I have a 2007 Hyundai Sonata with a problem that has baffled the local Hyundai dealership on three different visits. It has also baffled my local repair shop, who can usually diagnose the most esoteric problems.

I will be driving along and, as I slow down and the tach goes through the 1100-1200RPM range, the headlights and air conditioner (and sometimes the radio) will shut off. After several seconds, they will come back on, but the headlights come on as if they are being turned on by a dimmer switch. The problem is consistent and it doesn't seem to matter how fast I am going prior to slowing down. It also seems to happen after I have been driving for ten minutes or more.

The Hyundai dealership replaced the alternator and battery. They re-flashed the onboard computer. They replaced the aluminum positive battery connector made from a different material. They said that the anti-moisture grease on the brake light bulbs was causing the problem. They said that after-market brake light bulbs was causing the problem.

Another Hyundai dealer said that it sounds like a loose ground. My local repair guy says that it sounds like a "body control module". No one has been able to locate a loose ground connection.

Do any of you have any ideas?

When I brake all the lights go dim then brighten when I hit the gas. I figured it had to do with the alternator but it doesn't seem to cause any problems as it has done it for years.

Anyone with an Hyundai deserves a problem.

This has nothing to do with the brake lights, because it happens when the car slows down, brakes or not.

I should have said when I am not stepping on the gas, just removing the foot dims the lights.

On mine, letting off the gas alone doesn't make the lights go out... Only when the RPMs pass through the 1000-1200 range, and only when slowing down. Yours sounds like an alternator/battery problem.

Sounds like your voltage regulator.

Oh and don't mind, tommy. Everyone here knows he's a complete *************, but for some reason I'm the only one besides you who says anything about it. I think its because of his number of posts.

Without being very familiar with your engine and other engine room components, here are my thoughts...

Have you tried with just the headlights ON (air conditioner and radio turned OFF)? Does it do the same thing? Does it do this only while the vehicle is in motion or will it do it while parked and you just rev the gas then release?

I'm guessing you have halogen headlights, not HID... hence the slow recovery to full brightness after they cut out.

I'm guessing you don't have any voltage measurements from the battery or headlights as this is happening... but I suspect that the battery is taxed with a heavy load from something that is not running as smoothly as it should. My guess is that the AC compressor is worn (even though it's not that old), and even though it probably runs smooth at the other RPMs, it could have difficulty at the 1000-1200 range.

You also mention that it happens after ten minutes or more. I am guessing that after that amount of time the engine has reached full temperature as well as other surrounding parts have heated up. This heat causes expansion, and if the expansion has occurred on a part that has rotating components, there could be a specific speed in which it is most noticeable from the increased friction. This could cause a huge stress on the charging system by making it work harder to keep up with the load demand, but the failed load pulls the voltage down low enough to cause some systems to cut out or fail momentarily.

This could apply to any of the other rotating components connected by the serpentine belt (water pump, power steering pump, alternator which you already had replaced, etc...), if they are abnormally worn.

Page 232 of the owner manual I found online: http://pdfservicemanual.com/automotive/2......nual.pdf shows a view of the serpentine belt and components. I wasn't sure what size engine you had, but if this is the problem, then it could apply to either.

Only my guesses though... Good luck!

It happens only when the car is in motion. It will not do it sitting still.

It happens when only the headlights are on or only the A/C is on, also.

As for the voltage regulator, the dealer said that it is part of the alternator, which they replaced about a month ago.

Did it happen before they replaced the alternator?

Could be a problem with the door openig or rather unlocking and locking during driving. I guess one could test it by opening the door while driving to see what effect it has. It is a widely reported fault with this model in any event so it seems from reports.

Yes, the alternator/battery replacement had no effect.

And is there any interesting or unusual engine noise?

e.g. can you hear the serpentine belt?

John

Nope. Smooth as can be...

If there is not an odd smell, check the cable for a loose connection to the engine or altenator

To go the forums and Cartalk.com. Even if Click & Clack don't chime in, there will be 20 experienced mechanics who will. Good luck.

Do your self and sanity a favor. Go to an online Junk dealer and get an engine COMPUTER for that MODEL CAR. (used ones aren't that expensive...new ARE.)

Install it.

If it works it'll save you a fortune in diagnostics and repair ATTEMPTS. (we've been through it and ALL Toyota experts couldn't figure out what was wrong and it was costing an arm and a leg. I finally called a used parts guy and he said, out of the blue...change the computer. And it worked.)