FAULTY ELM 327 INTERFACES DE-MYSTIFIED (READ HERE)

[yani]

Hi,

A lot of people are having problems with some chinese ELM interfaces that cannot connect to CAN network even if they support all ELM 1.3 commands properly. There was one user here who bought 5 of these cheap interfaces with no luck and finally bought an expensive one....
I also have one of these "faulty" units and decided to play with it and see what is wrong.
Well, it seems that the problem is electrical and is not very hard to fix.
The problem is actually in the CAN termination method used in these interfaces - they use one 120 Ohm resistor connected between CAN-L and CAN-H.
According to the ELM327 datasheet the proper termination is to use 100 Ohm resistor on each of the 2 lines connected to ground through a 560 pF capacitor (see the picture below).
So i just got two 100 Ohm resistors and two capacitors of 560 pF and fixed my interface as shows below. And it WORKS flawlessly now.
The procedure is very simple:
1. Get two resistors of 100 Ohm and 2 capacitors of 560 pF. It should work without this. See the NOTE below.
2. Remove the single terminating 120 Ohm (or whatever) resistor from the interface
3. Solder the new termination circuit. I did it directly on the OBD connector as it was easier for me. I did it in a hurry just for the so don't blame for the soldering quality on the pictures. It should work without this. See the NOTE below.
4. Go to your car and enjoy....

NOTE: It is reported that most of the new interfaces now have this problem!
It was also reported by many users that the problem can be solved by simply removing the 120 Ohm termination resistor (which is connected between the two CAN lines, it is R14 on my photos but might be marked with different number on other interfaces) from the interface.

Please, someone with knowledge in the circuits to explain these better as I don't have time for more details.

PLEASE NOTE: This post is for interfaces that respond properly to all ELM commands in my "Test" which is in the Settings dialog (all commands should have response OK). If you have OK to all commands and can connect to engine, but cannot connect to anything on the CAN bus (like Body Computer, Dashboard, Service Reset) then this post may help you.

The original board of the "faulty" interface

elmfix1.jpg (92.53 KiB) Viewed 1025974 times

The bottom side of the board. The terminating resistor is circled in red

elmfix2.jpg (92.79 KiB) Viewed 1025974 times

Removed the old terminating resistor

elmfix4.jpg (79.36 KiB) Viewed 1025974 times

Soldered the new terminating resistors/capacitors on the OBD connector

elmfix5.jpg (92.79 KiB) Viewed 1025974 times

Proper termination as listed in ELM 327 datasheet

elmfix6.jpg (64.57 KiB) Viewed 1025974 times

Yani

[mino182]

Hi, please, where can i find ground? pin 4? Thank you.

[yani]

Pins 4 and 5 are both ground.

[temppu]

This really works fine!

Very big thanks yani!!!!
I'm happy man now

[mhowse]

Much Thanks for mod.
.been having no joy connecting stilo 1.9jtd 2003 to service reset and been holding off trying other elms as think they may be fakes also..got the parts in uk maplins store, resistors and capacitors for 82p bit of soldering , solderd both fly leads from capacitors together then single connection to ground onto pin 5 to save soldering problems to pins 4 and 5 and now fake elm connects to service reset and other options great..

much thanks for your effort in sorting out the annoying service reset for me as shall be doing oil filter change this weekend.

[JTD171]

Can anyone help me out my elm327 dosent seem to be the same layout as in Yani's pics so I dont know where to put the resistors and capacitors.

[steph48]

> To JTD 171

Hi JTD 171,
I have the same circuit as you, and the same problem as described by yani (connect to engine is ok, and others ECU KO).
After a look at this one, I think that the case described by Yani is not applicable for this circuit.
So there is also a difference between this layout and elm327 spec.
I have updated your picture, so you can see the correspondance between your layout, the layout described by yani and the elm 327 spec.
First, resistors R32 and R33 are just equals to 100 ohms. I don't have capacimeter to test capacities. So I think the problem is not the same as the one described by Yani.
So we can see that R33 resistor value is 21 kohm on the layout. The ELM327 spec wants a 4,7kohm resistor.
I wil test with a 4,7kohm resistor for thr next two weeks.

Steph48

CIMG0011.JPG (1.78 MiB) Viewed 1025648 times

[devc]

R1, R2, C1 & c2 are the components Yani is talking about. You interface is correct.

[JTD171]

OK thank you

[steph48]

Hi,The JTD171 interface seems to be correct but I have the same and:
- I have OK to all commands and can connect to engine, but cannot connect to anything on the CAN bus (like Body Computer, Dashboard, Service Reset).
My vehicle is a FIAT Croma 1.9 16v (150cv).

I have replaced the R5 resitor on the circuit (R31 on the ELM datasheet) by a 4,7kohm resistor but no change. The problem is the same.

If anyone can help me.:
In order to debug the problem I scanned the com port while multiecuscan tried to connect to 2 differents ECUs (Engine (succesfull) and body (failed) ):

BODY ECU :

Code: Select all

Port ouvert par le processus "Multiecuscan2.exe" (PID: 4068)

Requête:19/09/2010 16:24:02.01264 (+169.1406 seconds)
 41 54 5A 0A 0D                                    ATZ..           
Réponse:19/09/2010 16:24:03.93464 (+0.9219 seconds)
 0D 0D 45 4C 4D 33 32 37 20 76 31 2E 35 61 0D 0D   ..ELM327 v1.5a..
 3E                                                >               

Requête:19/09/2010 16:24:03.95064 (+0.0156 seconds)
 41 54 45 30 0A 0D                                 ATE0..          
Réponse:19/09/2010 16:24:03.99764 (+0.0469 seconds)
 41 54 45 30 0A 0D 4F 4B 0D 0D 3E                  ATE0..OK..>     

Requête:19/09/2010 16:24:03.99764 (+0.0000 seconds)
 41 54 4C 30 0A 0D                                 ATL0..          
Réponse:19/09/2010 16:24:03.05964 (+0.0625 seconds)
 4F 4B 0D 0D 3E                                    OK..>           

Requête:19/09/2010 16:24:03.05964 (+0.0000 seconds)
 41 54 48 30 0A 0D                                 ATH0..          
Réponse:19/09/2010 16:24:03.12264 (+0.0625 seconds)
 4F 4B 0D 0D 3E                                    OK..>           

Requête:19/09/2010 16:24:03.12264 (+0.0000 seconds)
 41 54 53 50 43 0A 0D                              ATSPC..         
Réponse:19/09/2010 16:24:03.16964 (+0.0469 seconds)
 4F 4B 0D 0D 3E                                    OK..>           

Requête:19/09/2010 16:24:03.16964 (+0.0000 seconds)
 41 54 53 30 0A 0D                                 ATS0..          
Réponse:19/09/2010 16:24:04.23164 (+0.0625 seconds)
 3F 0D 0D 3E                                       ?..>            

Requête:19/09/2010 16:24:04.23164 (+0.0000 seconds)
 41 54 43 41 46 30 0A 0D                           ATCAF0..        
Réponse:19/09/2010 16:24:04.29464 (+0.0625 seconds)
 4F 4B 0D 0D 3E                                    OK..>           

Requête:19/09/2010 16:24:04.29464 (+0.0000 seconds)
 41 54 43 46 43 30 0A 0D                           ATCFC0..        
Réponse:19/09/2010 16:24:04.34164 (+0.0469 seconds)
 4F 4B 0D 0D 3E                                    OK..>           

Requête:19/09/2010 16:24:04.34164 (+0.0000 seconds)
 41 54 43 52 41 20 37 43 30 0A 0D                  ATCRA 7C0..     
Réponse:19/09/2010 16:24:04.40364 (+0.0625 seconds)
 3F 0D 0D 3E                                       ?..>            

Requête:19/09/2010 16:24:04.40364 (+0.0000 seconds)
 41 54 53 48 20 37 42 30 0A 0D                     ATSH 7B0..      
Réponse:19/09/2010 16:24:04.46664 (+0.0625 seconds)
 4F 4B 0D 0D 3E                                    OK..>           

Requête:19/09/2010 16:24:04.46664 (+0.0000 seconds)
 41 54 41 54 31 0A 0D                              ATAT1..         
Réponse:19/09/2010 16:24:04.52864 (+0.0625 seconds)
 4F 4B 0D 0D 3E                                    OK..>           

Requête:19/09/2010 16:24:04.52864 (+0.0000 seconds)
 41 54 53 54 32 39 0A 0D                           ATST29..        
Réponse:19/09/2010 16:24:04.57564 (+0.0469 seconds)
 4F 4B 0D 0D 3E                                    OK..>           

Requête:19/09/2010 16:24:04.59164 (+0.0156 seconds)
 30 34 20 30 32 20 31 30 20 38 31 0A 0D            04 02 10 81..   
Réponse:19/09/2010 16:24:04.80964 (+0.2188 seconds)
 4E 4F 20 44 41 54 41 0D 0D 3E                     NO DATA..>      

Requête:19/09/2010 16:24:05.30964 (+0.5000 seconds)
 41 54 50 43 0A 0D                                 ATPC..          
Réponse:19/09/2010 16:24:05.32564 (+0.0156 seconds)
 4F 4B 0D 0D 3E                                    OK..>           

Port fermé

Thanks for any help.

steph48