2001 GMC Jimmy Stalling and Dying Out Mystery

Hi Brainoutmb. Welcome to the forum. On older vehicles they had a lot of trouble with loss of ground for the fuel pump due to rusty gas tanks and mounting straps. The fix was to drill a small hole on the flange joining the top and bottom sections, outside of the seam weld, and attach a ground wire with a self-tapping screw. On newer vehicles with plastic gas tanks, I've been reading a lot of stories about corroded ground wire terminals on the body. When the stalling occurs again, try to observe if you can hear the pump running. They're usually real easy to hear on GM trucks but have a helper turn the ignition switch on while you listen under the tank if you have to. It should only run for a few seconds when the ignition switch is turned on.

Caradiodoc

This is just a guess, but if we were to assume the two things are related, I would be looking at the generator. GM went from the world's second best generator to the worst pile starting with 1987 models. Even when they keep the battery charged, their design makes them prone to developing voltage spikes that interfere with computers and the signals coming from sensors.

If you can catch it while the problem is occurring, use a cheap digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it stays around 12.0 to 12.6 volts, the generator is not working. You would have most likely had starting problems by now. Computer problems typically don't show up until the voltage drops to 10 or 11 volts.

If the voltage is within proper limits, unplug the connector on the side of the generator towards the rear of the case. If engine performance improves, suspect excessive voltage spikes as the cause. Before you jump on a new generator, we should look closely at the battery. It is very common to go through four to six of these generators in the life of the truck, but what more and more professionals are finding out is the repeat failures can be reduced by replacing the perfectly good battery with a new one. As the battery ages and lead flakes off the plates, its "internal resistance" goes up. Basically that means it still has the capacity to start the engine but it loses its ability to dampen those voltage spikes. The battery will work fine in an older car or truck without so many computers or that generator design, but it won't dampen the voltage spikes that are produced even by a new generator.

The voltage spikes come from the operation of the voltage regulator built into the generator. To reduce heat, it turns full on and full off 400 times per second. VCR power supplies work the same way. They are called "switch mode" power supplies. Since the current flow through a coil of wire is being switched off instantly, it naturally develops a big voltage spike. That is how an ignition coil develops a spike that can be transformed into thousands of volts to fire a spark plug, but those spikes aren't so welcome when they transfer magnetically into sensor wires. The funny thing is the '86 and older GM generators and the Chrysler systems work the same way but they don't have voltage spike problems. This is strictly an '87 and newer GM thing.

I don't know if I'm leading you down the wrong road, but it is such a common, yet hard-to-diagnose problem, that it was the first thing that came to mind. One potential way to find it, besides the simple unplugging it test, is to drive it with a scanner connected that has record / playback capabilities. When the problem occurs, you press the record button. Since the live sensor data travels through the scanner's memory before it is displayed on the screen, the recording actually starts a few seconds before the button is pressed. That makes finding intermittent hiccups a lot easier. Later, when the recording is played back slowly, each sensor can be viewed to look for a glitch, spike, or dropout when the problem acted up. If a couple of sensors are affected at the same time, that would point to a generator problem.

Caradiodoc

Happy to hear you found a solution. I also chuckled that you are the first person to bring up the "generator vs. Alternator" story.

As a former instructor, I was very conscious of using correct terminolgy when explaining new or unfamiliar things. The humor and sarcasm were saved for when it would be understood and appreciated. To clarify, generators were big and clunky and their output came through brushes contacting the rotating member so you were lucky to get 30 amps on a good day. That often left batteries under-charged. Alternators take their output from the hard-wired stationary coil of wire and can easily put out over 100 amps. The terminolgy problem comes from the fact that Chrysler first used an alternator in 1960 and copyrighted the term. You will not find "alternator" used in GM or Ford service manuals. Since they produce alternating current, similar to what you'd find in a house, other manufacturers call them "AC generators". GM first used them in 1964 as I recall, and Ford around the same time.

To add to the confusion, beginning in 1996 with the government-mandated "On-Board Diagnostics" version 2, (OBD2), terminology is being standardized among manufacturers. Today every manufacturer uses the name "AC generator", or "generator" for short, so while your mechanic is correct, DC generators are long gone, the term "generator" is correct. Chrysler mechanics find the change hard to get used to because it implies this is just one more bit of changing technology they have to keep up with, but in fact the part is the same, just the name has ben changed.

In the future if you continue to use the term "alternator", every mechanic and parts salesperson will know exactly what you mean. Just because a team of industry thinkers tell us what to call something doesn't make it happen. I will be very happy if a new alternator solves your problem. :)

caradiodoc

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After rereading everything again, two things pop out. First of all:

"My dashboard voltmeter showed that the alternator was charging at about 12, not the necessary 14."

Has that problem been solved or does the voltage still drop when the running problem occurs? Since the generator has been replaced, if the voltage still drops, suspect the wiring to it. The "battery" warning light circuit sends voltage to the generator to tell it when to turn on. A common problem is loose connections on the ribbon cable connector on the back of the instrument cluster. Pressing on the cluster will often clear up the problem when the connector is loose.

The heavy output wire is pretty tough but look for a connector or fuse in the circuit. Really large fuses are bolted in and those bolts can become loose. Check any connectors for signs of looseness or corrosion.

If the charging system is working ok, we're back to the fuel supply system. Is it possible you would be willing to drive with a pressure gauge hooked under a wiper arm? I've done that a few times. It doesn't take long to become familiar with what is normal, and the pressure will change as intake manifold vacuum changes. The pump would be the logical choice but after two replacements, it's time to look somewhere else. Chrysler pumps almost never fail once they're running. They do sometimes fail to start up. GM pumps are noted for failing while driving but since yours has been replaced, if pressure drops when the running problem occurs, suspect a bad ground connection or a corroded connector in the wire feeding voltage to the pump motor. At the very least, if the pressure does not seem to be dropping to an abnormal level, that can be ruled out as the cause of the running problem.

Caradiodoc