The Garmin approach to technical fault finding...
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   17 years
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Background
About two years ago, I bought a Nüvi 310 from a UK main dealer. (The 310 is a 360 with less memory and the TTS disabled). A while later, I bought a GTM-12 Traffic Receiver on eBay. This was new and sealed. Here in the UK, there is a life-time subscription, that is tied to the receiver. I plugged in the GTM-12 and up came the Traffic Information. Though the quality of the data is sometimes suspect, this combination has worked well.
I recently bought another Nüvi 310. This was a used one and was sold as faulty (as it had satellite reception problems). It had originally been supplied as a Nüvi 310 Deluxe, so it included a GTM-12 as well. One of the first things I did, was test the GTM-12 by connecting it to my existing Nüvi. As previously, it was a straight-forward plug-and-play operation - it worked first time.
So far so good.
However ... I have now fixed the satellite issues on this second Nüvi 310, so I've been giving it some real-world 'field trials'. The first thing I noticed, is that there's no Traffic Information!
Having gone through numerous Resets, reloading of firmware, clearing of data, changing of PSUs etc etc, I came to the following conclusion: Both GTM-12's work fine with my original Nüvi 310 and neither work with the recently acquired one. So - there's some kind of issue with the 2nd hand Nüvi (which is actually quite a bit older than my original one).
If you sit the pair of them side by side and check the various diagnostics screens, you see all the same information; it's just that one of them never 'locks on' and starts using it. It's clearly unlikely to be a hardware problem.
Resolution
So, I rang Garmin.
The 1st line Technician accepted my reasoning and decided that there was nothing he could do. Fair enough, so what happens next? ... Would they be putting me through to 2nd line support? Would they be sending me some more detailed diagnostic software? Do they want anything sending by email, so they can analyse it?
er, no.
If I just send in the Nüvi, they'll replace it. Of course, it's out of warranty, so please include payment of £65 ($95).
and that's it. That's the sum total of their interest in the problem 
I have a friend with a Nüvi 350, so I'll hopefully test both GTM-12's with that soon - just to prove that they've not formed some unbreakable bond with my original 310 
Otherwise - does anyone have any thoughts as to how I might progress this? (technical or otherwise).
an idea...
I'm assuming both 310's are able to connect to the PC since you've updated the firmware to the same version on both units. The USB port on the side of the unit where the GTM plugs into pretty much turns into a data I/O port when the unit is connected to the cradle. The USB port may be defective in some way...maybe a bad pin in the connector or a bad solder joint from the connector to the main PCB.
Have you verified that the *GTM diagnostics* are identical with both units connected to the GTM's? If they are not identical or the "broken" 310 has no GTM diagnostics screen, this would be a sign that something is up with the connector on the "broken" 310.
From what it sounds like, if all things are equal with the software/firmware, the problem could very well be hardware related.
nuvi 760, nuvi 765T, nuvi 855, nuvi 3790LMT, nuvi 3490LMT - SoCal area
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I'm assuming both 310's are able to connect to the PC
Yes.
The USB port on the side of the unit where the GTM plugs into pretty much turns into a data I/O port when the unit is connected to the cradle.
That's my understanding too.
The USB port may be defective in some way...maybe a bad pin in the connector or a bad solder joint from the connector to the main PCB.
So, the theory being, that the USB circuitry is ok (because it can talk to the PC), but the RS232C circuitry (tx side) isn't. Presumably data does have to flow from Nüvi to GTM-12, to select the right frequency...
...I'll think about this...
Have you verified that the *GTM diagnostics* are identical with both units connected to the GTM's? If they are not identical or the "broken" 310 has no GTM diagnostics screen, this would be a sign that something is up with the connector on the "broken" 310.
At one point, I had both Nüvi's in the car (so I could try alternate psu's - as this apparently has been an issue in the past). Watching the diagnostics was quite amusing. One of them was clocking loads of errors - and couldn't read the version no. of the GTM-12. This was the one that was working!
------------------------ Phil Hornby, Stockport, England ---------------------- http://GeePeeEx.com - Garmin POI Creation made easy »
Garmin Support
That's kind of shabby treatment by Garmin support.
When I was having trouble, Garmin at least asked me to send my garmindevice.xml file.
They got back to me in a few days and said admitted there was a problem with the firmware which would be rectified by a later update.
The latest update did resolve the problem.
I wonder if the support may be different in the UK.
Anyway good luck and hope you can get the problem resolved.
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You've probably already done so, so if I'm stating the obvious...
On the *nuvi's end*, you may want to check the 4 connectors that contact the cradle bracket and verify they're clean & not worn or bent. It would seem likely that these connectors would come into play (by generating a signal or instance) telling the nuvi that it's connected to an externally powered cradle and to enable useage of the GTM. I do know that the unit won't use the GTM if it's not connected to the cradle & running off of battery power.
Just throwing out ideas...
Like I said previously, if all things are equal on the software end (firmware version), I would personally be looking for differences on the hardware end of the "broken" unit.
A question. What did you do to the "broken" unit to fix it's satellite issues?
nuvi 760, nuvi 765T, nuvi 855, nuvi 3790LMT, nuvi 3490LMT - SoCal area
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I do know that the unit won't use the GTM if it's not connected to the cradle & running off of battery power.
It pops up a banner to tell you that the Traffic Receiver requires external power, if you connect it without any.
I read through the 44 page thread at pocketgpsworld.com, which detailed problems owners had with these things when they where first released. Apparently, they are quite voltage sensitive and Garmin produced a modified car charger. (I checked, I've got the new one).
Just throwing out ideas...
That's fine - it's more than Garmin did
A question. What did you do to the "broken" unit to fix it's satellite issues?
I bought another Nüvi off eBay - (well, ½ a Nüvi actually: ie just the back case,battery,speaker,gps receiver) and swapped the receiver over.
------------------------ Phil Hornby, Stockport, England ---------------------- http://GeePeeEx.com - Garmin POI Creation made easy »