1998 GMC Sierra Truck cranks slower and slower, not wont st

Hi cman49ers. Welcome to the forum. Batteries run down slowly, as you described, because they are not being recharged during driving, not because they're defective. This is typical of a generator problem. A couple of things to note about your vehicle. GM went from the world's second best generator to the world's worst design starting with the '87 models. It is not uncommon to go through four to six of them in the life of the truck. One thing that more and more professionals are finding out is repeat failures can be reduced by replacing the perfectly good battery at the same time. That old battery might work fine in older cars and trucks but as they age, they have a reduced ability to absorb the voltage spikes that these generators produce.

Another thing GM loves to do is package things into large assemblies. This makes assembling the vehicle faster and more efficient but it costs owners a lot more in repairs because you're forced to replace things that aren't necessary. In this case, the voltage regulator is built into the generator. It is a high failure item but replacement is unbelievably difficult. In fact, it is almost impossible to get the unit apart without destroying it. Further, there is no way to test the regulator to be sure it is the cause of the problem. The diode block is another potential cause of the problem. The solder tabs are often broken off when trying to get the wires off. Now you have two parts that can't be tested. Replacing both of them makes repair impractical compared to just replacing the entire generator. Since you're buying the regulator too, expect to pay a little more for a rebuilt unit compared to other brands, but it is important to compare suppliers' prices. My friend found one with a lifetime warranty for half the price as a different one with only a two year warranty.

The place to start is by measuring battery voltage with an inexpensive digital voltmeter, first with the engine off, then with the engine running. Battery voltage, when fully charged should be close to 12.6 volts. You might find less but 11.0 volts or less suggests a defective battery. Next, with the engine running, you must measure between 13.75 and 14.75 volts. Less than 13.75 volts means the generator is not working properly.

Being able to restart your engine four times is probably because your battery is so new. It is running the electric fuel pump and a whole bunch of computers while you drive. Many years ago you could run half a day on just the battery, or around two hours with the head lights on. Today you're lucky to get an hour before the computers start to shut down or do weird things.

Caradiodoc

It's not the starter. There was very likely a bad connection on one of the jumper cables because of the side post terminals. They are not big enough to pass the amount of current required by the starter. Instead, put a battery charger on it and set it to slow charge for a couple of hours. You'll see it starts just fine. To be sure the connections are good, first turn on the head lights, (or watch the under-hood light), then plug in / turn on the charger. The head lights should brighten noticeably. If they don't, wiggle the charger's cable clamps. Watch out for sparks when you do that when the charger is on. Batteries give off hydrogen gas which is explosive.

Caradiodoc