Jim_H

If I were to disconnect my neg battery cable and ground the battery through my ohm tester, what should it read? I know zero would be great but what is the max I should see before I know I have a short somewhere? This is probably grade school but I gotta start somewhere.

onebad928s

why do you think you have a short?

Lou
83 928 5speed

Rich9928p

Jim,


You are correct that in this case to check the Amp reading you have the Amp gauge in series with the battery (either positive or negative, just have the polarity correct for your gauge).

There are several circuits that consume power while the ignition is off. The clock always runs, the radio and several control modules have battery powered RAM to store information. All of these items consume a small about of current, but if you add them all up it does amount to something. These are not “short circuits” in any manner of speaking. If you had a true short circuit, so much current would flow that the wire or connector would burn up (and potentially light a fire in the car). So, the “normal” key off power consumption varies by the year of your car and the number of do-dadds that are “alive” when the car is asleep. The current would be in the milli-Amps (mA) range, somewhere between 40 and a few hundred.

As any car ages, the electrical contacts that “open” to shut things off begin to get corroded or carbonized and a small amount of current will flow even when they are supposed to be turned off. The ignition switch gets a lot of use and can be a source of trouble. Our 928s have many relays that potentially do not fully “open” to turn the circuit off.

If you think the “total” rest current consumption is too high, you then need to test each circuit to see which one is the offending party. An easy start is to remove each fuse one by one and check the Amp reading. There are fuses that aren’t in the fuse panel, so you might have to do hunting behind the ash tray and elsewhere for those circuits. If you still don’t find anything, you then need to look at your schematics and figure out how to measure the current for relay switched circuits.

Another test that is more “deductive” is to measure the total rest current and then unplug the many modules and relays in the car one by one and note how much the current decreases.

Good luck.

BrianG

This question begs consideration of two different issues. If you really mean a "short", there will be a momentary, very high current draw. Since the battery can supply HUGE current on a limited-time basis, a short will "burn open" any circuit in the car. If the affected circuit is protected by a fuse, a short will immediately eat the fuse. If somehow the circuit is not protected by a fuse (or similar device) a short will burn off the wire of the affected circuit. So, if you are really looking for a "short", you might see a momentary indication of anything from a few amps to perhaps as much as a hundred amps, but that would only last as long as it took to burn open the circuit, so the ammeter is not a useful way of diagnosing a "short". You simply trace the burnt-open circuit until you find the problem.

If you are really looking for an unreasonable, battery draining load, you can do the total-battery current drain that you propose and see if the draining load really exists. There isn't much normally alive in the car, save for the clock, and a few other low-load electronic components. I suspect a draw of more than 0.01A would be unusual. Certainly anything approaching 0.1A would be suspicious and would indicate an interior light or something (check the glovebox light!!)

To my understanding, there are 2 separate "bus" circuits in the 928. One is always hot (+), and the other is key-switched hot (+). The culprit in this scenario is pretty much going to be in the "always hot (+)" bus. So, you place your ammeter like you suggest, noting the load value, and you start pulling fuses until you identify the offending circuit by the drop in load when the fuse is pulled. Then it's a matter of following that circuit through.

Hope this is of some use!

SharkSkin

If you do that, you won't be able to read anything unless you can read smoke signals! If you have your meter on Ohms setting and you hook it up this way, you will almost certainly blow the meter. Someone did this with one of my Fluke meters, and fortunately it only blew a $7 fuse that I had to hunt high and low to find. A cheap meter would be toast.

Make sure you have the meter set to the "AMPS DC" scale, the icon should look like a horizontal line with 3 or 4 short dashes under it. If the symbol looks like '~' that's AC and won't be of any use for what you're doing. Most meters require that you connect the leads to the meter differently when measuring amps or volts. Start on the highest scale on the meter, and put a fuse inline with a smaller rating than that of the meter.

for your meter if in doubt.

The safest way to do this check if you don't have a suitable ammeter is to hook up a light bulb between your ground post and the disconnected ground wire. Start with a brake light bulb; you can see changes in current draw as you pull fuses by the changes in bulb intensity. If it's enough to light the brake light bulb, it's a pretty fair sized short. If it lights dimly when you first hook it up, try a taillight bulb or a side-marker bulb. If you can barely get one of the mini pod-switch bulbs to light, then you have a pretty insignificant current draw.

HTH

Garth S

Everything Dave said !! regarding meter use/protection, and Rich & Brian have got the rest - except perhaps for the issue of disconnecting the wiring to the now open hatch when you are tail up - head down over the battery and meter: Even when you switch off all interior lights, the open hatch energizes the red indicator lights in both doors (even if they are closed). That will draw enough current to flatten many a battery overnight - and will mask any diagnostic attempts to .....

Jim_H

Great stuff, thanks everyone. I meant amps, and I will read the book

I am trying to run this warning down

Right after I start the 90 GT I get the following on my digital display

Type # 04

19,05

1988

It disappears after a couple of seconds.

I have replaced the battery, it was weak anyway but the warning stayed. Onto ground points after checking for excessive drain.
Thanks again

shaaark89

jim,
you may know this already, but that display is only showing you the info specific to your dash software, not really a warning. it does come up when the battery is low on starting as a default, so it would suggest that the battery is running low and therefore may be seeing a draw.