okay, i'm ready for the 3 wire plug test. i found one plug that has three colored wires. brown, yellow and green. ?

i stick the exposed end of the wire into the center terminal after it's attached to + on the battery.

tia

no luck.

but i may have tried the wrong three wire connection. i tried the three wire connector 6 inches from the battery's + terminal. the one with three different colors. i stuck the wire in the center from both ends to see where i get the best contact. it didn't work.

across the engine compartment, towards the drivers side, from the battery, there is a three plug set up going into the drivers side dash. they are big yellow connections. three of them. each one looks sealed in yellow plastic.

is this the three wire set up you are referring to?

tia

X27 plug location is in the Archives under X27 Plug.

But.... here you go ..

Do you see the brake booster/master cylinder Fluid Reservoir..??
If YES,
Then look to the left of the CAP on the reservoir....about 1 and 1/2 inches.
You will then see at that location a 3 wire plug..unplug it and bring 12v pos to the center terminal of the plug still connected to the firewall, NOT the plug you just unplugged.
That is feed for the starter and if you bring feed to that center teminal the starter will crank the engine over , verifying that part of the starter circuit as OK and the Battery capacity as high enough to crank the engine over.

You can also turn the key to RUN position and gear selsctor in P and start the car from this same test point in the event you have to move the car or you are stuck with a non-running car. This test bpasses the NSS and K38 relay, so if it does crank w.test. we know the problem is upsteam from this test point, which cuts the diagnosis time and direction in 1/2..as that is a convienent 1/2 way test point in that circuit. Anything downsteam from that test point is under the hood , and anything upstream from that test point is in the Cabin.

If you are not positively sure where a connector plug for testing is , ask first..b/c you bringing 12v to any 3 wire connector you see can wind up doing some serious damage.
Ask twice , test once.
The ATA module location is in a previous thread of mine...

thanks!

copied, pasted and printed.

the test worked. the starter cranked.

so the problem is inside the cab.

what's next? thanks!

Jumper K38 relay for starters...........
I think you may want someone to look at it as you do not seem to have a handle on elctronics.

Arthur, i took the car to the local mercedes dealer and had the problem fixed. 2 hours and $250 later, i'm on the road again.

Thanks to you, i didn't need to get it towed there.

Very Good.

The K38 relay jumper and ATA stuff can get a little tricky even for a Tech...
so you did the right thing.
But the X27 trick is something you now know and print it out and throw it in the glovebox b/c it will get you going if the problem comes again, and it also works if the NSS switch goes out on you [ another common problem]
Without knowing about the X27 trick, it is Tow Truck time....so a good one to remember......