Window Switch Contacts Burned, any one think this is the reason - PeachParts Mercedes-Benz Forum

Diesel911 · #1 06-07-2021, 05:02 PM

Supposed cause inductive kickback arch apark

https://www.benzworld.org/threads/prevent-inductive-kickback-arc-spark-on-power-window-motors.3075730/#post-18220971

I am 300Dman over on benzworld and I commented in the thread.

Has anyone owned or frequently been in a W123 to know if when the Cars were newish to know if they had the burned window switch contacts issues when they were newish? Or is it something that appears after the Car is old?

Diseasel300 · #2 06-07-2021, 05:58 PM

If inductive kickback (also known as reverse EMF) were the cause, ALL of these cars would be eating switches for breakfast. They don't. The OP in that thread has bad switches (silver plating worn off the switch points) or a high current draw situation due to a bad motor, bad window regulator, or gummed up window guides.

Diesel911 · #3 06-07-2021, 11:01 PM

Quote:

Originally Posted by Diseasel300

If inductive kickback (also known as reverse EMF) were the cause, ALL of these cars would be eating switches for breakfast. They don't. The OP in that thread has bad switches (silver plating worn off the switch points) or a high current draw situation due to a bad motor, bad window regulator, or gummed up window guides.

That is what I said in part of my response.

jay_bob · #4 06-08-2021, 07:43 AM

One thing to be careful of with the electrical contacts (not just the window switches, but just about anything else) is that there is silver plating on these contacts, plug pins, fuse clips, etc.

If you abrade the contact point with anything and wear off the silver plating, exposing the (usually) copper substrate, the contact will oxidize quickly. The silver plating protects the copper.

The resistance increases, the current heats it up more, the resistance goes up more, until the contact gets burnt.

barry12345 · #5 06-08-2021, 09:15 AM

As with a lot of things. I suspect after years of their lubrication being neglected. The current load on the switches exceeds the original design parameters.

If you spend a day dealing with the specific type lubricants for many situations. The car may even in general feel better to you. I suspect a full days effort to get most of it done.

The door hinges are robust on the 123s. Thankfully as few recieve lubrication as the tool to apply it is not on hand. Cars this old with no door hinge pin wear are not common. So some parts of the 123 are far above average.

I personally never neglect the hood hinges. Making certain I get to the parts way in that body mount. After repairing tear outs. I never want more of them.

87tdwagen · #6 06-08-2021, 03:11 PM

Quote:

Originally Posted by jay_bob

One thing to be careful of with the electrical contacts (not just the window switches, but just about anything else) is that there is silver plating on these contacts, plug pins, fuse clips, etc.

If you abrade the contact point with anything and wear off the silver plating, exposing the (usually) copper substrate, the contact will oxidize quickly. The silver plating protects the copper.

The resistance increases, the current heats it up more, the resistance goes up more, until the contact gets burnt.

Agreed. The reality is that given the age of these cars, the probability of electrical contact issues will only increase, due to mechanical wear, whether induced by man, time or grit.

I'm not a purist, so I modify my cars if I can improve them. With all vehicles of this vintage, MB or otherwise, I strongly recommend the addition of control relays for headlights and windows specifically as they have the highest loads and usually do not have individual control relays with lengthy marginal wiring.

Power your window motors directly and use your switches only for triggering your relays. If you do this, you will not have future brunt switch/contacts or wiring issues and your windows will zip up and down faster than ever before...including New.

Same thing for your headlights, high/low/fog. Wire a heavy gauge lead from the fuse box to the headlights, add the relays, use the headlight switch lead to trigger the relays and get a full 12V to the headlights.

The factory design has the current going through the entire system, wiring and switches before getting to the action end. The current drop is horrible, gets worse with time due to wiring corrosion leading to burnt parts of the system, all avoidable by changing the main feed line and making it short and direct. The long term concerns on wiring switches etc. is now trivial because that side of the system wont see more than 1A going through it to control the relays.

If it's a keeper, you can do this for under $50 and for a few hours of your time

n3nyong · #7 06-08-2021, 03:47 PM

FWIW we were cleaning non-op W123 chassis switches in the mid ‘90’s, so even then there were issues due to full current going through them.

Sent from my iPhone using Tapatalk

BillGrissom · #8 06-08-2021, 06:34 PM

A DC motor will surely give a kick-back voltage spike when turned off, due to inductance of the armature and the generator effect. But, many are switched off with a manual switch and no protective diode. Protection is more needed with semiconductor switches. I have to periodically open and clean my window switches on the console. I assumed that was due to spilled drinks by my offspring, who don't fess up. My switches still have shiny silver once cleaned. Silver can tarnish (oxidize), so not unexpected. If you take apart the mirror switch in the console, mark the body carefully and take photos as soon as you open it, as there are many ways to reassemble it wrong (how would I know?).

Texasgeezer · #9 06-08-2021, 07:01 PM

To 87TDwagen,

Do you mount the window relays inside the door cavity?

Is there a single relay available to reverse the voltage for the motor?

Any brand of relay and model you've had good success with?

Diesel911 · #10 06-08-2021, 08:31 PM

Quote:

Originally Posted by jay_bob

One thing to be careful of with the electrical contacts (not just the window switches, but just about anything else) is that there is silver plating on these contacts, plug pins, fuse clips, etc.

If you abrade the contact point with anything and wear off the silver plating, exposing the (usually) copper substrate, the contact will oxidize quickly. The silver plating protects the copper.

The resistance increases, the current heats it up more, the resistance goes up more, until the contact gets burnt.

The attached picture is what one of my Counsel Switches looked like back in 2007. Notice it is way beyond the silver plate issue. Since that time I have scraped it 2 times with a small pocket knife but it still works.

Diesel911 · #11 06-08-2021, 08:34 PM

Quote:

Originally Posted by Texasgeezer

To 87TDwagen,

Do you mount the window relays inside the door cavity?

Is there a single relay available to reverse the voltage for the motor?

Any brand of relay and model you've had good success with?

On my 84 W123 the single Power Window Relay is behind the Instrument Cluster. I have no idea what electrical components are inside of it.

ah-kay · #12 06-08-2021, 08:40 PM

You will be disappointed if you expect your switches to last forever. The rating of switch probably has 50% margin designed in by Mbz engineers but over time the load changes due to motor, fiction etc. I am ok with changing it out after 30 years.

As far as putting in a relay. It is doable but you just shifted the failure mode to the relay contacts. You increased the failure modes 2 folds as a minimum. I would not do it unless the switch in question is very very expensive or very very difficult to get to. Just be happy with your car, there are a lot of things you can improve on an old car if you have the time, money and energy. I don't. Life is too short.

87tdwagen · #13 06-09-2021, 08:21 AM

Quote:

Originally Posted by Texasgeezer

To 87TDwagen,

Do you mount the window relays inside the door cavity?

Is there a single relay available to reverse the voltage for the motor?

Any brand of relay and model you've had good success with?

I have usually installed them in the lower A or B pillars, where the wiring is before going into the doors. Usually there are kick panels in these areas to make access easier if needed. The goal is to use as much of the factory wiring but keeping the load side as short as possible to the motor. If your switch is on the door, then the relays should go in the door cavity, but in MB and most Euro cars of this era we are talking about center console mounted switches.

As to relays, you could get fancy and source a DPDT relay as a single relay to handle both directions, or use 2 single throw/pole relays one for each direction. I have a bunch of Bosch "Ice cube" relays, really old ones with the metal cover as shown below, but any will work. Those old Bosch ones are really reliable and very well built and I've probably only ever changed out a handful of them ever in 40 years of wrenching due to failure, plus these with the metal cover can be easily opened and serviced if needed. Available in 15,20,30Amp varieties.

You can pick what ever you are comfortable working with. I prefer the spade relays as they are far more common, and allow you to easily to wire in with regular spade connectors, or relay holders. The MB round tang relays would require obtaining specialized connectors which are harder to source.

As others have mentioned, cleaning window switch contacts for 1970-mid 80's to keep things working was and still is common practice to keep things working. Par for the course with these cars. Avoidable if you install the relays. A switch can only be disassembled so many times before it is destroyed from the process itself.

This mod came around namely thru Jaguar or any British car, enthusiasts, having to deal with Lucas "Prince of Darkness" electrics. I've done this to Series 1,2,& 3 cars, starting 25 years ago and have yet to experience a failure since.

I do not agree that you are introducing new failure points to the design. While theoretically you are adding more potential failure points, by correcting the original, most prevalent failure, current overload, you are reducing the overall failure of the system dramatically.

This mod is well documented, in this forum, and in many others, you can find schematics showing various ways to incorporate the relays on the interweb. You can also find more information in Kirby Palm's "The Book" on the Jaguar XJ-S which explains in detail the how to's. It is a mod that works, works well and is very reliable and has been applied by thousands of enthusiasts over the decades.

Hope that helps a little.

Mike D · #14 06-09-2021, 11:08 AM

These work on the round pole relays. "Bullet" connectors. Depending on the manufacturer you might need to trim the plastic for the best fit if you use the crimping method. I use the bare ones, solder and heat-shrink.