How to test and change alternator

How do you test alternator 87 xjs and what’s best way to remove it.

Testing is easy but requires a voltmeter. Start engine, rev to 1500 rpm or more, check the system voltage anywhere – cigar lighter is convenient, but it can be anywhere on the system. Voltage should be between 13.6V and 14.4V. If it’s either less or more, alt is malfunctioning.

Note that there’s a possibility that it’s malfunctioning even if the voltage is within that range. There are three circuits providing power within the alt and one can crap out, leaving a voltage that’s shaky but within range unless the loads are too much for it.

Also note that one way the alt may be malfunctioning is by not being driven properly. The belt could be slipping. Sometimes the crank damper has failed and is slipping.

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in the facelift model, the alternator has to come out UP past the right side, the sway bar will not let you pull it from below!

and if your car has the air pump (probably not a 87, though) , this has to go out first

as said above

  • check the voltage at the battery at 1500 or so : if too high, regulator is bad, if too low either the regulator is bad, or one or more diodes are gone, or the contacts are bad (or a combination of those 3)
  • then do the same test while increasing the load (ie : headlamps, rear demister…) : the voltage should stay the same, or the regulator is bad

So just air pump should I leave air pump out or put back?

I have no idea where in the world you live but I live in California, USA where our 1990 XJ-S convertible, and two other newer 1984 and 1990 Jaguars, must pass a visual and exhaust gas analysis Smog Test every two years. If I removed an air pump from one of these Jaguars, it would not pass the Smog Test and I would be unable to reregister it. Others live in locations around the world where the air pump was not required, where Smog Test are no longer required, or where there is no federal or state laws against emissions system removals. So one factor is where you live another is what laws you must comply with where you live.

You can not simply remove the air pump without dealing with it the associated piping, belt, and wiring to some degree. So another factor is you technical knowledge and mechanical ability.

I like to keep my Jaguars original, authentic and unmodified. Others seem to look for reasons to change things and do so at the drop of a hat. So another factor is personal preference.

I am certain that if you searched the Jag-Lovers archives for “air pump removal” you will find a lot of posts from people who have asked about this in the past and what they have done.


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If you decide to remove air pump, there is a rail of pipes going thru the intake manifold to the exhaust that are also worth removing. But it’s a bit of work, and the hardest part is trying to plug up the holes properly.

I did mine because I live in a state where there is no smog for my car, and i like how it de-clutters The Vee area.

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