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Garmin SL40 won't Transmit

My Radio is a Garmin SL40.

I Flew recently and with airfield information and a radio strength 5 took off.

Changed frequency to Shawbury Zone and requested basic service. Later I wanted to change frequency to London Information. Made my request to Shawbury but only manage a dull silence in my headset, although I could hear all incoming communications. The controller asked if it was my call sign trying to transmit, and if so, ask me to press the IDENT button on my transponder.

My radio will receive at radio strength 5 but transmitting is zero.

Regarding sidetone – When I press to talk there is a split second where everything is normal then the sidetone disappears and all is quiet and dull in the headset.

On the ground I tested the two way communications with a handset transceiver and the aircraft radio. When I try to transmit from the aircraft radio the handset gives out a loud buzz.

I would be most pleased if a solution can be found.

Regards John

Garmin support are quite helpful. Have you tried them?

EGTK Oxford

Do you have any audio panel, and do do you have in intercom? If so which units and does the intercom function fine all the time?

Do you have an emergency hand mike, does that work fine?

Did you try to transmit without any headset / microphone connected, did it generate a loud buzz as well?


The headsets go into an intercom PM501 and then on to the Garmin SL40.

The intercom works OK. I don’t have an emergency hand mike.

I will be at the field tomorrow and try the transmit without the headset connected.

Regards John

If transmit clearly you might have an issue with your microphone audio wiring. This wire goes from pin 12 on the PM501 to pin 8 of the SL40. You might also want to check pin 7 of the SL40 which should be a ground connection.


Back from my further investigations.

Looked at the copper pin which was protruding from pin 9 on the SL40Radio connector. Connection OK, the original installer has soldered 2 wires at this point.

Cut the ground from the PTT switch and rerouted directly to a good ground. Problem still remains. I could say the problem has developed in that when I press the PTT on either P1 or P2 there is a buzz in both headset and the handheld transceiver, whereas before the buzz was only from the transceiver and not the headset.

With the Headset plugged into P1 buzz is present in both headset and the handheld transceiver when I press either P1 PTT or P2 PTT.

Also – With the Headset plugged into P2 buzz is present in both headset and the handheld transceiver when I press either P1 PTT or P2 PTT.

I have checked the continuity of as many wires as I can. Headset jack sockets, various ground wires and PTT wires to the PS501 intercom and then wires from the PS501 onto the radio.

I have noticed for a very long while, but thought everything was OK, when I press any trim button there is a slight light shadow on either side of position indicator for both the roll and trim indicator. The operation of the trim and related bright indicator light is OK. It is the scatty light shadow that is a mystery.

I am starting to think that it could be the radio that’s on the blink.

Any thoughts would be most welcome.

Regards John

Try to transmit without ANY headsets AND the intercom in the OFF position. The intercom should work in emergency mode, and still transmit. Listen on your hand radio now. If it creates a hum, it is likely your radio needs repair or replacement.



I will try that on Sunday – My next visit to the field, (85 mins journey each way) after which, like you say a repair or replacement.

Thanks for the assistance- greatly needed as I am OK on mechanical but poor on electronics.


Your welcome. If you fly on a European registration you might need to consider that should you replace it, you should either repair or replace it again with an SL40, or upgrade to a new 8.33 kHz radio to stay legal. And that replacement by the same radio might not be worthwhile. Though there are quite some SL40’s around, so you might find yourself a bargain good for the next couple of years

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