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TIM MOHR, ASE Certified Technician
Category: Jaguar
Satisfied Customers: 14987
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Low voltage at tick over ~ 11v, 13.5 at 1000 rpm +. Suspect

Customer Question

Low voltage at tick over ~ 11v, 13.5 at 1000 rpm +. Suspect +12v main cable from battery in boot in boot has too much resistance in it, heard there is a joint that might need checking?
JA: Does the battery look like it's leaking? And does it have a strong smell -- like sulfur?
Customer: nope, battery is tip top nearly new
JA: What is the model of your '13 Jaguar?
Customer: early x300 xjr - 1995/6
JA: Are you fixing your XJR yourself? What have you tried so far?
Customer: Swapped battery with known good one. Used OBD2 analyser to read ecu voltage (even lower at tick over ~8v).
JA: Anything else you want the mechanic to know before I connect you?
Customer: Runs ok
Submitted: 11 days ago.
Category: Jaguar
Expert:  TIM MOHR replied 11 days ago.

Hi, my name is ***** ***** you checked the voltage at the back of the alternator, on the lead that goes to the battery, or did you check it at the battery?

Customer: replied 11 days ago.
They only checks so far have been from dash and OBD2 reader. At tick over you can here electrical items (e.g. windows) slow down as voltage drops. At 1000 rpm + all seems okay. No difference if battery is fully charged.
Expert:  TIM MOHR replied 11 days ago.

Ok, then you most likely have a weak alternator, and this can be checked very easily, if the voltage is the same at the back of the alternator, then its a bad alternator, if it is higher at the alternator, then the battery cable has to much resistance, can you check the voltage at the back of the alternator?

Customer: replied 11 days ago.
Looks like I need to be under the car on my back to get to it?
Expert:  TIM MOHR replied 11 days ago.

es, or locate the wire and probe it as close to the alternator as you can, but most likely this is just a weak alternator, if the voltage comes up with rpms,Tim

Expert:  TIM MOHR replied 11 days ago.

Please dont forget to rate me, so I can get credit for helping, after rating me, you can come back to this question and ask follow ups at no charge, thanks,Tim