Lights dimming when braking, why?

painemann · **Joined:** Sat Dec 20, 2008 7:02 pm **Posts:** 607

Well it's cold here so there are a numbe for things I can think of that might load the system and cause it. I have not noticed this before and this is my third winter with the jeep. My girlfriend drives it mostly so I can't keep track of how often it does it but I will see it once or twice when I drive it.
I notice it when I am decelerating slowing ane hit the brakes. What's would this be? The viscous heater? Or am I loosing the alternator? It has right around 100,000 miles now.
Thanks

mikey1273 · **Posted:** Thu Dec 16, 2010 10:58 am

How old is the Battery? I remember mine doing that just before the battery was found to be too weak to successfully start it on a cold morning.

painemann · **Joined:** Sat Dec 20, 2008 7:02 pm **Posts:** 607

mikey1273 wrote:

How old is the Battery? I remember mine doing that just before the battery was found to be too weak to successfully start it on a cold morning.

I wish that was it, it's New! Installed about a month ago.

geordi · **Posted:** Fri Dec 17, 2010 1:36 am

I would point at the alternator, with that much mileage. The clutch pulley could easily be slipping a bit and the alternator not putting out enough wattage to charge the battery sufficiently, or the total voltage might not be much above 12. Normal operating voltage should be 13.5-14 or so, with a working alternator. The diesel doesn't need much power to remain operating, but lights and the starter draw lots of power.

Alternators are available from Advance Auto for $150 or so, with a lifetime warranty.

onthehunt · **Posted:** Fri Dec 17, 2010 11:07 am

The same thing happens to mine. It's done it since new and it only happens when it's really cold. I just put up with it.

painemann · **Joined:** Sat Dec 20, 2008 7:02 pm **Posts:** 607

Thanks guys, I did replace the decoupler pulley last year but I think I over tightened it. But I think I will just replace the alternator and hope for the best!

Fulltimer · **Posted:** Fri Dec 17, 2010 12:28 pm

You might have a short somewhere that pulls down the juice where you hit the brakes.

Terry

1TDI4Me · **Posted:** Fri Dec 17, 2010 9:24 pm

Same thing on my F250 a couple of years ago. It has 2 batteries. Finally it wouldn't start. Batteries checked good, so did alternator. After a couple of visits to the dealer and no solution my dad did some old fashioned thinking and used a multimeter to determine that my terminal posts were dirty. Once cleaned and tightened I was good.

Radman · **Posted:** Sat Dec 18, 2010 9:46 am

Mine does it especially if the ABS is kicking in. My VW TDI's had a similar issue but it was the Glow Plugs cycling on/off. On TDI's the Plugs would cycle off above a certain RPM then turn back on when slowing down (only when cold of course and shortly after startup) It would cause the lighs to dim. Not sure if the CRD has same feature?

TJ2 · **Posted:** Sat Dec 18, 2010 5:07 pm

geordi wrote:

alternator not putting out enough wattage to charge the battery sufficiently,

Shouldn't we be talking amperage here? Or does the new science make them interchangeable?

geordi · **Posted:** Sat Dec 18, 2010 6:12 pm

Volts times Amps equal Watts. At 14 volts, a 100 amp alternator is creating 1400 watts of power. The output power is initially 3-phase AC, rectified into DC power and reduced to the desired voltage, which changes the other components in relation.

Drop the voltage, the amperage must go up in order to create the same wattage.

In the case of lights dimming when another power demand is brought on, the voltage is encountering extra resistance (more lights / devices) which then requires more amperage to accomplish the work. Raising the voltage reduces the amp loading, which also reduces resistance heating in wiring. Thats why the wires to the starter are huge (very low voltage, high amperage draw) yet high-tension power lines feeding thousands of homes are very small (extremely high voltage, comparatively low amperage for the same watt output)

Its physics.

Lights dimming when braking, why?

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