I've had an ECU bus problem for a few months and the problem is temporarily fixed by clearing the fault codes (saving for new car so no point replacing the ECU)

Today I tried to start the car and got the usual "gearbox fault" message and tried to clear the codes through the OBD dongle and it said the codes were cleared but still got the "gearbox fault" message

Took the battery leads off to discharge the computer for a hard reset and when I reconnected I get a clicking sound like the battery is dead and a high pitched whining sound coming from somewhere when the ignition is ON but the car is NOT running - also my battery tester does not even registering (it's an LED one that just tells you if the battery is good, medium or dead)

I don't know if the whining was there before the reset or not because the ignition was not turned on when I was fiddling with the battery - I also changed fuse 32 and that seemed fine as I was still getting the "gearbox fault" message afterwards

Got the battery on charge now just in case but when I tested it a few weeks ago there was a full charge and if it was dead the cd and air con wouldn't have worked when I first tried to start the car this morning

Any ideas?

EDIT: - battery was on charge for about an hour and I tried it and the car started - put the meter on the battery and it was barely registering LOW so obviously something has drained the battery since the last time I used the car last week - checked all the lights etc and nothing has been left on and you get an alarm if you leave lights on anyway so it's a mystery - can only assume it was either the cold weather or the battery just decided to fail - I'll keep checking the charge every week or so to see if it holds the charge

And the bonus is I found the charging lead for my emergency starter unit in the charger box so every cloud...

 

I think you located your problem with the dead battery. A weak will throw up all types of random codes, especially at start up. Running fans, and radio take very little amperage and are not the sign of a good battery.
The starter takes a heavy draw when in use and causes the voltage to drop and the modules are not receiving the correct voltage and signal faults.
You should probably have the battery replaced. At least have a load test conducted to see what state the battery is in.

 

I think I read somewhere that a dodgy battery can even cause the bus problem I've been having but discounted it because the meter always said it was in good condition up until a few weeks ago when I last tested and the on board diagnostics always gave a good voltage (around 14v IIRC) - a load test would be a good place to start - thanks Vern

It's a bit problem when you are saving for a new car and you have problems with the old one and have to decide just how much is worth spending on it - a new or even a used ECU is out of the equation because of the cost of getting someone to fit it but a new battery would be acceptable in the short term

 

UPDATE:

Battery was dead again today so ordered a new one but there was one weird thing I noticed

When the car failed to start I turned the ignition off and noticed the hi beam light was still on - the headlights are turned off though

Could that be draining the battery? I wouldn't have thought so but worth asking

Should I pull the headlight fuse and/or relay to see if that cures the problem? - I've only seen this today and it may be unconnected to the draining problem

I'd hate to stick a brand new battery in only for that to drain like the current one so hoping the hi beam light is just one of those faults thrown up by a dodgy battery

 

Kinda sounds like your high beam/turn signal stalk is broken and that is causing the high beams to stay on (killing your battery). A new stalk is fairly cheap and easy to install. I would take your current battery, get a charge on it and then go check on the headlights. If your high beam indicator is on, then your high beams should be on too.

 

Update - could be an alternator problem

New battery fitted and still the same old problem - today it wouldn't even start after clearing the code and check light - eventually I discharged the ECU by clipping the two battery leads together for about an hour - car eventually started after clearing the code again but the weird thing is every time I check the alternator I get a different reading - I jut get lights on this cheap tester so I either get good or fail or no lights at all - today was the first time I've had fail though

I suppose the next step is get a mobile mechanic out to check the alternator and get a quote to replace it with a 2nd hand one - there really has to be a point where I need to give up - used alternator is £35-£40 plus fitting so not too bad but it's all adding up

At least I'm getting pretty good at resetting the radio

 

Dave, I want you to try something for me. I want you to ensure you have a fully charged battery before starting this. With the fully charged battery, get into the car and depress the "TRIP" button and keep it depressed as you start the car. Keep the TRIP button depressed as you watch the message center on the instrument cluster. When you see it say "Engineering Test", release the TRIP button. Now, depress the TRIP button 34 more times (the exact number may vary slightly) and what you are looking for is the instrument cluster to say "BATT XX.X" where XX.X is 3 numbers. Depending on how fast you are, you may see it up around 14.3 or around 13.7. What number are you getting with the car idling shortly after starting and then after about 5 minutes of driving around?

Just to let you know, your dash may do some really interesting things. Do not worry about it. This will be normal for what I have you doing. As you near the battery portion, you will see the information section say "TRN-X" (where X is a number) and then "IVD XX" (where XX is 2 numbers).

If you are seeing say 12.5 after starting the car, then this would be a problem relating to the alternator, which may or may not be the actual alternator.

 

I did do this a few months ago and got around 14v with the old battery (with ignition on and engine off IIRC) - this is a brand new battery now

I'll give it a go in the morning - thanks for your help

 

Interesting reading reposted from another member on how the charging system works =


Also some extra reading material to charging system on X-Type from Topix.


Vehicles with 2.0L, 2.5L or 3.0L engines


The charging system consists of a 120 amp output generator and regulator assembly which generates current to supply the vehicle electrical system with electricity when the engine is running and maintain the battery in a charged condition.
The generator is belt driven by the accessory drive belt.
For additional information, refer to: Accessory Drive (303-05 Accessory Drive, Description and Operation).
When the engine is started, the generator begins to generate alternating current (AC) which is converted to direct current (DC) internally. The DC current and voltage is controlled by the voltage regulator, (located inside the generator), and then supplied to the battery through the main battery positive cable.
The generator is solidly mounted to the engine timing cover and is driven at 2.8 times the engine speed.
Vehicles fitted with manual transmission have a one way clutch fitted to the drive pulley, which prevents torsional vibrations from the engine being transmitted to the generator.
The engine control module (ECM) can switch the voltage regulator between two voltages to optimize the charging of the battery.
The low voltage regulator setting is 13.6 volts and the high voltage regulator setting is 15.3 volts, measured with the generator at 25°C (77°F) and charging at a rate of 5 amps. These values decrease with a rise in temperature or current flow.
The ECM determines the voltage setting of the voltage regulator. The high voltage setting is always selected by the ECM once the vehicle has started. The ECM determines the period of time that the high voltage setting is selected for.
There are three different time periods selected by the ECM which is dependent upon the vehicle conditions when the vehicle is started:The longest time period is selected if the ECM determines that the vehicle has been 'soaking' for sufficient time to allow the engine coolant temperature (ECT) and the intake air temperature (IAT) to fall within 3°C (37°F) of each other.The intermediate time period is selected when the ECT and the IAT is below 5°C (41°F).The shortest time period is the default time and is used to provide a short period of boost charge.At the end of these time periods the voltage is always set to the low voltage setting to prevent the battery from being overcharged.
The time periods are variable depending upon the temperature and the battery voltage. The target voltage of the battery varies between 14 volts and 15 volts depending upon the ambient temperature and the vehicle operating conditions. Once this target voltage has been achieved, providing the vehicle has been operating for at least the shortest time period, the ECM will reduce the voltage regulator to the minimum setting of 13.6 volts.
There are three wires connected to the generator from the ECM by connector EN16:EN49-1; RG wire, is the voltage regulator request setting from the ECM to the generator.EN49-3; OG wire, is a pulse width modulated signal (PWM) from the generator to the ECM which enables the ECM to monitor the generator load on the engine.EN49-4; U wire, is the charge warning lamp signal wire from the generator to the ECM.If the voltage request signal wire (EN 49-1) is open circuit or short to battery positive, the generator will charge the battery at a setting of 15.3 volts. If the wire is short circuit to ground, the generator will charge the battery at a setting of 13.6 volts.
A fault in the wiring or the connections from the generator to the ECM, will cause a fault code to be generated and stored in the ECM and the charge warning indicator lamp to be displayed in the instrument cluster (IC) after a short time.
The charge warning indicator lamp is operated by the IC after receiving a signal from the ECM through the control area network (CAN).
With the ignition switch in the RUN position the charge warning indicator lamp will be displayed in the IC when the generator is not generating power.
If a fault is detected with the generator a fault code will be generated and stored by the ECM. The charge warning indicator lamp will also be displayed in the IC.
Units should be repaired as an assembly and not dismantled for repair.
For additional information, refer to: Generator - 2.0L NA V6 - AJV6/2.5L NA V6 - AJV6/3.0L NA V6 - AJ27 (414-02 Generator and Regulator, Removal and Installation).




***Be careful getting the alternator checked by someone other than a Jag dealer, or someone who doesn't deal with Jaguar often. Here in NZ we, in the past, have had a lot of customers replace alternators because it was checked by AA, and because they saw 15.2 volts it was automatically assumed the alternator was overcharging. It is normal to see this voltage, but only at certain times. ***

 

@Thermo

Battery was showing 14.7 at idle (no accessories including radio, A/C, lights etc on) during drive stayed at a steady 14.7/14.8 (14.5 idling at traffic lights after a couple of minutes) hit the 5/6 minute mark and varied from 12.5 to 13

"If you are seeing say 12.5 after starting the car, then this would be a problem relating to the alternator, which may or may not be the actual alternator."

After reading Dell's post even with a low reading like 12.5 it could still be the ECU after all and it seems to me that the problem is made worse by cold/wet weather and most likely water damage to the ECU

Car started fine today but I did do around 20 miles yesterday after charging the battery and resetting/discharging the ECU so hopefully everything has reset but I suppose the real test is trying to start after the car has been unused for a few days

 

Looks like a working alternator. Battery may be good or bad.

 

dave, just because you have a new battery, do not put a lot of faith into that. I would say to first start with having a load test done on the battery at your local auto parts store. See what they say. It may be possible that you have a bad battery and it is starting to go, even though it is new. Would not be the first battery I have seen do this.

What has me interested is the fact that you are getting 14.5 reading. That is a little high and would tell me that you either have a bad battery or a bad battery cable. Another check that you can do is start up the car after the engine is cold. Take it out for a 5 minute drive and turn on everything electrical that you can (rear defroster, seat heaters, dash fan, headlights, etc). With the car still running, pop the hood/bonnet and access the battery. Cup your hand over the battery terminals (DO NOT!!!!! touch). Do you feel a lot of heat coming from one and not the other? The battery terminals should be warm to the touch because of the underhood heat. But, what I am talking about is something that you would consider cooking an egg on. If so, that battery cable is bad and that is the source of your issue. In short, what is happening is as you start the car, the battery voltage is dropping too low and it is causing the tranny computer to not boot correct, leading to the issues you are seeing.

 

Thanks Thermo - I'll try that in the morning

TBH I was going to replace the cables when I bought the battery but forgot - they look fine at the terminal end but not checked at the alternator/earth ends - I'll order a couple anyway - at £3 each it won't hurt

Actually I'll take it out now and see what happens

 

OK - been out again - battery at 14.5 when car started - dropped to 12.5 as soon as I put the full beam, A/C, blowers at full, front and rear demisters, CD player and even the wipers on intermittent - by the end of a 7 or 8 minute drive it was hovering around 12v - I popped the hood and the negative terminal was warm (maybe 10C) and the positive a little warmer but not frying egg hot (maybe 20C)

I'll ring round and try and find somewhere that will do a load test

EDIT - taking it for a load test in the morning

 

Should not be warm! May be corroded/bad connection.

 

Just a quick update - got a guy coming to fit new battery cables as soon as they arrive - like you said the terminals should not be warm/hot and a bad connection is more likely than a dodgy new battery

 

Probably the last update: The fault has been traced to somewhere between the ECU and instrument cluster - either a dodgy ECU or a bad connection somewhere behind the dash so we are getting there

One final question: How hot does the ECU get? - could I pack the area to stop water getting anywhere near the unit as it's pretty well known that the placement of the ECU could be better and gets hit by run off from the windscreen

 

Probably not very - but that water should be kept away anyway by the seal(s). Sounds like work to ensure a water-tight seal would be worthwhile.

 

Hopefully the next to last update (last will be a solution )

I've had the battery checked and it came out fine - this week I took it to another place and they checked the battery and the complete charging system too and even checked for a parasitic drain and no problems which does indicate that it is the ECU and/or wiring harness

Got a guy coming next week to whip out the ECU and check it for damage and if there is any I have found a place that will do a recon on it by mail order - if there is still a problem after that then the only other cause can be the connection between the ECU and dash so at least the options are decreasing and we are getting there

In the meantime I've been starting up the car nearly every day because it seems the problem only arises if the car is left standing for 4 or 5 days

 

G’day Dave,

I left a wifi OBD dongle plugged into my XJ12 a while ago and that caused the battery to drain. There is one pin in the dongle socket that is always hot and will slowly drain the battery with the dongle plugged in if the car is not driven every day.

Cheers,
Jeff.