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Alternator stops working once

1995 Mercury Cougar 4.6 w/ 175k miles.

Yesterday after leaving work I drove ~1 mile when my cabin fan suddenly slowed significantly. I had to run an errand, and after restarting the car the fan still ran slower than normal. As I traveled home (12 miles) I noticed the volt meter continue to drop as I drove. It usually runs ~13, but dropped to ~10. When I got home I checked the voltage at the battery (with fans off and engine idling) and got 12.4 Volts. Today I get 12.4 with the engine off and 14.5 with the engine idling. It seems that the alternator didn’t work for one trip, and has now started working again. Should I replace the alternator right away, or wait to see if the problem repeats? This model does have an external voltage regulator that I could replace seperately.

I should also add that my wife is 8.75 months pregnant, and though I won’t be using this vehicle to go to the hospital I also don’t want to be stranded on the road on my way home from work.

Based on the failure mode…and the mileage…I’m thinking brushes.

An alternator has 2 “brushes” that make contact with copper “slip rings” on the alternator. If they make contact, all’s well. If not, no alternator.

The brushes are made of a metal softer than copper, and wear with use. When they wear past limits, they start being unable to reach the rings and make contact. This manifests itself as an intermittent failure that gets worse with time.

They “might” be replaceable on your car without serious disassembly of the alt. (On my car, they were attached to the voltage regulator installed on the outside of the alternator.)

The first thing I would do is just pull all of the connections at both battery & alternator and clean them up really well. Then I’d have the system load tested (for free) at an auto parts store.

I checked the brushes, they were below tolerance on length. Minimum is 1/4" and even the long side is a little less than that (see attached picture). So…

I replaced the brushes and reassembled everything. I started the car, and the battery idiot light was on (it hadn’t been previously). I turned the car off and realized I hadn’t removed the little pin that holds the brushes back during assembly. While attempting to remove the pin in situ, I got a little spark. Ahh, I should remove the negative battery cable again. Did that, got the pin removed, reassembled everything and the battery light is still on. So now I’ve run the engine with the brushes retratced from the slip rings, but with both brushes electrically connected (via the metalic retainer pin) & after turning the car off I created a spark between said pin and the engine block.

With the car off, battery voltage is 12.5 V. Idling the battery voltage is about 12.1 V. I thought if I had destroyed the voltage regulator that it would over charge. It seems to not be charging at all. Increasing engine speed makes no difference. Do I have a loose connection somewhere or do I need a new regulator?

Before you go any further you’ll need to check for battery voltage at the alternator’s output. With the engine NOT running put your meter’s positive lead directly on the alternator output and the negative lead on the battery negative. If you don’t read battery voltage you’ve burned the fusible link in the wiring harness. If you do read battery voltage you’ve probalby burned the volage regulator.

My bet is on the fusible link.

With engine & switch off I get 12.5 VDC between the alternator output & the negative battery terminal. Looks like I need a new regulator.

Remove the regulator and take it with you to the parts store. They can usually test it for you.

By the way, double check your connections. You do have both the output and the field connectors correctly plugged in, right?

There are 3 electrical connections to the alternator; the positive battery lug, a 3 wire keyed connector, and a 1 wire keyed connector (that comes off of the 3 wire connector). Can’t hardly mess any of those up. Is there a fuse between the dash idiot light and the alternator?

See attached image. (This is not my regulator, but physically they look identical). The lug labeled ‘ground to test’ is already at ground (it is not floating). Touching it to ground has no effect (engine did not lug down, voltage at alternator did not improve). The other lug is also at ground. These lugs are connected to the 2 brushes. Supposidly grounding the test lug should cause the alternator to go to max output. Does that mean the voltage regulator failed the test, or the entire alternator failed the test?

Replaced the voltage regulator with no status change. Battery idiot light still comes on. Battery doesn’t charge. I’m thinking there must be a poor connection or fuse somewhere in that circuit?

OK, took the entire alternator with original voltage regulator to the local auto parts store, they tested it and it passed with no problems.

I suspect the initial field current supply isn’t getting to the alternator. If a fuse/connection was bad between the dash light and the alternator, would the dash light still come on? Are the letters on the triple connector A/S/L? (L for lamp?) What is the connector with the blue line?

Found the bad fuse, a 15A fuse in the under-hood fuse box. Replaced that and everything works perfectly. Hopefully NAPA will take back a voltage regulator with about 5 seconds of run time on it.

I think I would keep the alternator. In my experience, when I’ve had an alternator repair or replacement, the voltage regulator seems to go out next.