Author Topic: Work involved into a Wiring Harness restoration, hidden damages  (Read 1669 times)


  • Vendor
  • Gold
  • *****
  • USA, FL, Miami
  • Posts: 545
Dear friends

I am making this post to share some thoughts regarding what involves to fully repair a wiring harness and which things are always important to address in order to have reliability on the use of our loved Mercedes Gems.

Wire harness are rarely touched by any shop, since people tend to concentrate their efforts in what is visible to the public eye.

It s common that you see a beautiful looking Mercedes Pagoda or any other model that has been painted many times in its life, has received new upholstery, has a fully repaired engine to the bone, and still has reliability issues.

When you ask your mechanic he gives you all kind of reasons for that failure, changes many parts but at the end, the root cause was an aging wiring harness that has never seen the light of an ELECTRIC SPA. I mean has never been removed from the car and repaired in full extend.

That is how my journey on Mercedes classic electricity systems started.  My own Mercedes w111 220Se left me stranded many times because a drained out battery. My thrust fully  mechanic and friend expert in Mercedes  classic cars, Juan Carlos Perez Quintero, a rare example of what a dedicated mechanic to Mercedes is, made me change the alternator, advised me to bring the car to an electric shop driven by a German guy, and even so, after spending and spending money, the car still ran out of battery if driven for long periods of time with the air conditioning on.

In that time I made electric calculations of the total energy consumption vs. the alternator output and could not explain why system was failing. Reduced then, consumption by installing a smaller condenser electrical fan, etc.

Finally one day I decided to remove all the harness and see what was in there.....

Many surprises waited for me....  and finally a sum of various defects found under the external insulation explained the battery drain.  What I saw in my car electrical system, has been a common symptom in all the wiring harnesses I have serviced so far.

1. Damaged insulation on main T30.
2. Many splices and cable joints on cables going to or from alternator/starter.
3. Burned cable ends on headlights connectors, because of the use of higher wattage bulbs (halogen) of 70-100 watts)

Mechanics are not electricians and..... electricians are not engineers as well.   When you bring your Mercedes to and electric service shop, they normally do not have the wiring diagrams of your particular car, and they even do not understand in depth all the system complexities that your classic Mercedes has.

One classic example on these cars, is the COLD START CIRCUITRY, it is always bypassed or activated by a manual dash switch , because all the people that has serviced your car in its history is always in rush to get it running and has no patience to solve out all the mods that many people has made trough the years on it.

So my final advise, is that do not forget about that complex network on your loved Mercedes. It needs attention as well. No matter if you think that everything work's as a very nice customer I had who recently passed away due to Covid-19, use to say.

I am willing to help out those wanting to service their harnesses by themselves giving them the proper advise on what to do.    And also.... I offer my services for those who do not want to accomplish this complex work with their own hands.

There is also a world of possible modifications to fit more modern Mercedes original electrical systems in your car to make it more reliable if you deeply understand these systems.

The most classic example is modifying the harness to accommodate MERCEDES TRANSISTORIZED IGNITION which is an original mod that came on later 280SL (1971j to extend the lifespan of your ignitions points by means of using them as a triggering device instead of a high current element.


See this shared album of one of the works I completed for my friend and customer Vincent from

And that is all the story for today.

Waiting for your comments and experiences with the electricals  of your cars to guide you through the process.

Best regards
Eng. Leonardo Peterssen

Ps see the album link posted above, there you will see that no matter if everything work” there is always a hidden repair  that will leave you stranded sometime....
« Last Edit: April 23, 2021, 14:33:46 by lpeterssen »