Troubleshooting a battery drain - PeachParts Mercedes-Benz Forum

turbozeke418 · #1 11-02-2013, 12:31 AM

I have a 1980 300td with a WVO conversion. I bought an upgraded 80 amp alternator exactly a year ago and noticed that the battery light is always on. It's very dim sometimes and bright other times. I checked for battery drain by disconnecting the neg terminal and putting a test light between the terminal and cable and there is no draw until I turn on my fuel switch solenoid. So that wouldn't be the draw since it's off when the car is. I checked the glow plug relay with the car off and there is power going to it. There's power on both sides of the large metal fuse and the screw beneath it even when the car is off. Is this normal? I'd think that the power would be off to it when the key is in the off position. Any input would be greatly appreciated.

vstech · #2 11-02-2013, 01:29 AM

alternator.

turbozeke418 · #3 11-02-2013, 01:42 AM

That is what I've been thinking all along but they don't believe that is the answer. Been corresponding with him for the past few days trouble shooting. I have a 300SD with the same exact setup and purchased the alternator from him also for that car and there have been no issues at all.

1project2many · #4 11-02-2013, 09:11 AM

Quote:

I have a 1980 300td with a WVO conversion. I bought an upgraded 80 amp alternator exactly a year ago and noticed that the battery light is always on.

Are there any other lights on in the dash? Any chance the dim alternator light is really light bleeding over from another bulb?

Always on... as in never stops glowing when the key is off? That would indicate power is applied to an ignition sourced circuit. Maybe an added circuit is looking for ground and is finding it through the ignition wire to the dash light?

Always on... as in glows when the engine is running? That indicates a path to ground exists for the battery light after the dash.

If it glows when the engine is running, does it generally stay at constant intensity? IOW, if dim, does it tend to stay dim? If bright, does it tend to stay bright? If fairly constant, it may indicate the path to ground is through a component that has been inadvertently connected to the battery light ground wire from the regulator. If you've tied into any wires that are powered at key on, you may want to confirm the vehicle wires are not blue or white/blue before troubleshooting the alternator further. You might also have a short somewhere in the signal wire or a problem with the stator. Disconnecting the wire from the alternator will quickly tell you if the problem is internal or elsewhere in the circuit. With the wire disconnected there should be no path to ground and therefore, no light.

If the light glows with varying intensity and appears to brighten with increasing engine rpm there's a good possibility its related to the alternator.

I had trouble with brush misalignment on a Bosch alternator on my personal vehicle. Part of the brush overhung the edge of the slip ring and eventually made contact with the rotor. That one illuminated the light dimly at low rpm. Once the regulator was removed you could see the brushes were not worn evenly. It's a long shot but something I happened to remember. I couldn't get the regulator to align so every 3-4 months I would remove it, grind down the high spot, and re-install.

Sometimes when the light is dim and intensity varies with rpm it's because the rectifier in the alternator is allowing some A/C current to bleed through. It's caused by a faulty diode. You'll see proper charge voltage with a voltmeter and everything will seem to work, but with an oscilloscope there will be an obvious ac rider on the dc trace at the BAT wire.

A battery drain would cause the alternator to charge at a higher rate for a longer time than normal once the vehicle is restarted. This should only cause the light to illuminate if the alternator has been forced to recharge a very low battery frequently enough that heat has ruined the alternator. In this case the light is illuminated by a faulty alternator however, the cause of the alternator fault is external and needs to be corrected. If the supplier is reasonable then he's checking to ensure that a replacement alternator won't fail due to conditions out of his control. However, imo, if the car has been cranking fine and has not needed to be jump started, then the battery is not being drained enough to ruin the alternator and the "draw" test should be just a formality. On a similar note, add-on accessories that draw large amounts of current will force the alternator to operate at or near full capacity and ultimately the heat this generates can ruin the alternator. Do you have an idea how many amps you're using with the components in your conversion? Do you have any high current accessories added? Regardless, if high current output has killed the alternator the immediate problem is still the alternator. A decent local garage should be able to test your alternator on the car. They'll charge a fee, of course, but maybe you can get the alternator supplier to agree to cover it if the alternator proves faulty. Some of the chain stores have alternator test stations as well. If you bring in the alternator they'll test it for free. Ask if you can watch the test so you can see the results. Or you might have a local rebuilder that has equipment to perform a full evaluation on the alternator. They may be willing to test the alternator for free as well. Of course they'll try to sell you a gazillion amp, nuclear bomb proof, EMP resistant replacement if yours is bad but sometimes it's worth it to listen to the sales pitch.

Brian Carlton · #5 11-02-2013, 09:32 AM

Quote:

Originally Posted by turbozeke418

I checked for battery drain by disconnecting the neg terminal and putting a test light between the terminal and cable and there is no draw

Testing procedure is flawed.

A test light won't show any small electrical usage.

You need to put a small test meter in the same location and set it to read milliamps.

With everything shut down, you WILL see the meter give something above the usage by the clock (50 ma or so). Now, with the meter in position, you attempt to find the consumer by removing one fuse at a time. If nothing is produced by doing so, you remove the wires from the alternator. If that fixes the problem, the leakage is internal to the alternator.

Diesel911 · #6 11-02-2013, 12:15 PM

Quote:

Originally Posted by turbozeke418

That is what I've been thinking all along but they don't believe that is the answer. Been corresponding with him for the past few days trouble shooting. I have a 300SD with the same exact setup and purchased the alternator from him also for that car and there have been no issues at all.

Believing something is OK is not the same as knowing it is OK.
When you trouble shoot something if you do not go down the list of possible stuff starting with the most likely suspects what every you guessed was OK ends up being the cause of the problem.

It is normal for the GP Relay Fuse and that flat Wire in the Relay to have Voltage all of the time.

From what I have read on the Forums these drains are most often the Alternator (slow drain) or the Glow Plug Relay (fast drain).

If you had a Volt/Ohm/Multimeter You could tell us more.

Test light Bulbs have a resistance that needs to be overcome before they light.

Has the Battery been charged and load tested? Or swap in a know good Battery.

If the Charging Light is glowing with the Engine running it only indicates you are not charging; not that you have a drain somewhere.
That could even be simple things like a bad Ground or slipping Belts.

turbozeke418 · #7 11-02-2013, 02:24 PM

Thanks for everyone's input. Nothing was wrong with my car with the previous alternator that was 32 years old. I wanted to upgrade it and from the day I installed the new alternator I had the problems. No battery light was ever on before and nothing with my wiring has changed. I haven't done any electrical modifications since the new alternator. The battery light comes on bright when when I first start the car and will stay that way til I rev the car a little. Then the light stays lit but at varying intensities. Sometimes it's barely on and other times it gets brighter (usually at lower rpms). The drain on the battery is very slow. If I charge the battery up it'll last a few days of frequent driving. You can tell the battery isn't charging because it gets harder to start with in time and then has no power at all. Once again the battery is brand new and the battery it replaced was only a year and a half old.

rscurtis · #8 11-02-2013, 02:32 PM

BC is correct in using a multimeter to find a small draw; however, if it's too small to light a test light, it's too small to discharge your battery over a short period of time.

cooljjay · #9 11-02-2013, 08:09 PM

This sounds like a belt issue on the alternator. Did you install a new belt and tension it correctly? Did the pulley match the old alternator? It could also be that the plug on the rear of the alternator is dirty.....but it really sounds like a belt problem...

1project2many · #10 11-03-2013, 01:41 AM

Quote:

hanks for everyone's input. Nothing was wrong with my car with the previous alternator that was 32 years old. I wanted to upgrade it and from the day I installed the new alternator I had the problems. No battery light was ever on before and nothing with my wiring has changed. I haven't done any electrical modifications since the new alternator. The battery light comes on bright when when I first start the car and will stay that way til I rev the car a little. Then the light stays lit but at varying intensities. Sometimes it's barely on and other times it gets brighter (usually at lower rpms). The drain on the battery is very slow. If I charge the battery up it'll last a few days of frequent driving. You can tell the battery isn't charging because it gets harder to start with in time and then has no power at all. Once again the battery is brand new and the battery it replaced was only a year and a half old.

It seems much more apparent when described like that. If the vendor needs confirmation, pull the alternator and have it tested. Or swap with the SD and see if the problem follows the part.

turbozeke418 · #11 11-05-2013, 04:33 PM

I did a few tests today.... The car has 0 battery drain when shut off. When off the battery reads around 12.5 volts. When I start the car it goes somewhere between 12.5 and 13 volts. It doesn't go above 13 volts. If I turn the headlights, fan or anything else, the volts drop to around 11-11.5 (that's with a high idle) I would think that the issue is the alternator.

connerm · #12 11-05-2013, 04:51 PM

On my 85, I replaced alt, belts, battery, a couple of voltage regulators and was still getting wierd output readings.

Issue one: glow plug relay was malfunctioning.

Issue two: wire that excites the alternator field was almost completely broken at the connector on the alternator - almost.

Issue three: The two big wires from the alternator were rubbing on the chassis to the point that the wires were exposed. I guess after 28 years, the plastic cover of the wiring harness AND the insulation around the wires themselves just wore out.

I fixed all of the problems and no more wierd alternator readings. I include this only to get you thinking about other possible scenarios that cause electrical issues.

TnBob · #13 11-05-2013, 06:20 PM

With the car off, did you measure volts or amps / milliamps ?

turbozeke418 · #14 11-05-2013, 06:49 PM

yes...it was 0. I also checked the battery when I shut off the car and then the next day (went down to 30* here last night too) and the voltage stayed around 12.6.

TnBob · #15 11-05-2013, 09:58 PM

Voltage isnt what you should be measuring.
Amperage draw is what discharges a battery.