Winterizing a power antenna - PeachParts Mercedes-Benz Forum

donbryce · #1 08-07-2003, 03:15 PM

I'm posting this as a 'sanity' check of my reasoning, and to hopefully avoid frying my electrical system and/or power antenna circuitry.

What I want to do is conveniently be able to have my power antenna 'up' all the time (in winter), or go up/down with the ignition switch (rest of the year).

On my power antenna, there is a wire for a constant 12V source (or one that is that is 'on' with ACC and/or IGN), a wire that is supposed to see 12V (radio 'trigger') only when the radio unit is turned 'on', and a ground wire.

My stereo head unit doesn't have an antenna 'turn on' lead, so assume I'm working with a switched (ignition in ACC or RUN position) 12V trigger. I always have the radio on, so this is no problem. I also go to brushless carwashes, so having the mast always up when running the car is fine. I want the antenna to go down except in winter, though, to protect from vandalism (vandals don't like cold air, right?), and I also want to be able to retract it at the flick of a switch even in winter mode (putting car on a hoist, for example).

What I'm thinking of is connecting a constant power source with a toggle switch to the 12V constant input wire of the antenna, and the trigger (radio turn-on) wire to a switched ACC source. As I understand it, the 12V source, not switched by the radio turning on/off, is for the purpose of applying power to the antenna motor after the radio is turned off, to lower the mast. I believe the power antenna has a circuit that checks the current draw of the motor to measure it's load, and when it exceeds a threshold, stops it. So, when the antenna is down, the constant voltage wire should not be connected to the motor and no power should be drawn from the battery.

For summer season use, the toggle switch will be on, allowing the antenna to go up/down with the ignition on/off. For winter use, the toggle switch will be 'off', so no power can get to the motor to retract the mast. If I need to retract it temporarily, I'll turn the toggle 'on', then 'off' to resume winter (always extended) mode.

Is this too simple to be correct, or is there a flaw in my reasoning here?

wjm · #2 08-07-2003, 04:53 PM

Don,

Sounds crazy to me.. but you're the boss

Anyway, if I understand correctly, you want the antenna to be up ALL OF THE TIME in the winter. In the non-winter months, you want it to go up with ignition switch.

I'm no electronics expert, but were you planning on using a SPDT switch?

A toggle switch of this type USUALLY has two positions of the handle and three terminals on the back. One position of the handle makes a connection between two of the terminals on the back (normally the center and one end), and the other position of the switch makes a connection between another two terminals (normally the center and the other end). This is sometimes called a FORM C contact switch. This switch can also exist in the form where the handle has three positions where the center position makes no connection between any of the terminals on the back. Sometimes called a CENTER OFF switch.

Anyway, if you find switch as above and rated for 12 volts and reasonable amps, connect a constant power wire it one of the outer terminals on the switch. Connect an ACC wire to the other outer terminal and then connect the antenna trigger (from the antenna) to the center. That should give the the winter and non winter modes you are looking for. If you get the center off switch then you have a 3rd choice... always off (like for when you're just listening to the CD player.

Again, wouldn't that work?

Normally, a radio's antenna trigger wire is a constant 12 volts when the radio is on... so I would think that your premise about the antenna's "auto-shutoff" mechanism is valid.

donbryce · #3 08-07-2003, 08:53 PM

Thanks for taking the time to think about my set-up Willy. I assume your design connects an unswitched 12V source to the 'constant' 12V wire to the antenna (so the mast will retract in 'summer' position).
The connection of the antenna 'trigger' wire to the center pole of the SPDT switch is, then, to connect it to the switched source for up/down operation, or to the same constant (unswitched) source for 'always up' mode.
I see how the 'summer' switch position essentially connects the antenna normally for up/down mode, with the trigger wire only seeing 12V when the ignition is on.
The other side of the switch would act like the car was always running, or ignition on, by sending 12V constantly to the trigger wire. And as you point out, there should be no drain as the antenna circuitry should cut power when full extension is reached.
However, all I planned was a SPST switch to cut the 12V required to retract the antenna. Your design 'holds' it up with constant 12V to the trigger wire.
Hmmmmm, interesting option. Let me think awhile to compare and see if there are advantages/drawbacks in these 2 set-ups. For now, I'd say either should work.

wjm · #4 08-07-2003, 09:10 PM

Don,

I have not looked at the wiring of my antenna in ages... so I'm really not sure how it's powered... but I'm assuming that there is a ground, a constant 12v and a 12v trigger wire. Where the trigger has voltage, then antenna stays up.. otherwise it goes down. Again, that's my assumption and we'll have to see what others say.

If, for some reason, my idea does not work because it will put a drain on the battery when the antenna is up and the car is off... then we can come up with a slightly different strategy. ... maybe using a relay. Let me think about it and repost later.

wjm · #5 08-08-2003, 01:59 AM

Don,

Please tell me how your antenna is working now since your radio does not have an antenna trigger wire.

I suggest you give my original idea a try. First, though, you may want to test the current draw on that circuit (at the fuse) before and after. I would first test with key off while leaving the antenna wired as is. Then, connect 12v constant wire to the antenna trigger. This should make the antenna stay up. Now check current draw again. Hopefully is stays nearly the same. If you can, get a friend to connect the trigger wire the the 12v constant wire while you watch the current.

If there IS a current draw while the antenna trigger wire is powered, then we'll have to look at an alternative. Possibly check the current with the trigger wire powered and the antenna constant wire disconnected. If there is no draw, then we can use an SPDT switch and a SPDT relay to make this work. I have it drawn up and will share it with you if needed.

wjm · #6 08-10-2003, 09:51 AM

Don,

What's the latest?

donbryce · #7 08-10-2003, 11:52 AM

Here's a bit of background on the situation. The car in question is an 88 Mazda 929, which came to me with the power antenna mast kinked, so it won't retract, and the power lead disconnected, so the motor won't get burned up. These cars apparantly use a ground (- 12V) trigger instead of + 12V to power up the antenna, employing a fancy relay gazmo at the antenna connection. My replacement unit, on the way, has a standard 12V + trigger, hence my intention to use switched ignition source instead of that from the stock radio.
I think I'll follow your suggestions re. checking the current draw when the mast is 'up'. Frankly, though, I don't think the unit will draw any current after it's either all the way up or down, otherwise, the battery would go dead in a very short time, right? I guess what all this comes down to is the difference between disconnecting the constant 12V wire with a switch, so the motor won't retract, and doing this plus applying 12V to the trigger wire to make it behave like the radio is on.
I'm questioning why bother with the trigger wire when the mast is up anyway. and it shouldn't go down until the 12V constant wire sees current to reverse the motor and retract it?

wjm · #8 08-10-2003, 01:22 PM

Don,

Could you not have just replaced the mast and rehooked all the wires?

If not, you can still use the new antenna and have the entire system work EXACTLY like it did when the car was built.

Assuming you still have the original radio in that Mazda, the radio puts out a negative trigger when you turn the radio on.

Using a simple SPDT automotive relay (<$5 on ebay), you can convert the radio's negative trigger to positive. An SPDT relay is basically a switch (an electronic one).

How? SPDT relays have 5 connectors on it. 85, 86, 30, 87 and 87a. 85 and 86 power the internal coil, so when 85 has 12V + and 86 has active ground, power will flow from 30 to 87.

So, if you connect a 12v + constant to 30 and 85, connect radio - trigger (-) to 86 and connect the trigger from antenna to 87 (+) (87a is not used), then this should resolve this problem.

When your radio is off, antenna trigger IS NOT grounded (-), so the internal coil between 85 and 86 is not energized. That means that continuity is make between 87a... and since we're not using 87a, nothing happens except for the antenna going down.

When radio is on, antenna trigger IS grounded (-), so the internal coil between 85 and 86 IS energized. That means that continuity is made between 30 to 87 and since 30 has a +12volts, so will 87, which is now connected to your positive antenna trigger.

Looking at pictures of this will make it easier to understand.

See http://www.the12volt.com for more beginner info on SPDT relays. Site is sometimes down, so be patient. Also check out http://www.relayhelp.com/

wjm · #9 08-10-2003, 01:23 PM

Here is a picture (top one). Same as what I am saying except they flip 30 and 87. Also, replace "Car Alarm" with "Radio's negative antenna trigger"

donbryce · #10 08-11-2003, 07:00 AM

Thanks very much for explaining this so clearly. I'll have to test to find which pin is the - trigger, and the relay can be used to convert this as you describe. That would be preferable to using an IGN source.
As to the mast replacement, I had ordered a new unit complete at a bargain price, but it was defective, so it's been returned and a replacement is on the way. I'm too far into this now to not use it and simply replace the mast, which would be about the same price here anyway.
I'll post again when it arrives .