Car wouldn't start. Mechanic saying it's a costly repair

My car would not start at all. When I turned the key. Absolutely nothing. Dash lights would briefly come on, but the turn of the key would result in nothing and then the key not returning to the final stop and unable to take key from ignition. When jumped, it ran fine, but the moment it was turned off, there was again no power to anything, not even enough to use the key fob to lock the doors.

The complete battery ensemble was replaced four months ago, so shouldn't be the issue.

I took the car to the mechanic today. He called to tell me the throttle body needed to be replaced. He however told me that in replacing this, there was a chance a computer/sensor/component could have also gone bad, needing to be replaced also.

Wondering several things. Is this common, what sensor/computer part could go along with the throttle body that he's quoting at upwards of $500 and knowing this is his recommendation, does it seem to be what will fix the problem?
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Tuesday, February 10th, 2015 AT 8:28 PM

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My first reaction is the diagnosis doesn't make sense given the symptoms, however, ... The engineers have gone insane with all the computers that are not needed and cause a lot of trouble. Your car uses the very dangerous "throttle-by-wire" system that put Toyota in the news a few years ago. That replaces an uncommonly reliable three-ounce throttle cable with an unreliable and overly-complicated five-pound computer, motor-driven throttle body, and sensors on the accelerator pedal. A lot of independent shops won't work on these systems yet until they develop a track record because they don't want to be party to any future lawsuits. GM doesn't like to share anything with any shops other than their dealers so we can only go by tidbits of information that trickle out and from classes we attend.

Almost everything on cars today have a computer involved with them, and they all talk back and forth over a pair of wires called the "data buss". If anything shorts that data buss, none of the dozens of computers can communicate, and most of the electrical system will be dead. You could have a no-start condition due to a power window problem, frayed wires between the door hinges, or worse yet, a defective master computer which is the Body Computer that is built into the radio. Yup, you could be walking due to a dead radio.

My reason for sharing this insanity is to show why the throttle body could potentially cause a no-start condition. Most of what I've learned in classes suggests a defective throttle body will cause a completely different set of symptoms, but until we become more familiar with the problems these computers can cause, I can't rule anything out, regardless of how far-fetched it may seem.

Now, to back up for a minute, there's two other things you should check first before agreeing to what is going to become common / expensive repairs. The first is to follow the smaller positive battery wire to the fuse box and be sure that connection is clean and tight. Since all manufacturers began using this design, they all have had a common problem with those wires working loose and causing intermittent electrical problems. Next, follow the smaller negative battery wire to the body and be sure that one is tight. The positive wire causes 99 percent of the problems, but the negative wire shouldn't be overlooked.

This next problem only applies to GM cars and trucks, 1987 and newer. Due to their design, the generators develop huge voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. The battery is responsible for damping and absorbing those voltage spikes. It is common to go through four to six generators in the life of the vehicle. To reduce the number of repeat failures, always replace the battery at the same time when you have to replace the generator, unless it is less than about two years old. You have that covered. As a battery ages, it loses its capacity to absorb those voltage spikes.

What may have happened is one of the six diodes has been damaged already before the battery was replaced. That will cause the generator to only be able to develop exactly one third of its rated maximum current, and that is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks. The clue to both of these problems is jump-starting got the electrical system working again.

You can do the first part of testing the charging system with an inexpensive digital voltmeter. Measure the battery voltage with the engine off, then with it running. With it off, a good, fully-charged battery will measure 12.6 volts. If you find closer to 12.2 volts, it's good but discharged. It will need to be charged at a slow rate for an hour. Next, with the engine running you must find between 13.75 and 14.75 volts. If it is low, the generator must be replaced because the voltage regulator is built into it, and it's almost impossible to replace separately without damaging other parts. The generator was not designed to be easily-repaired.

If the voltage is okay, you'll need a professional load tester to measure the maximum output current and "ripple" voltage. You didn't list the engine size so I can't look up which generator your car uses, but if, for example, you have a 120 amp unit, with a defective diode you will only be able to get 40 amps during the full-load test. That is also cause to replace the generator. Ripple voltage will be high too, and that leads to excessive voltage spikes.

Be aware too that if anything related to the computers needs to be replaced, the car will have to go to the dealer to have software installed. They sure don't do that for free, but your mechanic will take car of that.
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Tuesday, February 10th, 2015 AT 9:23 PM

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