1994 Sentra starting problem

I have a 1994 Nissan Sentra w/ just under 150k miles on it that has had no mechanical problems until trouble starting the last 1 to 1.5 years. My mechanic initially put a new battery in it two separate times in less than a year (under parts warranty - he said living in FL, many cars need battery replacement in the summer heat here). He then said it needed a new alternator, which seemed to work only for a couple of months. Since July of this year, it has had failed to start and been taken to the same shop 3 times. In the last 2 months, they put in a second alternator, tightened a loose ground coming from the engine to the alternator (shouldn’t they have caught this a while ago?), and replaced the alternator again (under parts warranty). This last time, he admitted outright that they were stumped and replaced the alternator the second & third times in hopes that it was just a bad part. I took it directly to a friends mechanic to get a fresh perspective. The new mechanic kept it over the weekend to see how it sit w/out running. He then tested it earlier this week, telling me that the alternator was not putting out enough current to recharge the battery during & after running the car. This makes sense because the failures to start almost always happened after I had driven to work or a store & couldn’t get it home. His only suggestion was to ask my old mechanic what brand of alternator was being put in it (maybe a cheapo alternator is failing to perform up to specifications?). Is this a viable explanation? Since it’s been so frequent, the alternator is still under their parts & labor warranty.
Should I ask them to check for anything else specifically? Like I said, there has not been anything major wrong with it before (the power windows and lower speeds of the A/C blower have not been working for a few years now, but I’ve kept the windows up & A/C on high - it’s last working setting). When & how do I decide if this 14 yr old car is not worth fixing? Or is there a fixable explanation that makes it worthwhile to keep it running?

What is it doing when it doesn’t start? Have you measured the voltage across the battery?

Excessive resistance in the charging wire between the alternator output and the battery could cause the kind of trouble you are having, along with some other things. It shouldn’t be real difficult to find the trouble. You may want to take the car to a shop that specializes in electrical repairs and have them look things over.

Even though you live in an area with high heat the amount of battery and alternator replacements is way too much in my opinion. You should be able to fix the blower speed problem by replacing the speed control resistor pack located near the blower motor and mounted on the air ducting.

I had another mechanic look at it more closely, and he said that the alternator was slowing down to putting out an even 12-13 volts while running, not enough to recharge the battery. He said the previous mechanic probably only checked long enough to see that it was putting out more than 13. He checked belts and wires, etc. and found nothing else wrong. He said to ask the mechanic who did the warrantied work to try a different brand of alternator - I’ll also ask about excessive resistance in the charging wire. If this doesn’t pan out, he recommended a good alternator/electrical shop as you suggested. Thanks, I’ll let you know what happens.

To see if there is excessive resistance in the wiring between the alternator and the battery you just measure the voltage bettween the alternator output and the positive battery post while there is a good load on the charging system. If there is a problem then there will be an excessive voltage drop between those test points. You normally should have less than .25 volts of drop with a good load. The alternator output should be at least 13.8 volts with reference to ground. The battery grounding should be checked also to make sure it is ok.

On my Corolla the alternator puts out 14.5 volts when the engine is idling. 12-13 volts would be unusual indicating a problem with the alternator or the wiring harness, unless the battery was almost fully dead. It could be the battery is going dead overnight from a phantom current drain. Has your mechanic checked for phantom current drains to the battery when everything is off? That’s fast and simple to do.

If the alternator is putting out a low voltage to a good already charged battery, that’s usually a problem with the alternator itself. It may be you are just unlucky and getting poorly rebuilt alternators, one after another. If you can find a known good one, and install it as a test, that’d be something to consider. It’s not unusual to hear reports here of poorly rebuilt electrical parts, like alternators and starters. If all else fails, you could go to the dealership and purchase a new alternator.

Check the current drain. With these replacements repeated, it has to be a loss of power due to some drain. A high resistance should have been checked but I feel you just are drawing more power and someone did not check. Also verify that this car does not have a separate voltage regulator. It should not But…

I told these things to the first mechanic who did the repeated replacements - he looked again and said the AC was drawing too much. He said he disconnected the AC unit, put another new alternator in and made sure it was charging. I haven’t had any trouble starting since then - once in a while it’ll not do a thing when I turn the key, but it starts right up when I turn the key a second time.

There could be a problem with the safety switch in the starting circuit that is causing the no response issue. It may get worse over time. The ignition switch is another possibility also.