Hi All,
Suddenly, my LR3 won't start. More often than not, it will click and it almost turns over. Occasionally it will do nothing and other tries it starts after a few tries.

GAP tool says "U0100-87 (A8 ) Lost communication with engine control module/ powertrain control module 'A' - Bus signal/message failure - missing a message".

Whats odd, is if I clear the fault it starts right up. As soon as I turn the ignition off the issue is back. Any idea what to check first. I am handy but not great with electrical but willing to learn and give it a shot.

Thanks in advance!

*** I should add that I am also getting another fault: "U0159-87 Lost Communication with Parking assist control module - Bus signal/message failure - missing a message"

 

I would pull the battery and check the ECUs being it along the firewall. If the battery is not vented properly, the fumes can destroy things back there. The only other thing I can think is that maybe it is just a bad starter and the code is some erratic coincidental issue because they way you describe it "almost starting". If it was a legit ECU issue I would not expect it to impact the starter in that way, it would either turn over or not, not almost. Lastly I will suggest checking battery voltage before starting and after its been sitting for a while so the voltage stabilizes. Low battery will cause starting issues of course, but typically it causes a whole lot more than just one code.

 

When I plug in the GAP tool the dashboard screen says 12.2 to 12.1 volts. Then when started 13.7.

 

I try not to use the GAP tool for voltage reading because of accuracy, but those numbers seem okay although running voltage after start should be over 14. I will add that it is possible for a battery to internally short, show its over 12v, but still be weak on cranking amps.e If you think the battery and stater are good, check out the other stuff. You will have to remove the connectors to inspect. Not a hard task, just awkward space to work in a little.

 

"U" codes or "Lost communication with........." Is usually a broken/ corroded wire, poor ground connection or weak voltage.
Dakota Traveler is right, venting the battery is very important. Once the damage is done, it is often quite expensive to correct it.

Issues with the starter:
The battery can show acceptable voltage and the alternator can produce an acceptable voltage and still not charge the battery. Your battery may have the voltage but not the current required to make the starter work properly.

Try this:
Check the battery voltage as you engage the starter. If it remains at or near 12.0 volts, you may have a starter problem. if not, it may be the battery, battery cables or alternator.
If is below 12 volts when engaging the starter, remove the battery and charge it over night or longer.
While the battery is out, check the ECU connections and confirm no corrosion. Also check the ground cable to make sure it is not corroded and has a good electrical connection to the body. Check the large fusible link on the positive cable, it should not be brown or show signs of overheating. The negative cable is a known failure point. The cable ground point behind the battery is another. The ground lug behind the right front inner fender is another failure point for starter issues.
Reinstall the battery and check the voltage as you engage the starter. If it is good, it may indicate a weak battery or a bad battery cable not allowing enough current to get to it from the alternator for charging or enough current to get to the starter for well, starting (they often look fine but have high resistance to high current).

Like most, I like to start with the simple least expensive diagnostic test and work to the complex most expensive parts swapping exercise as a last resort.

General Disclaimer: I might be wrong.

Hope this helps
Jeff

 

Thanks Jeff for the detailed suggestions. Much appreciated.

How do I make sure the battery is properly vented. Just ensure the small hose is not kinked, etc..?

Also, does the fact that the vehicle starts just fine after clearing the fault help diagnose the issue or point in a direction. Seems like if it was a battery or starter problem that clearing a fault wouldn't do anything, but, I'm new to all this...

Thanks again for everyones help,
Michael

 

Yes and No.

Yes -just make sure the little hose is plugged into the front of the battery AND MAKE SURE THE BACK VENT IS PLUGGED OFF.

No- well actually - I don't (k)no(w) see what I did there?

As I understand it when communication is lost it tries to reconnect more often than normal. Since any module could miss a communication command in the right order nearly every time you turn the key on during startup (look for that little window that says systems check or some such) it is possible that there is a larger draw on the battery as it works harder to connect to the lost module and THAT is just enough to make the starter unhappy.
At least that is the way it was explained to me in layman's terms and I still did not fully understand what was being said.
Of course I am open to a better explanation when someone smarter than me comes along (and they will).

I can tell you with all confidence; every LR3 that I have put an aux battery in seldom has low voltage issues. I just got another LR3 last week. It is a 2008 HSE with 124,000 mile on it and the second battery is like third on the list after replacing the compressor drier, and tearing into the third row seats to get one of them to lay down flat

Hope that helps

Jeff