Chevy Silverado Charging System
Repair Advice for a 1994 Chevrolet Silverado 1500 That Keeps Dying
Your auto repair questions answered about a 1994 Chevy Silverado with a charging system problem.
This weekly Q&A session answers your questions about cars and trucks. This week explores a 1994 Chevy Silverado 1500 with a battery that goes dead after a couple of days.
Question:
Hi, my name is Sallie. I was searching the Internet hoping to find some answers and came across this site. My truck is a 1994 Chevy Silverado 1500, ½-ton extended cab.
The problem is that the gauges go crazy, fluttering all over the place. The speedometer will just jump 20 to 25 miles in a second even when there is no acceleration. The wipers go on and will not go off, even if they are turned off. The gas gauge and the other ones drop to near the bottom and the worst effect of this is that the truck has to be jump-started.The problem is mystifying.
The alternator has been replaced twice, plus other work, by a Chevy dealer and a couple of other local repair garages; it's suspected that the computer may be at fault.
We bought a charger and charge it till it is fully charged and if we are lucky it will run for a couple of days, but as soon as the gauges start acting erratically we know that we will need a boost. We unplugged the computer and were wondering if that would have the same effect on an auto computer as it does with a home desktop computer. Does unplugging the computer reboot it?
We are out of options of anything except buying another vehicle, which isn't financially possible at this time. Please email me to let me know if you think this is a computer problem and if it is what we can do about it.
Thank you,
Sallie
Answer:
The problem with the gauges is a result of a low battery charge, so once the charging problem is fixed everything else will be fixed.
The charging system is a relatively simple system. This is how it works.
Circuit Description
The generator provides voltage to operate the vehicle's electrical system and to charge its battery. A magnetic field is created when current flows through the rotor. This magnetic field rotates as the rotor is driven by the engine, creating an AC voltage in the stator windings. The AC voltage is converted to DC by the rectifier bridge and is supplied to the electrical system at the battery terminal.
This generator's regulator uses digital techniques to supply the rotor current and thereby controlling the output voltage. The rotor current is proportional to the width of the electrical pulses supplied to it by the regulator. When the ignition switch is placed in RUN, narrow width pulses are supplied to the rotor, creating a weak magnetic field. When the engine is started, the regulator senses generator rotation by detecting AC voltage at the stator through an internal wire. Once the engine is running, the Regulator varies the field current by controlling the pulse width. This regulates the generator output voltage for proper battery charging and electrical system operation.
The digital regulator controls the VOLTS Indicator lamp with a solid-state lamp driver. The lamp driver turns on the lamp whenever undervoltage, overvoltage or a stopped generator is detected.
At this point you should prbably start at the beginning with a complete charging system and battery test. First thing to do is turn the key RUN but don't start the engine. Check to see if the Charge Indicator Lamp comes on.
Be sure the alternator is putting out about 90 to 100 amps at 2500 rpm under load. Volts should be between 13.5 and 14.5 DC.
If this is correct then have a look at the battery. A weak battery or one with an internal short can cause the problem you are having. The battery has to be fully charged in order to test it accurately. First, do a load test. Milton makes a nice 6-12V Battery/Starter/Charging Tester.
Connect the load tester and hold the load switch for ten seconds while watching the needle on the tester. It should stay in the green zone. Ignore the results of the first test. Wait for the tester to cool down, it gets quite hot, and then repeat the test. This one counts. It should stay in the green zone while the load is applied. If it dips into the yellow or red zone, you need a new battery. You can use this tester to put a load on the alternator to check the amperage output also.
Now, disconnect the negative battery terminal and put an ammeter inline between the negative battery terminal and the negative battery post. Shut everything down and take the key out of the ignition and make sure all the doors are closed. Wait a couple of minutes and see what the ammeter reads. The maximum allowable draw is 0.050 milliamps. Any more than that you have a drain on the battery.
This will tell you exactly where the problem lies. Remember to make sure the Battery Light comes on. There is a reason for that. Remember in the description where it says, "When the ignition switch is placed in RUN, narrow width pulses are supplied to the rotor, creating a weak magnetic field"? Well the pulses come through the Charge Indicator Lamp. So if the bulb is burnt out or there is an open in the bulb circuit, the alternator won't charge. Some guys spend, literally, hours trying to figure out why an alternator won't charge simply because they assumed the bulb was good.
Another thing is if the first alternator was replaced by one from the same source as the first, it may be a bad batch of alternators. It happens at times. Getting another brand of alternator from a different source will eliminate this possibility.
There is a lot more information in the Electrical Trouble Shooting guide that will be of help to you.
That’s all for this week. Feel free to browse the archives for past topicsand other resources. And, as always, if you are stuck and need an immediate answer you can always call Vince on the phone.
