SLK 350 OBD2 Port - Power but No Signal

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SLK 350 OBD2 Port - Power but No Signal

Postby SpeedFerret » Fri Jun 09, 2017 3:15 pm

Both my scanner tools power up but wont connect.

I checked fuse 62 visually (fine) and resistance (0 ohms) -- I believe this is +12v power to the OBD2
I checked fuse 40 visually (fine) and resistance (0 ohms) -- I believe this is signal to the OBD2

I then got the multimeter out. Engine off key on:

Battery -ve to Pin 6 on the ODB2 port: 15mv (had expected between 2.5 and 3.5v)
Battery -ve to Pin 14 on the ODB2 port: 15mv (had expected between 2.5 and 3.5v)
Battery -ve to Pin 16 on the ODB2 port: 12.58v (same voltage as at battery so good result)

So that explains power to scanners but no signal to the scanners.

Battery off: 60ohms resistance between pins 6 and 14 ... so that seems OK.

With Key On Engine Off, the LCD panel in the instrument cluster now shows a number of 'fault' messages

SRS System Fault
ESP Unavailable
Vehicle Display Fault

The car starts OK and idles OK. The engine management light goes out but all other messages on the LCD instrument panel cycle as detailed above. After about 10 seconds the engine radiator fans go on full despite the engine being cold .. so looks as if the ECU doesn't know that the engine is cold (or believes the engine is hot). I put the car into drive .. I can feel the car wants to creep forward. I put the car into reverse and I feel the car wants to creep backwards. I try the radio .. it works fine. I try the windows and all the other accessories in the car and they all work fine.

I then try to open the roof and get the following msg in the instrument panel: "Vario Roof locked in drivers mode" . The roof wont recline.

Any ideas as to where I go next in terms of fault finding. I'm blind without gettind signal to that OBD2 port. Can I tap in on the other side of the central gateway to try and get a signal ?
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Re: SLK 350 OBD2 Port - Power but No Signal

Postby Dcunning35 » Sat Jun 10, 2017 6:34 pm

What voltages do you get with your dmm at pins 6,14 . The 60ohms confirms termination resistors you.need to see activity now .my product knowledge on merc isnt the best as my brand experience is another make however the can is an industry standard . A good wiring diagram of coms network will help work out a test plan
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Re: SLK 350 OBD2 Port - Power but No Signal

Postby SpeedFerret » Wed Jun 21, 2017 1:33 pm

Was up at the car last night .. its been too hot in the garage to do any work the last few evening. Took some meter readings from the Diagnostic Plug again which I think tends to confirm I have a canbus short to ground:

Pin 4 (Chassis Ground) to Chassis: 0 Ohms (Good)
Pin 5 (Signal Ground) to Chassis: 0 Ohms (Good)

Pin6 to Pin14: 60.7 Ohms (Good)

Pin4 to Pin6: 17.6k Ohms (Bad I assume .. thought it would be open cct)
Pin5 to Pin6: 17.24k Ohms
Pin4 to Pin14: 17.6k Ohms
Pin5 to Pin14: 18.98k Ohms

I have checked resistance between Pin 6 and 14 on the diagnostic plug with Pins 5 and 4 respectively on the Central Gateway and both lines are showing 0 Ohms (which is good).

So, I'm now thinking one of the nodes on the wider network is causing this short to earth. Need to find X30/5 and X30/6. I think they are at drivers door cill. I'm going to start by breaking the link to these two and eliminate all the nodes coming off 30/6. Then I'll pull the nodes one at a time off x30/5 and hopefully something will come up (or confirm the earthing issues is not Canbus B related). This is basically a resistance test so battery will be off as it has been for all resistance checks. Out tonight so may get a look on Thu night.

I hope I'm on the right track.
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Re: SLK 350 OBD2 Port - Power but No Signal

Postby Tivtech » Sun Jun 25, 2017 12:57 am

It is perfectly normal not to have an open circuit from the CAN pins 6 & 14 to ground (4 & 5), due to of the design of the CAN receiver chip design inside the various ECU's. These are differential receivers, but not with infinite input impedance.
On the other hand, The CAN but voltages must be in the vicinity of 2.5V to ground so maybe the various ECU's on the bus are not receiving power for some reason.

SpeedFerret wrote:Pin4 to Pin6: 17.6k Ohms (Bad I assume .. thought it would be open cct)
Pin5 to Pin6: 17.24k Ohms
Pin4 to Pin14: 17.6k Ohms
Pin5 to Pin14: 18.98k Ohms

I have checked resistance between Pin 6 and 14 on the diagnostic plug with Pins 5 and 4 respectively on the Central Gateway and both lines are showing 0 Ohms (which is good).

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Re: SLK 350 OBD2 Port - Power but No Signal

Postby Prince » Sun Jun 25, 2017 10:39 am

Question :?:
:idea:
If pins 4 and 5 are 0 ohhms they are a perfect connection to gnd.
Pin 6 to gnd is 17.4 ohhms.
Pin 14 to gnd is 17.6.

Pin 6 and 14 are connected through gnd as well. So remove the 60.4 ohms they would still be connected with a resistane of 35 Ohhms through gnd. But with the 60 Ohhms your resistance would acttually be 22 ohms.

Have I missed something, maybe. :roll: :lol: :lol: :lol:
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Re: SLK 350 OBD2 Port - Power but No Signal

Postby Tivtech » Sun Jun 25, 2017 1:17 pm

Take note he's talking about 17 KILO ohms, not ohms...that should be normal as input resistance to ground of each CAN line receiver input.
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Re: SLK 350 OBD2 Port - Power but No Signal

Postby Prince » Mon Jun 26, 2017 2:30 pm

Tivtech wrote:Take note he's talking about 17 KILO ohms, not ohms...that should be normal as input resistance to ground of each CAN line receiver input.


:oops: Sorry.My bad. Yes at 17 KOhms I would not be too concerned. After all one can use resistors to remove noise and 17Kohms is quite high.

I wil try to read better in future. :) Thanks.
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Re: SLK 350 OBD2 Port - Power but No Signal

Postby STC » Wed Jun 28, 2017 5:50 pm

You wont be able to tap in on the other side of the gateway.
You will need to establish communication with the Gateway or condemn it as faulty. I would be inclined to strip everything except the power supply and the 2 wires to the diagnostic socket from the Gateway.

(If you have Diagnostic CAN at 6 & 14, I don't think it will be Hi Speed so there may be no need for the 120 ohm resistors)

PS. What scanner are you using, might be an idea to try a different one.
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