Hi,

For the ones interested you may find the ELM327 reference design on ELM site http://elmelectronics.com/DSheets/ELM327DS.pdf. I don’t think that China manufacturer did more than this for the OBD interface, hope is usefull.

Regards,
Daniel

I don’t have ford but out of curiosity I’ve taken a look at my ebay adapter. It’s the larger of the two boards discussed here.

It is implemented largely as shown in the manual linked above except for three changes:

1. a transistor group (Q4, Q5, R13, R12, R36, D3) has been replaced with a comparator. A resistor is added between the comparator and the bus negative. (marked R12 on the board)

2. a pull up resistor has been added between bus- and 5v. the 5v is protected from the bus by a diode. (labelled R4 and D3 on the board). I guess this has probably been done to accommodate the the comparator, which would have a higher impedance than the group it replaces.

3. Resistor R6 is missing. This essentially means a digital output pin of the PIC is grounded

additionally the components are labelled differently. here is a list of equivalents:
(on the board vs in the manual)
Q5 Q2
Q1 Q1
U2 U4
R6 R7
R9 R8
missing R6
R14 R5
R3 R3
R1 R4
R2 R2
D2 D2
R11 R9
R7 R15
Q2 Q3
R13 R14
U4 Q5
U4 Q4
U4 D3
R5 R11
R12 added
(R=resistor, Q=transistor, U=Integrated circuit, D=diode)

I don’t see any problem with the comparator modifications so I think the next step would be to add in R6. It should be a 4.6k ohm and could be added as a surface mount to the track just below and right of decal Q1 on the back of the board. (scratch the track clear, cut it, solder the resistor over the cut)

I’m not going to try it myself as my car works just fine with the K-line communication.

Obviously, I take no responsibility if trying this fries your adapter, car or anything else. strictly at your own risk.

I have the same one and it doesnt work on my 02 Ford. No one’s found a fix for it yet?