Projectz28

Wierd one I am having a bit of trouble with. Got the motor runing and idling decent enough to start running some scan software with it. My buddy came over with his laptop and HP tuners software. We werent sure if it would work with an LT1 but since its OBD2 we figured it should. Well we were hooked up and reading fine. started the motor and we lost communications. We figured it was a software issue since we werent sure if the scan part of HP tuners would work.

So we moved on and hooked up my laptop with Autotap... same thing. We did a little investigating and we have communications and data transfer to the laptop Key on engine off. We have it when cranking... reading cranking rpm and what not... as soon as the motor fires we loose communications.

So we figured it was a ground isse and traced out the 2 grounds from the ALDL. I dont have my service manual in fron of me here so I might be wrong about these pin numbers.. but pin 4 ground goes to chassis ground just under the dash on driver side. Ground is good and the wire is good. Less than .5ohms. Pin 5 ground goes to the engine. Traced it out and thru the connector on the passenger side (interior harness to engine harness connector). From there thru the engine harness to the block. Bolted to the stud just above the starter. Again less than .5 ohms.

So we started looking at data wires. There are 2 data wires. tan and Purple one. I think the purple one in pin 2 is labled Class 2 data and I believe thats the one we read and flash thru. The other is tan and I forget what its called. Getting 4.5V on teh tan one but not getting any voltage reading on the other. Is it able to be measured with a meter? Not dure what kind of signal it is honestly.

Power supply is good 12v and its there KOEF and running..

Any idea what the issue might be?

BTW, some info about the car: it is a stripped down race car. Much of the wiring in the car has been removed. I did it personally and I took great car in doing it. Sat with a service manual in one hand and I made certain I knew what wire I was removing before i removed it. (Thats not to say I didnt make a mistake somewhere)... just saying it was not hacked up witout any care involved. I removed the simple stuff... HVAC, radio, abs, interior lighting, PDL, trunk release, BCM, all dash instrument cluster wiring also...

The other stuff is still there. headlights, fog lights ect...

Any help is greatly appreciated.

Seams like we are losing the data on the serial bus when running... and only running....

Could it be interferance?

battery has been relocated to the rear. Good chassis ground to the batery in the back with 1 gauge. Good ground from stock motor mont ground stud to the distro ground stud on wheel well. Also an aditional 1 guage ground strap from the driver side head (bolt/stud for ignition coil)to the chassis again...

Ricardo

This is very interesting.

I have the same scenario with a buddies car I do the tuning for.
I can scan it and clear trouble codes with autotap, with the ignition on and the engine off. I have even loaded several different tunes with LT1 edit with this condition. Communication is straightforward with the engine off. The moment I start it while scanning it I will loose it.

The car is a 1996 A4 383 stroker. CC 306 cam. Standard ignition. Original factory harness and wiring.

My car on the other side, shows no problem with communication. I can scan it with the engine off, crank it and it will continue to scan and log data untill I decide to end the session. My car is also a 1996 383. I run an MSD6AL box with no issues.

I never considered it to be interference, I always thought it could be some corrosion on the PCM terminals, but being the exact same symptoms with yours it might be the case.

Have you tried swapping the PCM to rule it out?
What spark plug wires are you running? I run the Moroso cut to fit and have them over the valve covers.

RG

Projectz28

exact same problem....

Running MSD 6AL box, stock opti and Taylor 8mm in the stock locations. I have not tried swapping PCM's as I have nothing else to swap it with. The car wont run on a stock tune and my edit is locked to my PCM.

I only pruned a few minor sensor connectors from the stock harness to clean it up a bit. Other than that I didnt really change anything.

Ricardo

My buddies car is also running Taylor 8mm spark plug wires.
Thats all I have till now.
I will check the resistance per foot of my wires to see if there is any difference in there.
RG

Projectz28

did a little research and seem there are several people who have this problem. All of them have said they now just start the car then plug in.... I'll have to try that also. Otherwise its either a data wire problem or interference.

Projectz28

Well, I am still having this problem. I did find something intresting though. I have the communications loss with the MSD hooked up. When I bypassed it and reconnected the factory harness directly to the coil the problem totaly went away. I have communication with teh engine running and not running. I figured it must either be the higher output spark being broduced by the MSD and probably just some EMI or RFI interference. I switched to Magnecor plug wires and that didnt help. I called MSD and all they told me was that I should run the battery + and - directly to the car battery using 1 size larger wire like their directions say to do. I tried that and rerouting them several different ways. I also hard wired the connectors for the coil. No good. I just bought a new Digital 6 Plus box and installed that. Still the same issue.

I have tried getting more serious about this and the only issue I see is that when I start the car I seem to have a ground problem on terminal 5 or 6 (cant remember right now...) of the ALDL. When the car starts the resistance to groung goes up. I still can read 12 volts from the hot terminal and 5/6 (ground) but the resistance for some reason goes high. i am going to try running new ground wires directly to the battery for the ALDL and see if that helps.

just thought I would post an update for anyone else who might run across this. Seems that nobody can figure it out.

I'm going to be forced to "borrow" an o-scope from work to look closer at the circuits soon.

96ZCamaro

I had this same problem. The battery was low but yet the car still started. But I put my battery on a charger, put it back in the next day and then the scanner wanted to work correctly.

Projectz28

definately not a battery problem....battyey was fresh off the charger. In fact this has been going on for so long it has happened with more than one battery. I'm using an Optima yellow top now.

I was really sure it was a ignition box problem since it only happened with the box hooked up. I just tested the new box last night and its still a problem.

I'm going to run a couple new ground wires tomorrow direct to the battery as a test.

Ricardo

I did also change the plug wires in my buddie's car to MOROSO, wich I use in my own car without any problem, and the issue went on. We can rule out the plug wires by now.
We have good communication with the engine off, ignition on and we loose it after a few seconds after starting the engine.

Projectz28

still the same problem.

Try checking something next time. Put a meter on the ground terminals of the ALDL I think its 4 and 5. Run a wire from the battery negative to the car so make this easier and check resistance from the battery to the ground pins on the ALDL, I noticed that whenoff I have less than 1 ohm. I turn the ignition on I have 20-30 ohms on 4 and 90 ohms on pin 5 resistance. When it starts for some reason it goes crazy but I can still read 12 v from either groung pin to the hot terminal pin (16 I think off the top of my head). I think thats where the problem is.

I am convinced its not interference from plug wires. Also not the ignition box.

TobyZ28

On the wire/connector I have there are two screws holding it together. These came loose over time and cause periodic outages with my logger. Tightening them back up fixed it. Just though I'd add this, however it doesnt sound the same as your issue

Ricardo

We also use an MSD 6AL box.
I still have not tried scanning the car while bypassing the ignition box.

Projectz28

little bit of an update. I quick ran a wire from the battery and just clipped onto both grounds and no change. Still lose communication when the car starts.

I am now thinking it is some sort of software problem. If I start the car, then connect the cable and open the software and then "connect to vehicle" it connects. Shut the engine off and start it back up and it will lose communication. I cant reconnect untill I shut down the program and the car, then reconnect like I did in the right order.

Not sure why. I am honestly at a loss on this now. But atleast I think I can data log with autotap now. I just have to do it in the right procedure. I am going to call Autotap next week.

jdob84

I know this is dredging up an old thread, but did this issue ever get resolved? Im having the exact same issues with my 95z28 right now and it's driving me up the wall! I have heard of plugging in after the car is started, but I've had no luck with that as of yet. I've grounded everything I can think of but still can't get good logging data. I bought the car with some hacked up wiring, but I thought I had it taken care of... Guess not. I'm running rich right now and I can't log my car to see what's going on. Kinda irritating!

Projectz28

yeah its defiately an old one

I dont even have the car anymore I sold it last year. I never figured out how to fix it but the problem is the MSD... I plugged the ignition back to stock and the problem went away. Hook the MSD back up and the datalog problem came right back. Down side was I didn't have my tach and I didn't have the rev limiter on the MSD....