Lights dim, radio shuts off

My 2006 does the same thing. Close observation shows this occurring at 1000 rpm's as the rpm's are falling. The entire electrical system is affected. The symptom is more dramatic with a heavier electrical load, such as with the A/C on high and the headlights on. There is a TSB on this issue that instructs the tech to check for a voltage drop in excess of 0.3 VDC on the wiring from the alternator wiring to the battery and engine bay junction box. Also to check all ground wires including the battery ground. Any wire that exceeds the voltage drop should be replaced. I do not know if this will actually fix the problem. I would be more inclined to investigate what electrical "event" is occurring at this engine speed. Sounds like a computer is programmed to perform an operation at this rpm and a relay is not responding in time. Perhaps the power control module or the body control module. Looking at a schematic of the system shows electrical input to the voltage regulator routed through the PCM. The BCM also is involved somehow in the circuit to the alternator fail light. Hyundai has a web site that provides maintenance manuals but dose not seem to provide enough info on what is occurring in the electrical system to account for this anomaly. I have just replaced the wiring harness from the alternator to the battery and junction box and will see if this is a lasting fix. Some of my previous attempts at repairing this only had temporary results. I will re-post if this is deemed an actual fix with the part number and how to purchase.

Having the same trouble. Please post if you find a fix to your problem.

I have replaced wiring harness part number 918503K111. It may be too early to tell but my car is performing flawlessly so far. This wiring harness came directly from South Korea. The cost is about half of what the dealer wanted. This harness is an exact fit for the car, however, the part number is slightly different from the original. The harness is a little bit shorter leading to the alternator. There is no detrimental effect to the installation. In order to change the harness you will need some mechanical skill. The installation is easy except for the lead that attaches to the starter. The starter wire is part of the harness.
To change the harness, first drive the car up on a pair of ramps, so that the starter lead may be accessed from below the car. Next remove the battery, (disconnect the negative lead first) so the wires that lead to the engine junction box may be routed. Remove the two wires at the junction box, remove wire from the alternator. The wire to the battery is already removed.
The tricky part is removing the starter lead. In order to remove this wire the shield on the starter must be removed. There are three attach points securing the shield. Two bolts, 10 mm socket size, secure the shield to the front of the starter. These bolts are accessed from underneath the hood. One of the bolts also holds a wire harness routing bracket. Next you must get beneath the car and locate the third securing point, which is a stud with nut. Once the shield is free it must be moved out of the way. I was unable to remove the shield so I just positioned it out of the way. Now one can remove the nut that secures the wire to the starter. There is a small gauge wire that leads to the starter solenoid that has a push-on type connector at both ends, disconnect this wire. Next, disconnect all of the harness routing clips (look at the new harness to see how to release clips). There is one bolt securing the harness next to the battery, remove this. Install new harness, enjoy!

I, too, am interested to hear about the longevity of the replacement harness solution. I have had my 2006 Sonata into the dealer a couple of times for this, and each time they reference the TSB, test voltage drops across the appropriate places, and tell me everything's well within tolerance.

But still, at night oncoming cars flash their headlights at me when my lights dim, thinking that I flashed my lights at them. For a car just over four years old, I have replaced more headlight bulbs, tail light bulbs, and just recently a high stop lamp bulb? Could it be the constant voltage drop below twelve volts is reducing the life of the bulbs in my car? It sure would be nice to resolve this once and for all even if the dealer is not willing to admit it is a problem.

I drove my car last night with all electrical functions on. The car electric system is still performing perfectly, this after replacing the positive battery/alternator harness. I did read the fix posted in the Hyundai forum 'linked" here by cwjohnst71. This might be a legit "cheap fix". I wanted to keep my car stock and opted for the harness replacement. One thing to consider, the wire run from the alternator goes directly to the engine junction box, not the battery, as in the cheap fix. I do not fully understand the charging system of this car and I wonder if Hyundai techs do either. Hyundai's on line manual does make a statement that the battery condition is monitored, does this fix conflict with engineering design? If one does use this as a fix I would strongly suggest checking the charging voltage under different engine RPM as well as electrical loads. The post on the Hyundai forum does state that the lights are brighter and stereo louder since his fix. A better fix may be to route the additional wire from the alternator to the correct stud on the engine junction box and perhaps to install another wire from the junction box to the battery, both in parallel to the stock installation. Good luck with this problem.

Thank you, Sonata, for your posts. And, you are correct that the cable shorting between alternator and battery is suspect. It has the right idea/spirit, but it bypasses any monitoring of that current/voltage. Hyundai does not want to admit they did wrong. I am going to follow your suggestions and take original and new cables to our lab and try to characterize any differences. Best wishes and Happy New Year!