Thread: P38 key FOB LED function does not mean the key is transmitting

I sent this to rangerovers.net to hopefully be added to their brilliant writeup on the P38 key FOBS/Remotes.
This should help others having trouble with troubleshooting the security system.


When you look carefully at the circuit inside the P38 key FOB you can see that the illumination of the LED has nothing to do with whether a signal via 433MHZ(Australia) is being transmitted. The LED is in series with the battery and lock/unlock micro switches only - so it just indicates whether battery voltage is OK or the switch is OK - certainly not that it is transmitting a burst of 433MHZ.

To test the handset it must be activated next to a RF detector set(available in some autoshops) in order to the observe an RF burst.


Without this information the diagnosis of a P38 locking issue can easily go astray. I.E. try to operate the locks - you notice the LED is operating - so you assume a valid RF transmission - so you assume it is not the key FOB and you start replacing other parts on the car. Therfore, using the LED to assume the FOB is serviceable leads you in the wrong direction.


My handset was illuminating the red LED on each press of the lock/unlock and yet there was no RF output at all - tested at an electronics workshop. I had an electronics professional solder in a new 433 MHZ SAW resonator(Australia). Before leaving his workshop we could observe that it was now transmitting a burst of RF each time we pressed lock or unlock - eventhough it was not synchronised to the vehicle. I returned to the P38 and carried out a synchronisation and now I have a fully functional FOB.

I have been going over your post in my head and was convinced I saw my remote led come on now and then but could not remember when...

My buttons have all but disintegrated and I still need to rebuild the remote (I got a kit laying around SOMEwhere) but I know the remote worked till the last button fell out. At first I thought it lit up during opening or closing of the door lock but that was not the case. I did notice recently though, and have confirmed it, that when I put the key in the ignition and turn it over to crank the led comes on for a fraction of a second. It's not a loose contact in the key as far as I can figure, I tried turning the key around in every way possible outside of the ignition or in the lock and it just won't flash.

So, perhaps it's just me but it seems that the car can activate the key somehow when cranking or I have this very peculiar behaviour? I do own the european version of course but afaik they operate on 432mhz as well.

Cheers,
-P