W140 Electrical Problems
 

Friend's '93 500SEL, with 142K miles, recently had a factory remanufactured transmission installed. The transmission R&R was uneventful. The day following the transmission service the starter self-activated while the vehicle was traveling at 60 mph. The vehicle was returned to the shop, where the wiring harness for the starter and alternator was found to have loss of insulation. The harness and starter were replaced. The day following the harness replacement the charging system warning light illuminated - maximum alternator output was 11.8 volts, so the alternator was replaced. Today, 10 miles after alternator replacement, the instrument cluster fuse (20 amp) blows everytime the turn signal or hazard warning switches are activated. According to the VMI, the engine compartment wiring harnesses were replaced at 63K miles. Any idea as to what the problem might be. Is there another harness that needs to be inspected? The vehicle was free of electrical gremlins before the transmission replacement and the repair shop is known to reputable... the four small gauge copper wires in the starter/alternator wiring harness were green in appearance and lacking most of their insulation.

jgl1, the earlier year MB's (not limited to W140) had wire insulation that was very poor quality. Rumor has it that the insulation was designed to be bio-degradeable. In any case, the insulation does not handle the heat of the engine compartment well. I suspect that during the trans removal, various harnesses were moved or removed. Any flexing could cause the insulation to crack off. This leaves bare wires which can touch each other and casue all sorts of electrical problems. I have a 95 S600, and I examined all the wiring very closely. The previous owner had changed a couple of them, because they look very new. Others had not been changed, so I rewired these myself. I'm including a link to a thread I wrote on another forum, you may want to look at it. There are some pictures of the insulation problem. Please note that I mistakenly refer to the MAF sensors as Throttle Actuators initially. I corrected that error later in the thread. Instructions for rewiring the Electronic (Throttle) Actuators are in my web page. MAF sensor rewire instructions are being worked on now.

http://www.mbnz.org/forums/forums/thread-view.asp?tid=1202133&start=1&fid=13

Fault turned out to be wiring located behing the instrument cluster... several wires lost their insulation due to overheating... and turn signal activation resulted in a short.