I bought Radio Shack branded a micro usb car charger for my N900 today. I tried to use the N900s FM Transmitter while it was charging. I was surprised that it did not work at all.

Has anyone tried the FM transmitter with an official Nokia car charger? Does the official charger have a FM antenna built into it?

The problem is the FCC. Plugging a wire can act as an FM antenna and then people in other cars might be able to hear you playing your Britney Spears albums.

Ugh.. well there goes one of my purposes of the n900. I wonder if there is a way to bypass that with a hack if it's not limited by the hardware.

There has to be a way around that, its completely unreasonable.
Who is NOT going to have their N900 on charge while in the car?

Its totally insane to expect people to drain their batteries while near an outlet. Especially as iPod FM transmitters come with car adapters, although I think they factor in the cable as you get no reception at all without it plugged in.

But that just proves the point, surely you just factor in the likely signal boost from the cable and drop the output strength accordingly while charging. There is always the possibility of interfering with other people, it all depends how well shielded your car is to begin with.

I use my n900 with the nokia dc-6 car charger and fm transmitter works fine.

Hmm the n900 doesn't charge with a power pin like the older tablets and other Nokia phones does right? So it's not a matter of plugging into USB vs. power pin since there is no power pin option.

It may not be the N900 itself. It might be the car recharge and the usb data cable which is the culprit. I have a nice little DC/AC power adapter which provices Fireware and USB powers. I use this to recharge my old phone and NITs. But when I tried using it with N900, it doesn't work. The N900 doesn't kick into recharge mode. And like the OP, it's kills my FM Transmitter. After I disconnect it, it seems the FM transmitter power output is also drastically reduced. I have to reboot the N900 to FM transmitter back to normal.

I suspect that the data connections in the USB data cable are confusing USB hardware or drivers. Does anyone know if the AC recharge only has the power connections wired on USB port? If not how are the data connections wired?

It [fmtxd] checks to see if a charger is in by listening to D-Bus (and, perhaps; a sysfs entry). Feel free to look at fmtxd's strings.

I have an original Nokia car charger lead and find that the N900 FM output works but is badly attenuated when the charger is plugged in.

This is a serious problem for me because I planned to use the FM transmitter to listen to podcasts while on long drives.

Until reading this thread I had assumed it was something to do with the power cord grounding the N900 to the car.

Repost:

I'm not sure if this has been pointed out in another thread or not but I thought I'd mention it since it's really helped me out.

Currently, the N900's FM Transmitter either deactivates or is reduced to an unusably low power level when and AC adapter is plugged in to the Micro USB port. This sucks since transmitting over FM burns your battery and there's usually a cigarette lighter socket nearby just begging you to charge off of while you listen & drive.

Well, I noticed that if you install fm-boost and execute it AFTER plugging in the power adapter, the N900 resumes FM transmission at the full power level of 118 even while charging!