Fixing a Mercedes Benz W140 S500
One of my relatives owns a Mercedes Benz W140 S500 below and it has
been running strong but recently it got a trouble. The electric cooling fan keeps
running and it never stops while the engine is on. The fan used to run only when
the car stopping at a traffic light or running slowly.
a Mercedes Benz W140 S500
The problem was caused by a short circuit of the wiring harness that was
connected to a sensor located at the front part of the 5 litters engine. As he
saw other wires, he was amazed at the terrible conditions of almost all the
wiring harness in the engine compartment room. The insulators were flaking
badly. The wire connected to the distributor was also flaked as below and
the wires were about to be touched each other. Pulling out the connector
from the distributor made it completely shorted.
After peeling the tape
The connector at the distributor
The more you touch, the more the insulator flakes
I searched the Interet and I found similar problems a lot there. Some people
are facing more serious situations. The problem is very inconvenient for us.
When insulation flakes, it will cause a short circuit and the engine might not only
stall but also damages the ECU. This is very dangerous. I wonder why
Mercedes had used such poor quality materials. I must say that their quality
engineering skill was not well developed. This type of problem could be found
very easily using equipments to test a heat cycle, a heat shock and burnings.
Below is the whole harness and at a glance it looks OK but once you remove
shown at above photos.
The owner decided to replace the whole wires. He has asked the dealership
The money was not a big problem for him but he was doubtful to let them do it
again since there are many people who suffer the same problem again
changing to a Mercedes new wiring harness within several years.
Then he asked me to advise. We discussed how to fix. My suggestion was to
use higher quality Japanese wires designed for the automobile application
instead of using a Mercedes wiring harness that might fail again. Some
information obtained said current harness is OK but those remain in the
market for reapair used materials not improved. My actual idea was to use
Thus we used high quality Japanese wires and rewired all. The 36 pin
connectors had to be wired like this below.
Above shows how the one side of the injectors and distributors wiring were
connected.
All the other connectors were reused as we did for 36 pin connectors attaching
the new wires by solder and covered that part by a heat shrinking tube that
withstand 125 degrees C. The job was not easy and it required us to spend
a whole week end but after completing the wiring and installing the harness,
the engine ran well again and there was no more problem. There will be no
more harness shorting problem again which is prone to cause a problem
using the original parts.
We enjoyed the process and now we are thinking how to spend the saved
money 115K Yen. The material cost was only 5K Yen. The labour cost was not
as saving money which can be spent for other good activities.
