I have a 01 suburban and I have recently noticed the lights will dim when I apply the breaks or the radio or heat/ac is on. I tested the alternator and they said it was fine but can the factory installed alternator not have enough power? Also my fuel gauge decided to quit working about a week ago I was reading that a gasket in the fuel pump might have rotten away cause of the heat has anyone else had this problem. Thanks for any advice 
It’s not that uncommon for a fuel gauge to either completely stop working and show empty all the time, or not register correctly. The cause is often what’s called the “sending unit”, a gadget residing in the fuel tank which senses the fuel level. Gasoline can damage the sending unit over time. Neither of my 40 year old truck’s fuel gauges are accurate, so I put a mark at the spot on the gauge where the tank is empty. That works for what I need. If you want yours to work accurately and correctly, that’s a problem any shop should be able to solve for you.
I presume you mean the headlights dim when you apply the brakes or turn the radio on. That’s unusual since those functions don’t use much electrical power, and indicates something is probably amiss. The factory alternator should have no problem keeping the headlights bright while you listen to some tunes on the radio. But I wouldn’t assume the problem is the alternator. Possible, but not likely. Have your shop check for a weak battery by performing a load-test on it. And ask them to clean and tighten the battery connections. If the problem remains, have them test the charging system.
If the blower motor is on its highest setting, it will load down an alternator at idle and that will dim your lights. If you have an aftermarket audio system with huge sub-woofer amps, that will load down a factory alternator as well. Other than that, I agree with George.
After I gave my 1984 Impala to my son, he retrofitted it with a killer stereo system with a huge boom box in the trunk. It overtaxed both the battery and the alternator and he had to upgrade to “police and taxi” equipment.
Do you sometimes hear a chirp when making sharp turns? Your belt might be slipping.
There’s a loose/corroded connection somewhere with high resistance.
Start by checking ground straps.
If you’ve just started having problems the alternator was big enough for 14 years. Did you have the battery tested? How old is it?
The factory installed alternator was strong enough for all these years, it’s strong enough now. Have the battery tested. If it tests weak, change it. If not, you either have a high resistance short to ground somewhere or the alternator was not tested properly under load.
Let us know how it goes. We do care.
Many modern alternator implementations use the existing on-board computer systems to regulate the alternator output. Not all of these implementations are created equal. Some have noticeable lag between onset of load and recovery of the charging system. This can lead to momentary dips in bus voltage that result in momentary lamp dimming. That does not mean the alternator is undersized. It is an annoyance only.
That being said, I echo the prior sentiments. You have a 14-15 year old vehicle. Many things can deteriorate over that time that need to be checked out. Poor ground bonds, other wiring issues, accessories drawing way more power than supposed to, even poorly connected head lamps that are now susceptible to slight dips in bus voltage. A proper diagnosis needs to be done…
The fuel sender on those trucks is pretty sensitive to contamination. If the fuel gauge is just sitting dead on empty then, yes, a wire may be broken. The only way to fix it is to drop the fuel tank and repair the broken wire or replace the sender. If the gauge is just wrong, i.e. showing 1/4 tank when you know it is full, the sensor has been gunked up. Run a big bottle of Techron through a fuel load and that usually cleans it up.
If the sender is broken, the sender and pump are a single unit and if the truck has over 100,000 miles its close to replacement time, anyway. It is living on borrowed time and WILL fail when the tank is full - Murphy’s Law! The replacements are about $100-$150 depending on quality.
I’ve only had the car for the truck for about six months there is an after market stereo but nothing with massive speakers and such. The fuel gauge and the diming problems started within a week of each other and that was about a month ago. I will take it to see about my battery and then go from there. Thanks all for the suggestions
If you have only had the truck for 6 months, how do you know it still has the factory alternator? It could have a used one from who knows what. That said, your symptoms are classic for a weak battery. Go have your charging system checked out. I have seen many time an owner takes his car for service to a national chain and is told “you need a battery” or “you need an alternator”. Then if whichever one ts replaced doesn’t cure the problem the answer is always, " Your bad alternator killed your battery" or " Your bad battery killed your alternator."
What I have never heard is "Oh, we guessed wrong, we will put your old battery or regulator back in and take off that charge.
I am not saying that they can’t both go bad at once, but I have never experienced it in 60 years of working at gas stations in the 50s and working on friends and family’s cars as well as my own.
Best of luck OP. Let us know what you find out.
With the dimming lights and the fuel gauge not responding, I would say that you have a poor battery to chassis ground. Or the battery cables or terminals are corroded too bad to carry a heavy load.
Because you do not mention that it is having trouble starting, I would first locate the ground from the engine to the chassis/body. Remove the bolt or nut fastening it and clean the contact area with a wire brush, then replace the bolt/nut and snug it down.
Yosemite
Do these symptoms happen while you have a trailer hooked up, or do they happen all the time?
Sometimes a vehicle will offer a towing package that includes a larger alternator.
This truck has a group 78 battery with those pathetic side terminals
that might be part of the problem right there. Maybe the terminals are corroded or stripped. There may be corrosion “hiding” in the battery cables themselves. Peel back the red and black rubber covers and have a look
I had one place check it out and was told it was my battery however I was told by the guy my battery was so bad that my truck wouldn’t even start to get out of the parking lot which has never been a problem as far as starting my truck so I will be trying to test everything at a different place. I have the same problems when I’m towing as well. I will have the battery check again at a different place , and go through everyone’s suggestions again. Thanks so much everyone
I don’t know whether or not your particular GM has it or not, but many GM cars have a junction terminal near the battery. All electrical power for the vehicle goes through that terminal except the starter motor windings.
Corrosion or scale can affect starting, function of all electrical items, and charging of the battery by the alternator.
That terminal is usually located near the battery positive terminal and is on the inner fender, etc.
Just follow the cable from the battery + terminal and clean the terminal which is generally located under a small cover. Just something for consideration if the battery and battery cable ends are clean and undamaged.
Some auto parts places will check a customer’s battery free of charge. No harm asking anyway. A load test on a battery is easily done and not ambiguous. I expect if one shop has already done it for you and found your battery is shot, it probably is in fact shot. How old is the battery? Do you know? I understand your confusion that the shop said the battery was so dead the truck wouldn’t start, but you know it does start. Did they have an explanation why you were able to start your truck after the test was done. Perhaps they were exaggerating is all. A battery that doesn’t pass a load test could still start the engine. For a while anyway. Of course you know when it doesn’t, it will be 5 degrees, dark, and snowing. No harm done though to have the load test redone. Best of luck.