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GPS issues (Magellan)

1K views 19 replies 3 participants last post by  AZgl1800 
#1 ·
So, I picked up a Magellan GPS for $59. I had it mounted in a waterproof case on the bike. It seemed to be working (didn't really use it, but it I seem to recall it working, but hadn't really been using it), but then the minute I left for the WCWRJAMR, it decided to stop finding any satellites. On the last morning of the ride, it started working again, briefly, then stopped being able to find any satellites. So, I exchanged it for a new unit (same brand/model.) The replacement won't find any satellites, so I took it out of the waterproof case, just in case it was causing issues.

So, it would seem obvious that the unit is bad, except, I take it in the car, and it works fine. Then, I tried my wife's Tomtom, which I'd used on my 88 GL1500 without any problems. It wouldn't find any satellites...

What are the chances that there's some electrical interference on the bike causing the GPS to not be able to find any satellites? I just realized that I have an MP3 player plugged in and it broadcasts over FM - I wonder if it's interfering? I'll have to try a ride without the MP3 player plugged in and see if that makes a difference. Really would be too bad if it does, since it's nice to have the music...
 
#2 ·
grrrr! had a post ready and it got lost.

The mp3 player has nothing to do with it.

The location of the GPS does, is there any metal around it, or above it?

Is there any digital noise in your mp3 audio ?

Do you have HID headlights installed? If so, the digital power supply might well be causing the interference.
 
#3 ·
Good thoughts. It's mounted forward of the radio, just behind the ignition lock, no metal around or above it. On my 88, I had the Tomtom suction cupped to the windshield, but was always concerned about it falling off while riding (I had a tether strap on it, but still concerned.)

I don't have any digital noise on the MP3 audio - regular radio reception is good - no interference.

I do have HID headlights. The ballasts are in the forward sections of the left and right dash pockets. The ballasts need some air circulation to keep cool, I'm assuming because the cases are finned like a heat sink, so I don't want to 'wrap' them in foil or something, but it sounds like they may be causing some interference. I did NOT have HID's on my 88 and the Tomtom (mounted in a different position) worked there.
 
#4 ·
I have my 7" Garmin GPS mounted to my windscreen with a vacuum cup. It has been there now for going on 10 weeks. That includes a couple of trips to Arkansas and back, and then the whole of my NASSIR8 trip. All of that totals well over 5,000 miles.

I have a couple of tethers mounted to the back of it. I use 3M sticky back zip tie cable mounts. They are 3/4" square and have little bubbles on their back edge with two crosswise holes to run a zip tie through. More of the zip tie cable mounts are stuck to the side of the shelter inside the opening.

It is not the cutest thing to be seen, but it is my ride and suits me. It is safe, I can leave the vacuum mount on the screen at night, and take the GPS inside a hotel in 30 seconds. The video camera is mounted with 3M sticky back to the center of the windscreen just barely above the vent. It has never fallen off.

In fact, that 3M sticky stuff won't come off at all unless you use dental floss and alcohol and use a sawing motion to pull the alcohol between the sticky stuff and the plastic of the windscreen. I once had the camera mounted to the left edge of the windscreen... that turned out to be bad, and the edge flopped so much in the wind that the resulting video is useless.

Take your vacuum mount and temporarily put the GPS back up on the windscreen and see if that is far enough away from the HID power supplies to let it work. But, I really feel like what you have is "digital noise" in the power lead to the GPS and that is the culprit.

Get a noise filter for any CB radio, should include a ferrite core and a capacitor. Install that in the power wiring to the GPS. That just might be the real culprit, and allow you to keep the GPS in its' current mounting position.
 
#5 ·
The vacuum mount.

I cleaned the back of the windscreen with alcohol. Cleaned the vacuum mount with alcohol. Then while damp, I pushed it hard against the screen and pulled the vacuum trigger and there it stayed. for 10 weeks it has stayed.

you can see the result of the vacuum tightness from the forward side of the screen. If it ever gets loose, the circle starts to get thinner where it is suctioned against the screen. My first attempt to mount it there resulted in it falling in about 3 days. I watched that circle get thinner and thinner.

So, thinking back to what mother Garmin said in their instructions, "clean the windshield with alcohol first" then stick it to the glass.

well duh, it works.
 
#6 ·
I'm unable to test in the garage (can't get a signal in the garage), and it's been raining tons here lately, but I got out for a short ride yesterday - turned the bike off after riding a bit and got a signal - bike back on and signal was lost. So, I'm thinking interference from the HID ballasts (as suggested by AZgl1800). So, I found an iron ferrite core to put around the power wire for the GPS. I'll have to wait for the rain to stop so I can get out and see if that fixes it. Next, I'm assuming would be ferrite cores around the power wires from the ballasts. If that doesn't work, then I'll try relocating the GPS to a suction cup mount on the windshield.
 
#7 ·
I know the power wires are already terminated, but if the power wires are looped thru the core a few times, it improves their effectiveness.

short of that, clamping several cores over the wires helps.
if the core is large enough to run the power lead thru it, make 3 turns thru the core.
Place the core as close to the ballast as feasible.
 
#8 ·
The ferrite core was just big enough to loop the wire through it, so the cable goes through it, wraps around it, then through it again.

With that said, I've wrapped it on the power wire to the GPS (it was more easily accessible than the ballasts). I understand that the ferrite works both 'from' a device and 'to' a device - so, here I've used it in the 'to' a device usage. It might not be enough - if it isn't, I'll pick up 2 of them (I found this one on an audio cable I had here - others I have are not clamp-on, so I can't remove them from the cable they're on), and put them on the ballasts.
 
#9 ·
I have several of the FM transmitter MP3 players. They plug into the 12V power adapter and transmit to the FM radio. I am using a Garmin GPS suction mount and do have 1 particular FM MP3 transmitter that interferes with the satellite reception of the GPS. I don't know why the one does and the others don't. It took me awhile to figure it out. Just unplug the FM transmitter and try your GPS. I tossed the offending FM transmitter in the trash.
 
#10 ·
I at first bought one of those 'FM radio transmitter' modules so I could hear the GPS audio from my cellphone. I thought that would be 'good enough'...

much to my surprise, that module dumps the transmitter 60 seconds after the GPS shuts up.... what? I had the radio volume cranked up all the way to max, same with the cellphone. just not enough audio there, but it was usuable.

then the module turned off and I was hit with full blown noise in my helmet speakers.
threw that expensive $16.95 POS in the trash.
 
#12 ·
Intriguing.

I would have bet money on the ballasts, and the MP3 player just being an 'antenna' bring the noise into the GPS.

I wrapped the audio adapter in aluminum foil instead of the ballasts. They were already installed with the sticky tape against the fairing. Even at that, it took a bit of moving audio leads around to minimize the noise enough that I did not notice it anymore.
 
#13 ·
"I would have bet money on the ballasts"
I would have too. It might be a combination of cheap eBay HID ballasts (they are metal casing type, rather than plastic, which I thought would have slightly better shielding), and the cheap MP3 player - both are probably electrically very 'noisy' (although I don't hear any of the noise in the audio output.)

I'm going to do some more testing etc. - I'll try the GPS plugged into the power socket in the right pocket, leaving the MP3 player in the plug in the left pocket, to see if the separation has any impact. This is the (cheap) MP3 player from eBay that I got: http://www.ebay.ca/itm/Car-Kit-Hand...hash=item20fd4210c2:m:m0xERaHep8qw5nzjVUv3NJA
It got it because 1. It's plugged into power - the one I had used batteries, and I kept forgetting to turn it off, killing the batteries. 2. it has a remote, so I can change songs etc. easily. 3. It has a built-in FM modulator, so I don't have to use the finicky cassette tape adapter with the audio plug input on it. Too bad if it's going to cause the GPS to not work...

I'll try different FM stations, but I highly doubt it's the FM transmitter, but you never know. I can't 'shield' the MP3 player, or I lose the FM transmitter's functionality.

I'm open to other suggestions and ideas, of course. One might be going back to the battery operated MP3 player, except I know I'll go through batteries like crazy... I could try a different MP3 player, but it was hard to find one with all the features this one had, even at a higher price (that doesn't necessarily mean a higher price is any better...)
 
#14 ·
I don't think that MP3 player is the culprit, I have an audio adapter that is just like it, but I powered it in a different pocket.

the problem with mine though, was it quit transmitting the FM signal when the GPS audio was silent for more than 60 seconds. that caused the bike's FM radio to go to full blown noise, or if on a station, to go to the full station's audio level.

whichever, it was hard on my ears, so I gave up trying to use it.

that was why I switched to an audio only adapter thru the Intercom input. I don't like that either, as it is unreliable switching ON to hear the GPS. it is going away, a waste of money for me.
 
#15 ·
I only had time for a short ride today (installed the new Progressive 416's in the rear and helped my neighbor replace an engine mount in his car), but I left the MP3 player unplugged and the GPS worked fine. Next, I'll try plugged them into separate locations, to see if it's feeding back through the power, or if it a 'radiation' interference issue.

The one I have is an MP3 player, with built-in FM transmitter - the FM transmitter is always on, so I haven't had any issues like you've had. My guess is that it's cheap e-bay stuff with no proper shielding etc.
 
#16 ·
yup, mine came from eBay, and IIRC cost me ~17-18 bux.

it was a gamble and at first, I thought it was going to work just fine.

Then two miles down the road, the FM transmitter turned OFF, and if you are on a long trip, that leaves a lot of blaring noise in your ears.

Or, if you try to listen to an FM station, the little adapter's volume output is so puny compared to the radio station, that you won't even notice it...

it was wasted money for me.
 
#17 ·
Tonight, I tried plugging the GPS into the outlet in the right pocket (MP3 is plugged into the outlet in the left pocket.) No change.

I can't believe I didn't try this before, but I changed the broadcast frequency of the FM transmitter in the MP3 player - I had it on 87.5 (lowest frequency it had, seemed out of the way of radio stations etc.) - I changed it to 90.1 and the problem (with the GPS) went away. I then progressively change the frequency back down and somewhere around 87.9 it started to deteriorate the signals to the GPS from the satellites (I had the status screen up on the GPS showing the s/n ratio, and could see the numbers change as I dropped the frequency), until by the time it hit 87.5 the signal was just gone.

Unfortunately, I did this all at the end of the ride tonight, so I'll have to take it for a ride this weekend and make sure that it continues to work...
 
#18 ·
Now that makes some sense...

the logic of it is, as you changed the FM frequency, you changed the operating frequency of the "Local Oscillator" that feeds the "Mixer block" that down converts the FM signals to the "I F" or Intermediate Frequency amplifier chain... this is where the Narrow Band filters are that prevent adjacent channels from interfering with the content you want to hear.

Now, why does this make a difference?

Because, the Local Oscillator is not a "Pure" sine wave.... It has harmonics that radiate upwards in frequency in multiples of the "LO" frequency.

Apparently, at around 87.5 mHz, the LO has harmonics that coincide with a portion of the GPS's own receiver circuits.

so, as you increase the FM radio's tuner upwards, you moved the "interfering harmonics" away from the GPS's receiver circuitry bands. And they "move away" at a much faster rate than the LO's change does.... by a factor of the "Multiple" harmonic.

Now, is that clear as mud?
sorry, my old electronics mind was awakened tonight.
 
#19 ·
My layman's guess was something to do with harmonics. Being that the FM transmitter is so close to the GPS receiver even very weak harmonics could have been interfering, the 87.5MHz must have been creating harmonic radio waves that happen to fall into the same wavelength. Interesting that 18X 87.5 = 1,575 - my understanding is that GPS satellites communicate on 1575.42MHz. So, I was getting an 18th harmonic of the base 87.5MHz signal basically wiping out the signal from the satellite. I wonder if other multipliers are generating strong enough harmonics as well, for example, 1575.42/16=98.5 (roughly), so I wonder if tuning the FM transmitter to 98.5 would block the GPS signal as well, or 18 is the only magical multiplier...
 
#20 ·
Actually, the local oscillator is running at 10.7 mHz ABOVE the FM radio station's frequency.... it has to in order to give the mixer's output at 10.7 mHz for the narrow band filters.

so, 87.5 plus the 10.7 = 98.2
The full span of the local oscillator is 98.7MHz to 118.7MHz

but that is just a matter of semantics, as your thinking is correct.

The actual harmonic value number is immaterial, as all you want to do is avoid one of the harmonics from interfering with the GPS's internal receiver circuits.


I had to double check my thinking by looking at a couple of references. IE, I had forgotten that FM radios use "high side" injection and not "low side". This is to prevent the LO from interfering with the desired FM stations.

https://en.wikipedia.org/wiki/Superheterodyne_receiver

http://www.eit.lth.se/fileadmin/eit/courses/eti041/VT2012/04a.pdf
 
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