From the Avionics West site:
The FAA Airworthiness Concern Sheet
Early this week I head a rumor from an AVweb editor friend of mine that there was a “Airworthiness Concern Sheet” circulating around that stated there may be an AD issued against the Garmin GNS 430. As you can imagine, this got my attention and I started to investigate the issue. Finally, I got a copy of the FAA Airworthiness Concern Sheet and talked to a person at Garmin about what was really going on and here’s what I found.
On or about September 10, 1999, Garmin International found under some conditions that the horizontal and vertical needles of the indicator that the Garmin GNS 430, GNC 420 and GPS 400 were attached to would at times deviate a couple of dots from the actual course. Garmin alerted the FAA on September 10, 1999 and asked them to consider an AD on the product at that time. The FAA has sat on this issue until now, almost two years later. I tried to call and emailed Mr. Roger Souter of the FAA for clarification on the subject. Mr. Souter is the FAA engineer in charge of this project; to date I haven’t received a reply back from him. There are 1,985 units that have the potential for this problem to occur according to the FAA ACS.
After telling the FAA of the potential problem, Garmin International came up with a fix called “Mod 1” which is covered under Service Bulletin 9905. This service bulletin came into effect September 17, 1999; seven days after Garmin told the FAA a problem may present itself in some installations. I wonder why it took the FAA almost two years to shed any light on this issue? On or about September 17, 1999 Garmin International notified the owners of the suspected units that a problem may exist and to have Service Bulletin installed to cure the problem. If you have an early model GNS 430, GNC 420 or GPS 400, check the serial number to determine if mod 1 is needed or has been completed. Hopefully you will have the serial number written in the logbook or on the weight and balance form. If not, the unit must be removed from the aircraft and the serial number verified to see if Mod 1 has been stamped off. All units manufactured after September 17, 1999 have this modification completed at the factory. DO NOT attempt to remove and reinstall your Garmin product; you could easily damage the pins trying to reinstall it. Believe me, this will make for a really bad day; I’d recommend getting your local Garmin dealer to remove the unit and check for Mod 1 if you don’t have that information already. Now let’s go over the Service Bulletin that describes Mod 1.
SERVICE BULLETIN NO.: 9905
DATE: 17 SEPTEMBER 1999
SUBJECT: GNS 430, GNC 420, GPS 400 MOD 1
The GNS 430, GNC 420 and GPS 400 units have been designed and tested to meet or exceed all TSO requirements, including susceptibility to noise being generated from other sources within the aircraft.
Recently, GARMIN received a limited number of reports regarding GNS 430 installations where the deviation indicator on the CDI/HSI was offset by a couple of dots from the actual course.
Investigation determined that these installations had electrical noise between 1 & 3 volts AC peak-peak induced into the GNS 430 CDI output which came from other items installed in the aircraft. This high level of noise caused an undesirable oscillation of the CDI outputs which resulted in inaccurate course deviation as seen in the CDI/HSI.
This modification incorporates circuitry changes to the unit deviation and flag outputs to eliminate the oscillation of the CDI outputs caused by external electrical noise.
This modification consists of the removal and replacement of six (6) capacitors, the removal of two (2) diodes, the removal and the replacement of two (2) resistors, and the addition of two (2) resistors and two jumpers to the main board. The removal and replacement of seven (7) capacitors and the addition of four (4) capacitors to the navigation board. the addition of four (4) capacitors to the glideslope board.
GNS 430, P/N 011-00280-00, S/N 96303639 & Below
GNC 420, P/N 011-00506-00, S/N 96800008 & Below
GPS 400, P/N 011-00504-00, S/N 96700007 & Below
Required for the following units: GNS 430 P/N 011-00280-00 S/N 96301296 through 96303200 and the following S/N’s
96300001 96300002 96300017 96300028 96300034 96300040
96300068 96300104 96300108 96300122 96300125 96300130
96300142 96300149 96300161 96300165 96300218 96300222
96300232 96300269 96300272 96300308 96300333 96300340
96300348 96300354 96300369 96300372 96300382 96300394
96300411 96300413 96300429 96300437 96300451 96300484
96300485 96300489 96300504 96300506 96300513 96300522
96300549 96300563 96300585 96300587 96300618 96300621
96300624 96300628 96300641 96300653 96300664 96300713
96300734 96300756 96300766 96300781 96300785 96300786
96300808 96300831 96300837 96300842 96300846 96300866
96300870 96300872 96300899 96300916 96300923 96300935
96300929 96300941 96300961 96300984 96300987 96301021
96301108 96301130 96301280 96301319 96301343 96301350
96301397 96301399 96301405 96301428 96301458 96301476
96301490 96301506 96301561 96301662 96301739 96301754
96301823 96301893 96301925 96301941 96301952 96302059
96302150 96302172 96302341 96302555
Recommended for the following units if the problem is exhibited in the installation.
GNS 430, P/N 011-00280-00 s/N 96301295 & Below and S/N 96303201-96303639
GNC 420 P/N 011-00506-00 S/N 96800008 & Below
GPS 400 P/N 011-00504-00 S/N 96700007 & Below
Garmin pays up to three hours warranty time when this service bulletin is completed by an authorized Garmin dealer. In some cases your Garmin dealer may want to return the unit to the factory for the installation of Mod 1.