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Voltage regulator causing dim headlights and slow windows

Post by FireFox31 » Tue Nov 19, 2019 9:21 pm

Hello, my 2000 V70 headlights are very dim and my power windows are slow to roll up and down. My headlight reflectors look ok, so I suspect there's a power problem robbing them of electricity. The battery is fairly new and fully charged.

How can I check the voltage regulator for proper output? What is the correct output range? How can I check the headlight voltage and what reading should I get? What else might cause decreased electrical output and what could I check to diagnose it?

This problem has been lingering for the past two years. I haven't driven the car much during that time. It seems to drive ok with proper power, gas mileage, etc. Thanks for your help.


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Re: Voltage regulator causing dim headlights and slow windows

Post by hausmeister » Wed Nov 20, 2019 2:48 am

Best install a voltage gauge. Or just hook up a multimeter.
You need to check the battery connections, the alternator connections etc. You can lose quite a bit of voltage on the crips on the battery terminals if they aren't that good any more.
The Alternator should put out 14Volt, in idle and during driving. Maybe slightly less.

I had to solder the battery terminals and replace the regulator on my 850 to get it back to ~14v.


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Re: Voltage regulator causing dim headlights and slow windows

Post by abscate » Wed Nov 20, 2019 3:27 am

Check the thick red wires feeding the fuse box for corrosion under the crimp, as well as the ground.

With a good battery, the windows should work well with the engine off, so the regulator isn’t the issue here


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Re: Voltage regulator causing dim headlights and slow windows

Post by Herb Goltz » Wed Nov 20, 2019 8:51 am

The battery + cable and the B+ cable abscate referred to are both good candidates for your symptoms, and both are common failure points in the electrical system particularly on the '99-00 cars. When my battery cable died it got hot to the touch. You can measure your alternator output easily by measuring at the big cable on the back of the alternator. You should see 13.7-14.1 VDC when running. Measuring voltage drops is easy, and a good way to diagnose which cables/connections are bad. Make sure your battery terminals are clean and tight.



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Re: Voltage regulator causing dim headlights and slow windows

Post by FireFox31 » Wed Nov 20, 2019 12:27 pm

To test these things, do I put the negative multimeter probe on the battery negative post? That relies on grounding cables, not revealing if the + or - wire is the issue. Sounds like I should start testing at the battery and work my way farther from it.


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Re: Voltage regulator causing dim headlights and slow windows

Post by Rattnalle » Wed Nov 20, 2019 12:32 pm

Start at the battery and measure wherever you can reach. Check the voltage level and if it's consistent or not. Also both while running and when not. The very first measurement you'll want to do is on the battery when it's been charged then rested a couple of hours, overnight or so.



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Re: Voltage regulator causing dim headlights and slow windows

Post by abscate » Wed Nov 20, 2019 12:41 pm

FireFox31 wrote:
Wed Nov 20, 2019 12:27 pm
To test these things, do I put the negative multimeter probe on the battery negative post? That relies on grounding cables, not revealing if the + or - wire is the issue. Sounds like I should start testing at the battery and work my way farther from it.
That’s right. Negative probe direct to the battery post


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Re: Voltage regulator causing dim headlights and slow windows

Post by FireFox31 » Sun Nov 24, 2019 6:23 pm

I think my alternator may be at fault. After getting into an accident, my battery was too weak to start the car. After the accident, the glove compartment was open with its light on for two hours and the four way flashers were on for over an hour. When I removed the battery to charge it, the charger read about 75% and took about three hours to charge. I had just driven the car 700 miles in four days, so I feel the battery should have been fully charged. I can't imagine some glove box and flasher lights could have drained it that much.

Once I get the body work fixed, I'll revisit this alternator testing. Until then, let's hope I can afford to have the body work repaired (if insurance doesn't pay, which I don't think it will).


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Re: Voltage regulator causing dim headlights and slow windows

Post by abscate » Mon Nov 25, 2019 5:26 am

That battery is probably just not accepting charge any more. If it’s more than 5 years old, it’s dead, Jim.


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