[xj] Alternator Randomly Not Charging

Randomly while driving the battery light will come on and the dash
voltmeter drops and the alternator stops charging the battery but
after continuing driving the voltage rises and the light goes off
and the battery is being charged again. I suspect the voltage
regulator unit but I might be wrong. I’m wondering if there was a
way to test the Voltage regulator to determine if it is still
functional so I don’t replace the regulator to find out its not
the problem.–
Tom96
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Tom96
The problem just may not be electrical in nature. Before assuming that it is, be certain to check the tension and condition of the belt driving the alternator. If it proves out to be okay listen to your engine first thing in the morning. Does it have a high pitched squeal that goes away as it warms up? If so, you just might have a bad harmonic balancer mounted to your crankshaft that provides the pulley that drives your alternator. If you have typical mileage on your '96, in the neighborhood of 125K or more, the balancer is due or past for failure.

Hope this is of help. You did request things to check prior to making parts expenditures.

Earl Kiker
Houston
95 XJS Conv. 4 ltr.
97 VDP===================================================
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In reply to a message from Earl Kiker sent Sun 8 Mar 2015:

Thanks for the quick reply, I was working on it earlier and
checked that the belt was properly tightened and the start was
smooth with no squeal. Sorry I forgot to mention in my first post
that it is a 1986 XJ6.–
Tom96
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In reply to a message from Tom96 sent Sun 8 Mar 2015:

It may just be the alternator brushes on their last legs.–
Dave Collishaw '79 Daimler Sovereign '96 X305 '56 BSA A10
Cambridgeshire, United Kingdom
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In reply to a message from Tom96 sent Sun 8 Mar 2015:

Absolutely the next step to take before taking things apart,
is to check the cleanliness and tightness of the connections
at the alternator. You will need to get dirty to do it, esp.
if you have an a/c compressor in the way. You will probably
need to check from under the car. First, disconnect the
battery. Second, take some brake cleaner or similar type of
quick drying cleaner to degrease the connections. You will
want to take a paper towel and clean each wire terminal and
connection spade at the alternator. Third, make sure each
female wire terminal is tightly crimped at the wire, and
firm and tight as it grips on the spade. You might need to
carefully use a pliers to gently pinch the female wire
terminal end to get it to grip tightly. Only after things
are all nice and clean and tight would I consider an actual
alternator problem. It sounds like if it isn’t the terminals
and belt/pulleys, then you will want to take the alternator
out to have a closer look at it. If the light and voltmeter
both agree that there is a charging problem, it probably is
truly happening.–
Tom Hishon, 69 E-type 2+2, 69 E-type OTS, 85 XJ6, '03 X-type
Wasilla, Alaska, United States
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In reply to a message from Tom96 sent Sun 8 Mar 2015:

Tom!

96, is that your age or the year of the Jag?

The problem sounds like the regulator, but if you
have the mileage on the car, it might be time to
replace the regulator, and rebuild the alternator.

                            Walter--

The original message included these comments:

Randomly while driving the battery light will come on and the dash
voltmeter drops and the alternator stops charging the battery but
after continuing driving the voltage rises and the light goes off
and the battery is being charged again. I suspect the voltage


Walter Schuster 78XJ6 FI Ser.II, 2002 xtype 3.0
Albuquerque/New Mexico, United States
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In reply to a message from Earl Kiker sent Sun 8 Mar 2015:

Thanks for the quick reply, I was working on it earlier and
checked that the belt was properly tightened and the start was
smooth with no squeal.

After the belt, Tom; Tom’s advice is very pertinent - it’s a fairly typical
symptom of bad connections…

You could manipulate the connections first, just to feel if there is any
looseness detectable - to focus on that area. Then, if cleaning/tightening
connections doesn’t work; while it is somewhat unusual for an alternator to
be intermittent due to an internal fault - it’s not impossible…:slight_smile:

An intermittent fault is difficult to catch - do the most yourself before
calling professionals…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)

Sorry I forgot to mention in my first post
that it is a 1986 XJ6.-----Original Message-----
From: Tom96
Sent: Monday, March 09, 2015 7:52 PM
To: xj@jag-lovers.org
Subject: Re: [xj] Alternator Randomly Not Charging

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In reply to a message from Tom96 sent Sun 8 Mar 2015:

did you get this resolved? I had a similar problem with my
84 XJ6, it ended up being a couple of things. a cooked
field wire, which killed my new replacement alternator.
wire replacement, and 2 alternators later it’s in good
order. this is how I figured it out.

Diagnostics

  1. with the car off, check the volts out of the battery.
    should be around 13.3 to 14 DC, and amps are somewhere
    around 30 to 40. I wear rubber mechanics glove for these
    tests. the volts won’t kill you but the amps will. 500mili-
    amps is all that is needed to stop your heart. BE CAREFUL!!

  2. disconnect the positive post BE CAREFUL NOT TO GROUND
    HOUR WRENCH ON THE HOOD LATCH, it will weld your wrench to
    the car and seriously get your attention (better yet
    disconnect the ground first). set your meter to Volts DC,
    connect the positive meter lead on the bulkhead lead and
    the negative meter lead on the positive post on the
    battery. (reconnect the ground after the positive lead is
    disconnected) fire up the car, check the voltage on the
    meter, switch over to DC Amps and do the same check, it
    should read somewhere around 13v and 25 - 40 amps.

if not, then on the drivers side (LHD point of view). next
to the brake booster there is another Battery/alternator
post. This post connects the two alternator (to the)
battery leads (brown wires) to the alternator, do the same
check there. the reading should be the same as the right
and left side of the car.

you can do the same test between the brown alternator wires
to the post. disconnect the alternator wire (brown) from
the left bulkhead posts, connect the meter between the
alternator wire (positive meter lead) and (negative meter
lead) to the post. fire up the car and check the voltage
(be sure the wires are reconnected on the right side to
avoid any sparks or shorts.

Do the same tests with the meter switched to DC Amps.
if any of these tests result in lower than 12 volts or 30
amps, check your wires for broken, cooked / discolored or
broken insulation, make sure the wire connected the clips
are is good order, Brakeclean works well for getting the
gunk off the wires. if the wires are in good order, then
check the field wire.
the field wire is a single brown wire running from the
alternator around the front of the engine (with the AC and
air pump wires) to a connection under the intake manifold.
the field wire drives the gauge in the car. make sure it’s
in good shape. This one is prone to getting cooked because
it runs along the block under the exhaust manifold along
the front of the head above the water pump, then between
down between the intake manifold and the block to a multi-
connector just behind the shock tower on the right side.

this is a hard one to test because it’s not real
accessible. with a bit of struggling, you can get to it
from under disconnect the belt tensioner and the belt,
force the alternator all the way to the left away from the
engine, and it’s just around the back on the right side of
the alternator.–
God made women in the likeness of a Jaguar
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In reply to a message from MLee sent Sat 11 Apr 2015:

Caution is a good thing, but a car battery is just not going
to kill you–unless it falls on your head, or sparks an
explosion. There aren’t enough volts to push that current to
your heart.

Thon–
The original message included these comments:

around 30 to 40. I wear rubber mechanics glove for these
tests. the volts won’t kill you but the amps will. 500mili-
amps is all that is needed to stop your heart. BE CAREFUL!!


tab a
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In reply to a message from MLee sent Sat 11 Apr 2015:

Good point. it was late, and well that was just stupid
12volts = death. seriously?!. thank you for pointing that
out.

Unfortunately my death comment kinda takes away any
credibility to what I wrote regarding the alternator.

The test is spot on. I had a multiple of things going on.

  1. Bad ground to engine / chassis. surprise!!
  2. cooked field wire / burned insulation, shorting out on
    the head, and cooked the air pump wires as well.
  3. dead diode in the alternator - replaced it,
  4. replaced the field wire, and double insulated it,
    replaced the air pump wires as well.

it’s all better.
now for the windows, door locks, sunroof, trunk release,
mirror, control arm bushings, shocks, ball joints, rear
brakes, carrier bearing, stake down kit. cam cover gaskets
and cam ‘D’ plugs, leather repair and conditioning / die.
head liner. I think that’s everything :D–
God made women in the likeness of a Jaguar
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