I can not locate the voltage regulator on my 1986 Jaguar XJ6 4.2. I have the symptoms

While I haven't examined an alternator for a Jaguar, most modern vehicles use alternators with the regulator installed INSIDE.
It's usually possible to rebuild an alternator if you have the right tools and are mechanically inclined. Some alternators require that you use a press to remove the pulley, and you'll need to figure out how to put the alternator back together once the brushes are out of their slot (there is usually a hole that aligns with the brushes; you just put an opened paperclip or other stiff wire in the hole to hold the brushes in place.)
It's usually just a matter of replacing parts; brushes, bearings and regulator takes care of most alternator problems, but you should have the diodes checked and replace the diode plate if you have any problems.
I don't know how difficult it is to find parts for a Jaguar alternator; you may need to contact a specialty parts supplier.
Good luck.

Hi, the regulator is integral to the alternator on your Jag.

Part Number: 14561 Weight: 13.0 lbs Warranty: Limited Lifetime Notes: 75 Amp alternatorPremium reman. Save the pulley, fan, shaftkey and/or fan spacer and mounting hardware from your failed alternator. You will need to install these parts on your replacement alternator. Unit Manufacturer: Lucas Amperage: 75 A Pulley Type: Sold without a pulley Fan: External Regulator: Internal Part Number: 14561 Weight: 13.0 lbs Warranty: Limited Lifetime Notes: 75 Amp alternatorPremium reman. Save the pulley, fan, shaftkey and/or fan spacer and mounting hardware from your failed alternator. You will need to install these parts on your replacement alternator. Unit Manufacturer: Lucas Amperage: 75 A Pulley Type: Sold without a pulley Fan: External Regulator: Internal Part Number: 14561 Weight: 13.0 lbs Warranty: Limited Lifetime Notes: 75 Amp alternatorPremium reman. Save the pulley, fan, shaftkey and/or fan spacer and mounting hardware from your failed alternator. You will need to install these parts on your replacement alternator. Unit Manufacturer: Lucas Amperage: 75 A Pulley Type: Sold without a pulley Fan: External Regulator: Internal