Alternator Rebuild

Alternator failure can produce many strange symptoms. For one thing, the tachometer gets its input signal from the alternator so will cease to work. Misfiring, stumbling and cutting out under power have also been observed in cases of alternator failure. If you are lucky, the ignition charging light will just come on dimly before the real trouble starts -- if this happens, waste no time in replacing or repairing the alternator to prevent a stranding.

Paul Dunn kindly contributes the following information on his experiences with alternator troubleshooting and rebuilding on the 1990 Range Rover which he and his wife purchased with 97,000 miles on the clock.

At 105,000 miles Paul's RR began experiencing idle instability and even stalling, as well as a ghostly little misfire which was anywhere from just a slight hint of a stumble to a complete cut-out. The misfire problem was usually associated with steady light throttle at highway speeds -- and especially at night when running with the headlights on.

Paul first suspected (and replaced, using an electrical tune-up kit from Atlantic British) the ignition wires, dist. cap, rotor and plugs. For good measure he replaced the fuel filter as well. Although this work improved the vehicle's running considerably, a few days later, the battery light seemed to be faintly illuminated, and the car was running rough. The light extinguished when the engine was revved, making Paul suspect the alternator.

If the alternator does go south, at least the options are more palatable than they were 8 years ago when it happened to me. For one thing, new and rebuilt alternators are much less expensive than the $700 they were then. The least expensive replacement options are now very inexpensive indeed. A rebuilt Classic Lucas alternator can be had from AutoPartsGIANT.com for $129, and a rebuilt unit for the P38 is only $142 from the same source. For other options including brand new alternators for under half the price they used to be, see our low cost generic Parts Sources Page.

If these prices are too high, you can tackle the task of rebuilding your own, using the instructions below.

Alternator Brush Replacement

So, Paul next tried a cure which he had once used for an old Chevy truck he once owned: alternator brushes. Out came the alternator and in went an alternator brush set (XK's Unlimited, $13). He was happy to know that the brushes were, in fact, replaceable on a Lucas product. The procedure for replacing the brushes is included in the step-by-step procedure section below). However, the old brushes still had plenty of meat on them, and after the replacement, a faint battery light illumination was still around at low rpm's. Thus, it was clear that the problem lay elsewhere.

Before really tearing into his alternator, Paul wanted to make sure there was no other electrical, vacuum or EFI problem causing the symptoms. Accordingly, he went through the following checklist:

-- Check to make sure the blue diagnostic plug under the right seat is well connected.
-- Check the "air gap" between the distributor pick-up base and one of the arms on the distributor shaft (it should be between .008 and .014").
-- Check the air volume plug connector at the air volume meter box half-way down the pipe from the air cleaner on the way to the plenum.
-- Also check the connections for the distributor pick-up module on the side of the distributor, the connection for the stepper motor at the back of the plenum, the throttle potentiometer at the entrance to the plenum (make sure there's a good seal for the unit, there may be a potential vacuum leak spot here).
-- Check all the vacuum lines, the T-junction piece by the pleum, and the larger diameter lines used for PCV purposes. Paul's were quite "powdery."
-- Check alternator belt tension.
-- Make sure the distributor wires are all well seated. A few of Paul's kept creeping out every now and then.
-- Paul suggests trying a couple of different rotors -- he has heard a rumor of a bad batch of replacement Lucas rotors.

Everything checked out OK, so Paul turned his attention back to the alternator and decided to attempt a home rebuild job rather than buying a new one.

Paul rebuilt his alternator by replacing the brushes, rectifier assembly, and the voltage regulator. This is an easy operation, but should only be performed if you're confident that the armature and the bearings are solid. The rebuild involved the following steps:

1.    Undo the battery leads.
2.    Take off the air cleaner. This is easy and provides much better access to the alternator connections and mounting bolts.
3.    Mark the and remove the electrical leads from the alternator so you know where they go during re-installation. Remove the Serpentine Belt (See separate article on this procedure).
4.    Undo the mounting bolts, taking note how they are situated.
5.    Remove the alternator.
6.    Undo the small screws holding the plastic cover on the back of the unit, and the screw that holds the two resistors in place.
7.   Take a good look at where all the wires go: from the voltage regulator to the rectifier and brushes, the ground leads, etc. Make a small diagram if you need to. Paul replaced each wire in turn so only one was unhooked at a time.
8.    To replace the rectifier, pry off the soldered leads and unscrew the three mounting screws. Remove the screw that holds one of the brush leads to the rectifier set.
9.     Take out the old rectifier.
10.    Install new rectifier with the three screws and solder into place the three heavy leads you pried off earlier. Attach screw holding yellow lead from brush.
11.     For brush replacement, undo the screws holding each brush assembly in place and remove the brushes by simply pulling them out of their channel.
12.     Clean the contact area on the armature shaft where the brushes make contact.
13.     Install new brushes, making sure the spring for each is exactly on the back of each brush, and pushes directly into the brush slot.
14.    For the voltage regulator, remove the screw that holds the bottom brush in, and the screw that holds the lead to the grounding screw in place.
15.     Switch out the unit, and re-install the screws from step 14.
16.    Make a final inspection and replace the plastic cover, along with the two resistors.
17.    Re-install the alternator into the vehicle.
18.    Reconnect the electrical leads. Be careful not to tweak the medium-sized post that is one of the main leads from the brush set. This can easily be over-torqued and will come apart from the small weld that holds it together. I did this and had to buy another brush set.
19.   Replace the belt.
20.    Re-install the air cleaner assembly and double-check your alternator belt tension, the mounting bolts, the heat shield that covers the back of the alternator, etc.
21.    Re-connect battery leads.

The total work time was two to three hours. For about $150 you have, at a minimum, greatly extended the life of your alternator. Who knows how long the bearings and the armature assembly will last?

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Page revised February 2, 2012