OBD2 socket testing

[phil clark]

Hi, been doing a little testing on OBD socket today (just trying to rule out car electrical problems) and ive noticed something rather strange. Using my lexia cable, without interface attached, if i test between pin 4 and the earth underneath the pedals without the ignition on, i get a reading of 0.9-1.0 on the the 200 ohms range, which isnt bad. NOW, if i turn the key to position 1 (so the battery light is on) and the systems are Energised, the reading multiplys by atleast 5, to between 5.0 - 6.0, on the same range. Any ideas guys, whats causing this to happen, because im having trouble with lexia, and i think this is effecting the system. Many thanks :-)

[cully]

i would go and clean the main earth points and the one by drivers pedal on the car!
pin 4 and pin 5 is ground

[phil clark]

i would go and clean the main earth points and the one by drivers pedal on the car!
pin 4 and pin 5 is ground

Respect your thinking on that, but how im testing it is basicley in a closed loop, going nowhere near the battery. Pins 4 and 5 join together, and unenergized the reading is really low. Its just when i fire the system up it changes, in quite a big way. Its as if once the dash/fusebox/incar systems are ready to work, there making the resistance value change. Never seen this before, and tommorow im planning on a quick test between pins 4 and 5, and if that does the same, im considering chopping the cable to pin 4, extending it, and putting it dirrectley on to the earth point. See lexia uses pin 4 only, not 5 (which is beyond me tbh)

[cully]

it would make the resistance value change!

think about how the meter works it uses a voltage to measure the resistance by reference
so if the car has a bad earth the meter is being effected buy the introduced voltage by the car feeding back through the high resistace joints

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pin 4 + pin 5 are wired seperate to a joint (EM34A) which goes to Joint (EM34) which goes to Earth point (MC34)

so maybe buybass all the joints go Pin 4+5 to earthpoint

im trying to locate the earthpoint but am having issues locating it

autodata says its in the drivers footwell

[phil clark]

it would make the resistance value change!

think about how the meter works it uses a voltage to measure the resistance by reference
so if the car has a bad earth the meter is being effected buy the introduced voltage by the car feeding back through the high resistace joints

Thats sounding better bud. Im not good at assessing values, but to me, the change in what ive found is way too high, and personally im begining to think that this has been effecting how my lexia works, and making it play up. I know that pin4 goes to a main earth cable, prior to screwing to the body, and pin 5 goes straight there. This is reminding me of the TPS earthing being effected by the MAF lol. :-)