Hi,

I am brand new to both this forum and my A4.

Here are the car details:

1999 A4 Quattro 2.8l Tiptronic 84k miles

Here are the symptoms:

The car was parked and driven to us about 100 miles with absolutely no issues. It sat over night and the next day I went to drive it and got a CEL. The transmission is in fail-safe mode.

I hooked it up to a regular ODB2 scan tool and am getting no communication from the port. I tried my cheapo bluetooth unit as well and got the same results.

From reading posts and looking around online it appears there may be a module someplace that is preventing data communication on the bus (k-line?).

There is no obvious other failure anywhere like the ABS module. As far as I can tell all other systems on the car are working fine.

So, with no codes to go on and only the above symptoms where is a good place to start? Using normal troubleshooting procedures I'd try simple things first like blown fuses or similar. But I need a general area to at least get far enough to get some codes pulled.

Summary:

CEL
Tiptronic fail-safe
No ODB2 data

Thanks for any suggestions,

JF

 

First thing I'd check is all the fuses. Has the stock radio been changed out or replaced with an aftermarket radio?

 

I will check all the fuses after work today and report back.

It is the stock radio.

Thanks,

JF

 

Okay, I got home and checked all the fuses. Actually tested each one.

Same result. Still no data on the ODB2 scanner.

As a test I pulled the fuse to the on board diagnostics (#12 in my car) and tried the scanner again. It immediately bitched that there was no connection and to turn the key on. So I shut it all down, put the fuse back, and was back to where I started. The scanner sees something there but I can't get any data.

Next idea?

 

I wished I had a list of relays and their locations on that car. It could be a relay not making up. I'll do some research and see what I can come up with.

 

What's the engine and transmission code for your car. Should be something like AMX or APR.

Also, there is a relay panel under the dash on the driver's side (US). You may have to remove the knee bolster on the bottom of the dash to get to it. There are a couple or three large fuses in that panel.

Still digging though.

Pull the fuse for the ABS module and see what you get. I'm reading that the ABS modules are known to short the K-line to ground for some reason. Also, does the factory radio work? Even if it does, you may need to pull it (it's VERY simple to do and takes 2 minutes, just get a set of radio pulling keys off eBay) and remove and re-seat all the the connections on the back of the radio .

 

I pulled the ABS fuse to eliminate that module and got the same results. Tried the same thing with the radio fuse. I can try that test again by physically disconnecting the ABS module but since it triggered the ABS warning light I think pulling the fuse disabled it for the purposes of this test.

I pulled the knee bolster and looked at the relay panel and tested the three fuses there. All good, no change.

I pulled the radio and checked the connectors and wire bundles. Nothing seems out of place, burnt, or worn.

The next things I am going to try:

Test the K-line for voltage. While the radio is out I will test the resistance of the K-line between the radio connector and the OBD2 port.

Next I'm going to dig out the TCU and make sure it isn't wet and that all connections look good. If I can find a pin-out for its connector(s) I'll check the resistance back to the OBD2 port there.

What other modules are on this bus that I can unhook or eliminate to narrow down the problem? I know of the ECU, the TCU, the radio, the ABS module. What else?

 

Actually, disregard my last question. I got the list of possible modules from the Ross-Tech Wiki for my car. I'll start hunting them down one by one tonight.

 

While I had the radio pulled I also tested the K-line for voltage between OBD2 port pins 4 and 7 and it was fine < 9V as indicated on the Ross-Tech wiki.

I tested resistance between OBD2 pin 7 and pin 3 on the back radio connector. It was pretty high with the key on - 196 ohms.

I pulled out the TCM/TCU and then the K-line resistance looked good and I was able to read from the OBD2 port.

While I had the TCM/TCU out the CEL went off and the only code I read on my scanner was a P1626 which is complaining about not getting data from the TCM/TCU.

As a test I disconnected the battery, touched the battery cables together, let it sit for a few minutes, connected the TCM/TCU back and reconnected the battery. No change

I ordered a used TCM/TCU from an eBay seller so we'll see if it works when it arrives next week.

Thanks for the suggestions and help. If anyone has anything else for me to try or do I'll be happy to. I'm assuming the TCU is not serviceable in any meaningful way.

 

Man I hope you get it sorted out. I think you may have found it. I'm going to have to study up on the K-line stuff. I'd be ready to burn the damn thing after a while. LOL