Mrmerlin

a weak battery makes it harder to find the hot spot,
a freshie changes the odds in your sleuthing favor.

Randy V

I already know that the current draw is 40 milliamps. That ain't gonna change with a new battery guys.

I'm good, really. Thanks.

dr bob

You may be right. Measure again with the new battery at full voltage.

Battery math: The battery typically has somewhere around 65 Amp/Hours storage capacity. You don't want to draw that capacity down by more than 10% or so ideally, 20% or so absolutely. So you have 10-20% of 65 Amp/hours to play with. With your 0.04 Amp/hour drain, it will take (((6.5/0.04)/24)/7) weeks to get to max drawdown if you plan to start the car. Calculator handy, carry the nine, you can leave the car sitting for a week before you infringe on the battery's ability to start the car without dipping the charge below that critical 80% level during cranking.

Drop the charge level below 10.5 volts resting, and you give up, for the sake of discussion, 7-10% of the remaining charge capacity of the battery. So do that twice and you have a battery that will deliver somewhere in the low 80 percent at its best, even after a "full" charge is applied.

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For most owners with a car that draws "stock" amounts at rest, do your own math to find out how often you need to drive the car. Else you'll want to disconnect the battery, and/or connect a good battery maintainer for the whole storage period. Handy battery disconnect options include a couple types of disconnect switch, less handy is lifting the ground strap at the rear apron under the tool panel. Wiring for a maintainer is not really difficult, but gets more interesting when you would like to charge the battery while the battery is isolated from the rest of the car. Plenty of examples of these options here in the forum.

Randy V

I really appreciate you and Stan sticking with me throughout this one.

Perhaps if you get a chance you could apply your fantastic knowledge base to this one:

https://rennlist.com/forums/928-foru...l#post13903387

dr bob

Visit that thread for a little info I gleaned from the wiring diagrams.

dr bob

Following up on some testing info in this thread. A member in another car forum shared an interesting link to a voltage-drop to current flow table, useful to quantify the current flowing through ATC fuses when testing in-circuit. There are some other variables that make these readings less than lab-quality accurate, but they should give users an idea of how much current is flowing through a fuse, using the method described above in posts 7, 15 and 16.

Fuse and Voltage Drop Charts to Download

j.kenzie@sbcglobal.net

Very neat Dr. Bob. I've been using this method qualitatively for years. Now you have current readings. Thanks.
I also have found that not all ATC fuses have terminal holes on top.
Thanks, again,
Dave

dr bob

I'm not sure that the chart is always going to be accurate, as fuse manufacturing quality "moves around" quite a bit. Still, it's a good rough guide that caught my eye.

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A quick follow-up on the little clamp-on DC ammeter I bought, with 100-amp limit. It is quite sensitive at the low range, and pretty darn accurate even at very low current flows. It's also amazingly sensitive to nearby and far-away magnetic fields, such that it needs to be carefully zero'd in the same relative position as it will be testing, and the earth's magnetic field causes readings to vary. So I set it next to the conductor I want to test, a few inches away and in the same relative position as it will be in when clamped around the wire. Then zero it a few times until zero is stable before moving it the few inches and clamping around the wire.

I'll do a separate write-up when I have time, including a few simple tests and demo's of it's use. It's not as handy as Jim C's Fluke ScopeMeter with the DC current probe, mostly because of the lack of that big 'remote' display. But at $40, it's a tiny fraction of the cost of his setup too, and may be easier for us mere mortals to get usable readings. More later on this.

M. Requin

Based on danglerb's report, I bought the Craftsman clamp-on meter he referred to. Like dr bob, I am pretty surprised at how sensitive it is, and his comments about zero-ing the meter are very important. In fact, probably due to operator inconsistency, I make several readings in a row that don't show much variation before I trust the numbers I get. I'm glad I added this to the tool collection.

928wolf

I believe I am having the same problem that this thread addresses.

However, now I have a much worse problem. After dis-connecting and re-connecting the battery ground strap to ground at the rear, the car is now completely dead. No connection. Zero volts. the whole car is Dead.

I measured the voltage from the battery positive to end of the battery strap and it shows battery voltage. So I know it isn't the battery strap (I replaced that a year or two ago when it was flaking out).

Before I start the arduous task of tracing every possible dead end where the battery power circuit could fail, is there anywhere I can start my search? There isn't a "main fuse" to check, is there?

1985 928S Euro, Manual

Thanks for any all help anybody can give!

davek9

Hi what's the voltage at the jump post in the engine compartment?
That is where it feeds the CE panel check and clean your connections there(disco bat first to clean)

Dave K

928wolf

OK, false alarm. Main power OK, car cranked over, but door lights and clock not working. Will work on that next.

Thanks for your help.

Mrmerlin

check the fuses
NOTE what was the last thing you fixed?

928wolf

Interior lights fuse blown. Replaced. But now back to the original problem which looks like the door light relay. This one already cost me a battery.

Does the door light relay cause all interior lights to stay on, or only the red light in the door? My interior lights won't go off.

FredR

If your interior lights will not go off have you checked to ensure the switch is not set to permanently on assuming we are talking about the lights in the front lower section of the door card? In the normal position the lights go out after the door closes.

Changing out those bulbs for an LED unit is a good upgrade.