1985 300sd w126 not charging at the battery - PeachParts Mercedes-Benz Forum

rwd4evr · #1 06-25-2019, 08:53 AM

I've search high and low and can't find an answer to this. Ive swapped voltage regulators and alternators and it's the same. No charge, 12.2 or whatever the charge left in the battery is, and I've used more than one. I've got a battery light on flickery, it almost goes out when revved but not quite. Tests I've found on here say the two larger blades on the alternator are battery power and the smaller should be one volt less key on not running, nothing key off, I've got 1.3v key off 2.3 key on. Then I have full proper charge voltage 14.4 at that terminal with it running. Could it be another bad voltage regulator?

Sent from my SM-J737A using Tapatalk

Diseasel300 · #2 06-25-2019, 09:09 AM

If you have 14.4V anywhere on the alternator but not at the battery, then the alternator is producing voltage. You have a bad connection somewhere between the blade terminals on the alternator and the battery. The heavy alternator leads run up to the 3-position terminal post on the false firewall by the battery, then by jumper to the positive battery connection as an auxiliary hookup. Be aware that the spade connections in the plug connecting to the alternator have been known to cause poor connections.

Sugar Bear · #3 06-25-2019, 09:41 AM

X 2 on the above...wiring between the alt and batt. Does the alt light work in the dash?

rwd4evr · #4 06-25-2019, 10:07 AM

Yes. Light is working as a dummy. On very dim when running. So the secondary wire on the negative post is the alt hookup? That was a little loose when I started screwing with it. I'll try cleaning that up and tightening it.

Sent from my SM-J737A using Tapatalk

Diseasel300 · #5 06-25-2019, 10:12 AM

The alternator/chassis wire is on the POSITIVE side of the battery, not the negative. The heavy positive cable goes to the starter, the pigtail goes to the 3 post terminal on the false firewall and thus out to the chassis and alternator. Pull up the ETM on Startek and it'll make it clear how the wiring should connect. The fact you have 14.4 on the field wire says the alternator works, power just isn't making the trip to the battery.

rwd4evr · #6 06-25-2019, 03:52 PM

Quote:

Originally Posted by Diseasel300

The alternator/chassis wire is on the POSITIVE side of the battery, not the negative. The heavy positive cable goes to the starter, the pigtail goes to the 3 post terminal on the false firewall and thus out to the chassis and alternator. Pull up the ETM on Startek and it'll make it clear how the wiring should connect. The fact you have 14.4 on the field wire says the alternator works, power just isn't making the trip to the battery.

Yes I meant positive. Ground obviously is negative and alt case. Gonna go over it in a few minutes after I eat.

Sent from my SM-J737A using Tapatalk

rwd4evr · #7 06-25-2019, 07:43 PM

Ok. I don't get this system. If I have power to the field and 14.4 coming out at the field running, how is it possible to only have battery power at the output? I just swapped voltage regulator into my running 450slc and it works fine. So now I'm going to swap the third alternator in from that car I guess. I ran a jumper cable from ground post to the alternator just to be sure that's not my issue. I'm really in the dark on how the bat light could be influenceing the system as well.

Sent from my SM-J737A using Tapatalk

Diseasel300 · #8 06-25-2019, 08:16 PM

The battery light is used to "flash" the field on the alternator and get it to produce voltage. With the alternator not functioning, the light bulb will glow since there is potential to the battery negative through the alternator. Once the alternator is "flashed" and is producing voltage, the potential to battery negative goes away and the bulb stops glowing. You've already proved the regulator is good since it works in another car. The alternator has already been replaced too, it's highly unlikely to have multiple bad alternators. There's most likely a bad connection somewhere. Verify what you have against what the FSM shows. It's easy to get lost in the weeds if you're not familiar with electrical wiring.

rwd4evr · #9 06-25-2019, 09:13 PM

Ok. Wtf. Known good alternator. Key on engine off, the line from the field has zero when separated from the two blue on the same stud, which both have battery power. Running with all three connected I have 14 .2 at rpm and 14.3-5 at idle. Battery voltage everywhere else. If I disconnect the one blue the idiot lights go out but field remains 14.xx. dissconect the other blue lights stay and so does 14.xx at the field connection. I checked with ground on body,battery and engine all the same reading I'm at a loss here.

If I understand this correctly both large teminals on the alternator are the same and the field is just an on switch with the key, correct? So it operates like a one wire alternator, if it has power to field it should be generating power to the main line. I'm gonna throw a match at this thing.

Sent from my SM-J737A using Tapatalk

rwd4evr · #10 06-25-2019, 09:42 PM

I don't have a fsm for a 126.. And either way how is that possible that a connection is lost? Could it not be seeing a load so not charging? So if I connect the output of the alt directly to battery it should charge without running all over the car first? If someone has a fsm wiring diagram I'd really appreciate it.
You can email it or whatever.
BirchsGarageMB@gmail.com

Sent from my SM-J737A using Tapatalk

rwd4evr · #11 06-25-2019, 09:50 PM

13.2 volts coming out of the alternator with the battery dissconnected while running. 12.5, same as **** off connected while running.

Sent from my SM-J737A using Tapatalk

Diseasel300 · #12 06-25-2019, 09:59 PM

See drawing 108:
https://www.startekinfo.com/StarTek/outside/12253/program/ETM/126td83-85.pdf

Seems the SD has the alternator wiring going to a common terminal that connects down to the starter motor. Make sure everything is connected, if all else fails, check continuity from the alternator plug to the battery positive terminal.

rwd4evr · #13 06-25-2019, 10:09 PM

Won't let me go to it.

Sent from my SM-J737A using Tapatalk

Diseasel300 · #14 06-25-2019, 10:15 PM

The mobile site doesn't work. You have to go directly to the link posted, or view on a computer. It should load fine.

rwd4evr · #15 06-25-2019, 10:19 PM

Ok so Houston we have a problem. The solenoid actuator (purple starter wire) is grounded, pretty certain it shouldn't be, as it is only to trigger the starter solenoid when you turn the key. So. I dissconnected it and hooked my meter to ground and the purple wire and turn the key to start, voila! 12 volts to start the car like normal. Any other time I hook my meter to positive and the same wire and I have 12 volts(bad). What the he//? Could it be ignition switch issue and causing the charging issue too?

Edit
Sorry I lost a post somewhere. Green circled is the starter wire on my junction box in the frame rail. The three wires twisted are the ones for the second stud, including field wire. Like that when I got it, not the issue.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode