1992 400SE.
The radio works fine while in Key is in the ON position (vehicle off) or while idling, but as soon as you put it in gear, the speakers go mute. The radio is still illuminated and I can change stations, but no sound is comming through the speakers.

I have to restart the car to get it working again.

Before I owned the vehicle, some already removed the stock AMP and put in an aftermarket Pioneer. The wiring harness has been changed, and the charging voltage off the battery reads 14.4V while idling and 12.4V while off.

Any ideas??? Thanks,

Sounds like that Pioneer amp is having a problem.

I have a '92 300SE, and if I'm not mistaken, the Bose systems that came in our cars had a speed-related volume change ability. As the car speeds up, the volume increases - when it slows down, the reverse is true.

I'd look to see if that particular wire (and I think it's a wire separate from the main radio harness) is the source of your problem, especially since the radio effectively goes mute when you put the car into gear.

According to AllData, there is are two wires, that are Green/Black stripe in color that connect to the vehicle speed sensor. They go from the radio to a terminal block X46/5 in the right footwell. I've attached a picture of where it lives.

Again, according to AllData, a square wave is transmitted along this wire - 200hz at 20mph and 330hz at 35 mph. If you disconnect the wires that go to the radio (don't know if other systems pull from that point, so beware what you take off), the radio should "think" that the car is at rest all the time.

There's also a cell-phone mute circuit, which is a Yellow/Black stripe wire that plugs in on the radio adjacent to one of the Green/Black wires mentioned above. There wasn't any information as to what type of signal is present on this wire if the cell phone is in use.

1992 400SE.
The radio works fine while in Key is in the ON position (vehicle off) or while idling, but as soon as you put it in gear, the speakers go mute. The radio is still illuminated and I can change stations, but no sound is comming through the speakers.

I have to restart the car to get it working again.

Before I owned the vehicle, some already removed the stock AMP and put in an aftermarket Pioneer. The wiring harness has been changed, and the charging voltage off the battery reads 14.4V while idling and 12.4V while off.

Any ideas??? Thanks,
Your battery/altenator system is working well since 14.4V is fine; so is 12.4V when off.

Check MENU#14 for the pin I/O wiring for a BECKER radio. Pin 1 is usually the speed/radio volume signal.

Sounds like Pin 3 is active (ground) so lift the wire and see what happens.

I was on my way to get the computer scanned (other idling problem), and the speakers would come on intermintently, then shut off for 3-4 sec, then come back on. I'm thinking its a loose connection either at the aftermarket Pioneer Amp or aftermarket Pioneer Deck.

I had a bunch of codes in the computer, which were cleared, and I will return in a few days to see which have come up again. (As many may be old, and never been cleared).

I didn't make note off all codes, but I remember one was the Left and Right Front Speed Sensor. (The mechanic said this is common, due to when Emission Test are done, the rear wheels are spun, and not the front, therefore causing this code to store.) But it may tie into the radio problem.

I'll report back when I go to have the codes read again.