Hi, I have had a new alternator installed, but the voltage keeps going down.
When the car is cold all works well, lights on, A/C, radio on I get about 13.8 volts enough for electrical system and a little to charge the battery.
When the engine warms up after 4 -5 miles the voltage goes down to 13.25 and when I come to stop sign the volts go down to 11.6 to 12.25 and when the engine fans come on because A/C is on and I'm not moving the voltage goes down 11.25.

I am getting a MAF code of P0105 all the time and others and I even get ABS light on and No -Traction but that goes away after new engine cycle, so I am sure I get the ABS and no-Traction because of low voltage.

Of course at the 12.25 or lowerI am not getting any charge to battery.

Like I said the morning cool engine everything seems to be good and with a warm engine I loose a lot of voltage especially stopped.
Of course I realize there will be some voltage lose with the accessories on, but a healthy alternator should always throw 13.5 volts with accessories running should it not?

Thanks for your help and ideas and

Happy Motoring...

 

Where are you connecting your meter to measure the voltage?

What is the battery voltage after being left overnight - without engine running? Will give an idea of battery condition. Also measure the battery voltage during cranking.

At 1500rpm I would expect about 14.2-14.5v

I would check that your battery and engine/alternator positive and earth/body connections are good.

There are temperature compensation systems built in to most alternators, that do alter the voltage, warmer batteries don't need as much voltage to charge.

I will do some measurements to confirm, but I am sure someone else will get back to you before I do!

 

Loose belt?

 

Battery shot (internal short)?

 

check the feedthrough cable connector on the ECM housing

 

check for slipping belt or a bad ground cable. Corrosion on the battery cables maybe?

 

I'd always suspect the battery first. So many issues come down to failing batteries. Five years appears to be normal lifespan. You can check manufactured date for Jaguar OE batteries by the code stamped on the top of the negative terminal:



It's a 4-digit code where WW/YY stands for week number and year.

Next in line, the ground connection doesn't get many reports for coming loose:



The positive cable is a known weakness:



Several reports of the terminal working loose in the black junction box behind the spare wheel.

Graham

 

I would absolutely replace, or load test the battery.

Some voltage drop, I would think, would be normal for 2 reasons:
1. You just used a bunch of power from the battery starting the car, so as the battery is replenished, total voltage draw would go down.
2. The voltage regulator (probably with input from the ECU in these cars) is going to regulate voltage based on some (or probably many) factors. Ambient temperature should be one of those factors, a colder battery will get a larger voltage and a warmer battery will get a lower voltage. Something like this would be for no load on the battery. Throw in loads and all sorts of other algorithms probably apply.

So before troubleshooting something that would seem normal to me (like the voltage drop), I would absolutely check that something isn't wrong with the battery first as they just seem to cause so many problems in these cars if they are not in top shape.

 

From the JTIS
Generator
The charging system consist of a 130 amp output, SC1, generator and regulator assembly.
The generator and regulator assembly generates electrical power to the vehicle electrical system with electricity when the engine is running and maintains the battery in a charged state.The generator is belt driven by the accessory drive belt. 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 battery charging voltage is determined by the temperature of the generator. In cold conditions, starting the vehicle from cold the battery voltage will be between 14.2 volts and 15.1 volts and will reduce as the engine warms up.
In hot conditions starting the vehicle when the engine is already warm the battery voltage will be between 13.5 volts and 14.3 volts.
A fault in the wiring from the generator to the battery junction box, 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 after a short time.
With the ignition switch in the RUN position the charge warning lamp will be displayed in the instrument cluster 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 instrument cluster. Units should be replaced as an assembly and not dismantled for repair.

There are indeed some voltage related P codes so worth plugging in an OBD reader.
Also - where are you measuring these voltages?

 

Vettegood,

A test you might want to perform: Start the car cold and measure the voltage. Then let it run with something like a fixed electrical load, let's say everything shut off that can be for starters.

Monitor the voltage as the car warms up. Does the voltage decline in a continuous sort of way, or is it more like a step function at some point?

This might help rule some things in or out.

(As others have asked: where are you measuring the voltage?)

 

I have checked the values for my car. (New alternator (noisey) 3000 miles ago and new battery this year.)
I have looked through the parts list and electrical diagrams and there don't appear to be any major differences between the charging circuits on our 2 cars.

Though yours is listed as having a suppression module connected to the alternator terminals, I wonder if that could cause any problems - if faulty.

As suggested, maybe as simple as needing a new belt.

Anyway the voltage I get when left overnight before starting 12.5V
when warm with a/c and radio on, are: 1500rpm 14.3v Idle 13.7v

Like others have said these cars do not like suspect batteries. Caused all sorts of warnings, when starting, before I changed mine. Though I would not expect the battery to cause these with the engine running. Though as in all things, I could be wrong!

 

Ok Ok, I have not answered this particular question because I am a little embarrassed.


I am not a mechanic and do not have professional tools, so I am using a meter that you insert into the cigar lighter in the console.

I know it is not a true blue reading, but I figured once I got a baseline on this meter it would tell me the drops in voltage as it reads it hot or cold, even though it may not be perfect.

It did seem to do a pretty good job as the pro's numbers were almost exact to mine and it gave me something to tell the shop about what I was experiencing.


I took the car to shop today (not Jaguar) and he has verified several things. The alternator is ok and has ruled out other ideas. Ie. Harmonic Balancer, loose belts etc.


He does say that there is a 1 volt drop between the main cable from alt. To battery and says that is not good.
Now he is looking for the cars specs.


In the morning he said they will do hot and cold amperage reading.


I am not sure where this main cable goes, does it go thru the firewall to trunk via the inside of car by center console under carpet or on the outside of cabin under the carriage to the trunk.

It must be a healthy size cable I assume.
is it protected with a plastic coating?
Any description of what this cable looks like or located.


How could this cable go bad?
What would make it go bad?
Does this sound plausible?

Any other thoughts?


Thanks for all the ideas and responses. I will keep informed as things progress.


Happy Motoring...

 

See Pic 3 in Post #7 for the positive cable battery connection and harness junction box.

Graham

 

Diagnosis Complete:

It is said that the main wire harness from the alternator to the main fuse box is bad. The cable which is 10 gauge and about 4 ½ foot long looses about 1 volt between the two points. All under the hood in engine compartment.

Once again my investment with my 2nd after market warranty will pay off. Best investment of my life.

The shop wants to make their own wire harness from 8 gauge wire instead of buying Jaguar harness.

They say it will be cheaper, which will make warranty company happy and not be picky, plus I will get a better quality job.

Don't know how true this is, but my shop has never led me down the wrong path before.
It would seem 8 gauge cable is thicker and better than
10 gauge.
Surprised it is only 10 gauge from Jaguar in the first place!
8 gauge cable will be better for a hot location like Arizona.

Question: is this a common fault with Jaguars?

Will need a day for the warranty company approval and 1 maybe 1½ days for repair Job.

Let you know outcome next early next week.

Until then
Happy Motoring...

 

Glad to hear you have found the problem.

That is not good.

If it was correct, using ohms law, there is about 250A flowing through that cable. Have you gone in for electric traction?

 

FYI -
Here are pictures of the Alternator wiring harness that goes to the main fuse box on the right side of engine that needs replacing. Looses 1 volt between alternator and fuse box.

I still do not understand how this can goes bad without any physical damage to it!

Is this a common problem with Jaguars? has anyone heard of this before?

Repair cost 850.00.
169.00 of the part and 630.00 for labor that includes the diagnostic time as well.
The Warranty company first refused the repair because they said it was a battery cable, but I was able to persuade them that it was not a battery cable and was part of the wiring harness for the electrical system , which they cover in whole.

Happy Motoring...

 

I would still check the ground strap from the engine to the frame. A quick test is to put a meter from the engine to the shock strut bolts or other good frame ground while it's running. You shouldn't have more than 0.1 volts or so difference. My car had over 1 volt loss. Cleaning up the corrosion fixed it.
The XJs are known for this problem, and I had it myself.