With some advice I put in a new battery to my xk8 .Also bought a plug in volt meter.
Reading is 11 .7 with ignition on and 13.7 with motor .This sounds a little low.I tried it on another vehicle and that was .5 volts higher on both .does anybody have a thought on this.
Cheers Stu

 

stumo, keep in mind that the ECU does not always read the voltage at the same place your plug in device does. So, there is a possibility of there being a voltage difference. For example. If your ECU is measuring the voltage at the battery, but you are measuring at the cigarette lighter, you have 20ish feet of wiring that is pulling current. You have current flowing through a wire, it will drop some voltage. Not to mention the voltage drop of a fuse. So, having a 0.5 VDC difference in this case is not unlikely. This is where I would tell you to measure directly on the battery and see what that says. WIth the car off and only access to the battery being open, your meter should read 12.6 VDC (not reading this says there is either a bad battery, discharged battery, or a battery that is not being properly charged). Then, as you start the car, if you car follows the standard Jaguar "recharge" philosophy, you will see the battery go from 12.6 VDC (engine off), to 11.5 VDC (engine cranking), to 14.5 VDC as the engine first gets up to fast idle, maintains 14.5 VDC for a calculated period of time to "replace the power removed from starting the car, to then slowly ramp down to 13.7 VDC where it will maintain the voltage.

As a reference, per most standards for wiring in a low voltage application, there is an allowable 1 volt drop between the source (alternator) and where you are measuring (this case, the cigarette lighter). Anything less than that is acceptable. So, seeing 13.7 VDC in the cabin of the car shortly after start is considered acceptable assuming that your car does the recharge program that I mention above.

 

Thanks Chris,
Very helpful and in depth explanation,Gratefully Received.
Cheers
Stu

 

should be 13.8 to 14.2 vdc running. engine off 12.6 to 12.7 if all modules are asleep. test must be done at the battery.

 

How old is the battery?
11.7 VDC is replacement time.
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clubairth, keep in mind, he was reading the voltage most likely from the cigarette lighter, not the battery. So, there can be a voltage difference between the two spots. You get something pulling a bit of current inside the car (dash fan) or you have a wire that is developing a little bit of resistance (fractions of an ohm), that can easily account for the voltage difference. Like was said, all voltage measurements should be taken at the battery, preferable on the lead post of the battery, not the battery terminal. The X-TYpes are well known for having a slight voltage difference between the wire attached to the battery terminal and the lead post of the battery. Seen many a people pulling out their hair because of this difference.

 

Battery is New ,reading was from lighter,and Vet Member 'Thermo' had some useful info on that.I will get a reading from direct from the Battery.
Thanks for your input

 

What does it read now?
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Unfortunately it is back at the garage .but it was a brand new battery I was hoping might solve the issue but didn't ,my local garage is at a loss and decent auto electricians are like hens teeth in my rural area.I saw a video on utube that mentioned that ignition coils can be an issue with these.I would be grateful if you had any knowledge in that regard.
Cheers Stuart

 

Stumo, if you are feeling like getting ot know your car a little more, I may be able to help, if not atleast get you down to a circuit or two. If you are game, lets get some more detail about what is going on so I can get a good idea of what we are dealing with.

First things first, you have an XK8, but what year? I may have the diagrams for the car.

Second, what specifically is your issue? Battery drains after sitting for 2 days, some component does not work, lights seem to work but the car will not start, etc. Try to be as descriptive as possible. Sometimes it is a small detail that is the key to things.

Third, how would you rate your ability to use a multimeter? I only ask this to know how detailed I need to be in describing where the probes go.

Lastly, any work done on the car just before the problem started?

We can get you taken care of. Might surprise you.

 

Thanks Chris,
Really appreciate you trying to help me out. Firstly I am retired after 50 years as a craftmen woodworker . Any mechanical issues I encounted I always considered best left to those skilled in that specialty . Consequently I am as ignorant of these matters as can be imagined .
A few pointers ,my car is an early 1996 XK8,Good Nick for it's age ,85,000kl, mechanically great .Love it . However it has developd this issue that after 40 to 50 kl it goes into 'poor performance,' mode.This has happened on every occasion since it started ,4 times in a row.,initially tow truck,limp to garage,or lastly I have learnt to do a hard reset and drive to tthe garage on a hope and a prayer.I jjust happened to see a guy on utube describing the same issue and explaining that these cars are suseptable to problems with the ignition coils ,he replaced them,and solved the problem. I live in a rural area and have a great long term relationship with my local garage,but auto electricians they are not.My mechanic has as reader but it always displays a diferent code,not helped by me doing a hard reset last time.
I would fully understand if you considered me a lost cause
Thanks awfully
Stuart "stumo'

 

Always post what the code is. The actual numbers.
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stumo, I can understand the sentiment about "I haven't done it, so I am going to bow out gracefully". But, I am also hearing that you are not being given many other choices though. I will say that if you want to give it a go, I can give you a few recommendations and we can take things from there.

Also, have you been in the XK8 forum for your year car and seen what the people there say? They would be more "technical experts" on the car than me. I am just a guy that likes to take on the "mission impossibles" and see what he can get fixed. Call me a glutton for punishment. LMAO.

 

Thanks
I just ordered a Reader so I can do that myself .Garage has not kept a record.

 

Thanks Chris, I am still learning how to navigate the site . I have been looking through the XK8 XKR forum ,but wasn't sure if you could be year specific.
Stu