W124 (S124) Charging issues - PeachParts Mercedes-Benz Forum

FrodeS · #1 04-23-2018, 05:14 AM

Hello all.
I am having some electrical trouble, and hope some one can help me. Sorry for long post, I am trying to explain the problem thoroughly.

I live in Norway. English is not my spoken/written language, so please let me know if I make no sense.

The car is a 1992 300 TD Turbo S124 (124 estate). This model/year was not sold in the US, it is a Euro model with the OM603.960 3.0 6-cylinder turbo diesel. The charge system is however the same as in all 124's. It has done 620k km / 387k miles. I've had it since 2003, doing 330k km / 200k miles with it.

I have a fairly good understanding of the charge system, voltage, ampere, resistance and how to measure.

Problem: The car is not charging the battery as it is supposed to.

What happens: From a cold start, voltage is around 12,4-12,5 (draining battery). The alternator delivers around 10-12A. (this is after the pre glow lets go). After a while, like 2-3 minutes at high idle (1000-1200 rpm), voltage and ampere increases. A raises to around 25-30A, and voltage will eventually hit the 14,5V it is supposed to with the regulator currently sitting in it.
After it hits 14,5V, and with revs a bit over idle (1000+), I can activate all consumers (lights, fan, heated rear window, heated seats), and the alternator charges like cracy (60A), keeping the voltage at aroun 14,3V (as normal).
Now, if the revs drop to idle, both A and V drops and does not keep up with the demand. Now, if this happens, revving the engine is NO help, except if I rev it to like 3500-4000 rpm, then it will slowly start to rise again, hitting the 14,3V after halv a minute or so. Then, it can rest at 1000+ rpm, and charging fine.

What I have tried, with absolutely no change in behaviour:

-Replaced the regulator/brushes (old ones were fine)
-Replaced the alternator for a brand new one.
-Replaced the battery
-Tried the old alternator in another car (1985 W124 260E) (works perfect there)
-Tried the battery in another car (works perfect therer)
-Removed, cleaned and lubed all connecors in the small block in front of the battery.
-Had a buddy wiggle the cable from the alternator a lot.

Grounding alternator/motor/battery is fine, with no resistance and no voltage drop.
Charge lamp works as supposed.

The car has been sitting still most of the winter because of a noise in the gear box.

The small junction block in front of the battery have 3 terminals, were 2 are connected. The upper two ones are connected, and distribute the power to/from the battery, from the alternator, and to the fuse box.

The last one have 4 cables on it. One goes to the alternator (thin cable), that is the pre charge, telling the alternator what to do. The three others I don't know were goes. I suppose one goes to the charge lamp, but what does the other two do, and were do they go? I noticed the 1985 260 have only 3 cables, as the 1992 300 TD Turbo have 4. Those are all thin cables. On both cars, this terminal shows around 2V with ignition on (engine not running). After engine started, it is around 0,2V above charge voltage when things are working as supposed. When charging goes down on the TD Turbo, it is only around 0,05V above charge voltage.

Yesterday, when taking the car out of the garage first time in a while, I noticed all the lamps in the dash board (except pre glow light) would light up for a very brief moment. This tells me there is a loose connection some were. Either a loose connection, or a short to ground, I suppose.
Is it the cable going to the battery light that also "tells" the car engine is running and shuts the lamps off after start? They go out as expected after start, they only lit up very briefly a couple of times as it was moved around the yard.

If any one have some clue on were to go from here, I'd be happy for any advice or thoughts.

Hirnbeiss · #2 04-23-2018, 07:14 AM

I would check all ground (jord) connections. If no change, I would consider replacing the positive cable from the alternator. I have seen internal corrosion on them cause problems, although I would expect it to occur when you wiggle the cable.

Mxfrank · #3 04-23-2018, 07:48 AM

Has it always been like this or did this a new problem?

FrodeS · #4 04-24-2018, 05:26 AM

Thanks for the replys.

It has not always been like this. It has not been used mouch this winter, only 2-3 times. All of them, I've noticed very poor charging. It used to have superb charging.

Ground connections to the alternator, battery, motor etc are all fine. I am considering replacing the cables from the alternator to the junction block, but as you say, with absolutely no change when wiggeling, I don't expect the problem to be there. Also, it does charge 60A under the right conditions. It just takes ages to get there, and it will stop when the revs goes down.

I think the problem is some were from the junction block, and inside the car. It would be nice to know what the three thin cables does, and what makes the lamps in the dashboard come on briefly.

optimusprime · #5 04-24-2018, 05:44 AM

Check battery leads .And all earth points you can find ,remove the earth leads clean up the body it contacts and put a little grease on the body before clamping it back up the earth leads . Another thing is the fuses check them out and make sure they are clean on the ends before refitting them ..Do this one at a time .And dont put them back in the wrong places . The ovp relay plays a big part in the system ,can you swap it with another to try it .

FrodeS · #6 04-24-2018, 06:22 AM

Hello, Optimus.

OVP was new a couple of years ago. No other signs of it failing.
All fuses have been out, cleaned and inserted again.
Ground in engine compartment and battery is fine.

lsmalley · #7 04-24-2018, 08:02 AM

Did you try checking the tension on the belt? If the belt is somewhat loose, then you may possibly experience charging issues as the alternator is not able to provide adequate charging unless you keep the RPMs high (3000-4000) to provide more output.

FrodeS · #8 04-24-2018, 08:14 AM

I have been thinking about the belt too. Tension is fine, but I know it is old.

I tried turning the alternator to see were it gave up and started slipping first. The alternator then drives the belt with it, and it will slip on the output wheel from the engine, not the alternator.
I tried operating the steering, putting load on the servo pump. That did no difference to the charging at all.

I would expect to have problems with the servo as well, if the problem is in the belt?

Mxfrank · #9 04-24-2018, 09:17 AM

I think this is just an aging battery issue.

FrodeS · #10 04-24-2018, 09:24 AM

Well, thank you for your input. If you read my entire post, you'll probably agree it is not.

Dubyagee · #11 04-24-2018, 10:56 AM

I have a 124 doing the same. I am about to install a GM single wire and be done with it.

Diseasel300 · #12 04-24-2018, 11:12 AM

Typically when you see the idiot lights glowing dimly, it is because the alternator is not charging. Since you already established that the flashing circuit works (alternator light and the idiot lights), that is not likely to be the issue. The idiot lights have diodes to feed the flashing circuit as a backup to the charge light.

Since the alternator works in another car, the alternator itself isn't likely to be the issue either.

That leaves cable connections as about the only thing left. The original push-pin design of the alternator connection is pretty piss poor. If you suspect issues with it, you may try re-terminating it or cleaning, tightening, and reinstalling.

Next is to remove and examine the engine/chassis ground strap. It is possible that it LOOKS ok but has corrosion under the face preventing a good connection.

Lastly, check the chassis to battery ground strap. I bring it up because it looked fine in my SDL, but when I moved it, it disintegrated. Clearly no good. The metal under the tab was oxidized as well.

The very last thing I'll bring up is to inspect the tabs inside the alternator that the regulator connects to. On my SDL, I had issues with the charging voltage routinely falling below 13V, regardless of RPM. The problem turned out to be a gunk buildup on those tabs. Since cleaning, I have never had an issue again.

FrodeS · #13 04-27-2018, 06:16 AM

Thank you guys. I took the car for a longer spin today, thinking maybe it just needs to be used some.

The problem might be as stupid as it is easy to fix, and maybe Ismally is on to it.

What i noticed:
1. Cooling issues. Temperature is stable and fine, until suddenly it raises VERY fast from ~80C (normal) to 100 and above. It may only take the time I need to accelerate from 20-50 mph, and goes slowly back down. On the highway it was fine too, until I did a sudden manuver overtaking. I've had radiator issues before, but when that happen, it takes way longer for the temperature to go up. This fast raise can only happen due to too low water level (it is fine), thermostat shutting off (not inspected, but fairly new, original one), water pump issue (hmmm), or some kind of blockage. Thermostat being the main suspect as the car have been sitting for a long time, but water pump, maybe.

2. After visiting a customer, starting the car half-warm on dry tarmac, now for the first time on summer tires (grips a lot better than winter tires), as I turned the steering wheel, the lamps (idiot lamps??) started flashing. I've tried several times. Wheen I turn it all the way over, the lamps come on. I think, this can only be because the belt stopping under load from the servo pump, and the alternator stopping together with it.

With this in mind, I've come to think of something else I noticed last time: When standing more or less over the engine compartment, revving it slightly and waiting for the voltage to raise. As it reached maximum voltage, there was a sudden breeze of air comming from the front. I inspected the electrical fan, but it was not moving. I am starting to believe that as the load of the alternator came down, and the grip of the belt got better due to temperature, at a point the belt picks up speed and revvs the main fan faster.

May it accually be this easy?

I tried ordering a new belt at the steeler today. After 15 minutes of waiting, they wanted $120 for it, so guess I'll just order else were and wait for a couple of days.

Diseasel300 · #14 04-27-2018, 09:51 AM

New belt is ~$20 at the auto parts store of your choice. Shelling out $120 for a serpentine belt at the stealership should get you a free CT scan.

Check your tensioner spring and shock on the tensioner pivot. Also make sure the belt is routed correctly! It is possible to route the belt incorrectly on the 602/3 and still have it sort of work!

FrodeS · #15 04-27-2018, 10:13 AM

I know. Accually, I've got one for free from a friend who run a small store for Mercedes-parts. It is 2 cm too long, but we'll try. Unfortunately, it won't be sent until Monday, and Thuesday is a day off so I'll have it Wednesday at best.

Tensioner and the shock is fine. Checked them when the new alternator was put in.
Good point about the routing. I noticed it does not have mouch contact with the bottom drive pulley. It goes directly from the AC compressor on the left side, under the drive pulley, to the alternator if I remember correctly. I know it is listed in the operators manual, so I'll have a look later today.