Skolacz1961

Hello Folks,
Have a new to me 02 TT Coupe, 225 bought from original owner a month ago. Has not been registered since taking ownership, waiting better weather in New England and while I perform some TLC maintenance, in other words, it hasn’t been driven since acquisition.

The car was started multiple times over a couple of weeks, with no issues observed, but in hindsight I now realize I did not allow it to run long enough to fully recharge the battery based on my reading on it’s electrical needs - recirculating pump running every time after a start/key is turned off, etc.

Leading up to the error scenario I had removed the Instrument Cluster to replace/repair the Pixel Display. Cluster was repaired, reinstalled and worked perfectly. VCDS was used to perform the output tests and everything worked as designed for a couple days thereafter.

Up to two days after installing the cluster, everything was fine, however, with the battery in a weakened state, I attempted a start and it became evident the battery was too low of power and I aborted start, but not before the typical clicking of starter associated with inadequate power.

Battery terminals were disconnected, cell fluid levels were topped appropriately with distilled water and charged overnight. Reattaching the battery, key to ACC and running Window Calibration procedure, everything was normal, car and instrument cluster operated normally, cluster clock/odometer would light when door opened.

Hours later, when opening the door, I noticed the Instrument Cluster Clock/Odometer did NOT light when door was opened, however window still drops. Trying to start, the ABS, ECS???? And Battery light come on and after self tests, all but the Battery light extinguish.

Records indicate the battery is two years old and with a fresh charge indicates over 12V, hours later, it was still above 12V.

Clock/Odometer do not light when door opened, window goes down
The Instrument Cluster Battery Light is ON when key turned to ACC
Gauges do not function in any capacity if car started - albeit until Immobilizer disables running
Disconnecting Battery, shorting terminals together, does not restore proper behavior
Have tried door closed, door open, key in ignition, key ON when reconnecting battery - various methods I’ve read to try.

VCDS indicates the following:
17072 ECM Power Relay Load Circuit P0688 35-10 Open Circuit - Intermittent
17978 Engine Start Blocked by Immobilizer P1570 35-10 - Intermittent
18058 Powertrain Data Bus P1650 35-00 Missing Message from Instrument Cluster
18010 Power Supply Terminal 30 P1602 35-10 Voltage too Low - Intermittent

Reading the forum it sounds like 18010 is caused by battery disconnect.

Reading suggests if Clock/Odometer do not turn on, that engages Immobilizer

I’ve inspected connections and relays, all are in exceptional condition - previous owner claims car was garaged, I see no corrosion indications of extended periods of outdoor exposure. Taking spot checks of voltage at various locations, 12V is observed, with minimal voltage drop as would be expected. Attaching a battery charger to the fully charged battery does not clear the Battery Indication on the Instrument Cluster - I would expect with the added voltage provided from the charger if there really was a weak battery situation going on, it would clear the indicator, leading me to rule out the battery.

I’m suspecting a Power Issue to the ECM, Instrument Cluster or both, is the cause of 17072 & 18058 as the real culprit, I’m just at a loss to understand how a low battery kicked off this chain of events.

Any thoughts or troubleshooting suggestions will be greatly appreciated. I have access to VOM, Oscilloscope, VCDS and Bentley Manual/Schematics.

Thank you!

Skolacz1961

Issues appears to be fixed.

Battery light on is red herring, until car is started and alternator is in the game, the battery light will be illuminated, this is true for all cars, but was so focused on trying to figure out the issue, didn’t stop to consider that is normal ops.

Issue was residual flux on the circuit board from the pixel screen repair on the OTHER side of the circuit board. When I replaced the pixel display I cleaned up the top side of the board, but didn’t think to look at other side. Though the unit worked fine for two days after reinstall, all I can figure is gravity and sun warming the interior allowed residual flux to migrate around the processor chip - which is on the other side of the PCB from pixel display connector. Removing the white backing of Instrument Cluster and examining under microscope, I noticed a lot of flux around the processor, cleaned it up with alcohol, dried it off and retested. Unit worked as expected. Of course, the other possibility is, I have a cold solder joint and the process of slightly flexing the PCB to remove it from the White Back reconnected enough for everything to work - only time will tell. As circuit boards I used to work on from that era were prone to excess flux interfering with signal flow, my assumption is the power up diagnostic test was failing, thus the cluster did not power up correctly and as such did not communicate with the ECM.

Once it was functioning correctly, it was easy for me to notice that many of the cluster lights we NOT working when it was in the error state, no EPC, no Check Engine, no Seat Belt, no Brake, no Air Bag, no Immobiler Key light, all that did light was the Yellow Triangle with the Counter Clockwise arrow, the ABS and Battery light.

Posting here in case it helps someone else who might have same experience. Curiously, the same fault codes listed in original message are still posted in VCDS. As I can now communicate with the Instrument Cluster and everything functions correctly, and the car runs, I’m ignoring them.

Skolacz1961

One last bit of information: I wasn’t aware that the DTC’s don’t clear when battery is disconnected, which explains why the codes were still present, though everything was functional again. After using VCDS to clear the DTC’s, no codes are present.

03_AR_CO

I was about to suggest clearing the codes and see if any return. Good job :-)