1992 Mercedes benz 400se system not charging but has new alternator and new battery

Asked by May 29, 2014 at 05:25 PM about the 1992 Mercedes-Benz 400-Class 4 Dr 400SE Sedan

Question type: Maintenance & Repair

3 Answers


Here's the answer, one side of the "battery" light in cluster, connected to the ignition positive. The wire on the other side of same lamp bulb is connected to the small wire on the alternator. If both sides of the lamp are positive the light won't light, good condition. I'd either side drop down to a lower voltage beyond the normal differerence of the two voltages the light will will light. So the lamp is in series with the way the alternator small gets a sample of the battery voltage when the ignition switch is on. I found that in my 1992 W140 that the wire that travels down from the one side of the battery lamp, through a metal guide tube along with the little starter little wire, goes direct to the little wire one the alternator. The insulation on the little wire will degrade in side the tube and short the inside metal wall of this guide tube, thus putting a low on one side of the lamp, the alternator side causing the light to light. At the same time, this short cuts off the alternator, and the alternator will no longer have a clean +12v sample of the battery voltage that would allow the alternator to function properly. Well here is how I fix it, I finally got a good accurate schematic of all this circuitry. So, remove the cluster from dash board. On one of the two plug/connector there is a wire that is blue in color, it is wire "15"on the connect. This is the answer. I cut the wire about 3" from the connector, and spiced a new wire in about 8' long. Leave the old shorted wire alone. Since I can read schematics that blue wire originally made its way directly to the little terminal on alternator, (D+). Ok, so I ran the extended cluster wire out my car door, across the right front fender down under the car. At the alternator I cut the (D+) wire on the alternator about 10" from the alternator. Then like magic after I connected that wire to the alternator, the light went out, the voltage back at my truck battery went up to 13.4 , so there it is you need to replace the wire between the alternator (D+) and wire #15 at the wiring harness jack between the instrument cluster. Clearly after I did this I re-routed my new wire nicely from behind the cluster, through the firewall, and over to alternator (D+) terminal. Thus, I did not have to buy a new wiring harness, I just left that shorted wire in that metal routing tube forever. P. S., the other little wire in that tube is the starter wire from the ignition. If you are having starter problems, you might want to check for ground fault in the the wire from the ignition to the starter. That wire shares that tube with the alternator wire. I hope this helps somebody. This shorted wire was also making my instrument cluster light up like a Christmas tree. My oil analog gauge did not work, sat, light kept coming off and on. All of that went away. Some of these problem were associated with the wire grounding, also some of these problem were from the fact that ecm's don't like low voltage condition, they can operate correctly in low voltage conditions, I.e. The alternator was being forced to turn its self off from the 0v condition being feed to it from this grounded input wire signal. This alternator is not a self exciting type. It need a sample ov the battery voltage to operate properly. If the voltage at the (D+). Terminal is too low or too high it will simply turn its self of. I would say good luck , but in this case luck had nothing to do with it. My "Big Goldly" 1992 mb 400se is rolling nicely ! William (510) 759-5181

2 of 2 people found this helpful.

I have a 1992 Mercedes benz 400se

2 of 2 people found this helpful.

About the Benz,is there's specific way the aernator comes off cause its locatedbottom riight, any advise thanks

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