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Ask Lou P. master tech Your Own Question
Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 4276
Experience:  15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
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My battery went dead in my Mercedes s500. Replaced the

Customer Question

My battery went dead in my Mercedes s500. Replaced the battery know all light come on when key is turn but will not start
Submitted: 8 months ago.
Category: Mercedes
Expert:  Lou P. master tech replied 8 months ago.

Hello, my name is***** and i will do my best to assist you.I am going over your story now and will reply back soon with some info,thanks!

Expert:  Lou P. master tech replied 8 months ago.

at any point was the battery hooked up backwards or the jumper cables hooked up backwards?

Customer: replied 8 months ago.
No
Expert:  Lou P. master tech replied 8 months ago.

ok, are you sure the old battery was dead?

Customer: replied 8 months ago.
Expert:  Lou P. master tech replied 8 months ago.

if so then the next step is going to be checking for available voltage at all the fuse boxes , its possible that a prefuse blew or the issue was not a battery to begin with. how many keys do you have?

Customer: replied 8 months ago.
One
Expert:  Lou P. master tech replied 8 months ago.

ok, if the key turns then it will most likely not be a ignition switch or key issue , for the most part at least. you need to have all the fuse boxes checked out to see if power is present at all of them . not sure how much you want to do or if you work on the car yourself and if you are good with electronics and what not. let me know and ill do what i can to help you.

Customer: replied 8 months ago.
Power is present q
Customer: replied 8 months ago.
Any other suggestions?
Expert:  Lou P. master tech replied 8 months ago.

ok i will need more detailed info from you, i can help for sure, but general statements are not going to help either of us, power is present where?

if power is present at all 3 fuse boxes then we need to see if we can jump the starter to see if it works.

the relay for the starter is on the passenger side of the engine bay fuse box. do you know how to jump terminals?

Customer: replied 8 months ago.
Sorry yes power to all three. Do not know how to jump terminals
Expert:  Lou P. master tech replied 8 months ago.

ok, on the bottom of the relays are numbers ON THE RELAY near the terminals, there is 85-86-87-30 and sometimes 87a

when power and ground are supplied to 85 and 86 then it will energize a magnet that will close contacts to allow power to go from 87 to 30, so what we need to do is use a wire and insert in on the fuse box where 87 and 30 are to connect them which in turn should crank the starter, if it does then we know the starter is good. if it doesnt we need to find out why.

the relay location on the passenger side fuse box is in this diagram i am sending you.

Expert:  Lou P. master tech replied 8 months ago.

relay L is the relay we need.

Customer: replied 8 months ago.
I inserted the wires in the spots you told me to left the wires in a tried to start the car it started to crank before the key was also the way to the start position
Expert:  Lou P. master tech replied 8 months ago.

Correct that is what we were trying to do. That means that the starter works. The issue is further up in the system. So now you need codes checked to see if the correct gear is displaying or if the communication system is functioning.

Customer: replied 8 months ago.
Ok so how do I do that (get codes)
Expert:  Lou P. master tech replied 8 months ago.

You need it diagnosed with a scan tool. If you do not know how to access codes then your best bet is to take it to a shop to get diagnosed. From here it's only going to get more complicated. Cost you time and money compared to having the dealer check it out.

Customer: replied 8 months ago.
Lou
It is giving me codes P0500 and P1747
Expert:  Lou P. master tech replied 8 months ago.

ok let me check those.

i think those are transmission fault codes, if so , when the key is on, does the dash show the gear is in P in the right corner?

Customer: replied 8 months ago.
The dash shows no indication if the car is in park
Expert:  Lou P. master tech replied 8 months ago.

ok then that is the issue, either the shifter is bad or the communication system in the car is down due to a failed module or water. i actually have another customer with the same issue,. we found water in the passenger side engine bay fuse box and also inside the transmission control module.

Customer: replied 8 months ago.
There was no water in the fuse box, i will check the transmission control module
Expert:  Lou P. master tech replied 8 months ago.

ok it is the little guy tucked between the fuses and the side of the fuse box housing. has two connections on top, you most likely will find trans fluid filled inside. but we also want to look for corrosion or loose connections or milkshake looking color indicating that there is water intrusion. what kind of scan tool are you using?

Customer: replied 8 months ago.
Ok I will check that. U380 Memo Scanner OBDII/EOBDll Memo Scanner
Expert:  Lou P. master tech replied 8 months ago.

ok that scanner will only check the codes in the engine module. so it wont help a whole lot. sorry.

Customer: replied 8 months ago.
What kind of scanner should I get?
Expert:  Lou P. master tech replied 8 months ago.

we need one that can read all the mercedes modules, depends on how much money you want to spend. but a mercedes star diagnosis is the best.

Customer: replied 8 months ago.
Lord you are right the star is expensive
Expert:  Lou P. master tech replied 8 months ago.

yes, BUT once you learn how to work it , man it saves you in the long run with diagnosis, especially when you have a tech on your side lol

Customer: replied 8 months ago.
Lou I was going back over everything you told me to check had my son sitting in the car while I checked the starter again I inserted the wires where the starter relay goes my son turned the key to the position right before the start position starter engaged then he turned it to the start position the car started I immediately pulled the wires out and the car stayed running. Not sure what that was about any idea?

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