WillyC4S

Does anyone know what the current draw is for a 993 when the car has only the immobilizer active?

I'm having some battery longevity issues w/ my Optima Yellow Top now. I usually have the car on a battery maintainer and have noticed that the last few times when I've had the vehicle just sitting around w/o the charger, the vehicle loses charge in several days (about 3-5 days and I can't start it anymore). The vehicle did not exhibit these symptoms half a year ago. The battery is about 18months old.

Here's what I've measured in my car and I don't know if this falls within acceptable standards or if my car is drawing too much current (i.e. a short someplace in my 993's electrical system):

1) Front hood open (so trunk light and interior lights are on) and immobilizer is off. Measured current is 2.46 amps.

2) Front hood closed (so trunk light and interior lights are off) and immobilizer is off. Measured current is .27 amps.

Car has not been started or run in several days. Readings were taken under these "cold" conditions. Ignition off (i.e. no keys in car) in all readings.

I tried activating and deactivating the immobilizer and it seems that the current draw is negligible (reading doesn't change on meter).

I took the reading by inserting the multimeter in series to the battery. One probe on the negative battery terminal and the other probe on the ground cable. Current will flow through the meter and this is what's measured as consumption.

I'm trying to diagnose whether it's my battery not holding a charge or if the car is drawing too much current during "no load" situations.

I believe that the Yellow Tops have a capacity of 55amp-hours. At a draw of .27 amps, this would mean that the battery could sustain about 203 hours before expiring. This is the equivalent of ~ 8.5 days.

At .27 amps this would almost concur with what I'm seeing in reality (i.e. 3-5 days before a no-start situation). I'm guessing that at this point it's a short.

Any help or comments highly appreciated. Any ideas where common places where a current draw like that would happen?

Thanks,

*****

yank

easy way to find the draw is just to start pulling fuses intill your meter changes.

WillyC4S

I was just thinking about pulling those fuses. When morning rolls around here that's what I'm planning to do. Thanks for the input.

Has anyone checked out their no-load current draw though? What should it be consuming?

Thanks,

*****

Blizzard

*****,
Keep us posted on your findings. I am having the same problem - new car to me - not sure how old the battery is.

WillyC4S

Paul,

Have you measured how much current your 993 consumes at rest? Seems like most people haven't done this one yet for their 993's.

I actually got more info from the 911 boards, seems the older 911's were drawing down about 12mA to 20mA, not like my 250mA.

The pre-993 911's had issues with the door switches, trunk light switches and other utility light switches not functioning and causing the lights to stay on and drain the battery. It's probably a good guess that our cars have similar issues as well.

I'll be checking those items out and posting my results.

*****

Blizzard

*****,

You are right. On my 86 I have finally pulled the bulb out of my trunk light.

How do you test the draw on the 993?

Thanks,

WillyC4S

Paul,

Judging from the lack of response must be a bunch of software guys on the board.

To check the current draw you have to insert your meter in SERIES to the electrical circuit at the battery. To check the voltage drop, you just measure the voltage across the positive and negative terminals.

These are 2 different things. Measuring the voltage is an operation that is performed in parallel to the circuit, but measuring current is an operation that is performed in series to the circuit. The latter is by far the more riskier operation since the meter actually has potentially large amounts of current passing through it (2-5 amps). It's acting as a short (zero resistance) when current is measured.

To do the check you have to set your meter to measure DC current and then make sure that your probes are plugged into the correct jack that can sustain up to 10 amps of current. There are usually several jacks on the meter with one jack that will accomodate this high current capability. Usually black line (ground) doesn't change, but the red line (positive) is the one you have to correctly plug into the jack. This is important since you'll blow the fuse of your meter otherwise.

Remove both battery terminal leads to prevent electrical surprises. I also removed the spare tire, tire pump & jack to give working room inside the well of the trunk. Also unbolted the battery mount so battery was free to be pulled out. Be careful of the vent tube on the battery to not snap it off or lose it. Optimas don't have this since they're totally sealed.

Then connect one of the probes to the just removed battery line (say negative ground cable) and then the other probe to the corresponding free battery terminal (negative terminal). You're meter is now in series to the circuit. Careful here since once the positive lead is connected to the battery your meter could have several amps of current going thru it.

Complete the circuit by connecting the positive battery clamp to the positive terminal. Now you're meter is reading your current draw. You might have a negetive reading, but just take the absolute value of it and that's your consumption.

I'd make sure all the meter connections to the battery and cable are solid before attaching the positive lead back.

If you plan on trying this, let me know what you come up with.

Thanks,

*****

WillyC4S

OK, here's a very interesting update:

I found out which fuses are causing the current drain. It wasn't easy to find this one and was almost by accident. It takes 2 fuses to be removed before the current drain goes away; not obvious at all.

Fuse #11 (engine and luggage compartment light, glove box lamp). Removing this fuse dropped the current consumption by ~10mA. Not significant, but at least it showed up on the meter.

Fuse #29 (interior light).
Removing fuse #29 and having fuse #11 installed (like you would normally do when testing a circuit to precisely isolate the cause, removing 1 at a time only), the meter shows no drop whatsoever (slight dip and then back to previous reading, but no permanent drop).

Removing fuse #29, without fuse #11 installed, the current drop went to zero (this is good!). Somehow both circuits are shorted together is how I'm interpreting these readings.

I'm going to look at the electrical schematics to see what's going on. Anybody else interested in this? Anybody else had these problems before?

Thanks,

*****

neunelf

*****,
I just read your first post! This is *EXACTLY* what is happening to me!! I feel like I just read my thoughts.

I have a (new)yellow top and in 4 or 5 days it gets drained flat. As I am sure you know don't let a yellow top go completely flat; they are a pain to recharge.

I made a post a couple of weeks ago suspecting that my alternator/resistor was going bad but I have since come to the belief that it is caused by an excessive current draw because I can duplicate the end result of a dead battery just by letting the car sit.
The timing of your post is perfect!

I cant give you my voltages because I lost this.


My MIA cigarette lighter plug in voltmeter.

To answer the question in your last post, "Yes, I am very interested in this"

I know little about the electrical system of a car so I can't add any thoughts on how to solve this but I could buy a handheld voltmeter and play with it.

WillyC4S

Brad,

Good to know that I'm not the only one having these problems.

Did you try to remove the two fuses? I've taken my #11 and #29 fuses out and am trying to see if this will make the problem go away. I'm almost certain this will temporarily fix the battery drain issue since current consumption is down significantly. I was measuring about 25mA to 30mA with the immobilizer turned on. Actually there wasn't a measurable difference between the immobilizer on or off, just a current spike to about 120mA when you turned it off and then the spike went away after 20 seconds and settled to the above readings.

I haven't checked the schematics yet, but I'll do that tonight and see what's up.

*****

Randall G.

Hey *****,

Got out my wiring diagrams, looking to pitch in and help. Surprise, surprise, on the 964, fuse #29 is unused. The engine, luggage compartment, glove box and interior lights are all supplied by fuse #11. It looks like fuse #29 is also unused on the '95 993 (the latest model 993 for which I have a complete set of electrical diagrams), as well. This according to sheet #17.

So ... it looks like sometime after '95, Porsche decided to add a separate fused power source (#29) for the interior lights.

I also think your test results indicate some sort of short between the power lines (but why?). It looks to me like fuse #11 continues to supply whatever high draw is otherwise being provided by fuse #29. Or vice-versa.

Here's a quick schematic that--in theory--might explain why current drops 10mA when fuse #11 alone is pulled, but not when fuse #29 alone is pulled:



12V/.27A = 44 ohms
12V/.26A = 46 ohms

With fuse #29 removed, fuse #11 still provides 12V to node A--no drop in current. With fuse #11 removed, 12V from fuse #29 must pass through an extra 2 ohms of resistance, dropping the current by approx. 10mA. Like I said, just a theory.


It will be interesting to hear what you can figure out from the post-95 wiring diagrams.

WillyC4S

Randall,

Thanks for all the help. Good to see somebody else that's done some EE stuff as well. I think your conclusion is identical to mine, although I didn't get down to do all the calcs on it like you did.

The schematic you drew does emulate the logic of what I'm seeing. The question in my mind is how these two independent circuits were bridged to allow this electrical behavior to happen in the first place. That's really puzzling me. I think the immobilizer system could be the culprit here.

I took a look at what I had as far as schematics go and I don't have the '97 year electrical diagrams so I'm stuck as well.

I did notice some very strange behavior though. After I took out the 2 fuses, I was checking the current draw again and then put the fuses back to get a final reading. Guess what? The damn circuit is functioning correctly w/o producing the 250mA draw I saw earlier. Now I can't reproduce the drain!

Also some more gremlins associated w/ the immobilizer system. Lately I remember that turning on the immobilizer would cause the LED lights to double blink and there were no initial flashing for 10 seconds; the 993 manual indicates this is an indication of a fault. I believe the car used to flash the LEDs for 10 seconds and would NOT double blink the LEDs, only a single blink; can't remember exactly since I don't drive this car daily. The 993 manual indicates the later is the correct indication of a fault-less state.

After I removed the 2 fuses, the immobilizer system would indicate a fault-less state instead of the double-blinking LEDs I saw earlier. This is expected.

On top of this, now that I can't reproduce the current drain, the immobilizer LED's are indicating a faultless system, even when the 2 fuses are replaced in their original slots. This is unexpected.

I'll have to think about this some more, but w/o a '97 schematic I can't do much. My suspicion is that the immobilizer system is at fault somehow since it's the single element that sees all the inputs sources which are fused by #11 and #29 (interior lights, luggage compartment light, engine compartment light and glove compartment light). Assuming that #11 and #29 are truly independent circuits, then logically the immobilizer is the bridge between the two.

I'm actually contemplating just running the car w/o the 2 fuses. It's just the lighting that gets affected. The alarm still works when a door is opened, but I'll need to totally verify if all the other inputs still trigger the immobilizer as if the 2 fuses still are in circuit. Wish I had that schematic, would make it very black and white.

*****

Ray Calvo

*****, the '97-'98 wiring diagrams are available in Suppl. 21 of the shop manual. I bought mine in March 1998. For info they're up to no. 25 now.

As you probably know, the supplement no. is shown at the bottom of the page.

Might go to MY-Porsche and see if you can get all the updates for your shop manual.

Good luck.

For info, I also have a voltmeter like Brad - safely tucked away in my toolchest. Tried looking at it for any info for y'uns to search and find your own, but all it says is "Battery meter".

<a href="http://www.my-porsche.com/technical/tech2.htm" target="_blank">MY Porsche Technical Publications</a>

neunelf

I have bought my multimeter and have now got some readings.

I don't know a lot about what I am doing but I am learning.

With the meter set at 20m and the trunk and interior lights on (trunk open,alarm Not set) I got a reading of 2.21.
With everything off (trunk closed,alarm Not set) and the meter still set at 20m the reading was .06
For anyone in the know is this excessive? I would not assume so if compairing it to *****'s first post, but why then does my battery drain in 4 or so days.

Should I start pulling fuses randomly?
FYI when I pulled fuse 11 AND fuse 29 the meter dropped from .06 to .03
The two fuses pulled alone didn't change the reading. Is this normal?

*****, glad your problem is fixed but sorry you haven't figured exactly why.

Randall G.

Hi Brad,

Any chance you can post a picture of your multimeter's face? I'm not sure what setting you had your meter on, but ... I'm going to guess your initial reading was 2.21 amps, consistent with *****'s reading with the interior and trunk lights on (2.46A). If this is the case, your .06 reading would equate to .06x1000 = 60 mA = too high.

&gt;Should I start pulling fuses randomly?

Seems like a good idea ...

&gt;FYI when I pulled fuse 11 AND fuse 29 the meter dropped from .06 to .03 The two fuses pulled alone didn't change the reading. Is this normal?

Doesn't seem normal to me .... but, fuse #29 is unused on my car, and I don't have the appropriate wiring diagram for your '96. Any chance you have the diagram? I went to Ray's link above, and it says the '94-98 wiring diagrams are no longer available.