Hi all,
After extensive reverse engineering (together with AI, Claude) of the PNDNavigator binary on the Carminat TomTom SD card, I've found the root cause of the --:-- clock issue that many users are experiencing since around 19-20 April 2026, and more importantly, a fix.
What's happening - the root cause
The Carminat TomTom uses a SiRF Star III GPS chip (GSW3.2.4TT_3.1.00.12-C35B1.02) that stores the GPS week number in a 10-bit field, which can only count up to 1024 weeks before rolling over to zero. Around 19-20 April 2026, we crossed GPS week
2415. This is exactly
1024 weeks after GPS week
1391, which corresponds to
6 September 2006.
This means the chip's 10-bit counter has overflowed once, and the chip now thinks the date is somewhere in
September 2006 — not the classic 1024-week rollover (that won't happen until November 2038), but rather the chip losing track by exactly one full 1024-week cycle.
We can confirm this because when the loopdir folder was deleted from the SD card during testing, the system recreated it with a timestamp of
6 September 2006 — exactly what the GPS chip was reporting as the current date at that moment.
Why the clock shows --:--
Through reverse engineering the PNDNavigator ARM binary using Ghidra, I found the function CClock:
oCurrentDateTime. This function retrieves the current date and time and returns it to the clock display. It contains a hard-coded year threshold check:
c
if (year < 2007) {
// return invalid / show --:--
}
Because the GPS chip reports a year of 2006, this check fails and the clock display always shows --:--. The navigation itself continues to work because position fixing doesn't rely on the absolute date.
The fix
The fix is to lower the year threshold from 2006 to 1977 (before the GPS epoch of January 1980), so the software accepts any year the chip reports — including 2006.
On cards where PNDNavigator is a standalone file on the SD card (this applies to cards that were modified using the FastActivate tool):
- Make a backup of PNDNavigator on your SD card first
- Find the following bytes at offset 0x402928 in the file:
d6 07 00 00
- Replace with:
b9 07 00 00
On Mac, using Python:
python
data = bytearray(open('/Volumes/TOMTOMDISK/PNDNavigator','rb').read())
file_offset = 0x402928
assert data[file_offset:file_offset+4] == bytes([0xd6, 0x07, 0x00, 0x00])
open('/Volumes/TOMTOMDISK/PNDNavigator_backup','wb').write(bytes(data))
data[file_offset] = 0xb9
data[file_offset+1] = 0x07
open('/Volumes/TOMTOMDISK/PNDNavigator','wb').write(bytes(data))
print('Done!')
On Windows, using a hex editor like HxD:
- Open PNDNavigator
- Go to offset 402928 (hex)
- Verify you see D6 07 00 00
- Replace with B9 07 00 00
- Save
Important notes
- This fix applies to modified/unlocked SD cards where PNDNavigator exists as a standalone file
- On original unmodified cards, PNDNavigator is embedded inside loopdir/loopback.ex3 (a Linux ext3 filesystem image) — patching that requires Linux tools to mount the filesystem and is more complex
- The navigation itself continues to work correctly even without this fix — only the clock display is affected
- The fix has been tested and confirmed working on a Renault Laguna with Carminat TomTom firmware version 8.843
Additional findings
We also patched the ephemeris files (packedephemeris.ee, ee_meta.tlv, ee_meta.txt) to correct their timestamps. While this did not fix the clock display by itself, it may help with GPS fix acquisition time.
