Donald

Recently noticed that the new battery is discharging in short period. With the DMM I am reading 14.4 at the jump terminal after starting the car, and the dash indicator also is positive. A few minutes later, before much opportunity for heat soaking, dash indicator goes flat and the terminal reading is negative 16.6a.
What is that about?

GlenL

Ignore the dash gauges and refer to the DMM. Or clean all the grounds and harness connectors in the car.

14.4V is a bit high for running. 13.6V would be nicer.

Donald

Glen:
Ignoring the dash is not the answer to the problem: negative amp output on the DMM at the terminal.

WICruiser

Why are you looking at amps on the DMM? Initially you are reporting voltage, which makes sense.

It sounds like the alternator is charging so your battery drain problem is a current draw when the engine is not running. You need to isolate what circuit is drawing power when the car is not running. A typical place people find problems is the lights in the back edge of the door and/or interior lights not shutting off when the doors are closed.

GlenL

Reverse the leads and you'll have a positive reading.

Is there an actual problem with the car?

88porsche928

lol glenl, what if he actually does have a negative reading? Im sure you are 100% correct but its funny to think about.

Donald

Okay, reading my OP this morning in a calm state- it obviously makes no sense.
I am using a borrowed induction meter, did not realize it shows + - depending on the loop orientation, and must have flipped it between readings. Re-did this morning and it is a bit higher (18.6amp, + or -, take your pick).

As to volts vs amps, the readings were all amps so my alternator is "slightly" below spec- I don't know if the car uses 16-18 amp running or if that is alternator output. Last week at the alternator shop it pegged close to 140 amp, and yes I have put the engine rpm up over 4k for the benefit of the little green wire (and because I do anyway).

Glen- the biggest problem w/ the car is me, I am really weak on all things electrical. The observed problem is that the battery seems to have been discharging while driving.

-Is reading amps at the jump terminal a bad way to measure alternator output?
-Could the alternator be putting out more and I am reading it wrong?
-I have another freshly rebuilt alternator, best way just to swap and take readings? (maybe find a 10 year old to do it for me...)

WallyP

Alternator output goes to two places - running the car, and charging the battery.

The battery charge current goes from the alternator to the starter connection, and from there to the battery. Never gets close to the jump start post. What you are measuring is just what it takes to run the car, and that has very little to do with what the alternator will put out.

The best place to measure current is at the battery ground strap.

Donald

Wally- Thanks.

Voltage not running:
@ jump post: 12.95v
@ battery: 12.98v
Voltage running:
@ jump post: 14.0v
@ battery: 13.9v
Amps not running:
@ battery: .59
Amps running:
@ battery: 5.0 dropped to 1.0

GlenL

Those voltage are fine.

Your gauge is just showing that the battery is charing and current is flowing around the car differently as the battery and alternator come into balance. Crusty grounds and connectors are playing with your mind.

Try cleaning the connection from the battery that is inside the tool panel and both ends of the engine ground. That's on the right side of the engine. You'll have to get it from underneath.

I've seen this declining voltage on my 928 and 944. Worrisome to see, but if you've got 12.95 after driving it then you're in fine shape.

Donald

Glen-
Thanks. Engine ground & battery grounds were cleaned last month, will have to start on other connectors & secondary grounds.
TTFN-

Mrmerlin

So Don just to make things clear are you trying to measure voltage or amperage?? You should be looking for voltage so make sure you heve the correct buttons pushed on your volt meter.
I would suggest that you reclean the battery and the ground points as well as the hot post and the 14 pin connector above it open the back of the 14 pin and see if any of the wires are shedding insulation . Clean the ground points at the top of the CE panel and the main engine ground on the right side of the block under the exhaust manifold.
If the car has gotten wet since or prior to the charging issue then check the bottom of the blower box for water tell tales the water can drip down and start corroding the fuses and the relay pins and also fill a relay with water if it goes to the right spot any relay that has corroded pins should be opened and inspected smell them for burning.

Also What was the last thing you fixed?? sometimes that can point you to the direction to follow. . open the windows and close the hatch and doors see if you can see the rear warning light staying on open the door to find out what the light looks like when its on also with the doors closed try removing the relays one at a time till you find one that clicks when removed , you could have a stuck blower relay or defroster relay or any other number problems.
you may also have to go check the connections at the alternator , remove the brush assembly and inspect the brushes as well as checking the starter connections

Donald

Stan:
Thanks, I was trying to check both, initially amps to see if the alternator was charging, then measured volts at the direction of the Roger, Glen & Wally. I actually used one DMM for amps, another one for volts (because I would do something like switch to the wrong function, given the chance). I felt the car was losing charge while driving, it is on a trickle charger most of the time in the garage.
I will check those grounds & connections. Last thing I did was drop the alternator to have it checked at the shop after I found it losing charge; and I went through the a/c refrigerant system. Before that, it was with a mechanic for over a year for a clutch, shifter bearings and replacing the nose cone.

Mrmerlin

Ok so lets just drop the amperage word for now and concentrate on voltage.
Follow all of the things i suggested,
do every one even if you have already done it , so you will be retracing some steps.
I think you may find the issue your having

jon928se

Those figures look sensible and reasonable but.........

Amps @ battery running - 5.0 dropped to 1.0 After starting the engine there will be some current flow back to the battery to "re-charge" it. This current flow will decrease as the battery reaches a higher state of chrage. You don't mention if this was at idle or higher revs. If this measurement was at idle, by comparison if you kept measuring and went for a drive you would expect the 1.0 figure to increase for a while with the slightly higher voltage available from the alternator at higher engine revs then after some time the current tot he battery would reduce.

Amps not running:
@ battery: .59

If this was with ignition off and all doors closed and locked - wait 2 minutes after locking then measure. There is your problem - depending upon alarm system fiteed (or not) you should expect to see absolutely no more than 0.05A and ideally around 0.02A. However I am not sure how accurate clamp type ammeters are when measuring very low currents on very big cables.

For reference , to avoid the pitfalls and vagaries of the 928 elec system the only way to reliably measure current draw at the battery in the "parked and locked up at the airport" condition is as follows.

With ignition off and key out.
Connect DMM on the 10Amp current setting between the battery -ve post and a clean ground on the body (Battery earth cable remains connected at this point)
Close all doors and lock. Then disconnect the battery earth strap leaving the DMM connected between batt -ve and ground. wait 2 minutes and read the current flow.