New alternator and battery, now what!? 300D Electrical Injustice... - PeachParts Mercedes-Benz Forum

scoodidabop · #1 06-01-2007, 08:51 AM

1982 300TD

So I thought my battery was bad. Bought a new one at autozone, BAM! Didn't fix the problem.

I pulled the ashtray out and hooked my multimeter to the white plug there to monitor voltage during driving. With my old alternator, it never went above 11.5. So, I tracked down the Saab 115 amp alternator at Autozone, and bought it. I then proceed to spend the next 4 hours in their parking lot trying to get the old one out, and the new one in. The belts aren't quite tight enough, but it's working for sure. I highly recommend the alternator. It fits perfectly!

I drove it around for about 20 mins, and the battery kept discharging. By this time it's about 1am so I found a 24 Autozone and got them to put the battery tester on it. The guy there determines that it's not the battery, but there seems to be a short somewhere that's causing the alternator to not keep the system charged at idle.

SO, the big question; where do I start? I guess there's a short somewhere, but where should I begin the search? Ever heard of this issue before?

Thanks...
-Chris

vstech · #2 06-01-2007, 09:18 AM

scoodidabop, did you ever measure volts with the key on? I figured you had a huge short. I would start by pulling ALL fuses out of the car and measuring battery volts. then one by one plug in the fuses, when you find the one that dropps the volts down, youhave the circuit to start chasing.
John

scoodidabop · #3 06-01-2007, 09:29 AM

Well, that's the tricky thing. When I put the key to position 1, everything is okay (when the battery was actually charged). I think it was 12.2 or something. The real issue seems to be when the car is running. When I look at the voltmeter, I feel like I'm watching a bomb about to go off. The battery just starts draining (10.45, 10.44, 10.43, 10.42 about .01 per sec and levels off around 4v). When I have it at 2500rpm it'll jump to 13v, but it still drains...

vstech · #4 06-01-2007, 09:33 AM

how about in position 2?
position 1 doesn't turn very much on.
John

Douglas.Sherida · #5 06-01-2007, 09:44 AM

I had a very similar problem recently on my euro 84 300TD turbo. System not charging, swapped in known good alternator and battery, system still not charging.

Aside from the not charging, I was most bothered by the fact that the idiot light on the instument cluster (charge indicator lamp) did not light up when the ignition was keyed prior to starting. I pulled the instrument cluster and determined that the bulb was functional. I then tested the continuity of the thin blue wire that runs from the plug on the alternator (the other two big red ones run to the starter) to the big round plug on the fuel/temp/oil pressure gauge on the back of the instrument panel (pin #6 on my car), and sure enough there was an internal break. So, I spliced in a new wire from the alternator to the idiot light and viola, light works = charging system works.

The reasoning behind all this (as I understand it from reading the forum archives) is that in order for the alternator to generate an output charge it must first have a "priming current" to generate a magnetic field between the rotor and the windings. That low voltage current comes from your battery and the circuit passes through the idiot light. No light = no priming current = no magnetic field in the alternator = no electrical output from the alternator.

Once the alternator is spinning (in the presence of the priming current) it generates its own self sustaining magnetic field and the higher voltage current it produces somehow shuts off the idiot light. Hence the idiot light staying on when your alternator is not charging and the weak glow from the light (especially at idle) when the alternator output is sub par. That idiot light is not so dumb after all. It can tell you alot about your charging system, if you understand how it works.

BTW, I learned all of this by searching the archives of the forum. If I mispoke, please correct me (but blame the forum).

vstech · #6 06-01-2007, 09:48 AM

yup. you got it DS, that is why I wanted him to check voltage drop with key in on postion. still, voltage dropping to 4 volts during a drive, cannot possibly be just a non charging alternator. totally disconnect the alternator altogether, and the voltage on these old cars will hardly drop for days. it's gotta be a drain on the system.
John

#7 06-01-2007, 10:07 AM

For what it's worth...I had a similar problem...ended up replacing the pigtail lead from the back of the alternator to the junction block on the fenderwell......problem was solved. This was cheap and easy to do. Hopefully your issue is just as easy.

scoodidabop · #8 06-01-2007, 10:16 AM

Thanks for the help guys.

VS, it drops down to low 11's in pos. 2, but it doesn't drain. It only drains when running.

Doug! woohoo! I think my idiot light wasn't working.... hmmm... I'll go check it out.

Are you guys familiar with the Saab alternator? I want to make sure I've got it hooked up correctly.

javascript:showImage('Alternator','http://img.eautopartscatalog.com/live/F400010140BOS.JPG',0)

I matched up the B+ on my old one to the large bolt, the other big red wire to the smaller bolt, and the blue wire to the small spade terminal. I think that's right because it is generating charge.

Anyway, I'm quite sure it's just a wire somewhere. I'll check that point in the cluster first. Where on the fenderwall does it terminate?

Mustang_man298 · #9 06-01-2007, 11:02 AM

Quote:

Originally Posted by Douglas.Sherida

Once the alternator is spinning (in the presence of the priming current) it generates its own self sustaining magnetic field and the higher voltage current it produces somehow shuts off the idiot light.

Just FYI, if you were to look at an electrical print (at least for a GM car) you would see the light does work exactly as you said, and the reason the light behaves this way, is that the circuit is designed with (depending on make and year) either a diode or a given amount of resistance in it, and basically works by either reversing electrical flow so the diode blocks it off, or providing a less resistive path when the alternator is excited. (electricity will always flow on the least resistive path).

I agree it sounds like you have an issue with not properly charging, maybe that mysterious drain only when running is from it trying to "excite" the alternator, as that circuit would only be enabled in the igntion switch in the run position.
Here's another test idea to try. Because this is a diesel, once running there should be no difference in electrical load in the run position whether running or not. If they use the resistive system to operate the light, you will see voltage whether or not current actually flows on that circuit. So, grab an ammeter and measure load on the battery with it in the run position not running, then do the same running, that will tell you if you really have a sizeable short causing the drain. You might see a difference from the alt trying to charge it, to eliminate that doubt you might do a 3rd test with the alt disconnected and running. Just some more ideas.

Kuhlrover · #10 06-01-2007, 11:48 AM

If you think you have short that is draining the battery but not blowing a fuse, take a close look at the glow plug relay. A while back mine went bad. While the engine was running it would come on by itself and there is nothing to indicate this is happening other then there is a big drain and the alternator. I had to hook voltmeter to one of the glow plugs and drove around. When I the battery would start doing a heavy discharge, the voltmeter said 12 volts at my glow plug. I new this should not happen. Also, while this was occuring, the glow plug light acted like it normally would and never came on when the engine was running.

TimK

scoodidabop · #11 06-01-2007, 02:47 PM

Bad news... My battery discharged to 1.8A!

So when I set my meter to milliamps, disconnect my + on the battery and put my meter between the lead and the battery terminal, what should it read?

Right now, it's reading 16.0. I tried pulling a few fuses, and disconnecting the glow plugs, but everytime I took a reading, it increased! I'm up to 21.1 now!

NC Benz · #12 06-01-2007, 03:39 PM

HUH??? 1.8A? You should be reading VOLTS. AMPs is the current flow.

With nothing connected you should show very little or no draw. Also in order to measure AMPs you need to have the meter in series with the circuit.

Go back to volts and tell us what you measure in OFF and running positions.

Quote:

Originally Posted by scoodidabop

Bad news... My battery discharged to 1.8A!

So when I set my meter to milliamps, disconnect my + on the battery and put my meter between the lead and the battery terminal, what should it read?

Right now, it's reading 16.0. I tried pulling a few fuses, and disconnecting the glow plugs, but everytime I took a reading, it increased! I'm up to 21.1 now!

scoodidabop · #13 06-01-2007, 04:46 PM

Quote:

Originally Posted by NC Benz

HUH??? 1.8A? You should be reading VOLTS. AMPs is the current flow.

With nothing connected you should show very little or no draw. Also in order to measure AMPs you need to have the meter in series with the circuit.

Go back to volts and tell us what you measure in OFF and running positions.

Whoa! My fault! I was reading VOLTS. Just a typo, sorry.

MORE DISCOVERIES!!

With the car's + lead disconnected, the battery reads 10.5V. As soon as I connect the +, BAM 1.5V. So, I disconnected the fender well terminals and it fixed the reading! Picture time...

Those are my connections on the passenger fender well, just in front of the battery. When the left two terminals are disconnected, the problem goes away. Where do all these guys lead? I'm kinda sure the left two have something to do with the starter, right? It's brand new, and functioning so I don't think that's the problem.

scoodidabop · #14 06-01-2007, 05:13 PM

I just did some forum searching and found someone with my same problems. I'm going to double check my alternator, just to be safe.

Douglas.Sherida · #15 06-01-2007, 07:50 PM

Quote:

Originally Posted by scoodidabop

Whoa! My fault! I was reading VOLTS. Just a typo, sorry.

MORE DISCOVERIES!!

With the car's + lead disconnected, the battery reads 10.5V. As soon as I connect the +, BAM 1.5V. So, I disconnected the fender well terminals and it fixed the reading! Picture time...

Those are my connections on the passenger fender well, just in front of the battery. When the left two terminals are disconnected, the problem goes away. Where do all these guys lead? I'm kinda sure the left two have something to do with the starter, right? It's brand new, and functioning so I don't think that's the problem.

Just a couple of thoughts: A battery at 10.5V is dead. Absolutely will not start the car. And, you will get all sorts of funny things happening with such low voltage, not to mention that the draw on your alternator (if its charging) may just kill it. A fully charged battery after it has rested over night should read about 12.6V. Search around online, but IIRC at 12V its only about 40% charged.

So make sure your battery is fully charged and then make sure the idiot light comes on when the ignition is keyed (same position as the glow plug light).

When I had problems a while back, I finally concluded that my el cheapo multimeter was garbage. It was a good opportunity to buy a new tool.

I more than got my money's worth out of it when I determined that I did not need a new alternator.