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.