Any ALDL and ECM experts on here

[nsxxtreme]

I am having a new issue from the list of never ending issues with this damn truck. I purchased a usb ALDL cable from aldlcable.com

The problem is as soon as I plug that cable in, my truck goes into limp mode and the check engine light starts flashing. My truck is an 87 blazer with ECM service number 1227747 which I believe should NOT need the 10k resistor they have. So I have the jumper completely removed.

If I remove the cable from the truck and measure the impedence between points A&B on the cable it measures 1.2k ohms. I think this is what is causing my truck to go into limp mode. This should be and open circuit I believe. I can still read the ECM with this cable connected but the ECM itself is in limp mode and my headers get red because the timing is way off in limp mode.

My question is, is there anything on the ALDL connector on my truck that could be causing the cable to get screwed up? From looking at the connectors it seems like a dumb simple connector with a ground and a serial data out pin.

 

[4X4HIGH]

I have the USB ALDL cable from the same people and the way they send it is the way it NEEDS to be to work properly.

 

[nsxxtreme]

I have the USB ALDL cable from the same people and the way they send it is the way it NEEDS to be to work properly.

my understanding is that little jumper just changes the impedence between pins A&B.
No jumper or jumper across the top is open circuit between A&B
jumper placed on the 2 right pins should be 10k ohm between A&B
jumper on the two left pins is a short between A&B.

Measure your cable like mine and my guess would be yours would meaure open circuit between those 2 pins not 1.2k ohms.

Funny thing is I measured this last night and it was screwed up like this. Then I measured it today when I got home and to my amazment it measured correctly!! open circuit, short, 10k ohms based on where the jumper was. So to prove I wasn't crazy I plugged it back in the truck with no jumper. I took it back out of the truck and remeasured and its 1.2k ohms!!!!! I can't explain that one without seeing the circuit how they have it designed but they have it sealed up pretty tight so no chance of that happening.

 

[4X4HIGH]

I'll go run out to my blazser and grab my cable and see what I get and also tell you where the jumper is. I'll post back in a few.

EDIT: I just checked my cable and I get an open between the two same pins you have pictured (the other end not hooked to anything). Also the jumper in on the pins closest to the USB end.

 

[nsxxtreme]

Also the jumper in on the pins closest to the USB end.

Ya if you remove the jumper you should get the same measurement. Basically those upper 2 pins connect the circuit on the lower two pins. So removing the jumper or putting it across the two upper pins is the same thing.

 

[mudbuggy]

I don't know which connection, but it also reads from another connections inside there. I don't think it goes on a and b, I believe it a and x or b and x. x meaning whichever one it actually connects to!!!

 

[nsxxtreme]

I don't know which connection, but it also reads from another connections inside there. I don't think it goes on a and b, I believe it a and x or b and x. x meaning whichever one it actually connects to!!!

Not sure what your refering to but the "connection" is definately from A&B I have confirmed that with the manufacturer of the cable. Only the upper 6 pins are actually connected to anything. And even then I think only 3 of the 6 pins are actually used.

I contacted them previously and they just stated to mail it back.

Why mine is acting the way it is I have no clue unless they have some circuitry between those two pins. Should be a simple resistor or short between A&B. But I supect they tried to be fancy and put a circuit in there.

I emailed them pictures of the cable and the measurements yesterday but no response. I don't mind mailing things back but it is kinda annoying. I would like to get this truck up and running correctly soon.

 

[mudbuggy]

So was it running before you monkeyed with the aldl?

 

[nsxxtreme]

So was it running before you monkeyed with the aldl?

Truck runs fine without the aldl and no check engine light. But I want to do some data logging with the truck running so adjustments can be made to the tune. I however can't connect that aldl cable because it puts the truck in limp mode.

 

[mudbuggy]

the pic that you put up, why are you testing those pins. If I am looking correctly to your picture you are at the opposite end of the connector of the a,b connections (the ones that you jump with a paperclip)

I have included two pictures; the first is the one of the vehicle end of the cable, as you know, but the two connectors that are covered with my thumb are the "a,b" connections. I think this is where your and I difference in testing caused an issue.

I had to pull codes on another gm vehicle just today and eventhough this plug is opposite of the original, but the location is still the same. Notice the notch on the bottom of your original pic.

The second pic I added for your benefit, I don't know where your thingy is right now, but I just came inside from testing my cable for you and this position is working correctly for my system.

The third pic is the ones that you were testing in your original picture. Did I understand you were testing backwards or is there no notch in your original picture.

Not trying to start anything just trying to get a handle on it.

 

[nsxxtreme]

I can't see your pictures. But if you go here http://www.aldlcable.com/ and look at the pin out of the 12 pin aldl you will see A&B are on the right and a slot on top to make sure you plug it in correctly. If you look at my picture closely you will see that the slot is on the top and the pins that I am measuring are on the right.

 

[mudbuggy]

I don't know what the hell I'm thinking of, might be the OBD2. I was thinking that the wide part goes to the top, but I just walked out to the truck to look, then on the way back in walked by the vette and it has the OBD2 but it is upside down. Might have been the car and not the truck I was looking at.

What position is your resisitor in? Is it closest to the usb end or the OBD! end?

On mine, which I just tested to make sure I was connecting with the laptop, (but I sure as hell wasn't paying attention to which way the plug goes in) the resistor goes on the 2 pins closest to the usb end

 

[nsxxtreme]

It's not a resister just a jumper. It sets the resistance between A&B. The two upper pins closest to the USB end are the same exact pins. Meaning they are shorted together regardless of if you have that jumper on there. Think of it as a storage location for the jumper. The real purpose is so you can turn the jumper sideways and make a connection with one of the bottom 2 pins. Jumpered all the way to the left causes a short between points A&B. Jumpered between the right 2 pins is 10k ohms between point A&B. No jumper attached or across the top closest to the USB end is the same thing and should cause an open between A&B.

The reason I left the jumper off was to try and lesson the confusion to people who asked where I have the jumper. But I see now that didn't work.

 

[mudbuggy]

Nope, sure didn't and to add to it I'm drunk!!!!!!!!!!!!!!!!!!!!!!!!

Call it practice for Friday

I just thought of something, do you have access to another vehicle that will hook up to the OBD1 connection. Thinking that if that one goes into limp mode, then well um, you know!

 

[nsxxtreme]

Nope, sure didn't and to add to it I'm drunk!!!!!!!!!!!!!!!!!!!!!!!!

Call it practice for Friday

No worries I appreciate the attempt to help regardless. I'm an electrical engineer so most of the time I can figure out how most electrical things work. There is no documentation on this cable though so I have to make an educated guess.

 

[mudbuggy]

Well I'm glad you do that, I don't have the patience for it at times. Only when I have my spices of life does it get interesting..

Is there a way to test it? I mean from an idiot to an expert?

 

[nsxxtreme]

Well I'm glad you do that, I don't have the patience for it at times. Only when I have my spices of life does it get interesting..

Is there a way to test it? I mean from an idiot to an expert?

Without knowing how thing are connected inside there isn't much I can do other then what I already have done. If I had another car with an aldl port I could try it on another car. But this is the only GM I own.

 

[mudbuggy]

what a basic check of continiuty from one end to the other? If 3 or 4 aren't getting a 0 then might be an issue inside the cord? At least then a basic understanding of the overall understanding of the cord layout? Might not work but what the hell can't hurt.

I mean that's what a diesel fabricator would do. How come an electrical engineer didn't do that?

 

[nsxxtreme]

what a basic check of continiuty from one end to the other? If 3 or 4 aren't getting a 0 then might be an issue inside the cord? At least then a basic understanding of the overall understanding of the cord layout? Might not work but what the hell can't hurt.

I mean that's what a diesel fabricator would do. How come an electrical engineer didn't do that?

What are you continuity testing? Without knowing what the circuit is you can't say anything with any degree of certainty about what you are measuring. If that jumper connects to a transistor then your results will be meaningless.

If it simply ran through a resister from jumper to A&B then ya I could measure resistance from jumper to pin at the other end. But mines not working correctly so that would be meaningless. Secondly I don't know that a resistor is actually in there since I don't have a schematic. Many electrical items, capacitor, transistors, IC's all have a measurable resistance. I suspect it's a little more involved then a simple resistor. Resistors don't change there resistance.

 

[Keitha]

Pull the little jumper on the usb end then try again.