I hate to advertise my mistakes but it might help some future people. I just bought a newer X-164 GL550 after selling my 2008 . Changed all the fluids, brakes etc brought all the maintenance up to current. It had the push button start and I removed it. Occasionally the key will turn and nothing happens on the first turn but so far has always started on the second turn. I decided to install a couple of relays .. starter and compressor . I put the part number into RockAuto and bought the Bosch.. MISTAKE THEY DONT HAVE THE DIODES!!!
Result pictured blown ground foil right off the relay/fuse board. After watching a few videos and realizing the older boards are not forgiving … I did repair my board but am going to put a new one in since I am all over the country with my bird dogs and vacationing a lot .

 

If after replacing a compressor relay your SUV..R,ML and GL…starts runs and dies when the compressor turns on.. pretty likely you burnt the ground file as in my pictures. Here is a good Youtube video with a better explanation than mine.

 

Unclear.

I don't think the diode has anything to do with it.

 

Thanks for taking a minute and replying. My results were exactly what the video tried to explain. My ground strap was blown in two different areas. I drove about 100 miles today with the repaired ground strap.. everything was ok. Still for $220 going to put the new box in. Still trying to digest the info about the diode.. my board toasted as soon as i plugged the relay in without the diode.. key was not on for the compressor to run… it acted like a direct short?? That foil was definitely not made to take much current flow.
If I had the complete circuit diagrams in front of me I am pretty sure the diode would stick out as a problem area.
Being an old retired electrician/medical electronics guy I will get a better answer BUT since someone posted that he had 4-5 boards all blown by putting a relay in without the diode into the compressor with the same results I had… I tend to believe him. Bill

 

Now I am really wondering if some of the 164 fires we have all heard about are from these boards going up in smoke? I witnessed my board go up in smoke in a few seconds and instantly pulled the relay.
OK for electronic people out there .. I will post a few diagrams from WIS and you can see if you put in the “same” relay missing the diode you are basically hooking up ground thru a few ohms of the compressor motor right to the 40 amp fuse ..positive 12v.Look 87 & 87A with the diode. (Absolutely a horrible design IMO) 87A goes to the ground foil in the relay/fuse board…87 comes right off the 40 Amp fuse…if the diode is missing as is the yellow relay (that had the same part number at RockAuto..002-542-76-19.. it has been removed since I wrote to them yesterday.
Basically 12v positive flow thru the NON DIODE RELAY (yellow in my example)to the SIGNAL ground( signal ground is for the relays small current to pull in the contacts.. high current flows thru the relays thru the wiring and fuses…. NOT THE SIGNAL GROUND on the board.(line 31)!!
kG on the diagram is the proper relay (usually green) with the diode. So when replacing the compressor relay make sure it has the 87a diode on the relay cover. thanks for reading my long winded post

 

That is the wrong relay! Looking at the wiring diagram: It is double throw, unenergized 30 to 87a, energized 30 to 87.


Relay G is supposed to be single throw double pole, energized 30 to 87 and 87a - with a diode on 87a.


The diode is on 87a presumably to drain the voltage surge when the motor windings are de-energized. Or vice versa. Something like that. I'm not a good circuit guy, so don't quote me.

87a connects to 31, which is ground. 30 is a supply rail, coming from the battery.

When you plug in the yellow relay, you are connecting the supply directly to ground. No wonder the trace burns up. Now, the fuse should have blown instead - but the fuse is not on the supply line but rather on the line out from 87. We had seen the trace burning but I assumed it was just a poor design with inadequate heat dissipation; it never occurred to me people might be plugging in the wrong relay. Not that it's anyone's fault; the two relays fit the same pins.

It is not a fault tolerant design, requiring a special relay that, if not used, burns up the circuit board underneath.

The part I don't understand is that you should have seen a nice spark when plugging in the yellow relay, as 30 appears to have constant power even when the ignition is off.

So yeah, good observation that the diode is not present in the relay you received. I guess it sorta was the diode after all, in that the absence of the diode means it's the wrong relay.

I'm still not sure this is the whole story, but it's something.

 

P.S. As you figured out, that trace can be repaired (I guess you soldered a bare copper wire on top of it?). But if the wrong relay is in there, the board will only have problems.

P.P.S. Moral of the story: Inspect the wiring diagrams on replacement relays! It is not just diode versus not; those are two very different relays.

It is good to finally get to the bottom of this PCB trace burning issue.

 

No doubt like I state in the beginning I screwed up with the help of RockAuto… I put in the part number and was given a few choices…
OK being a electronic guy.. I built and tested MRI’s and Catscans (in another life a few years ago)
ANOTHER REASON YOU WILL SEE SIGNAL GROUND TRACES BURN.
1. Diodes do fail and not always burn open but they can short closed… then the trace will burn!
2. Not many relays can be put into the wrong socket.. back into my steel mill electrician days…
…Horrible design and it really makes me wonder if fires have been caused.
It was instant smoke when I plugged the relay in.. no spark just smoke and pulled it out quickly but you saw the damage in my original pictures.
I googled more today and there are many many post about “after I replaced the relay the car will start than shutoff with the compressor turns on.”
Now for the really bad news… almost all sites state Part not available .. not made Obsolete!!!
We shall see if my order goes thru… if not I will take it apart again and really spend time building a better trace and insulating it !! thanks Eric for sharing your thoughts… Bill
Eric the small ground trace is just for circuit grounds small current application … that is why I call the circuit horrible… to blow a 40 amp fuse you need a big thick ground not a paper thin circuit trace… up in smoke!

 

Just to show and talk a little more about the importance (Eric definitely pointed that out) of looking over the relays when replacing and just NOT trusting part numbers . Here are some pictures from RockAuto
AND I DEFINITELY SHOULD HAVE KNOWN BETTER… I am posting this stuff to help others from not making the same mistake or if you did how to fix it.
Notice all the relays came from a search on 002-542-76-19
Today I made them aware and hopefully RockAuto gets that page corrected.

 

Here's the fun part: That 40A fuse is on the output of the relay, between the relay and the compressor, not on the +12V rail. So when you installed the yellow relay, it was running nearly unfused current through 31, the ground rail. 30, the +12V rail, is almost unfused, except for a 100A fuse at the battery. So the only thing protecting 31, which I assume is the copper trace, is a 100A fuse. Insane.

On the bright side, you don't have to be particularly careful about insulating the wire used for the repair, as it's ground, but maybe you have to be careful to not touch it to the metal bars in there, which might be 30, the 100A fused +12V.

This WVE relay does not have the diode protection but it does appear to be the correct function:


Still, I'd get a configuration that matches. The Vemo relay has the diode and looks like it's the correct configuration:


The MB part number you are looking for is A00025422319. Incidentally, that also goes in relay socket I (starter). Amazon sells generic. I think they are a standard automotive motor control relay. Maybe they're so common that MB decided against trying to sell them as OEM.




Anybody could look at the relay wiring diagrams and figure out they are different. Rock Auto should know better.

 

From my understanding of the diode is too take a little of the breaking energy of the contacts giving the contacts a little longer life.. I did order OEM’s on Amazon… will know more when I get them. The pathetic part is that the grounding should be on a decent size wire NOT the signal ground to the energizing coils. Really horrible design. All the years I was around design circuitry you just always keep the signal/control circuitry separate from the power side of circuitry thus preventing melt downs. Fuse protection should save meltdowns.

 

When you have current flowing through a coil (i.e. a motor winding) and you shut off the current, the voltage across the switch rises - the current tries to keep flowing. The diode shunts that excess energy to ground. It has a "breakdown voltage", not meaning that it breaks the diode but the voltage overcomes the diode's resistance. It stops the voltage across the switch from rising past a certain point. Otherwise the switch contacts will absorb the excess energy by burning up.

There are lots of flaws to the MB electrical system. The fusing, for example, is weirdly placed. In many cases, the path to ground is completed, not the supply. It is counter intuitive and annoying. In this case, downright dangerous.