Hello all,

I've been chasing down a problem with time and money for the past 4 months. I'm on my 3rd alternator, 2nd battery and 3rd time through all of the fuses to check for draws.

I had Autozone check the voltage to the battery a few times and it reads between 12.8-13.2 or so. What's the "normal" acceptable range for this? Again, I'm on my 3rd alternator in as many months.

I've checked the draw on the fusebox and I disconnected the 1984 original alarm system which was making a very bright light when I checked that fuse.

New battery at the same time and it lasted for a month. Same problem... so, checked the fuses again and it seemed that the stereo might be still drawing a power (an aftermarket unit, about 7 years old, installed by previous owner).

Besides checking the battery ground (tip taken from another forum... thanks guys), and the glow plug relay, any ideas?

Tired of throwing money, time and patience at it!

I'm a teacher and need to get to school reliably... Other than this issue it has been a fantastically reliable car :-)

__________________
1982 300SD - 195Kmi, B-100 Biodiesel (>10K miles)

Get a proper ammeter and check the current draw through each fuse holder (fuse removed). Identify circuit/s with greater than single digit milliamps draw, check the fuse circuit card or wiring diagram for devices in the circuit, use the ammeter on the power supply wire to each device to find the culprit.

Or get one of these things -



and develop the habit of disconnecting and reconnecting the battery when you park the car for a long time. Don't bother with the little clock and radio memory fuse because you'll burn it when you get in and start the engine without first connecting the battery.

Sixto
87 300D

What is that thing called?

__________________
Julio Garcia
1987 300 SDL (mine)
2006 Jeep Liberty CRD (mine/kids)
2008 Hyundai Sonata (wife)
2000 Saturn SL AKA "Gocart" (everyone)

Battery disconnect switch.

Sixto
87 300D

Hello all,

Update on the issue.

Replaced the alternator again. Got a decent multimeter and here are the stats.

This is all at idle:
13.3V with no load, just idle
12.8V w/ only A/C
12.28V when electric fans kick on
12.6V when all are on and I rev to 1800 RPM or so

I've been leaving the multimeter connected all weekend and going out every 30 min or so to check the voltage. I teach science so I've been viewing this as a real-world, real-frustrating, controlled science experiment!

So:
With the battery still connected in the car, ran it for about 10 min
12.29V @ 6:30 PM
12.25V @ 8:00 PM
12.18V @ 2:00 AM (set an alarm to check it!)
12.14V @ 10:30 AM next morning

Then I decided to disconnect the battery entirely after having ran it for about 10 min again to charge.
12.45V @ 10:45AM
12.33V @ 11:45 AM
12.29V @ 12:30 PM

So, battery then not holding a charge? It's completely disconnected from the car, so no draw from any accessories.

It's the second battery... an 84 month NAPA legend.

Could a bad battery ground strap or a stuck open glow plug relay cause a battery to go bad? Don't want to replace the battery and have to do it again in a few months... NAPA will eventually start to get suspicious :-)

As always, thanks for all of the good advice all!

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1982 300SD - 195Kmi, B-100 Biodiesel (>10K miles)

Hmmm ... How frustrating. I really have no idea. One painfully slow way of troubleshooting might be to remove all the fuses, but one and run your test. Then replace them one at a time, "rinse and repeat" ;-) You may be able to at least isolate the circuit.

__________________
Julio Garcia
1987 300 SDL (mine)
2006 Jeep Liberty CRD (mine/kids)
2008 Hyundai Sonata (wife)
2000 Saturn SL AKA "Gocart" (everyone)

As long as the alternator is able to supply a voltage above what the battery shows when disconnected from the electrical system and enough current, you should be able to get it to charge. That is if the battery is indeed good.

You will get lackluster charging when you have the alternator under alot of load like AC, Headlights, Fans, Glowplugs, and all the other electrical systems in the car.

When the current draw is high, the voltage will tend to drop. A higher output alternator may be needed to maintain charging while under a high load.

__________________
Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

Divide and conquer!

Try a known good battery and see if you get the same results. That should rule out a bad battery.

The alternator is pretty easy to test.
To work, it pulls energy from the battery to activate it's armature coils. More energy to up the power generation; less when it's not needed. With the turning of the armature, a large output of energy is generated in the stator, rectified and fed into the electrical system - hopefully charging the battery and running the rest of the system. The voltage regulator alters the amount of energy being fed into the armature/electromagnet in turn regulating the amount of energy appearing on the output of the stator.

You should test unloaded voltage output from the alternator - DC voltage. It should be in the Mid 13VDC to low 14VDC range for good charging. If the voltage is too low, you wont get a charge and likely there is a problem with the regulator or the grounding.
Then you should check AC volts - there might be a little ripple but nothing substantial. If there is too much AC then the rectifiers are bad.

__________________
Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)