Usb-c charger overheat when I plug my Garmin edge 1040 solar why?

I suggest contacting Garmin. That it happens on different chargers with different cables and a computer USB port leads me to believe it is a problem with the 1040 USB-C port.

The same thing happens to me if I plug the 1040 solar into a high power usb-c charger and a cable capable of handling the power. The 1040 also gets too hot. 

Now I charge it on a low power charger only.

Same thing started with my 1040 today. The bottom of the 1040 and the USB connector plugged into the charging port were too hot to touch. I called Garmin and they have already shipped a replacement. I would not charge your current unit anymore because of possible fire danger.

Garmin wants back the old unit, as well as the cable that I was using for charging. (A high-quality cable.) They emphasized that I should only use the Garmin-provided cable for charging, which I will do, but that seems like a way to deflect responsibility. USB is a standard, and it shouldn't be necessary to use a particular brand of cable, as ling as it is a quality cable.

Yes, there is a USB standard (multiple, in fact). That doesn't mean the quality of all USB cables is equal. The quality of my Garmin-supplied cables is less than my pricier Satechi cables. So those are the ones I use with my Garmin, and I have had zero issues. They also work well with my Karoo 2 and Cycliq cameras. If it overheats with the Garmin cable, I'd send it back with that one. 

Same thing happened to me 10 days ago.

The edge 1040s had the battery at 30%, I decided to charge it as usual, after a few seconds I smell a burning smell, and very quickly unplug the cable, I put it near my nose to smell the cable and burns the tip of my nose, the garmin usb-c cable connector was hot impossible to touch.
I tried with my samsung usb-c cable, 2 different adapters and with the computer, the problem remained the same, in a few seconds there was overheating.
Funnily enough no problem charging my samsung phone with the usb-c garmin and samsung cable, everything works with the phone.

Needing the gps the next day, I decided to use it that way, the battery was enough for the gravel day.
Then the battery completely discharged to 0% because I couldn't charge it anymore, I decided to charge it again, and there, miraculously, no more overheating, it charged to 100%.

However, what I see with a magnifying glass in the usb-c socket of the edge 1040s, a black overheating mark and the sealing cover of the usb-c connector which no longer holds firmly since the overheating, I suppose it has suffered a deformation.

I contacted Garmin, I have to send it back

This is a concern as it raises a couple of questions.  A while back chargers circuits were poorly designed and couldn’t correctly limit the current of larger capacity chargers. By now all suppliers should have sorted this. Either Garmin hasn’t done a good enough job or we just have a few unlucky faulty devices.

i have regularly used an Anker USB high power charger capable of 45w. It can charge a laptop. I use it mostly for charging an iPad on fast charging and the Anker cable is more than suitable for the higher current.  I have never had an issue charging the 1040 this way.  

For reference people need to understand the chargers ability to deliver current should be irrelevant as it’s the device that sets how much it takes (the charging circuit in the device). With USB C this also dictates voltage (look up Power Deliver PD charging).  

My advice is always use good quality chargers and cables. There are a lot of cheap ones around and their voltage and current regulation circuits may be poor. This can often lead to issues with the device being charged. 

My 1040 Solar overheated while charging and melted the plastic surrounding the USB port and cable - fusing them together. Was left on charge for a couple of hours while I was out, only discovered when I later tried to unplug the cable but couldn't.

Same PD branded power supply I've used for a while, including charging this Garmin and many other devices. 95% certain same cable too, at the very least this cable will have been used at least some of the time charging this and other USB-C devices from this charger.

Garmin UK shipped out a replacement straight away and the damaged one was sent back for investigation. I had assumed it was just a one-off hardware defect, though worryingly now looks to be not an isolated case.

wabeck said:

They emphasized that I should only use the Garmin-provided cable for charging, which I will do, but that seems like a way to deflect responsibility. USB is a standard, and it shouldn't be necessary to use a particular brand of cable, as ling as it is a quality cable.

How would Garmin know it's a "quality cable"?

Garmin can't take responsibility for a random cable they can know nothing about.