Audio Systems: bmw 528e power antenna, power antenna, metal chassis

Question
Hi,

I just replaced my OEM power antenna with a Hirschmann.  It's new and it worked fine when I first installed it - two incoming +12V lines, and one ground wire.  The red incoming is always hot, and the white one switches on and off with the radio, initiating the up-down sequence.  

Bizarrely, a few minutes after I'd installed it and cycled it up and down a few times with the radio power button, I turned off the car and the antenna stayed up.  I checked the voltage on the red and white wires coming from the radio, and instead of approx. 10.5V (where they were before installation) they had gone down to less than 3V, not enough to power the antenna.  Checked the antenna with a 12V aux battery, still runs fine.  What could be causing this strange low voltage situation in the leads from the radio?  Turning the car on doesn't change anything.  And I checked the voltage at the fuse under the hood, that was 12.8V.  Any ideas?

Thanks!

Answer
First thing; check the ground connection. Make sure it has a good metal chassis ground.

Does your vom have a current measuring capability? If so, you could measure the current being drawn.  Because the voltage at the antenna is only 3 v it is likely there is a voltage drop somewhere; such as the motor of the antenna drawing too much for the line you have it connected to.

Disconnect the wires to the antenna. Does the voltage go back up?  Check the fuse port to make sure good contacts.

If you still haven't found the culprit, try running a power wire directly from the battery and cycle it to make sure it is fine.  If so, use the direct wire along with the turning ON wire.

Hope these ideas help.

C