Charging problems

[ALVW3]

My 1990 TD will slowly lose charge over the period of a week, and unfortunately I'm no too good when it comes to electrical issues. I assumed it was the brushes/volt reg and replaced the unit, to no avail. The alt light doesn't stay on after starting, it goes out as normal.

So I've tried taking a few measurements. I'm wondering if anyone can offer some suggestions, given the following I've found:

Key Off, Engine Ground to:
Pos Batt Term: 13.07V
Big Bolt on Alt: 13.04V
Little (blue wire) bolt on Alt: 0V

Key On, Engine Off, Engine Ground to:
Positive Batt terminal: 12.5V
Big Bolt on Alternator: 12.5V
Little (blue wire) bolt on Alt: 1.4V

Engine On (blipped and revved up), Engine Ground to:
Pos Batt Term: 12.7V
Big Bolt on Alt: 14.4V
Little (blue wire) bolt on alt: 14.4V

I tried taking some resistance measurements between the Bat Neg and engine block, alt housing, car body, and got the same value for all: around 0.3-0.4 ohms, which from what I understand is a bit high?

Tried cleaning the connections at the batt and alt without any change in the numbers.  Not sure where to go from here ... ?

[MizpahPAH]

From what you have done so far it sounds like you are pretty good at electrical

One thing you may want to check also is if you have a parasitic drain when the key is off pull the negative cable from the battery put inline a test light
if everything is off in the car the test light should not light. If it does then there is a draw / drain.

From you reading so far you checked using engine ground what about at the battery

do the same tests but this time use the negative battery cable as ground

If you still get the voltage drop / between alt bolt and pos battery terminal when engine is running
check the starter solenoid main terminal comes from the battery and feeds the alternator. maybe that or maybe a bad ground.

0.3 to 0.4 ohms is low just touch your meter leads together that will show you low resistance depending on the meter you have
analouge may need to be adjusted / zeroed. digital should not

[Zulfiqar]

You have a bad wire coming from the big alternator terminal - the voltage should remain the same on the battery +ve post and the alternator big post.

[ALVW3]

Ok, thanks guys.

I'll try checking the alt-batt cable first.

Is it possible it could be a ground connection too?

Weird that I get the same voltage (12.5V) when the key is off, but such a difference in voltage (14.4 vs 12.7) when running. You'd think a bad cable would knock down the current consistently ...?

[burn_your_money]

You have a bad wire coming from the big alternator terminal - the voltage should remain the same on the battery +ve post and the alternator big post.

+1 for this

To test this what you need to do is put your multimeter on volts, put one end on the stud on the alternator and the other end on the positive post on the starter. The car needs to be running and the throttle needs to be blipped so the battery light comes off.

So basically you will have one lead on the multimeter at each end of the wire. Take a voltage reading. You are looking for a low number, like 0 to 0.4 volts. Anything higher then that and it's either a dirty connection or a bad wire.

From what you have posted I'd expect you to get a reading of 1.7v

If that reading checks out, then leave your one lead on the starter post and move the other lead to the positive terminal on the battery. Take a voltage reading like above.

If either wire shows more then a 0.5v drop then it needs to be replaced, or the ends cleaned up and then retested.

Weird that I get the same voltage (12.5V) when the key is off, but such a difference in voltage (14.4 vs 12.7) when running. You'd think a bad cable would knock down the current consistently ...?

The reason why you are seeing a drop in voltage (not current) is because the alternator is creating the voltage. Since it's working it is making full voltage (14.4 volts) The wire that runs from the alternator to the battery has a problem and is eating up some of that voltage. That is why you no longer see 14.4v at the battery.

The reason why you can't just slap your multimeter on ohms and test a wire for resistance is because the wire is not under any load. Even the smallest strand of wire would show virtually no resistance. Now the second you try and load that wire up with some amperage then you will notice if it is defective. Since you can't use a multimeter to check for resistance on a live wire you have to check for voltage drop, which is what the above test is doing.

Make sense?

[ALVW3]

Yep, that was the problem. Upon close look the connection at the starter solendoid was rusted and corroded. After much penetrating oil I squeeked the nut off, cleaned up the connnections as best I could, and now I'm getting 14.3V at the alt and 14.27 at the batt positive.

Appreciate the education on the electrics - I wish they made a "Automotive electrical systems For Dummies" book for guys like me.