Hi Guys, need some help please. My SL500 2003 with 78K is in wonderful condition, however has just recently developed and problem were it starts normal and runs fine, but shuts down the engine after exactly 20 minutes every time. It does this every time and so below is a couple points I’d like to show what have established so far.

1. Both batteries are sound, correct type, ratings and load tested - fine
2. I have no battery warnings
3. Since this problem started I now can’t scan normally with my Foxwell NT710. The quick scan stops at approx 60%, so I have to scan the individual modules. I have established that CAN 1 & CAN 2 will not communicate. After the car it stops, it won’t restart immediately, as it displays symptoms of a flat battery - i.e slow cranking but the battery voltage is approx 13v
4. The car will restart after about an hour and run again for 20 mins - then shuts down.
5. When running - voltage checks on each battery shows 14.3v at each battery.
6. I have checked all battery connections - all are immaculate.
7. BCM - inspected and is immaculate - no burning or leaking capacitors. All BCM wiring are tight and immaculate and no faults with this when scanned.
8. Relays K57 & K75 both electrically checked and appears fine.
9. Footwell pre fuses F52f2 200A & F52f1 100A both checked ok.
10. Fuses F32 & F33 have not been checked.
11. No other systems have stopped working
12. Car has Key-Less Go and I have I have been using the normal key since this problem has developed as the key is able to be switched off immediately when it won’t crank.

With the car cranking & starting normally, I don’t believe it is related to the crank sensor on the account it doesn’t crank after the 20 minutes auto shut down.

From what I read in the Mercedes dual battery guidance material - the system has an emergency mode. With the consistent 20 min shut down, it appears that this may be a clue to this fault and perhaps also the lack of OBD scanning too, as the car has always scanned prior to this fault.

So I would appreciate any suggestions guys and have attached the dual battery guidance material for info and useful info.

Regards
Joe
Cairns Australia

 

I would change the CPS as a “maintenance item, and check battery grounds and frame to engine transmission ground under the car. The starters do tend to turn slow after a while.

 

polish all battery terminals and connectors. Coat with dielectric grease.

 

...go easy on the the dielectric grease, it is not a conductor...

 

I stand corrected. see ai recommendations:

3. Battery Terminals (Sometimes)

Not on the contact surfaces—only after the connection is tightened, a thin layer on top helps prevent corrosion.

💡 4. Bulb Sockets

Especially on:

• Headlights
  
• Taillights
  
• Turn signals
  

Keeps moisture out and prevents seized sockets.

⚠️ What Dielectric Grease Does NOT Do

• It does not improve electrical conductivity
  
  
• It does not fix a bad connection
  
• It should not be slathered on metal contacts—just a thin film around them
  
• Because it’s non‑conductive, too much can actually insulate the connection.
  

  🛠️ How to Apply It (The Right Way)

• Use a very thin film
  
• A pea‑sized amount is usually enough for an entire connector.
  
  
• Apply to rubber seals, plastic housings, and around the metal contacts
  
• Do not pack the connector full of grease
  

 

Maybe call around to some auto repair shops and ask if they have a scanner with "Topology Mapping" which shows how the various CAN buses interconnect with modules. Scanners include Topdon Phoenix, Launch X431 PRO3 and just a few others.
Since you cannot get a complete scan, this a problem that needs to be solved. Sounds like the ECU has a 20 minute timeout built-in when it detects a problem with the CAN buses.
Have you checked for corrosion on the connectors to all the SAM modules?

 

Hi Guys,
Thanks for your replies. When the car shuts down I agree it could be a CPS heating up and failing, but that theory does not explain why the car will not crank over after it stops. My experience with CPS failure is that it will still crank over at normal speed, but won’t fire. I will go ahead and purchase & replace my CPS, but am pretty sure this is not my issue. Something has faulted which is why CAN A & B is only recently not communicating when scanned & I’m sure this is a clue to my fault.
Does anyone know how specifically check for CAN bus A& B data feeds please…?

Thanks
JD