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1991 Mercury Capri never ending alternator problems!

This car is about to drive me crazy! My 1991 Mercury capri is having problems with its charging system. I’ll go out and start my car and the alternator will be charging for a couple of minutes or about half an hour, but then it’ll just stop charging and my headlights will get dim and my wipers will get sluggish. I replaced the alternator a few days ago because it was doing this, and it worked perfectly for a couple of days and then just went right back to doing it. I replaced the plug yesterday because it was corroded and i thought maybe it was making an ill connection but that was not the case. I thought about hard wiring the alternator to the battery with a replay, just to test if maybe the wiring had a shortage or anything, but i’m not exactly sure if that’s a good idea. My battery is only 3 months old, and i had it tested and they said it was fine. i ran a million test with my mutlimeter…

I dont know, i need help



I used to own a 1989. Mercury Sable. The volt regulator was external to the alternator. I had the same problem and had to replace both the alternator and volt regulator. I don’t know if this is the way the Capri is set up.

“million test” ? what is this?

I’d check all the battery and alternator cables. There could be a bad ground connection, or a cable with internal corrosion.

Thank you for getting back so quickly!
Ive checked my battery terminals, and the ground on my alternator. Both are fine One of the wires that go to the plug on the alternator was black with corrosion but i cleaned it up pretty well, and traced it back to another plug and checked for corrosion there. i plan to replace this wire tomorrow, and see how it goes. my alternator has an internal voltage regulator, so both are new.

Does the battery voltage measure between 13 and 15 volts when the engine is running? With it being closer to 15 when the battery is partially discharged, like when it has sat in the cold outside overnight before starting the engine? And does the battery voltage measure around 12-12.5 volts with the engine off, measured after the car has sat overnight?

If so, and the voltage regulator is part of the alternator, I think what I’d do in this situation since pretty much everything is new, I’d take the alternator back to the place you bought it from and ask them to put it on their test jig, to see if it is working correctly. Most auto parts stores have a test jig for this purpose. If not, an auto-electric shop would have one.

Just a fyi, for the charging system to work correctly the cabling between the battery and alternator positive has to be very low resistance, on the order of 0.005 ohm. The only way to get that low of resistance is if every connection is perfectly clean and tight. Any corrosions or oxidation, no way will it work. Likewise for the connections between the battery negative to the chassis ground, and the alternator negative to the chassis ground. And since the alternator is probably grounded to the engine, not the chassis, the engine has to be well grounded to the chassis too. This was all done at the factory when the car was built, which is why it doesn’t cause problems in most cars for the life of the car. But if any of those connections have ever been removed, positive or grounding cables changed, etc, definitely something to suspect.

Edit: I should add one time my Corolla had similar problems, and I discovered a bad splice in the positive wire between the alternator and the battery. It wasn’t easy to find, buried in a thick wiring harness down in the middle of the engine. Not a fun job. I had to use a sharp knife to cut away the outer wrapping of the harness, then I just followed that wire as it wound its way from the battery toward the alternator. Lo and behold, there at the splice it was down to just 3 or 4 strands out of 20. What happened, the battery was leaking a little which I didn’t notice, and the battery acid was slowly decanting down that cable, since the cable was touching the battery. It followed the cable down, staying inside the harness so I couldn’t see it, until it hit that splice, where it arrived at bare metal and promptly ate the soldering gadget that joined the two thick wires. I replaced that gadget, properly re-joining the cables with a low-ohm connection, and the leaking battery, and it fixed the problem.

no battery cable is going to have 0.005 ohms of resistance

That’s not realistic

0.2 - 0.3 ohms is more realistic, and it would be acceptable

See if the engine is grounded to the body properly. I solved a few electrical problems by correcting that situation. If you can’t find the wire you can make one.

#0 aluminum is 0.0016 Ω/10 ft.
#2 aluminum is 0.0025 Ω/10 ft.

multiply by 2 and add in a bit for the clips, and you can hit 0.01Ω

0.3Ω and 200 amps starting current gives you 60 volts drop, obviously not workable.

0.01Ω and 200 amps starting current gives you 2 volts drop, which is good.

There’s no point in measuring ohms when dealing with electrical battery, charging, or starting problems. The results are useless. Volts is all that matters here.

When the trouble happens again check the voltage on the wires on the back side of the alternator. There is most likely two wires going there and they both should have close to battery voltage on them. One wire supplies power for the exciter of the alternator and is called the ‘L’ or lamp lead. Power comes from the ignition switch and passes through the battery warning light and then to the alternator. It is possible that the light socket may be dirty and is causing a resistance to the circuit. The other wire is the ‘S’ or battery sense lead that tells the regulator what the charge status of the battery is. When the battery voltage goes low the regulator tells the alternator to produce more current to compensate for the load on the battery.


With all due respect, you will NEVER encounter a battery cable that only has 0.005 ohms of resistance

I’m not a scientist, engineer, researcher, etc.

But I do work on vehicles for a living, and I know what I’ve seen and measured

I believe you’re thinking about this in a hypothetical sense

Whereas I’m talking real life

Don’t take this the wrong way, but if YOU were measuring resistance of wiring in a vehicle, you’d condemn the whole wiring harness, front to back, according to your unreasonable expectations

Is the old school method of testing voltage drop and residual voltage obsolete these days? I seem to recall learning to operate a VAT-28 quite a few years ago and the volt meter could find most charging and starting problems by moving the leads to significant locations in the system being tested.

Is the old school method of testing voltage drop and residual voltage obsolete these days?

Apparently. Now we have people talking about ohms testing wiring, which is useless. A VAT28 or VAT40 or other carbon pile tester would be nice here but all that’s really needed is a simple voltmeter.

A VAT 40 was much simpler to connect when testing for starting and charging amps but I preferred the VAT 28 for most other testing because it seemed much easier to handle. The only thing handier was the red/green LED test light but actual voltage was often needed.

11:55AM #0 aluminum is 0.0016 Ω/10 ft. #2 aluminum is 0.0025 Ω/10 ft.

Maybe I am way off base here, forgive me if I am. But aren’t the battery cables and wiring to the alternator copper? What does the resistance of aluminum have to do with this?


Resistance of anything–copper, aluminum, gold plated-- has nothing to do with this problem as resistance values are meaningless. All that matters is available voltage and current flow. Measuring resistance will only lead you down blind alleys and give you false hopes. It’s a waste of time.

Go to the auto parts store and buy a roll of 14ga copper stranded automotive wire. Strip 3 feet of that wire, untwist it, and remove just one small strand of that wire. Measure the resistance of that length of wire, it should be close to 0. Now remove the positive cable that runs from your battery to your starter and install that one strand in its place. Try to start the car. It won’t start, will it? But since the resistance of the wiring is almost 0 ohms and the battery is known to be good, the car must have a bad starter, right?

I was looking at the wire resistance numbers and something didn’t seem right to me either. Let’s say there is only a 10 amp draw on the battery with a wire loss of .2 ohms of resistance. That means a 2 volt drop on the wire. Not real good for use on a 12 volt charging system.

The specs for an 8 gauge wire (pretty reasonable size for the charging wire of a car) is about 0.63 milliohms per foot. So a 10 foot length would be about 6 milliohms. A much more practical value for wire loss.

The problem is resistance through a poor connection changes with the load carried. The wires have crimped or soldered connections at the ends of them, which are subject to wear and corrosion over 40 years. A funky connection may show very low resistance unloaded, but have a significant increase in resistance when trying to carry a 30 amp load. That’s why all automotive electrical testing is done with components on the car and the system operating.

db4690: it takes special equipment to measure resistances less than 1 Ohm. an ordinary ohmmeter will not do. Using a voltmeter to measure voltage drop is a good alternative.