trojanman

I've had a continuing saga over the past few months regarding the at-rest discharge of my battery. It's been a frustrating thing because of the somewhat intermittent and inconsistent nature of the problem (sometimes takes several days, sometimes overnight, etc.).

I replaced an older battery back in June and was generally worry-free up until about a month ago when the problem began recurring with increasing frequency. In the past week, the battery would drain overnight to the point that I couldn't start the car in the morning.

I have been the poster child for "search is your friend" here on Rennlist and have printed many threads dating back to 2003 dealing with this problem in both 993s and 964s. Because of the dreaded "CCU fan staying on" issue being so prevalent, I kind of assumed that once I got around to troubleshooting the problem with my own car I'd find that it (the CCU) would in fact be the culprit.

So I bought a multimeter and set out yesterday afternoon to confirm my educated assumption. Keep in mind that at-rest (key out, alarm set, all systems "off") current draw (in milliamps) expected for our cars is around 20 - 30. The CCU fan that is designed to stay on for several minutes after ignition shut-down adds somewhere between 150 - 200 mA to the at-rest condiditon.

I observed both of these readings myself, and my CCU fan turned off after ~15 minutes as designed. So now what? I began turning systems on and observed the changes. With doors open (all interior lights on, including boot and deck lid), draw jumped way up to ~ 2.5 amps... yes folks, your little interior lights draw a lot of power.

Adding to the condition above a turn of the key to the "on" position, draw went up to ~ 8 amps. So now I turn the key off, close the doors and set the alarm... at this point the CCU fan is NOT running and I'm seeing around 400 mA... hmmmm.

So now I start pulling fuses one at a time and nothing happens until I get all the way down to no. 38. I pull that one and the draw drops immediately to 20 mA. Ah-hah!! Fuse no. 38 is a 15-amp "radio/booster" fuse. Keep in mind that not only was my CDR-210 off when I was checking, but the radio faceplate was removed. Obviously the radio was getting power even with the faceplate removed and the unit turned off.

This was not completely shocking to me as I had noticed that my radio would sometimes be on when I returned to my car after exiting and setting the alarm... I assumed that there was a short somewhere but figured that popping out the faceplate each time I exited the car would solve the issue. Obviously I was wrong.

So bottom line? My car started right up this morning for the first time this week. My radio doesn't work (obviously), but at least I feel I've isolated the problem. So where do I go from here and further diagnose the radio/amp problem? Do I assume it's the head unit and replace it? Would it be easier to just hard wire the power to the radio so it only works when the ignition is on?

Advice sought please, and if anyone needs help with setting up your multimeter in line in order to read current draw let me know (it's a little tricky and can be dangerous).

Thanks.

brucec59

Interesting... First of all, putting power to the radio only with the key on would mean you'd loose your station presets every time you turned off the car. Do you have the hi-fi option? Maybe the amp is staying on. You should hear a click from under the passenger seat when the amp turns on and off as you turn the head unit on and off. Or perhaps your antenna booster is always on? Either of those conditions would be the result of incorrect wiring of the stereo, where the accessories (amp and/or antenna booster) are not wired correctly to the wires that tell them to turn on and off.

trojanman

Good point Bruce, although I think I would not lose the presets but would need to enter the security code (this is what happens when I've disconnected the battery in the past).

Yes, car has hi-fi. It does stand to reason that the amp is what is remaining "powered"... what else would draw ~.4 amps? Although I don't understand why the radio would come on intermittently then.

hoggel

You have an arena red S? In that case I'll take a look at the schematic tonight and see what is powered by that fuse under what conditions. Do you have any aftermarket electronics in the car, radar detector, amplifier, ... ?

trojanman

Ha, yes I do! Only aftermarket electronic stuff are HIDs and a radar detector... I installed the detector and it is hard-wired into the hot side of the sunroof switch (only hot when ignition is on).

Thanks in advance for your help.

hoggel

Fuse 38 only powers the radio (connector pin 4) and the amplifier (connector pins 10 and 11).

From the diagram I see no key switch control to the amplifier. The key switched power to the radio is on connector pin 7.

The schematics show no power connection to the antenna amplifier. The DSP in the door is powered by a different fuse.

If the radio is sometimes on when the key is off then the electrical part of the ignition switch may be intermittent? The radio contacts on the ignition switch don't control anything other than the radio.

trojanman

Thanks brother... maybe the ignition switch is the issue.

geolab

what do you mean by ignition switch?
I do not know if both versions are identical (US-ROW), but ROW even with key in the pocket,
you can turn the radio on.

hoggel

The power for the radio is on with the first turn position of the key (by the schematic).

There are contacts labeled "BUZZER CONTACT IGNITION LOCK" that are made when the key is inserted. From the diagrams that set of contacts goes to the tiptronic, the gong (key reminder), and the anti-drive-off.

trojanman

Same here in the US, however radio should shut off and stay off when ignition is turned off.

brucec59

I cannot turn my radio on with the key out. The key has to be on at least the first click to the right before I can turn on the radio. Mine is a '95, which might be different.

trojanman

Must be different. Radio on both my former '96 and my current '98 will power up without key.

Chuck W.

Jeff .. I have an extra CR-220 radio. How about I loan you the radio and you swap out the radio to make sure that's not the problem. It's a plug and play with you model.

hoggel

Can you turn the radio on without the key in the ignition? According to the schematics in the factory repair manual the key in/out contacts have nothing to do with the radio.

Since the radio does have power all the time from fuse 38 there may be a logical turn-on with key turn OR the radio ****? The head unit will eject a cd without the radio turned on so it's not completely asleep.

I'd check to see how mine operates but it's stored in my sister's garage for the winter.

ltc

Modern headunits are never fully asleep. They require a continuous connection to B+ and the car manufacturer usually has fairly tight specs on quiescent (dark) current for any module given a permanent connection to B+

(FWIW, modern MOST DSP amplifiers are spec'd at 250uA or less, modern headunits are spec'd at 2.5mA or less).

When connected to a slave amplifier (by wire, not plastic optical fiber as is used in all MY03+ Porsches), a remote turn on line is routed from the headunit to the amplifier. The headunit usually receives an input called SWB+ (or ignition in), as a signal to begin full powered up operation.

If you have a draw on F38, it could be a sneak current path between the 2 modules, likely due to a defective component on either one (amp or headunit).

(FWIW, I designed these for a living....for Porsche)