1996 855 T5 - Input on schematic for OBDII LED code reader?

[Sommerfeldt]

Hey!

I thought I would build a plug-in code reader that will flash the codes old-school, for those of us who have Volvos w/o the box under the hood and either can't find a tool that works, or don't want to shell out hard earned bucks for one. I've seen Ben850's small (but excellent) project in building a tool like this, and wanted to refine that a bit. I already built this thing in 10 minutes yesterday:

3.jpg (22.98 KiB) Viewed 1337 times

... but I wanted to make one that will actually read everything, without having to move a lead from pin to pin. The idea is an instrument to plug in, that can switch pins to the necessary ones w/o having to remove the thing from the OBDII-connector or do anything but push the momentary switch.

So my question to you guys, who in general are smarter and more tech savvy than me by far - will this work:

Any suggestions are welcome - it's the first time I've drawn one of these things, so pleasce ignore the arrowheads - I don't know how to get rid of them.

- S

[jreed]

It looks like it will work. The schematic doesn't show a current-limiting resistor in series with the LED. You will need to add one if you are not planning to use an LED with a built-in current limiting resistor designed for 12V applications.

You might want to check that you can get useful information from all the pins you've outlined. On my '97, I was successful only with pin 6 (the ECC unit). I couldn't get the other pins to respond.
I wrote up my circuit here:
https://www.matthewsvolvosite.com/forums ... 43#p251189

Good luck!

[Sommerfeldt]

Resistor!? God, I hate electrical stuff.
Thanks for that - I would have overlooked it and gone into a blue rage (ultra-blue) when the thing burned out on me.

- G

[MrPc]

With the circuit you show, you run the risk of blowing the driver transistors for pins 3, 6, 12, and 13 if more than one of the pin selector switches is closed when you read codes. This is because more than one output would be connected directly together, and large currents could flow if the buffers drive the signal in opposite directions.

It is quite possible that the car's OBD-II circuit has current limiting built into it already, in which no damage would result. Anybody know?

To be sure to avoid damage, you can add current limiting resistors between each of pins 3,6,12,13 and their switches. Then if more than one pin is selected, the conflicting drivers would have two of these resistors between them to limit damaging currents.


With either built-in current limiting, or your own limiting resistors, reading codes with more than one pin selected, would get no blink or 1/2 blinks when/if signals conflict. This would indicate that you need to check the switches and set them properly.

[esl_97_850_T5]

Sommerfeldt,

1. I've been able to use my "jonesrh flashing LED tool" (~6th link here) on my USA '97 850 T5 using all 3 of these pins:

- pin 3 for Motronic 4.3,
- pin 6 for Climate Control, and
- pin 13 for Cruise Control.

There is no lead inside my USA '97 850 T5 OBDII socket's pin 12.

I believe pin 12 is used by ABS for Volvo 900 and S90/V90.

2. In your diagram only one of the pins 3, 6, 13, 12 should be switched on at any given point in time. You could do that with:

- a rotary switch (like this rotary switch example #1 which uses pin 4 and/or 5 as ground, or this rotary switch example #2 which uses pin 4 as ground), or

- a 4-pole DIP switch where you just make sure that you don't switch more than 1 DIP switch on at any given point in time -- (on this simple flashing LED diagram for use with pin 3 I would draw in the 4-pole DIP switch on the initial downleg to pin 3, then have that 1 input line to the 4-pole DIP switch to branch out on its output side in 4 directions to pins 3, 6, 13, and 12). I'm thinking that's what you meant by your diagram. Correct? To make it clearer in your diagram, I'd draw a box around the 4 switches for pins 3, 6, 13, and 12 and label it 4-pole DIP switch. Of course, if you have 2 of the DIP switches on at the same time, the problem that MrPC mentioned would presumably still be possible.

3. The easiest way to deal with the resistor is to buy a 12V LED that already has an in-built resistor (if that option is available to you). In the USA, Radio Shack used to have a 12V red LED like that.

esl_97_850_T5

[Sommerfeldt]

Thanks guys!
I was thinking that I would just have 4 small on/off switches and be careful not to have more than one on at any one time, but I think that was a bit optimistic...accidents do happen. A rotary switch is a pretty great idea, and perhaps the current limiting is a good idea no matter which solution it comes down to.

As for the LED, I think one ready made for 12V is a good idea as well...it should save some space, since I'm rubbish at soldering and I want to make this thing as small as possible.

Thanks again, guys - this is helping a lot, like I knew you would!

-S