Lack of electrons
#1
07-24-2013, 12:48 AM
Lack of electrons
I have been having issues with keeping the battery charged on the XJS for a while now.
The voltage gauge on the dash is lucky if it ever reads more than 12V, and as soon as I load it up, it drops to closer to 9V
Voltage measured at the cigarette lighter by a digital volt meter never gets above 12.8V. I think from research I need to be 13.8 to 14.4V
I have already replaced the alternator. It's a Bosch 115A.
I'm pretty sure the belt is not slipping because the tops of the pulley grooves are not shiny
Next on the list is probably going to be to test the resistance of the big fat wire that runs from the alternator to the starter. I was also pondering the idea of rigging a temporary connection via jump leads from the alternator power line to the battery +ve terminal directly. I have also seen on other posts reference to "+ve terminals on the bulkhead". This could be another place for me to direct connect the alternator to as part of the troubleshooting. Does anyone have a picture or a description of these +ve terminals ? I've looked around my car and they are not jumping out at me. Are they inside the firewall under the dash ? or in the engine compartment ?
Any additional inputs welcomed
The voltage gauge on the dash is lucky if it ever reads more than 12V, and as soon as I load it up, it drops to closer to 9V
Voltage measured at the cigarette lighter by a digital volt meter never gets above 12.8V. I think from research I need to be 13.8 to 14.4V
I have already replaced the alternator. It's a Bosch 115A.
I'm pretty sure the belt is not slipping because the tops of the pulley grooves are not shiny
Next on the list is probably going to be to test the resistance of the big fat wire that runs from the alternator to the starter. I was also pondering the idea of rigging a temporary connection via jump leads from the alternator power line to the battery +ve terminal directly. I have also seen on other posts reference to "+ve terminals on the bulkhead". This could be another place for me to direct connect the alternator to as part of the troubleshooting. Does anyone have a picture or a description of these +ve terminals ? I've looked around my car and they are not jumping out at me. Are they inside the firewall under the dash ? or in the engine compartment ?
Any additional inputs welcomed
#2
07-24-2013, 12:54 AM
Make sure all electrical connections are clean. I use a wire brush to make sure the battery terminals are free of corrosion and able to make clean, conductive contacts.
Instead of reading at the cigarette lighter, try to read directly on the battery with your voltmeter. Maybe you can pinpoint if the problem is upstream or downstream of the battery that way.
I take it you are sure your battery is not defective?
Best,
Instead of reading at the cigarette lighter, try to read directly on the battery with your voltmeter. Maybe you can pinpoint if the problem is upstream or downstream of the battery that way.
I take it you are sure your battery is not defective?
Best,
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Sarc (07-26-2013)
#3
07-24-2013, 03:53 AM
Veteran Member
The voltage gauge on the dash is lucky if it ever reads more than 12V, and as soon as I load it up, it drops to closer to 9V
DO not rely on the Voltage gauge as these are not reliable
Voltage measured at the cigarette lighter by a digital volt meter never gets above 12.8V. I think from research I need to be 13.8 to 14.4V
Is that 12.8V when the car is running or off? This makes a HUGE difference. A car batter is fully charged at 12.6Volts and discharged at less than 12.4V, this is the voltage with NO load . When the car is running the voltage across the battery could climb to 14V due to the charging circuit.
Next on the list is probably going to be to test the resistance of the big fat wire that runs from the alternator to the starter. I was also pondering the idea of rigging a temporary connection via jump leads from the alternator power line to the battery +ve terminal directly.
Do not worry about this at the moment. I would be looking at the battery connections, check for corrosion and bad connections also check the earth from the battery to the body.
DO not rely on the Voltage gauge as these are not reliable
Voltage measured at the cigarette lighter by a digital volt meter never gets above 12.8V. I think from research I need to be 13.8 to 14.4V
Is that 12.8V when the car is running or off? This makes a HUGE difference. A car batter is fully charged at 12.6Volts and discharged at less than 12.4V, this is the voltage with NO load . When the car is running the voltage across the battery could climb to 14V due to the charging circuit.
Next on the list is probably going to be to test the resistance of the big fat wire that runs from the alternator to the starter. I was also pondering the idea of rigging a temporary connection via jump leads from the alternator power line to the battery +ve terminal directly.
Do not worry about this at the moment. I would be looking at the battery connections, check for corrosion and bad connections also check the earth from the battery to the body.
Last edited by warrjon; 07-24-2013 at 03:56 AM.
#4
07-24-2013, 06:46 AM
Veteran Member
The big wire from the alternator goes to the starter, and from the starter to the post on the firewall RHS. It is well worth cleaning all these connections, and also checking and cleaning the earth straps from the motor to the body. This goes from the LHS sump sandwich plate to the subframe and from there to a bolt on the LHS lower chassis rail. All these connections are libale to be duff unless you have cleaned and greased them recently.
Some people, notably the GREAT XJS PROPHET Grant Francis, recommend an extra, second, big wire directly to the firewall from the alternator and an extra earth strap from the alternator directly to the frame.
That is what I did, as Grant recommended, and together with a 115 amp alternator all problems disappeared straight away.
Greg
Some people, notably the GREAT XJS PROPHET Grant Francis, recommend an extra, second, big wire directly to the firewall from the alternator and an extra earth strap from the alternator directly to the frame.
That is what I did, as Grant recommended, and together with a 115 amp alternator all problems disappeared straight away.
Greg
#5
07-24-2013, 07:24 AM
Your battery in the boot has a HUGE +ve cable that terminates at the LH side of the engine bay, just behind the LH camcover, and has a rubber cover on it so sparks do not upset the Prince of Darkness.
From this LH +ve terminal an equally large cable travels across to the RH side +ve post. This also has a cover on it, AND, a second large +ve cable going to the starter motor solenoid.
All these joints need regular cleaning, and grease coating to assist electickery path.
The largish cable from the starter solenoid TO the alternator is also a suspect item now its old.
The earth cables alongside the battery are a well documented trouble spot.
As mentioned, the engine earth straps are a wonder of engineering at its absolute best, and a new single "eye to eye" battery cable from the engine direct to the chassis will be one of the best "upgrades" that you can do.
It sounds like your alternator is the newer Bosch unit, with a multi V belt driving it. Make sure it is tight, as they will slip with very little noise in some cases.
These Bosch are a good unit, and at 115amps gutsy as hell, but do take some driving.
Bad earths are the biggest issue with these cars so look there very carefully.
From this LH +ve terminal an equally large cable travels across to the RH side +ve post. This also has a cover on it, AND, a second large +ve cable going to the starter motor solenoid.
All these joints need regular cleaning, and grease coating to assist electickery path.
The largish cable from the starter solenoid TO the alternator is also a suspect item now its old.
The earth cables alongside the battery are a well documented trouble spot.
As mentioned, the engine earth straps are a wonder of engineering at its absolute best, and a new single "eye to eye" battery cable from the engine direct to the chassis will be one of the best "upgrades" that you can do.
It sounds like your alternator is the newer Bosch unit, with a multi V belt driving it. Make sure it is tight, as they will slip with very little noise in some cases.
These Bosch are a good unit, and at 115amps gutsy as hell, but do take some driving.
Bad earths are the biggest issue with these cars so look there very carefully.
#6
07-26-2013, 12:15 AM
Thank you gentlemen. I think that clearly defines my weekends work. I had a quick look under the hood and I see the +ve posts now. They are really well tucked away. From a first quick inspection they look 100% "original" so cleaning and greasing will be a good thing to do anyway.
I will go and buy some nice fat wire and add both the 12V to firewall and the ground to frame as recommended.
Just when you thought you knew your XJS after owning her for 4 years, I get taught lots of new lessons :-)
I will go and buy some nice fat wire and add both the 12V to firewall and the ground to frame as recommended.
Just when you thought you knew your XJS after owning her for 4 years, I get taught lots of new lessons :-)
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Grant Francis (07-26-2013)
#7
07-28-2013, 09:38 PM
So, I did the alternator wiring mod...... big fat 4 gauge cable straight from the alternator to the +ve post on the firewall. Also added a big thick earth from the alternator to the frame.
Also cleaned the +ve posts and the earth straps in the trunk.
Outcome....... 13.8 to 14.0V with the car running at idle. Doesn't drop more than 0.2V when loaded up (A/C, lights, etc). Starter motor has never spun so fast in its life!
I bought an amplifier installation kit from amazon as it was cheap (<$20) and it had plenty of 4 gauge cable, plus the connectors.
Happy camper, cheers !
Also cleaned the +ve posts and the earth straps in the trunk.
Outcome....... 13.8 to 14.0V with the car running at idle. Doesn't drop more than 0.2V when loaded up (A/C, lights, etc). Starter motor has never spun so fast in its life!
I bought an amplifier installation kit from amazon as it was cheap (<$20) and it had plenty of 4 gauge cable, plus the connectors.
Happy camper, cheers !
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AllanG (07-29-2013)
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#8
07-28-2013, 10:48 PM
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07-29-2013, 02:27 AM
#10
07-29-2013, 03:57 AM
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#12
07-29-2013, 07:57 AM
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#13
12-29-2013, 01:53 AM
Member
If I may be so bold as to jump on this thread....
When I start my Jag (XJ-SC V12) after it has been sitting awhile, the alternator does not come on line.
Sometimes I can coax it by loading the electrical system up, ie headlights and air cond.
The car has only done 60,000 MILES, so nothing should be worn out.
Any thoughts and suggestions appreciated.
Thanks. Rod.
When I start my Jag (XJ-SC V12) after it has been sitting awhile, the alternator does not come on line.
Sometimes I can coax it by loading the electrical system up, ie headlights and air cond.
The car has only done 60,000 MILES, so nothing should be worn out.
Any thoughts and suggestions appreciated.
Thanks. Rod.
#14
12-29-2013, 05:29 AM
Rod,
Your '86 is more than likely a Lucas 75amp alternator, and very tired by now.
They were a bugga to get excited sometimes, and that is a Lucas thing, and is usually the first sign of diodes going flaky.
Pull the small exciter wire off the back of the alternator, attach a volt meter, switch ON the ign (DO NOT short that wire please), and ensure you have 12v in that wire, if not, find out why and fix that. The usual cause is the idiot light bulb on the dash is blown.
I usually upgrade to the Bosch 110amp "kit" from H&H Prestige is Melbourne. Usually works out cheaper than getting a Lucas rebuilt.
Your '86 is more than likely a Lucas 75amp alternator, and very tired by now.
They were a bugga to get excited sometimes, and that is a Lucas thing, and is usually the first sign of diodes going flaky.
Pull the small exciter wire off the back of the alternator, attach a volt meter, switch ON the ign (DO NOT short that wire please), and ensure you have 12v in that wire, if not, find out why and fix that. The usual cause is the idiot light bulb on the dash is blown.
I usually upgrade to the Bosch 110amp "kit" from H&H Prestige is Melbourne. Usually works out cheaper than getting a Lucas rebuilt.
#15
12-29-2013, 05:42 PM
Member
Just found this on "experience in a book"
I live and learn, so will check out that idiot light today.
Also thanks Grant. Will check wiring too.
ALTERNATOR EXCITATION: The field on the XJ-S alternator is excited through the warning light, so if this light is burnt out or disconnected charging may be intermittent. According to Randy Wilson, "A common enough design. Lots of alternators will respond like this... some would never charge. Here's what happens: the alternator needs a signal to indicate the key is on, and a power source to boost itself into a charging condition... bootstrapping, if you will. This power requirement is low enough that a lot of alternators use the power flow through the charge light for this purpose. This is called the exciter. And, in reality, key on is not really an important condition to the alternator. You just don't want to be driving the exciter full time, lest your battery will go flat overnight. Once the alternator is charging, it becomes self-exciting, and no longer needs or uses this external power source (oversimplification). Thus the light goes out.
"The exciter is used to induce enough of an electromagnetic field to start the current generation. However, there is always some residual magnetism floating around in a used alternator. With this residual, the alternator can generate a little bit of current, if you spin them fast enough. In the case of Lucas (and others) the current needed to self-excite is fairly low -- low enough that the current from the residual magnetism can meet the demand. This usually happens at 2500-3000 engine rpm. And as soon as the alternator is up and charging, operation is as normal. You don't have an outside exciter source, but you don't need it any more.
"The moral of all this is that silly charge system idiot light is a double-sided test. Everyone knows it is bad when the light is on with the engine running. Few people know that it is equally bad if the light stays off with the engine not running, but the key is on."
"The exciter is used to induce enough of an electromagnetic field to start the current generation. However, there is always some residual magnetism floating around in a used alternator. With this residual, the alternator can generate a little bit of current, if you spin them fast enough. In the case of Lucas (and others) the current needed to self-excite is fairly low -- low enough that the current from the residual magnetism can meet the demand. This usually happens at 2500-3000 engine rpm. And as soon as the alternator is up and charging, operation is as normal. You don't have an outside exciter source, but you don't need it any more.
"The moral of all this is that silly charge system idiot light is a double-sided test. Everyone knows it is bad when the light is on with the engine running. Few people know that it is equally bad if the light stays off with the engine not running, but the key is on."
I live and learn, so will check out that idiot light today.
Also thanks Grant. Will check wiring too.
#16
12-29-2013, 05:53 PM
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#17
12-29-2013, 06:32 PM
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#18
12-30-2013, 12:40 AM
OK, step #2A, increase the revs, see if the alternator excites that way. Mine did for years, until the thermo fans required a gutsier unit.
Those Lucas units were NEVER any good at idling charging, and/or exciting, always needed a helping "blip" of the throttle to activate them, even in the 6 cylinder cars.
Addition to any step above.
Check ALL the earths, especially the engine earth strap fiasco, the many alongside the battery, the bunch on the rear of "A" bank inlet manifold.
I strongly suggest adding an addition earth strap, engine to chassis. Anywhere that is convenient will do. I used the rear bolt on the LH "V" mount as it bolts direct to the chassis rail, and then one of the sandwich plate bolts, it eliminates the nightmare Jaguar earth strap/s around the LH engine mount, and is down there out of sight.
I still reckon the Lucas is testing your resolve. Interesting who will win here??>
Those Lucas units were NEVER any good at idling charging, and/or exciting, always needed a helping "blip" of the throttle to activate them, even in the 6 cylinder cars.
Addition to any step above.
Check ALL the earths, especially the engine earth strap fiasco, the many alongside the battery, the bunch on the rear of "A" bank inlet manifold.
I strongly suggest adding an addition earth strap, engine to chassis. Anywhere that is convenient will do. I used the rear bolt on the LH "V" mount as it bolts direct to the chassis rail, and then one of the sandwich plate bolts, it eliminates the nightmare Jaguar earth strap/s around the LH engine mount, and is down there out of sight.
I still reckon the Lucas is testing your resolve. Interesting who will win here??>
Last edited by Grant Francis; 12-30-2013 at 12:42 AM.
#19
01-02-2014, 05:24 PM
Senior Member
How long should the battery stay charged? If my wife doesn't drive her XJS every 4-5 days
the battery doesn't have enough charge to start the car. The battery is a 1 year old Sears brand. Previous batteries had the same problem. I now keep a battery minder on full time to\keep the battery up.
I've wondered how much juice the offline electronics are using? You know, the security system and such. Would they put a battery down after 4-5 days?
the battery doesn't have enough charge to start the car. The battery is a 1 year old Sears brand. Previous batteries had the same problem. I now keep a battery minder on full time to\keep the battery up.
I've wondered how much juice the offline electronics are using? You know, the security system and such. Would they put a battery down after 4-5 days?
#20
01-02-2014, 07:45 PM
How long should the battery stay charged? If my wife doesn't drive her XJS every 4-5 days
the battery doesn't have enough charge to start the car. The battery is a 1 year old Sears brand. Previous batteries had the same problem. I now keep a battery minder on full time to\keep the battery up.
I've wondered how much juice the offline electronics are using? You know, the security system and such. Would they put a battery down after 4-5 days?
the battery doesn't have enough charge to start the car. The battery is a 1 year old Sears brand. Previous batteries had the same problem. I now keep a battery minder on full time to\keep the battery up.
I've wondered how much juice the offline electronics are using? You know, the security system and such. Would they put a battery down after 4-5 days?
You need to do an amp draw test to establish exactly how much juice is draining when the car is meant to be "asleep".
Ideal figure is <400 milliamps.
I have "word" somewhere on how to do if you are unsure, so ask and I will attach.
Our XJ-S's and other models sit for weeks on end, and fire up just fine.
I reckon you got 1, maybe 1.5 amps draining, and security systems can do that.
Last edited by Grant Francis; 01-03-2014 at 02:51 AM.
