Hi guys, still working the electrical gremlins out of my new acquisition (03 XJ8) and have repaired the ABS module (thanks LOADS for the pix!!!), replaced the security and lock module due to the brake lights staying on all the time and making the last remaining transistor VERY hot (which obviously means that the PO lost a brake light and spliced the two together), but the last remaining bug seems to be that the radio will not always accept input from the controls (either at the head unit or from the wheel switches). When this occurs, the only thing I CAN change is the volume via the face plate knob...I can't even shut it off! Then suddenly, everything will function normally again. I'm a bit mystified by this as I am used to losing one function (due to a bad/dirty switch or something) or losing the whole unit due to a broken wire, bad plug etc...but the fault is usually there or it's not. There are no funny noises from the radio when it plays, the tape unit functions normally and sounds good. There is no static or other evidence of loose connections, etc...sometimes it's fine and other times I'm stuck with the local classic rock station here in Indy. Some thoughts would be appreciated if anyone else has experienced this. I searched the threads here but didn't find this specific problem. Thanks for all the great help and pix so far.

 

The radio buttons are multiplexed, which means that a power source is sequentially applied to groups of switches in a matrix, and the power is sensed among groups of switches along the othe raxis of the matrix. That means that if one switch sticks on, like if it has contaminated contacts, (cold beverage from a tall glass in the cupholder in my case), or a mechanically stuck switch, none will read. Try "tapping" each button vigorously when this occurs and see if you can identify the stuck one. Disassembly and contact cleaner is the best solution.

 

Thanks sparkenzap! I will try this in the future, but today I noticed that the radio was playing hard to get again when I came out for lunch. It is currently over 90F here in Indy and the interior was quite warm. After vigorous application of A/C for about 10 minutes or so, the interior was fairly comfortable and the radio began accepting commands again...temperature related or coincidence?! I'll have to pay more attention to the temp in the cabin when troubleshooting and I'll get back to you on what I find out. Thanks again for the input.

 

Good observation! well, the aformentioned muxes are IC circuits and certainly have potential for thermal failure modes....

 

My wife's VW New Beetle currently has this issue. She splashed Coffee on the buttons a while back and now once in a while one sticks. I have yet to disassemble and clean her radio, but I've had this type of error before, and contact cleaner (or I've used rubbing alcohol - Be SURE to allow to dry before reassembly) did the trick. Tapping vigorously like mentioned by sparkenzap will usually tell you which one it is, although I end up cleaning them all while I'm in there, it still gives you confirmation of the correct diagnosis.